Numerical Models An Overview and Tutorial. Types of Models ► Short range models These tend to be more suitable for more specific features such as fronts,

Numerical ModelsNumerical Models

An Overview and TutorialAn Overview and Tutorial

Types of ModelsTypes of Models

► Short range models Short range models These tend to be more suitable for more specific features These tend to be more suitable for more specific features

such as fronts, temperatures, and convection. They are such as fronts, temperatures, and convection. They are considered non-hydrostatic. considered non-hydrostatic.

Forecasting for as little as the next 1 hour to as long as 3 Forecasting for as little as the next 1 hour to as long as 3 ½ days. ½ days.

► Long range models Long range models These are hemispheric or global models and are highly These are hemispheric or global models and are highly

skilled at wave patterns within the jet stream. However skilled at wave patterns within the jet stream. However they also have skill at synoptic features and an outperform they also have skill at synoptic features and an outperform the short range models at times! These are usually the short range models at times! These are usually hydrostatic or isentropic. hydrostatic or isentropic.

Forecasting out to as far as 15 daysForecasting out to as far as 15 days► For an understanding of terms such as hydrostatic For an understanding of terms such as hydrostatic

or isentropic, we encourage you to look outside this or isentropic, we encourage you to look outside this tutorial (google it!) to get an overview. tutorial (google it!) to get an overview.

So many models!So many models!

►RUC – Rapid Update CycleRUC – Rapid Update Cycle►NAM – North American MesoscaleNAM – North American Mesoscale

WRF-NMM, WRF-ARW and WRF-HRWWRF-NMM, WRF-ARW and WRF-HRW

►GFS – Global Forecast SystemsGFS – Global Forecast Systems►ECMWF – European Center for Medium ECMWF – European Center for Medium

Range Weather ForecastingRange Weather Forecasting►NGM – Nested Grid Model (being NGM – Nested Grid Model (being

phased out.)phased out.)

So many models!So many models!

►GEM – Global Environmental Multiscale GEM – Global Environmental Multiscale (Canada)(Canada)

►UKMET – United Kingdom UKMET – United Kingdom Meteorological ModelMeteorological Model

►NOGAPS – Navy Operational Global NOGAPS – Navy Operational Global Atmospheric Prediction SystemAtmospheric Prediction System

►Hurricane models such as the HWRF, Hurricane models such as the HWRF, GFDL, and WW3GFDL, and WW3

Let us narrow it downLet us narrow it down

► For the purposes of this tutorial we will only For the purposes of this tutorial we will only concentrate on the models most used here concentrate on the models most used here at NEXLAB. at NEXLAB.

► And they are ….And they are …. RUCRUC WRFWRF GFSGFS And to a certain extent the ECMWF and GEMAnd to a certain extent the ECMWF and GEM Oh, and there is ensemble forecasting, too. But Oh, and there is ensemble forecasting, too. But

we’ll get to that later we’ll get to that later

Rapid Update Cycle (RUC)Rapid Update Cycle (RUC)► The RUC is a very short range model with forecasts as short The RUC is a very short range model with forecasts as short

as 1 hour. as 1 hour. ► Many versions of the RUC run at places such as NCEP, FSL, Many versions of the RUC run at places such as NCEP, FSL,

local weather offices, and even colleges. The operational local weather offices, and even colleges. The operational model comes out of NCEP and is run every hour. model comes out of NCEP and is run every hour.

► Every three hours (0Z, 3Z, 6Z, etc.) it runs a forecast out to 12 Every three hours (0Z, 3Z, 6Z, etc.) it runs a forecast out to 12 hours. During the hours in between it forecasts only for the hours. During the hours in between it forecasts only for the next 3 hours. So, at 0Z you get forecast through 12Z but at 1Z next 3 hours. So, at 0Z you get forecast through 12Z but at 1Z you get a forecast valid through 4Zyou get a forecast valid through 4Z

► Generally used for the 1, 2, 3, 6, 9, and 12 hour forecast Generally used for the 1, 2, 3, 6, 9, and 12 hour forecast period but as mentioned before, many versions and suites of period but as mentioned before, many versions and suites of the RUC are run so this is not always the case. But this is what the RUC are run so this is not always the case. But this is what you see on our website here at COD. you see on our website here at COD.

► RUC Analyses often used for “analysis” data such as the SPC RUC Analyses often used for “analysis” data such as the SPC meso-analysis page so understanding the RUC can be vital in meso-analysis page so understanding the RUC can be vital in modern day forecasting. modern day forecasting.

Rapid Update Cycle (RUC)Rapid Update Cycle (RUC)

►The lastest version of the RUC (as of The lastest version of the RUC (as of September 2008) has a horizontal September 2008) has a horizontal resolution of 13km and 50 layers resolution of 13km and 50 layers through the depth of the atmosphere. through the depth of the atmosphere.

►Much information can be found about Much information can be found about the RUC at the RUC at http://http://maps.fsl.noaa.govmaps.fsl.noaa.gov//


This is the domain of the RUC-13km model. Data is taken a grid point spaced every 13km throughout this entire region (but only

this region!)


