Severe Thunderstorm Summary METR 361

Severe Thunderstorm Severe Thunderstorm SummarySummaryMETR 361METR 361

The Pre-storm EnvironmentThe Pre-storm Environment• On the Surface map, look for:On the Surface map, look for:

1. Warm air, T > 50 F2. High dew points > 50 F3. “Triggers”

a. Cold frontb. Drylinec. pressure troughd. Outflow boundaries

850 mb map:850 mb map:

• Temperature RidgeTemperature Ridge

• Humidity RidgeHumidity Ridge

• Low level jetLow level jet

700 mb map:700 mb map:

• Dry air (intrusion) for convective instabilityDry air (intrusion) for convective instability

500 mb:500 mb:

• Cool air “pockets”Cool air “pockets”

• Difluent areasDifluent areas

300 mb, 250 mb, 200 mb:300 mb, 250 mb, 200 mb:

• JetsJets

Ageostrophic Ageostrophic flow sets up flow sets up capcap

Cap is broken Cap is broken when jet moveswhen jet moves

SoundingsSoundings

• T1T1Humid at low Humid at low

levelslevels

Dry above 700 mbDry above 700 mb

Capping InversionCapping Inversion

• Indices (CAPE, Indices (CAPE, LI, TT, SWEAT, etcLI, TT, SWEAT, etc …

Composite Charts are very helpful to Composite Charts are very helpful to assess the areas of likely severe assess the areas of likely severe

weather:weather:• By only plotting symbols of features By only plotting symbols of features

favorable to severe trws, “clutter = severefavorable to severe trws, “clutter = severe

NWS Warning ProcedureNWS Warning Procedure

• Convective outlook early in day (from Convective outlook early in day (from SPC)SPC)

Not for public warning purposesNot for public warning purposes

Often seen on the Weather Channel.Often seen on the Weather Channel.

•Watches (from SPC)Watches (from SPC)TornadoTornadoSevere ThunderstormSevere Thunderstorm

Warnings come from the local officesWarnings come from the local offices

Verification is done by local offices and Verification is done by local offices and reported to SPC and NCDCreported to SPC and NCDC

For straight-line wind only, look for For straight-line wind only, look for a. uni-directional shear (speed shear)a. uni-directional shear (speed shear) b. dry air to create downward b. dry air to create downward momentummomentumFor large hail, look for For large hail, look for a. large lapse rates in the hail growth a. large lapse rates in the hail growth zonezone b. Wet-bulb zero between 2200 and b. Wet-bulb zero between 2200 and 2800 m2800 m

(about 770 hPa to 720 hPa)(about 770 hPa to 720 hPa)For tornadoes, look forFor tornadoes, look for a. both speed and directional sheara. both speed and directional shear b. large lapse rates (CAPE)b. large lapse rates (CAPE) c. more ingredients to increase c. more ingredients to increase severityseverity