#10 Postby polarbear » Fri Jul 11, 2003 4:47 pm
As others stated, CAPE is a measurement of the positive buoyancy (upward motion) of air parcels in the lower atmosphere. The Lifted Index (LI) and Showalter Index (SI) are two other ways of measuring instability. Today, CAPE is probably the most popular method of measuring instability, with the Lifted Index not far behind.
As far as where to issue a watch, that depends on several factors. For both types of watches, meteorologists (the lead forecaster(s) at SPC) will almost always already know the general area of thunderstorm development early in the day (from looking at their convective outlook). They will monitor satellite and radar data, as well as hourly surface observations (lots of hand-drawn charts, looking at pressures, pressure tendencies, temperatures, dewpoints, wind streamlines) to determine where exactly storms may develop. There are no *magic* numbers when it comes to temperatures, dewpoints and other factors. Combinations of shear and instability that lead to severe storms in the winter, may be too weak to work in the summer.
Early signs will be in the visible satellite imagery and indications from surface observations and upper air data. Boundaries (fronts, outflow boundaries, sea breezes, troughs, windshift lines) play a big role in determining where the storms will develop (they provide focii). SPC likes to issue a watch at least ONE hour before the stroms form, and at least one hour before storms move out of an existing watch.
What type of watch? This depends on many factors, probably too many to mention here. For severe thunderstorm watches, you'd normally expect multicell or single cell thunderstorms, organized in some manner (squallines, clusters, mesoscale convective systems). In a severe thunderstorm watch, we generally expect a low tornado threat, but that doesn't mean that they aren't possible. Supercells sometimes do occur in these watches, or certain small scale (mesoscale) processes in and around existing storms could develop tornadoes. In these cases, it may be impossible to determine if tornadoes may occur, several hours before hand.
Tornado watches are issued on a similar basis that severe thunderstorm watches are issued, with one exception. This watch is usually issued when there is an enhanced threat of tornadoes (3 or more). You generally have much more directional (chance of wind with altitude) and speed shear (increase of wind speed with altitude) when tornadoes form, as well as small differences in the temperature and dewpoints at the surface. There may or may not be a great deal of instability in either type of watch.
Both watches may have valid times of 3 to 8 hours, with an average of 6. With tropical cyclones, tornado watches may have valid times of up to 12 hours.
There is also PDS (Particularly Dangerous Situation) watches. For severe thunderstorm watches, these are issued when there is an enhanced threat of widespread damaging wind gusts, usually greater than 65 knots (75 mph). These almost always are issued in association with a Derecho. For tornado watches, these will be issued when conditions support an enhanced threat of F2 to F5 tornadoes.
The text "in and close to the watch area" is included, since thunderstorms (which are out of our control) do not tend to follow man-made boundaries that we've made on a computer screen. SPC will try their best to put all areas where an organized threat of severe weather exists in a watch, but that does not always work out.
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I can litterally recite a severe thunderstorm watch ariel outline for all of NE KS. 