This is the 12Z RUC, 3 hour This is the 12Z RUC, 3 hour forecast of surface theta-e, valid forecast of surface theta-e, valid

at 15Zat 15Z.

North American Mesoscale North American Mesoscale (NAM)(NAM)

► The NAM is the operational short range model run by NCEP. The NAM is the operational short range model run by NCEP. ► The actual model itself is called the WRF (Weather Research The actual model itself is called the WRF (Weather Research

and Forecasting), and more technically the WRF-NMM and Forecasting), and more technically the WRF-NMM (Nonhydrostatic Mesoscale Model). (Nonhydrostatic Mesoscale Model).

► The WRF, like the RUC, is run different ways by different The WRF, like the RUC, is run different ways by different places. You may see references to the WRF-ARW (Advanced places. You may see references to the WRF-ARW (Advanced Research), too. Research), too.

► With all that being said, the NAM is the operational run and With all that being said, the NAM is the operational run and uses the WRF-NMM model for the basis of operational uses the WRF-NMM model for the basis of operational forecasting. If you see NAM or WRF, it simply means you are forecasting. If you see NAM or WRF, it simply means you are looking at the WRF-NMM model. looking at the WRF-NMM model.

► Confused yet? Don’t worry because the models are always Confused yet? Don’t worry because the models are always changing names. changing names.

► I encourage you to look at the WRF homepage at I encourage you to look at the WRF homepage at http://www.wrf-model.org/index.phphttp://www.wrf-model.org/index.php


► The NAM/WRF is run four times a day at 00Z, 6Z, The NAM/WRF is run four times a day at 00Z, 6Z, 12Z, and 18Z. 12Z, and 18Z.

► The NAM/WRF does a forecast for every 6 hours out The NAM/WRF does a forecast for every 6 hours out to 84 hours from the time it is run, or 3 ½ days. to 84 hours from the time it is run, or 3 ½ days.

► The NAM/WRF is set up as a non-hydrostatic model The NAM/WRF is set up as a non-hydrostatic model run at 12km of horizontal resolution and has 60 run at 12km of horizontal resolution and has 60 layers through the depth of the atmosphere. layers through the depth of the atmosphere.

► Further learning about the NAM resolution can be Further learning about the NAM resolution can be found at found at http://www.meted.ucar.edu/nwp/pcu2/namhres1.hthttp://www.meted.ucar.edu/nwp/pcu2/namhres1.htm m


This is the domain of the WRF-12km model. Data is taken a grid point spaced every 12km throughout this entire region (but

only this region!)


This is the 12Z WRF, 6 hour forecast This is the 12Z WRF, 6 hour forecast of QPF (precipitation), valid at 18Zof QPF (precipitation), valid at 18Z

Global Forecast System (GFS)Global Forecast System (GFS)

► The GFS is a short, medium, and long range The GFS is a short, medium, and long range model with forecasts out to 384 hours. model with forecasts out to 384 hours.

► The GFS forecasts for every 6 hours (00Z, The GFS forecasts for every 6 hours (00Z, 6Z, 12Z, and 18Z) out to 180 hours, then 6Z, 12Z, and 18Z) out to 180 hours, then every 12 hours out to 384 hours. every 12 hours out to 384 hours.

► At 180 hours the resolution is not as fine so At 180 hours the resolution is not as fine so the user will see less detail in products the user will see less detail in products beyond 180 hours. beyond 180 hours.

► The GFS has 64 layers throughout the depth The GFS has 64 layers throughout the depth of the atmosphere with a horizontal of the atmosphere with a horizontal resolution of 30km (though 180hrs). resolution of 30km (though 180hrs).

Global Forecast System (GFS)Global Forecast System (GFS)

The GFS is a global model run for the northern hemisphere (the domain is shown above) and for the southern hemisphere. This is one reason the GFS

is very skilled at wave patterns and overall jet stream flow.

Global Forecast Systems Global Forecast Systems (GFS)(GFS)

This is the 00Z GFS, 9 day This is the 00Z GFS, 9 day forecast of 500 speeds, valid at forecast of 500 speeds, valid at

00Z of day 900Z of day 9

The ProductsThe Products

►Speeds (jets)Speeds (jets)►Vertical Velocity and vorticity productsVertical Velocity and vorticity products►Temperature fields Temperature fields ►Humidity and moistureHumidity and moisture►Thicknesses and precipitation (QPF)Thicknesses and precipitation (QPF)► Instability and shear fieldsInstability and shear fields

The Products -- SpeedsThe Products -- Speeds► GFS (in this GFS (in this

case)case)► 500mb speeds500mb speeds► Jet maximaJet maxima► HeightsHeights► Upper lowsUpper lows► RidgeRidge► Wind barbsWind barbs► Color bar (key)Color bar (key)

LL

Products like this Products like this can be found on just can be found on just about every model about every model for major layers of for major layers of

the upper the upper atmosphere such as atmosphere such as

250mb, 500mb, 250mb, 500mb, 700mb, and 850mb. 700mb, and 850mb.

