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 4 th 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?
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 ”.
TROUGH RIDGE A 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 0Stable 0-1000Marginally Unstable 1000-2500Moderately Unstable 2500-3500Very Unstable 3500-4000Extremely 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 PPolar Source (also, A=Arctic) TTropical Source cContinental Source mMaritime Source These deal with temperature These deal with moisture content Cold Hot Dry 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.
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
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”
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/