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Severe Thunderstorm Summary METR 361

Severe Thunderstorm Summary METR 361. The Pre-storm Environment. On the Surface map, look for:. Warm air, T > 50 F High dew points > 50 F “Triggers” a. Cold front b. Dryline c. pressure trough d. Outflow boundaries. 850 mb map:. Temperature Ridge Humidity Ridge Low level jet.

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Severe Thunderstorm Summary METR 361

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  1. Severe Thunderstorm SummaryMETR 361

  2. The Pre-storm Environment • On the Surface map, look for: • Warm air, T > 50 F • High dew points > 50 F • “Triggers” • a. Cold front • b. Dryline • c. pressure trough • d. Outflow boundaries

  3. 850 mb map: • Temperature Ridge • Humidity Ridge • Low level jet

  4. 700 mb map: • Dry air (intrusion) for convective instability

  5. Cool air “pockets” Difluent areas 500 mb:

  6. 300 mb, 250 mb, 200 mb: • Jets Ageostrophic flow sets up cap Cap is broken when jet moves

  7. Soundings • T1 Humid at low levels Dry above 700 mb Capping Inversion • Indices (CAPE, LI, TT, SWEAT, etc …

  8. 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

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

  10. Watches (from SPC) Tornado Severe Thunderstorm

  11. Warnings come from the local offices

  12. Verification is done by local offices and reported to SPC and NCDC

  13. 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

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