Some models will Some models will include a few layers include a few layers in between, as wellin between, as well.

The Products – Vorticity The Products – Vorticity ► Vorticity is a complicated thing to understand. For the basis of Vorticity is a complicated thing to understand. For the basis of

this tutorial we will only discuss the basics. You are this tutorial we will only discuss the basics. You are encouraged to learn more about vorticity on your own!encouraged to learn more about vorticity on your own!

► Essentially, vorticity is the spin of the air. The more the air is Essentially, vorticity is the spin of the air. The more the air is sheared, or spun, the higher the value of vorticity. sheared, or spun, the higher the value of vorticity.

► Vorticity is generally displayed on a 500mb map due to the Vorticity is generally displayed on a 500mb map due to the fact that 500mb is in the middle of the troposphere. fact that 500mb is in the middle of the troposphere.

► When looking at a vorticity map, the center of vorticity (the When looking at a vorticity map, the center of vorticity (the maximum value) is generally associated with the heart of a maximum value) is generally associated with the heart of a shortwave trough. It might be referred to as a “vortmax”shortwave trough. It might be referred to as a “vortmax”

► Areas downwind of this vorticity center (where vorticity is Areas downwind of this vorticity center (where vorticity is being advected positively) can be understood to be an area of being advected positively) can be understood to be an area of upward vertical motion. Consequently, areas upwind of the upward vertical motion. Consequently, areas upwind of the shortwave trough where vorticity is decreasing can be shortwave trough where vorticity is decreasing can be understood to have sinking air. Again, this is a simplified understood to have sinking air. Again, this is a simplified understanding of vorticity but does have practical understanding of vorticity but does have practical applications. applications.

The Products - VorticityThe Products - Vorticity

► GFS (for this case)GFS (for this case)► 500mb Vorticity500mb Vorticity► 500mb Heights500mb Heights► Vorticitiy Maximum Vorticitiy Maximum

(vortmax)(vortmax)► Shortwave troughsShortwave troughs► Color BarColor Bar

XX

X

X

X

The Products – Vertical The Products – Vertical VelocitiesVelocities

► The Vertical Velocity product (or VV’s or UVV’s for The Vertical Velocity product (or VV’s or UVV’s for upward vertical velocities) is simply a measure of upward vertical velocities) is simply a measure of how much the air is rising or sinking. how much the air is rising or sinking.

► This is a measure of overall rising air (synoptic) so This is a measure of overall rising air (synoptic) so even the fastest of vertical velocities is very small. even the fastest of vertical velocities is very small. This is not a measure of strong forcing as in a This is not a measure of strong forcing as in a thunderstorm. The measure is done in thunderstorm. The measure is done in microbars/second which is close to centimeters per microbars/second which is close to centimeters per second. second.

► Two main ingredients go into measuring vertical Two main ingredients go into measuring vertical velocities – temperature advection and vorticity velocities – temperature advection and vorticity advection. Warm advection and positive vorticity advection. Warm advection and positive vorticity advection are associated with lift, or rising air. Cold advection are associated with lift, or rising air. Cold advection and negative vorticity advection are advection and negative vorticity advection are associated with subsidence, or sinking air. associated with subsidence, or sinking air.

The Products – Vertical The Products – Vertical VelocitiesVelocities

► Like the other Like the other products, we are using products, we are using the GFS model here. the GFS model here.

► Areas of rising airAreas of rising air► Areas of strong sinkingAreas of strong sinking► Also, as with previous Also, as with previous

maps and slides, the maps and slides, the heights and color bar heights and color bar are located on here. are located on here.

Products like this can be Products like this can be found on most models found on most models

and at many layers and at many layers such as 500mb, 700mb, such as 500mb, 700mb,

and 850mb. The and 850mb. The default, or standard default, or standard layer, is usually at layer, is usually at

700mb for this product. 700mb for this product.

The Products - TemperaturesThe Products - Temperatures

►There are three main types of There are three main types of temperature products found on most temperature products found on most model outputs. model outputs. Basic air temperatures (T)Basic air temperatures (T) Wet-bulb temperatures (TWet-bulb temperatures (Tww) – used for ) – used for

precipitation type. precipitation type. Theta-e (Theta-e (ΘΘee) – a measure of total heat ) – a measure of total heat

(basically a combination of air (basically a combination of air temperature and moisture)temperature and moisture)

The Products - TemperaturesThe Products - Temperatures► GFS (in this case)GFS (in this case)► Surface (2 meter) Surface (2 meter)

temperaturestemperatures► Mean sea level Mean sea level

pressurepressure► Highs and LowsHighs and Lows► Wind barbsWind barbs► You can find fronts, You can find fronts,

troughs, and ridgestroughs, and ridges► Tropical systems, Tropical systems,

tootoo

H

L

L

L

All models have All models have some form of some form of temperature temperature forecast plot. forecast plot.

