Transcript File

Thunderstorm
Ingredients
EarthStorm 2009
©Oklahoma Climatological Survey
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
By “convection”, we
mean thunderstorms.
Bork! Bork!
1.Moisture
2.Instability
3.Lifting (a boundary)
A 4th ingredient helps
with long-lived,
rotating storms:
• Shear
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
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 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
Cold•
Hot•
Dry•
Moist•
P
T
c
m
Polar Source (also, A=Arctic)
Tropical Source
Continental Source
Maritime Source
These deal with
temperature
These deal with
moisture content
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
Stronger, westerly
winds up here!
Weak, southerly winds down low
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”
Down
draft
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/