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

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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/