Generally you will Generally you will find this for the mid find this for the mid and low levels (500, and low levels (500, 700, 850, 925) and 700, 850, 925) and

the surface. the surface.

The Products – Temperatures The Products – Temperatures (Theta-e)(Theta-e)

Cool and dry air advecting

southward

Southeasterly winds

advect warmer and more moist air

An understanding theta-e is An understanding theta-e is moremoreeasily attainable when one has easily attainable when one has studied soundings and the studied soundings and the concept of parcels. What one concept of parcels. What one should realize, though, is that should realize, though, is that theta-e is a measure of the total theta-e is a measure of the total heat. The higher the value of heat. The higher the value of theta-e (measured in Kelvin in this theta-e (measured in Kelvin in this picture) the higher the heat picture) the higher the heat content. In this picture to the right content. In this picture to the right it is easy to see that a very cool, it is easy to see that a very cool, dry, continental air mass is taking dry, continental air mass is taking hold of the Great Lakes region. hold of the Great Lakes region. It is important to note that theta-It is important to note that theta-e values are often detailed at the e values are often detailed at the surface, 850mb (as in this surface, 850mb (as in this picture), and 700. For severe picture), and 700. For severe weather, we look for theta-e to weather, we look for theta-e to decrease with height. In other decrease with height. In other words, very warm/moist at the low words, very warm/moist at the low levels and much cooler/dryer air levels and much cooler/dryer air aloft. aloft. Concepts of theta and theta-e Concepts of theta and theta-e also involve motions of air such as also involve motions of air such as isentropic lift. We encourage all isentropic lift. We encourage all students to look further into these students to look further into these concepts on their own (or in a concepts on their own (or in a class!)class!)

Moisture ParametersMoisture Parameters

►Relative Humidity – generally done for Relative Humidity – generally done for upper air products, 250mb-850mb. upper air products, 250mb-850mb.

►Dewpoints – generally done for the Dewpoints – generally done for the lower layers of the atmosphere lower layers of the atmosphere (850mb, 925mb) and near the surface. (850mb, 925mb) and near the surface.

Moisture Parameters - RHMoisture Parameters - RH

► Relative Humidity plots are done all upper levels of the atmosphere. Relative Humidity plots are done all upper levels of the atmosphere. 250 – One might look for areas of RH to see where thick cirrus is being 250 – One might look for areas of RH to see where thick cirrus is being

forecastforecast 500 and 700 – The model world’s version of looking at a water vapor satellite. 500 and 700 – The model world’s version of looking at a water vapor satellite.

One can see dry intrusions, areas of vertical ascent and subsidence. And a One can see dry intrusions, areas of vertical ascent and subsidence. And a careful observer can note areas of convection blowing up within a dry careful observer can note areas of convection blowing up within a dry intrusion. intrusion.

850 and 925 – Forecasters can use this to see areas of thick stratus and 850 and 925 – Forecasters can use this to see areas of thick stratus and warm advection as well as general areas of vertical motion (which creates or warm advection as well as general areas of vertical motion (which creates or impedes cloud development.)impedes cloud development.)

► Streamlines, a snapshot of the wind flow, are also plotted along with the Streamlines, a snapshot of the wind flow, are also plotted along with the RH to help the user in the analysis. RH to help the user in the analysis.

Moisture Parameters – Moisture Parameters – Dewpoint Dewpoint

► Dewpoint temperatures (TDewpoint temperatures (Tdd) are generally plotted for the lower ) are generally plotted for the lower levels of the atmosphere such as 850mb to the surface. levels of the atmosphere such as 850mb to the surface.

► Surface dewpoint plots can be calculated in different ways of Surface dewpoint plots can be calculated in different ways of which the user needs to be aware. Here at COD, we do it two which the user needs to be aware. Here at COD, we do it two ways. ways. 2m T2m Td d – The “skin” layer of the surface. This is the model prediction of – The “skin” layer of the surface. This is the model prediction of

what the surface observations will record the dewpoint to be at each what the surface observations will record the dewpoint to be at each hour. hour.

0-30mb T0-30mb Td d – An average dewpoint of the lowest 30mb. This takes into – An average dewpoint of the lowest 30mb. This takes into account some mixing that goes on in the lowest part of the boundary account some mixing that goes on in the lowest part of the boundary layer. layer.

► Do not be surprised to see other parameters plotted along with the Do not be surprised to see other parameters plotted along with the dewpoints such as streamlines, wind barbs, lifted indexes, C.A.P.E, dewpoints such as streamlines, wind barbs, lifted indexes, C.A.P.E, etc. Dewpoint temperatures plots are vital to severe weather etc. Dewpoint temperatures plots are vital to severe weather forecasting hence the severe weather overlay plots and wind plots. forecasting hence the severe weather overlay plots and wind plots.

► Dewpoints are also very important in winter forecasting, Dewpoints are also very important in winter forecasting, temperature forecasts, general forecasting (clouds, etc.), among temperature forecasts, general forecasting (clouds, etc.), among other things. other things.

