Severe Thunderstorm Summary METR 361

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Severe Thunderstorm SummaryMETR 361
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The Pre-storm Environment

• On the Surface map, look for

• Warm air, T gt 50 F
• High dew points gt 50 F
• Triggers
• a. Cold front
• b. Dryline
• c. pressure trough
• d. Outflow boundaries

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850 mb map

• Temperature Ridge
• Humidity Ridge
• Low level jet

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700 mb map

• Dry air (intrusion) for convective instability

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500 mb

• Cool air pockets
• Difluent areas

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300 mb, 250 mb, 200 mb

• Jets
• Ageostrophic flow sets up cap
• Cap is broken when jet moves

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Soundings

• T1
• Humid at low levels
• Dry above 700 mb
• Capping Inversion

• Indices (CAPE, LI, TT, SWEAT, etc

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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

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NWS Warning Procedure

• Convective outlook early in day (from SPC)
• Not for public warning purposes
• Often seen on the Weather Channel.

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• Watches (from SPC)
• Tornado
• Severe Thunderstorm

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Warnings come from the local offices
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Verification is done by local offices and
reported to SPC and NCDC
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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