STORM2K

When the NWS issues a Severe Weather Watch, is there a specific combination of weather conditions that need to be in place before they issue it. I guess I'm looking to see if there's a minimum dewpoint/humidity/temperature level before it is issued.

My county is in the northeastern quadrant of a Severe Thunderstorm Watch box that's up until 9:00 pm. It made me wonder why the next county to the north of me is not included.

I figured with all of the great pro and amateur forecasters that we have on this board, I'd get some pretty good answers!

Thanks!

Remember what the weather radio says... "In and close to the watch area".

Stephanie, when SPC issues a severe weather watch (for severe thunderstorms or tornadoes) ... parameters such as shear at the different levels in the atmosphere, CAPE (convective available potential energy), lift, etc...play a factor in determining the best area of potential severe weather ... they'll draw basically a straight line from points to point which will include bisecting a county ... they'll have the entire county within the watch box area even if only a small percentage of the county is covered within the box that is drawn by the SPC ...

Since severe thunderstorms can and do occur outside of watch box areas during certain situations because the watches are placed within the best areas where severe weather is most likely, that's why the NWS/SPC always have a header that people in and close to the watch box area(s) should monitor for rapidly changing conditions ...

Hope this helps out.

SF

It would be impossible to issue a weather watch box to include every single location that will see a severe thunderstorm.

The highest risk in a certain area are outlined.

Thanks gang! I do remember the "In and around the watch area" announcements!

I guess that for an average person that doesn't understand CAPE values (or have access to look up), I was just wondering if they could reasonably assume that a watch area would be issued on that day if the other ingredients such as dewpoint, humidity and temperature were at a given level. I know that a coldfront could (and probably will) provide the energy needed to help destabilize the atmosphere and help them form. Are CAPE values somewhat associated with a coldfront?

Thanks for that answer...SF! You sure do know your stuff...

And David, I just noticed your radar avatar...looks like your area is getting pounded! :o

Very good answer Storms. The outline the SPC makes is a guidance only for the watch, or the area with the greatest threat so to speak.

One of the most common misconceptions I encounter are those who think that if you are under a Svr. T-Storm Watch that you can't have a tornado. This is certainly not true.

wx247 wrote:Very good answer Storms. The outline the SPC makes is a guidance only for the watch, or the area with the greatest threat so to speak.

One of the most common misconceptions I encounter are those who think that if you are under a Svr. T-Storm Watch that you can't have a tornado. This is certainly not true.

REMEMBER, A SEVERE THUNDERSTORM WATCH MEANS THAT SEVERE THUNDERSTORM WATCH MEANS THAT SEVERETHUNDERSTORMS PRODUCING LARGE HAIL, DAMAGING WINDS, DEADLY LIGHTNING, AND SOMETIMES TORNADOES, ARE POSSIBLE IN AND CLOSE TO THE WATCH AREA, KEEP ALERT FOR THREATNING WEATHER CONDITIONS AND BE READY TO MOVE TO SHELTER.

Yes, I remember that from listening to my Wx Radio for too many hours. I can litterally recite a severe thunderstorm watch ariel outline for all of NE KS.

Stephanie, CAPE is the amount of potential energy available for thunderstorms to develop ... of course, other factors are involved ... Lift, shear, CAPE by itself doesn't mean anything without other parameters favorable for thunderstorm development ...

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.

Thanks guys for all of the input! I appreciate it and I knew I'd get some very well thought out, intelligent answers from the members on this board!

Stephanie, for a Severe Thunderstorm Watch...there may not be a minimum temp/ dewpoint...if there's enough instability.

Here in metro Atlanta and West Georgia, I've witnessed severe storms with temperatures in the upper 40's...causing significant wind damage and hail. A few years ago, I watched large hail falling from the sky with a dewpoint of 45° (temp was 73°).

Tornado producing supercells in the southeast U.S. normally require a greater degree of low level moisture to produce significant tornadoes. In this area, very rarely have I ever observed tornadoes occur with dewpoint values below the 59-60° range. I have seen F3/F4 tornadoes occur on a couple occasions with temps and dewpoints in the 60-61° range...especially in January, February and March when upper systems and winds aloft are very strong.

However, the large tornado outbreaks such as occurred last November 10th, and on April 8-9 1998 most ofter occur when temps and dewpoint values are much higher....temps well into the 70's, and surface dewpoints well into the 60's (at the time of the 4/3/74 F5 tornado in Xenia, Ohio the temp/ dewpoint was 75°/64°. When the F5 occurred west of Birmingham, AL on 4/8/98 the temp/ dewpoint was 80°/70°).

Perry

Gotcha Perry!

I also remember last year when it was so hot I would wonder why we didn't get any thunderstorms even though it was pretty humid. That was mainly because all of the LP's and fronts stayed way to the north of us and that Bermuda High kept cranking. Nothing could come close to help stir the pot so to speak!