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TORNADO FORECASTING In the U.S., only the National Weather Service (NWS) issues tornado forecasts nationwide. Warnings come from each local NWS office. The Storm Prediction Center issues watches, general severe weather outlooks, and mesoscale discussions. NOAA National Weather Service Chicago,
IL Weather Forecast Office This is a very simple question with no simple answer! Here is a very generalized view from the perspective of a severe weather forecaster: When predicting severe weather (including tornadoes) a day or two in advance, we look for the development of temperature and wind flow patterns in the atmosphere which can cause enough moisture, instability, lift, and wind shear for tornadic thunderstorms. Those are the four needed ingredients. But it is not as easy as it sounds. "How much is enough" of those is not a hard fast number, but varies a lot from situation to situation - and sometimes is unknown! A large variety of weather patterns can lead to tornadoes; and often, similar patterns may produce no severe weather at all. We just don't know. Tornado forecasting today and tomorrow is quite difficult already. Specific severe weather forecasting more than days in advance is little more than guessing, or using tornado climatology for the forecast area and time of year. For that reason, there is no such thing as a long-range severe storm or tornado forecast. There are simply too many small-scale variables involved which we cannot reliably measure or model weeks or months ahead of time; so no scientific forecasters even attempt them. Our farthest convective outlook is for day-3, and can be found on the SPC Forecasts page.
Each NWS forecast office uses output from at least one Doppler radar in the area to help to determine if a warning
is needed. Doppler radar signatures can tell warning meteorologists a great
deal about a thunderstorm's structure, but usually can't see the tornado
itself. This is because the radar beam gets too wide to resolve even the
biggest tornadoes within a few tens of miles after leaving the transmitter.
Instead, radar indicates strong winds blowing toward and away from it in a way
that tells forecasters, "An intense circulation probably exists in this
storm and a tornado is possible." Possible doesn't mean certain, though.
That is why local forecasters must also depend on spotter reports; SPC forecast
guidance on the general severe weather threat, and in-house analysis
of the weather situation over the region containing thunderstorms, to make the
best-informed warning decisions. By most measures, SPC has improved its tornado forecasting over the past few decades. There are many ways to objectively gauge forecast performance - for example, verifying tornado watches with tornado reports and both watch types by all severe reports. The general trend from 1985-2003 has been for a greater percentage of tornado watches to contain tornadoes. |