View
215
Download
0
Category
Tags:
Preview:
Citation preview
Thunderstorm Ingredients
Thunderstorm Ingredients
©Oklahoma Climatological Survey©Oklahoma Climatological Survey
EarthStorm 2009EarthStorm 2009
What will you (hopefully) get out of this?
• Learn about the ingredients to a severe thunderstorm
• How to locate these ingredients on weather maps
• How you can forecast from the classroom or home
Ingredients for Convection: MILS
1.Moisture
2.Instability
3.Lifting (a boundary)
A 4th ingredient helps with long-lived, rotating storms:
• Shear
By “convection”, we mean thunderstorms.
Bork! Bork!
Ingredient #1: Moisture
• Moisture is the “fuel” for the storm
• Moisture for thunderstorms comes from lower levels– Surface, 850 mb are
typically checked• Typically looking for sfc
dewpoints above 50-55F• Some buzzwords:
– “tropical moisture”, “dew points”, “low-level moisture”, “theta-e”, “potential temperature”
(wxscript)
What areas are favorable for storms?
Where is the moisture coming from?
Again, we need a deep pool of moisture…
Ingredient #2: Instability
• Does the structure of the atmosphere support rising motion?– Relatively cool air aloft and relatively
warm air at the surface is unstable!– Large instability can mean stronger
updrafts
• Lots of buzzwords:– “CAPE”, “unstable”, “approaching
upper-level storm”, “shortwave”, “upper level system”, “piece of energy”, “vorticity max”, “height falls”, “trough”, “impulse”, “disturbance”, etc.
You know, they’d save you a lot of trouble if they just
said “trough”.
What is Instability?
TROUGH TROUGH
RIDGEA 500 mb map shows the state of the atmosphere “half way up”
Ingredient #2: Instability
• It doesn’t have to be a huge upper-level trough – “Shortwaves” that rotate
around a trough can sometimes be enough
– Very moist (and warm) air at the surface goes a long way to set up instability.
NWS products will provide a lot of guidance with this ingredient.
One Measure of Instability: CAPE
CAPE Value
Stability
Below 0 Stable
0-1000 Marginally Unstable
1000-2500 Moderately Unstable
2500-3500 Very Unstable
3500-4000 Extremely Unstable
CAPE values are usually calculated using sounding tools
Ingredient #3: Lifting Mechanism
• Instability is the engine, and moisture is the fuel, but we need a spark or trigger to get it started.– Boundaries are where storms form– A front, dry line or other surface boundary
can be an initiating “push” to get things started
– Mountains are good at this too!
• Some buzzwords:– “outflow boundary”, “front”, “convergence”,
“dry line”, “surface low”(wxscript)
Air Masses
• Source Regions– Generally flat and uniform composition– Light surface winds– Dominated by high pressure
• Arctic plains, Subtropical oceans, Desert regions
• Air Mass Types (named by their source region)– Four General Categories
• P Polar Source (also, A=Arctic)• T Tropical Source• c Continental Source• m Maritime Source
These deal with temperature
These deal with moisture content
ColdHotDry
Moist
Air Masses in N. America• cP (and cA) Continental Polar (or Arctic)
– Cold, dry, stable air (like after a cold front!).– Source: Canada and polar regions.
• mP Maritime Polar– Cool and moist (like Seattle or Maine).– Source: northern oceans.
• mT Maritime Tropical– Very warm, very moist (like a miserable July day in OK).– Source: Gulf of Mexico, also Caribbean & eastern Pacific.
• cT Continental Tropical– Hot, dry air (like after a dry line has passed).– Source: Deserts of Northern Mexico and SW United
States.
Air Mass Classification
A Front is:
• A boundary or transition zone between two air masses– Moisture content is often different across front– Wind characteristics are typically different– Temperature can change across the front
• Located in a pressure trough
• A focal point for generation of disturbed weather
Fronts
• Cold Front: Blue with sawteeth– Cooler air is advancing into a region of warmer air
• Warm Front: Red with “bumps”– Warmer air is replacing cooler air at the sfc
• Stationary Front: alternating warm/cold front markers.– Front is moving slowly. These are typically structured
like warm fronts
• Dry line: Brown with lots of bumps– Boundary between dry air and moist air
Cold Front
Can you spot the front?
Warm Front
Special Ingredient: Shear
•The first three ingredients give potential for storms … but shear helps them survive longer, and possibly rotate
•Shear: the turning and/or increasing of winds with height
Weak, southerly winds down low
Stronger, westerly winds up here!
Special Ingredient: Shear• Shear means winds that change as you go up
in the atmosphere– Speed shear: wind speeds change rapidly with
height– Directional shear: wind directions rotate with
height– Strong speed or directional shear (and especially
both) can support long-lived, rotating storms
• Some buzzwords:– “shear”, “veering”, “hodograph”
updraupdra
ft
DownDowndraftdraft
How can this be used at home?
• Build a composite map of ingredients– Look for dewpoints of 55F or greater– Find upper level (500mb) troughs and ridges– Look for lifting mechanisms (cold fronts,
drylines, sea breezes)– Is there shear for long lived storms?
How can this be used at home?
• Compare your answers to the “experts”!
http://www.spc.noaa.gov/products/outlook/
Recommended