Moisture Parameters – Moisture Parameters – DewpointDewpoint

This is a great This is a great example of “moisture example of “moisture return”. At image one return”. At image one (at 12Z Nov 20) one (at 12Z Nov 20) one can observe a surface can observe a surface high pressure sitting high pressure sitting along the coast of along the coast of Louisiana and Texas. Louisiana and Texas. This is noted by the This is noted by the anticyclonic anticyclonic (clockwise) flow of air. (clockwise) flow of air. With time, this high With time, this high pressure moves to the pressure moves to the east, allowing the east, allowing the southerly flow on the southerly flow on the back side of the high back side of the high to bring moisture from to bring moisture from the Gulf of Mexico the Gulf of Mexico back into the eastern back into the eastern half of the US. half of the US.

Thicknesses and PrecipitationThicknesses and Precipitation► Most sites that provide model data, including here at NEXLAB, tend to display Most sites that provide model data, including here at NEXLAB, tend to display

thickness plots with a QPF (Quantitative Precipitation Forecast) overlay. thickness plots with a QPF (Quantitative Precipitation Forecast) overlay. ► What is thickness?What is thickness?

To properly understand thickness plots, students are encouraged to further look into To properly understand thickness plots, students are encouraged to further look into the hypsometric equation. A basic understanding is as follows: “The thicker the the hypsometric equation. A basic understanding is as follows: “The thicker the warmer”. Air expands when it is heated, and the atmosphere responds accordingly. warmer”. Air expands when it is heated, and the atmosphere responds accordingly. Higher thickness values mean a higher average temperature within that layer. Higher thickness values mean a higher average temperature within that layer.

A standard thickness plot is done for the 1000 – 500mb layer. However there are A standard thickness plot is done for the 1000 – 500mb layer. However there are many others and can be seen on our models’ page. many others and can be seen on our models’ page.

The “540” line is a first guess, rule of thumb, for rain vs. snow. 540 is short for 5400 The “540” line is a first guess, rule of thumb, for rain vs. snow. 540 is short for 5400 meters. That can, often, represent the thickness associated with an average meters. That can, often, represent the thickness associated with an average temperature from 1000-500mb that is cold enough for snow or ice. But beware – it is temperature from 1000-500mb that is cold enough for snow or ice. But beware – it is not very precise. not very precise.

► What is QPF?What is QPF? Be careful to understand that precipitation forecasts are based on precipitation that Be careful to understand that precipitation forecasts are based on precipitation that

has fallen has fallen and and not not precipitation that precipitation that is falling is falling !! The precipitation plot is for precip that has fallen either for the previous 3 or 6 The precipitation plot is for precip that has fallen either for the previous 3 or 6

hours. Be aware of which one you are looking at!hours. Be aware of which one you are looking at! Just because a model says it will have precip doesn’t mean it will have it. Model Just because a model says it will have precip doesn’t mean it will have it. Model

world does not equal reality. Models are tools, not gospel! Do not get caught up in world does not equal reality. Models are tools, not gospel! Do not get caught up in looking at QPF and basing your forecast completely on those plots. They are good looking at QPF and basing your forecast completely on those plots. They are good indicators but not perfect prognostications. indicators but not perfect prognostications.

Thicknesses and PrecipitationThicknesses and Precipitation► QPF along with QPF along with

color bar. color bar.

► 1000-500mb 1000-500mb thicknessesthicknesses

► The “540” line The “540” line (5400 meter (5400 meter thickness) is solid thickness) is solid yellowyellow

Using thicknesses to forecastUsing thicknesses to forecast► There are 6 There are 6

thicknesses plotted, as thicknesses plotted, as well as the 850mb, 0 well as the 850mb, 0 degree line.degree line.

► Also the average RH Also the average RH from 850-500mb. from 850-500mb.

► Each thickness rule of Each thickness rule of thumb rain vs. snow thumb rain vs. snow involves a different involves a different slice of the slice of the troposphere. troposphere.

► Again, line is based on Again, line is based on statistics that say statistics that say “given this thickness, “given this thickness, for this particular layer, for this particular layer, the average the average temperature generally temperature generally produces wintry produces wintry precipitation.”. precipitation.”.

► This is NOT the same This is NOT the same as doing an analysis as doing an analysis and not absolute fact. and not absolute fact. This is only one of This is only one of many tools for rain vs many tools for rain vs snow. snow.

Instability and Shear Instability and Shear ProductsProducts

►Many of these products involve a more in Many of these products involve a more in depth understanding of meteorology. depth understanding of meteorology.

►However, limited knowledge of these products However, limited knowledge of these products can allow a user to at least get a feel for the can allow a user to at least get a feel for the severe weather conditions possible. severe weather conditions possible.

►One is encouraged to further study severe One is encouraged to further study severe weather paramaters such as shear, instability, weather paramaters such as shear, instability, sounding analysis, hodographs, and other sounding analysis, hodographs, and other severe weather analysis tools and concepts. severe weather analysis tools and concepts.


