The Forecast Process

The Forecast ProcessThe Forecast Process

ATMO 4300ATMO 4300Spring 2010Spring 2010

Scales of MotionScales of Motion

Global (Planetary) ScaleGlobal (Planetary) Scale

Synoptic ScaleSynoptic Scale

MesoscaleMesoscale

Figure from www.atmos.washington.edu/2003Q3/101/notes

ITCZ

Average Pressure in January

Average Pressure in July

The Polar Front

Jet Stream Figure from www.crystalinks.com/jetstream.html

Seasonal Position of Jet Stream

Figure from www.earth.rochester.edu/fehnlab/ees215/fig17_8.jpg




MesoscaleMesoscale

Synoptic Scale

Synoptic Scale – 500 mbImage from www.rap.ucar.edu/weather




MesoscaleMesoscale

Mesoscale

Forecast MethodsForecast Methods

Forecasting Methods

Climatology

• Using long-term averages to forecast a particular weather element

• Example:• What’s the problem with using

Climatology?

Persistence

• Current Weather = Future Weather• Accuracy depends on: 1. Time scale 2. Progressive vs. stagnant weather pattern• Example:

Trend

• Extrapolating current weather out in time based on steady-state conditions

• Best for approx 0-6 hours (Nowcasting)• Example:• What’s the problem with this method?

Analogue

• Also called “Pattern Recognition”• Today’s weather similar to a past event• Forecast based on what happened with

the past event• Example:• What’s the problem with this method?

Analogue - Example

• Go to the following web site:• http://tropical.atmos.colostate.edu• Click on Forecasts• Click first “new” link• Scroll down to No. 5

Variations on Analogue Method

• Teleconnections – Relationship between weather in one part of the globe to events in another part

• Used in long-range forecasting

• Example: El Nino – connection between tropics and North America

El Nino

• What is it?

• An event associated with significant warming of sea surface temperatures in the eastern tropical Pacific Ocean.

Normal Sea Surface Temps

• Ocean temperatures are normally warmer in the west and colder in the east.

• Figures from www.cpc.ncep.noaa.gov

El Nino EventFigure from www.cpc.ncep.noaa.gov/products

Effects on AtmosphereFigure from www.cpc.ncep.noaa.gov

Misconceptions about El Nino

• El Nino or La Nina does not directly cause any one particular weather event.

Variations on Analogue Method

• Decision Trees / Checklists• Using common parameters associated

with many past events • “Rules of thumb”• Example:

Example of Decision Trees

• Class Home Page• Click link to All NWS Offices• Click Midland on the map• Click Office Information• Click Local Research• Click on 5th article

Numerical Weather Prediction

• Using computer models of the atmosphere to predict weather variables

• Model Output Statistics (MOS)• Ensembles

The Forecast ProcessThe Forecast Process

The Forecast Process

• With all the data and the different forecasting methods, how do you decide what to forecast?

• Rain or sunny• Snow amount

• Graphic from www.lib.umassd.edu/graphics/judge.gif

A Systematic Approach

• Start with the Big Picture

- Look at the hemispheric (or partial hemispheric) upper wind flow pattern

- Look at a large scale water vapor satellite animation

(current & recent history)


• Review Synoptic Scale - Surface map – cyclones/anticyclones, air

mass boundaries - Upper air maps – troughs/ridges,

PVA/NVA, temp advection, meridional vs zonal flow

- Radar/satellite – precip/clouds in and near forecast area

The Forecast Funnel


• Associate current weather with causes. Example: Evaluate moisture and lift for precip.


• What is the time period of your forecast? Example: 0 – 6 hours (Nowcast) Radar, satellite, profilers, detailed surface

analysis Example: Days 1 to 7 Numerical Weather Prediction models


• Choose a forecast methodology(s)• What weather elements am I forecasting

and what will affect those elements?• When forecasting different weather

elements, usually best to forecast temps last.

Summary

• What happened and why?

• What is happening and why?

• What is going to happen and why?

• Adapted from Lance Bosart, SUNY, Albany

Documents

The Forecast Process