SPC's policy on watches?

[dmckemy]

So today has been a really active day across much of the southern/southeastern U.S. today, especially the Carolinas. The SPC had most of the Carolinas under a slight risk for the day, as well as many other portions of the U.S. However, if you take a look at the storm reports, there are a ton of wind/hail reports in the Carolinas (and several severe t-storm warnings have been issued during the day). One thing to note, though, is that there has been no watches issued at all for the Carolinas...and I don't even think an MD was issued either (please correct me if I'm wrong though). Parts of Texas and Oklahoma have had watches issued, as well as Colorado, and they have had their fair share of storm reports....

So I guess my question is, why didn't the SPC issue a watch for the severe weather going on across the Carolinas today? I believe that the local NWS offices work together with the SPC in issuing the watches (I think), but there was no watch issued at all! Just by taking a look at the meso-analysis data before the storms started firing off, CAPE was through the roof (over 4000+ in some areas), LI's were -8 (even saw one area with -10), moisture was plentiful, and there were several other favorable parameters...The one thing that lacked was wind shear in the atmosphere, but I guess all the other parameters made up for it.

This thread isn't aimed to bash the SPC...I was just interested in knowing how/why they issue watches and what the criteria must be. This has happened more than once, and I figured there must be some sort of guideline they follow before issuing a watch for an area.

Thanks for the help!

 

[Gerrit Gulden]

From experience, you wont always get a severe watch issued by the spc when severe is in fact occuring. The point that matters is that your local NWS issues the warnings. Was there any Mesoscale Discussions posted? Usually they will discuss whether a watch is warranted or not. Keep an eye on this page http://www.spc.noaa.gov/products/md/ when your having severe weather or expecting. You will find info is there is one posted for your area about what to expect and also if a watch is warranted or not. Usually if you see "Watch likely" then you can expect one, "watch unlikely" means they know severe is expected or occuring but they dont expect it to be too widespread to issue a watch.

Hope this helps...

 

[Jeff Duda]

That is a good question, dmckemy. Indeed, no watches or MDs were ever issued for anything close to the Carolinas yesterday, and the number of severe reports (and a look at the radar imagery for the area) seems to indicate a watch should've been issued.

I looked through the old mesoscale analyses, and I can possibly see why they may not have issued a watch, however. Despite very large CAPE, wind shear was nil. In fact, flow itself was very weak across the area, so possibly the forecasters thought that would limit the severe threat.

 

[Benjamin Rock]

Here is a link to a study done a couple years ago. It explains some of the methodology for issuing the alerts. You might be able to find a lot of answers to your question here..It's in PDF.

http://www.caps.ou.edu/reu/reu08/Final Papers/Keene_final_paper.pdf

 

[dmckemy]

Gerrit: Nope, no MD's were issued...just checked the archive and couldn't find anything.

Jeff: I do agree that their was a lack of wind shear in the area...I remember looking through the 0-6km and 0-8km shear, and there was practically nothing over the Carolinas. So in a way, I could see why they could have been hesitant to do anything at first...but a lot of the convection started firing off around 1-2ish, and the severe t-storm warnings kept going all afternoon through the evening. It wasn't like that only a few storms popped up and a couple of them were severe...storms were popping up everywhere and severe t-storm warnings were being issued like crazy. I would think that the SPC would at least have some sort of MD mentioning what was happening and what their thoughts were on the matter (like if they were planning to issue a watch or not), but like you said, there was absolutely nothing.

Benjamin: I skimmed through the article you posted, but really couldn't find their methodology for issuing MD's/watches (and the article mainly concerned tornadoes warnings/watches and not severe t-storm warnings/watches). The tornado lead-time information was very interesting though!

On a side note...I noticed that an MD was issued for SE Virginia/Eastern N.C. on the possibility of a watch being issued....what made them issue one today but not yesterday?

 

[Rich Thompson]

The most difficult watch decisions are those involving environments that are perceived to be "marginal" by the SPC forecasters, and the NWS forecasters agree with that assessment. We're usually less aggressive in the marginal cases, and occasionally we misjudge an event by deciding not to issue a watch early in the event.

I didn't work yesterday's event during the day, but I can confirm that the SPC forecasters weren't particularly impressed with the severe potential across the Carolinas area. Plus, I don't believe any of the local NWS offices contacted SPC to discuss the need for a watch.

One final note - the SPC reports page treats every report the same. Many of these summer "pulse severe" events consist of hail reports less than 1" in diameter and downburst winds that cover just a few square miles. In other words, it's tough to tell the difference on our reports page between a derecho with huge swaths of damaging winds, and a series of wet microbursts spaced about 1 per county.

These sort of events are a good reason to take an extended vacation during the summer