► Instability products – how fast can air rise?Instability products – how fast can air rise? Convective Available Potential Energy (CAPE) – Measure in Joules Convective Available Potential Energy (CAPE) – Measure in Joules

of energy per Kilogram of air. The higher the number, the more of energy per Kilogram of air. The higher the number, the more unstable. Usually ranges from 25 (very low) to 5 or 6 thousand unstable. Usually ranges from 25 (very low) to 5 or 6 thousand (very high). (very high). ► (SBCAPE) Surface Based – air lifted upward from the ground(SBCAPE) Surface Based – air lifted upward from the ground► (MLCAPE) Mixed Layer – accounts for mixing within the boundary layer(MLCAPE) Mixed Layer – accounts for mixing within the boundary layer► (MUCAPE) Most Unstable (parcel) – lifts many parcels from within the (MUCAPE) Most Unstable (parcel) – lifts many parcels from within the

boundary layer then gives the value for the most unstable lifted air. boundary layer then gives the value for the most unstable lifted air. Lifted Index (LI) – the difference in the temperature within the Lifted Index (LI) – the difference in the temperature within the

cumulus, or thunderstorm, vs. the temperature outside the cloud. cumulus, or thunderstorm, vs. the temperature outside the cloud. The more negative the value, the more unstable. The more negative the value, the more unstable.

Lapse Rates or Delta-T’s – shows how fast the atmosphere is Lapse Rates or Delta-T’s – shows how fast the atmosphere is cooling with height. The higher the number, the faster the cooling with height. The higher the number, the faster the atmosphere is cooling off with an increase in altitude. atmosphere is cooling off with an increase in altitude.

(CINH) Convective Inhibition – how much will the air be (CINH) Convective Inhibition – how much will the air be suppressed? Values that are higher point to air that will be suppressed? Values that are higher point to air that will be prevent air from rising due to warm air in the mid levels of the prevent air from rising due to warm air in the mid levels of the atmosphere. Also referred to as a cap or a lid on the atmosphere. atmosphere. Also referred to as a cap or a lid on the atmosphere.


► Shear products – how much can the air spin?Shear products – how much can the air spin? Helicity – measures shear. The higher the number, the higher the Helicity – measures shear. The higher the number, the higher the

shear. shear. ► 0-6 km – an idea of “total shear” within a storm0-6 km – an idea of “total shear” within a storm► 0-3 km – concentrates on the lower part of the atmosphere0-3 km – concentrates on the lower part of the atmosphere► 0-1 km – can help forecast the possibility for tornadoes0-1 km – can help forecast the possibility for tornadoes

One should never take these products as gospel. Shear and One should never take these products as gospel. Shear and instability are very difficult to forecast and models are not very instability are very difficult to forecast and models are not very

skilled or precise when it comes to the placement or exact skilled or precise when it comes to the placement or exact numbers of instability or highly sheared areas. One should do a numbers of instability or highly sheared areas. One should do a very good analysis before ever looking at a model! Other model very good analysis before ever looking at a model! Other model packages such as BUFKIT (which we have here at the lab!) and packages such as BUFKIT (which we have here at the lab!) and RAOB do a wonderful job of showing a point by point vertical RAOB do a wonderful job of showing a point by point vertical

analysis of the atmosphere, and show CAPE and shear far better analysis of the atmosphere, and show CAPE and shear far better than just simple plots.than just simple plots.

A Convective ForecastA Convective Forecast

►There are three basic ingredients for There are three basic ingredients for convection (showers and thunderstorms). convection (showers and thunderstorms). Lift – fronts, mountains, sea breezes, Lift – fronts, mountains, sea breezes,

shortwave troughs, etc. shortwave troughs, etc. Instability – high CAPE values, large negative Instability – high CAPE values, large negative

values of LI’s, strong lapse rates, etc. values of LI’s, strong lapse rates, etc. Moisture – High values of dewpoints or mixing Moisture – High values of dewpoints or mixing

ratio in the low levels (the boundary layer). ratio in the low levels (the boundary layer).


First, lets us understand this is the GFS and it is a 5 day forecast so there are reasons First, lets us understand this is the GFS and it is a 5 day forecast so there are reasons to doubt this. For the sake of this tutorial, let us just assume this is going to be correct to doubt this. For the sake of this tutorial, let us just assume this is going to be correct or what we call “perfect prog” (perfect prognostication). or what we call “perfect prog” (perfect prognostication).

The above is a forecast for SBCAPE valid at 18Z and 00Z. This is a fairly low instability The above is a forecast for SBCAPE valid at 18Z and 00Z. This is a fairly low instability forecast, but there is enough instability for thunderstorms and severe with CAPE’s forecast, but there is enough instability for thunderstorms and severe with CAPE’s above 1000 forecast. What do I notice here? I see a bull's-eye of CAPE in the Texas above 1000 forecast. What do I notice here? I see a bull's-eye of CAPE in the Texas Panhandle at 18Z, then in southwest KS at 00Z. Clearly, the area of instability is in a Panhandle at 18Z, then in southwest KS at 00Z. Clearly, the area of instability is in a north-south line in the western high plains of the US. This should make a forecaster north-south line in the western high plains of the US. This should make a forecaster aware that thunderstorms are certainly possible in that region. aware that thunderstorms are certainly possible in that region.


