Transcript Severe Thunderstorm Summary METR 361

Severe Thunderstorm Summary
METR 361
The Pre-storm Environment
• On the Surface map, look for:
1. Warm air, T > 50 F
2. High dew points > 50 F
3. “Triggers”
a. Cold front
b. Dryline
c. pressure trough
d. Outflow boundaries
850 mb map:
• Temperature Ridge
• Humidity Ridge
• Low level jet
700 mb map:
• Dry air (intrusion) for convective instability
500 mb:
• Cool air “pockets”
• Difluent areas
300 mb, 250 mb, 200 mb:
• Jets
Ageostrophic flow
sets up cap
Cap is broken
when jet moves
Soundings
• T1
Humid at low levels
Dry above 700 mb
Capping Inversion
• Indices (CAPE, LI,
TT, SWEAT, etc …
Composite Charts are very helpful to assess
the areas of likely severe weather:
• By only plotting symbols of features favorable to
severe trws, “clutter = severe
NWS Warning Procedure
• Convective outlook early in day (from SPC)
Not for public warning purposes
Often seen on the Weather Channel.
•Watches (from SPC)
Tornado
Severe Thunderstorm
Warnings come from the local offices
Verification is done by local offices and
reported to SPC and NCDC
For straight-line wind only, look for
a. uni-directional shear (speed shear)
b. dry air to create downward momentum
For large hail, look for
a. large lapse rates in the hail growth zone
b. Wet-bulb zero between 2200 and 2800 m
(about 770 hPa to 720 hPa)
For tornadoes, look for
a. both speed and directional shear
b. large lapse rates (CAPE)
c. more ingredients to increase severity