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Severe Weather FAQ Below are some
frequently asked questions designed to give you help for products in
the AccuWeather.com Severe Weather section. If you have a question
that we didn't answer, let us know! Note: for a lengthy list of
technical meteorological terms you may wish to visit the Weather
Glossary on AgriWeather
(click on Help/FAQs then pick "Weather Glossary" under "The
Weather.")
SPC
Products: These technical products are issued by the
Storm Prediction Center (SPC) in Norman, Oklahoma.
- Severe
Weather Today & Tomorrow - The Day-1
Convective Outlook, and Day-2 Severe Weather Outlook are
guidance products issued by the Operational Guidance Branch
(OGB) unit of the Storm Prediction Center. Both outlooks outline
areas in the continental United States where severe
thunderstorms may develop during the time period.
- Mesoscale
Discussion - When conditions actually begin to
shape up for severe weather, SPC sometimes issues a Mesoscale
Discussion (MCD) statement. If there is no current MCD
statement the last issued one is shown.
DIFAX
Products: These black and white fax maps come from the
National Weather Service. Because of the way they are sent, some of
the maps may be oriented incorrectly. DIFAX maps and charts are very
large and detailed images, so you will have to use the scroll bars
at the right side and bottom of your browser window to view the
entire image. You can
print
each map from your browser, but remember to reduce the size to fit
your paper.
- U.S. Radar Composite Chart - Depicts shaded
areas of precipitation over the continental U.S. Precipitation
cells and expected movement is shown. Weather Watch areas.
Administrative messages from National Meteorlogical Center.
Coverage Area: Continental U.S.
- U.S. Surface Plot- Chart shows surface high
and low pressure centers and frontal systems. Each reporting
station shows wind direction and speed, cloud cover, visibility,
temperature, dew point, precipitation type and amounts, and
barometric pressure. Outflow boundaries, the boundaries
of thunderstorm outflow air, which may travel far from the
original storms and could contribute to new storm development,
are sometimes noted.
- NGM 0 to 36 -hour Lifted Index - Four-Layer
Lifted Index. Four panel chart shows current and NGM forecast
12-hr, 24-hr and 36-hr maximum/minimum lifted indexes. See the
section below for detailed information on Lifted Indices.
AccuWeather
Severe Indices: Many indices can be useful for predicting
severe weather. The importance of one particular index varies from
situation to situation. The following are explanations of US maps
produced; these and other indices can be shown for each individual
Upper Air Reporting Station from across the world by viewing the
SKEW-T's.
For detailed information on SKEW-T's, visit the
Upper Air Sounding FAQ.
- Current Total Totals Index- This US map is
produced every morning and evening from upper air data. This
index determines the potential for thunderstorms and is very
helpful in diagnosing the difference between weak and strong
thunderstorms. The relationship between the TTI and Thunderstorm
Acitivity is as follows:
44 - Spotty, usually light
thunderstorm.
46 - Spotty, usually moderate
thunderstorm.
48 - Usually heavy thunderstorm.
50 - Usually heavy thunderstorm with hail possible.
52 - Usually heavy thunderstorm with a tornado
possible.
56 - Heavy thunderstorm with tornadoes
likely.
- Current/Forecast Lifted Index- The Current US
map is produced every morning and evening from upper air data.
The forecast maps are produced twice a day from the ETA and NGM
forecast models. This index helps predict overall atmospheric
stability. The lower the number, the more unstable the
atmosphere. Guidlines for interpreting the numbers are as
follows:
>+3: Stable atmosphere.
+3 to
+1: Slightly unstable, supports showers
+1 to -1:
Unstable, supports showers and isolated T-storms
-1 to
-3: Moderately unstable, supports widespread T-storms
-3
to -6: Very unstable, supports severe T-storms
<-6:
Extremely unstable, supports widespread severe weather.
- Current K Index- This map is produced every
morning and evening from upper air data. This stability index is
useful in predicting non-frontal thunderstorm situations.
Relationship of K-Index to probability of thunderstorms:
15-20:
< 19%
21-25: 20-39%
26-30: 40-59%
31-35:
60-9%
36-40: 80-89%
>40: >90%
- Other Severe Weather Indicies - Explanations
of other indices shown on individual SKEW-T's can be found in
the Upper Air Sounding FAQ
Text
Products:
- National Excessive Rainfall Outlook - This
product is issued by the Hydrometeorological Prediction Center
(HPC). It discusses the potential of Flash Flooding in the
United States based on current and forecast conditions. It is
usually in plain language, but standard US Postal Service
abbreviations and directional abbreviations are sometimes used
(SEWARD = Southeastward, ERN = Eastern, etc).
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