This is the MLCAPE and Delta-T’s for 700mb to 500mb (one takes the temperature at a This is the MLCAPE and Delta-T’s for 700mb to 500mb (one takes the temperature at a point at 700mb then subtracts the 500mb temperature from that same point to get the point at 700mb then subtracts the 500mb temperature from that same point to get the Delta-T, or change in Temperature. Delta-T, or change in Temperature.

In this case, there isn’t much difference from the SBCAPE and MLCAPE. Sometimes In this case, there isn’t much difference from the SBCAPE and MLCAPE. Sometimes there is. However, at 18Z you can clearly tell that the extent of CAPE above 500 J/kg is there is. However, at 18Z you can clearly tell that the extent of CAPE above 500 J/kg is far smaller than in the SBCAPE plot. By 00Z, the MLCAPE and SBCAPE are far more far smaller than in the SBCAPE plot. By 00Z, the MLCAPE and SBCAPE are far more similar. Why would this be? Advanced students should really being to think about similar. Why would this be? Advanced students should really being to think about

why/how this could occur.why/how this could occur.


Another way to look at instability is by using the LI forecasting. This plot also can easily Another way to look at instability is by using the LI forecasting. This plot also can easily show you fronts based on where the LI gradient is located. Notice how packed the lines are show you fronts based on where the LI gradient is located. Notice how packed the lines are of positive LI’s (stable air) in Nebraska, for instance. Again, this plot can confirm where the of positive LI’s (stable air) in Nebraska, for instance. Again, this plot can confirm where the instability is. You can see the bulls-eyes and area of most unstable air. One can also note instability is. You can see the bulls-eyes and area of most unstable air. One can also note the bountiful southerly flow brining warm and moisture air into the Plains. the bountiful southerly flow brining warm and moisture air into the Plains.


The question now is … “Why is the CAPE so concentrated along the western high The question now is … “Why is the CAPE so concentrated along the western high plains? What is going on there? At this point one should have done an upper air plains? What is going on there? At this point one should have done an upper air analysis. For the sake of this tutorial we will simply show you the overall pattern at analysis. For the sake of this tutorial we will simply show you the overall pattern at 500mb. An analysis of this shows a very large, somewhat closed, upper low centered 500mb. An analysis of this shows a very large, somewhat closed, upper low centered in Montana, and lifting to the northeast. A strong jet max extends from the desert in Montana, and lifting to the northeast. A strong jet max extends from the desert southwest into the northern plains. One should being to get a picture of what is southwest into the northern plains. One should being to get a picture of what is going on. Notice the southern part of that jet is co-located with the area of CAPE you going on. Notice the southern part of that jet is co-located with the area of CAPE you have seen in previous slides. Now you know you have an area of maximum winds have seen in previous slides. Now you know you have an area of maximum winds aloft over an area of instability. aloft over an area of instability.


Now that we see what is going on in aloft we should take a look at the surface. Based on the Now that we see what is going on in aloft we should take a look at the surface. Based on the temperatures alone, we can see some sort of frontal moving through Nebraska, Colorado, and New temperatures alone, we can see some sort of frontal moving through Nebraska, Colorado, and New Mexico. An area of low pressure (a fairly weak one with pressures above 1005mb) exists in southwest Mexico. An area of low pressure (a fairly weak one with pressures above 1005mb) exists in southwest KS/southeast CO. A fairly elongated area of low pressure extends from that low down into New Mexico KS/southeast CO. A fairly elongated area of low pressure extends from that low down into New Mexico and extreme southwestern Texas (a surface trough). Temperatures are maximized at 18Z along the and extreme southwestern Texas (a surface trough). Temperatures are maximized at 18Z along the front back into the Low and in southwestern Texas. This front, low, and trough are serving as an area front back into the Low and in southwestern Texas. This front, low, and trough are serving as an area of convergence where air is forced upward. In other words, the area where CAPE existed, and there of convergence where air is forced upward. In other words, the area where CAPE existed, and there was a jet max, also has lift. was a jet max, also has lift.

Do you notice the surface winds in the Texas Panhanldle? At 18Z they are mostly southerly but at 00Z Do you notice the surface winds in the Texas Panhanldle? At 18Z they are mostly southerly but at 00Z they back (shift in a counter-clockwise direction) and our now out of the southeast? This increases the they back (shift in a counter-clockwise direction) and our now out of the southeast? This increases the shear as winds aloft, as you saw in the previous slide, are from the southwest! Those winds are shear as winds aloft, as you saw in the previous slide, are from the southwest! Those winds are turning with height. turning with height.

At this point, we now we have lift and instability. Now we need to look for moisture!At this point, we now we have lift and instability. Now we need to look for moisture!


Very strong southerly to easterly flow from the Gulf of Mexico and southern US is very Very strong southerly to easterly flow from the Gulf of Mexico and southern US is very evident on the streamlines plotted on this image. On top of that, dewpoints above 50F evident on the streamlines plotted on this image. On top of that, dewpoints above 50F exist in a large area (anything in the brighter green is 50+ Fahrenheit.) Dewpoints exist in a large area (anything in the brighter green is 50+ Fahrenheit.) Dewpoints exceeding 60F are actually quite abundant along the area of convergence along the fronts exceeding 60F are actually quite abundant along the area of convergence along the fronts and trough. It appears a feature known as a dryline is located from southwest Kansas into and trough. It appears a feature known as a dryline is located from southwest Kansas into southeastern New Mexico. Students are encouraged to look further into what this feature southeastern New Mexico. Students are encouraged to look further into what this feature is. Regardless, it is easy to tell where winds are “piling up” or converging, and that is what is. Regardless, it is easy to tell where winds are “piling up” or converging, and that is what is important. Those areas are located where the strongest instability exists, as well as is important. Those areas are located where the strongest instability exists, as well as under that 500mb jet stream maximum. under that 500mb jet stream maximum.


Cinh is simply the measure of stable air, or negative-CAPE, that exists which will impede Cinh is simply the measure of stable air, or negative-CAPE, that exists which will impede the progress of upward moving air. You will notice that at 18Z the cap (areas where cinh the progress of upward moving air. You will notice that at 18Z the cap (areas where cinh exists) breaks in the Texas pahandle into western Kansas. You will also notice this is exists) breaks in the Texas pahandle into western Kansas. You will also notice this is where, as noticed in previous slides, convergence at the low levels exists, there is plenty where, as noticed in previous slides, convergence at the low levels exists, there is plenty of moisture, shear, and instability. Is it possible that we could get convection? Could it of moisture, shear, and instability. Is it possible that we could get convection? Could it be severe? It appears that, while the instability is weak there is at least the possibility of be severe? It appears that, while the instability is weak there is at least the possibility of convection and could even be severe in nature. The cap gets stronger at 00Z but still convection and could even be severe in nature. The cap gets stronger at 00Z but still shows some weakness with low values. Advanced students should be thinking about shows some weakness with low values. Advanced students should be thinking about why this might be going on. why this might be going on.

So now we must find out how the model is handling all this information!So now we must find out how the model is handling all this information!


We have seen in the previous slides all the ingredients for thunderstorms that exist in We have seen in the previous slides all the ingredients for thunderstorms that exist in the western high plains. Now we check to see if the model is indeed forecasting the western high plains. Now we check to see if the model is indeed forecasting thunderstorms to develop. Can you see what the model is doing here? Precipitation is thunderstorms to develop. Can you see what the model is doing here? Precipitation is already ongoing in Nebraska at 18Z (remember that precip is accumulated from 12Z to already ongoing in Nebraska at 18Z (remember that precip is accumulated from 12Z to 18Z). Remember that front moving through Nebraska? Meanwhile, from 18Z to 00Z 18Z). Remember that front moving through Nebraska? Meanwhile, from 18Z to 00Z precip moves down the trough into Texas and New Mexico. Knowing what you know, precip moves down the trough into Texas and New Mexico. Knowing what you know, you can infer that this precipitation is likely in the form of showers and thunderstorms you can infer that this precipitation is likely in the form of showers and thunderstorms – convection. – convection.


► Convection that may be severe - S.L.I.M.Convection that may be severe - S.L.I.M. Shear – a change of wind speeds and/or direction over Shear – a change of wind speeds and/or direction over

some distance. This is best done by some sounding (model some distance. This is best done by some sounding (model or real data) analyses. But forecast helicities, or just an or real data) analyses. But forecast helicities, or just an analysis of the jets and low level wind flow can be analysis of the jets and low level wind flow can be sufficient as an overview of the potential shear. sufficient as an overview of the potential shear.

Lift – an ability to force the air upward. Fronts, sea breezes, Lift – an ability to force the air upward. Fronts, sea breezes, mountains, etc. Generally speaking with severe, lift will be mountains, etc. Generally speaking with severe, lift will be associated with fronts, drylines, surface troughs, and associated with fronts, drylines, surface troughs, and shortwave troughs. shortwave troughs.

Instability – conditions in place to allow air to rise on its Instability – conditions in place to allow air to rise on its own (air lifted must be warmer than the surrounding own (air lifted must be warmer than the surrounding preexisting air in place). Don’t forget to look at CINH, too, preexisting air in place). Don’t forget to look at CINH, too, which is an area of stability supressing convection. which is an area of stability supressing convection.

Moisture – sufficient moisture in place in the low levels of Moisture – sufficient moisture in place in the low levels of the atmosphere for storms. the atmosphere for storms.

Documents

Numerical Models An Overview and Tutorial. Types of Models ► Short range models These tend to be more suitable for more specific features such as fronts,