3/21/10 DISC: FL

[Jason Harris]

Glad you were there to get those pics Mike--that's the same cell I noted on the Nowcast that I referred to as "west of Fellsmere, south of Kenansville has exhibited meso, tvs" . . . did the storm ever get warned? I noticed before I left my computer that GR3 was suggesting 2 inch hail, though I guess especially in FL one has to reduce that by 1/3 to 1/2 or more.
*and Mike said it didn't get warned by the way. Now, that's something I'm curious about too. What's the likely reason for the weather office not putting a warning on that storm?
If Mike had called in the rotating wall cloud would that have made a distance? Was it the lack of a couplet? I know Mike didn't see a tornado, but seems like some storms have been warned based on less. What about the hail parameters? How relevant are GR3 hail estimated size with regard to NWS decisions to give warnings on hail? Maybe I'll email the Melbourne NWS office to see what they say.
**Went ahead and emailed the guy who did the spotter training here. He's usually willing to give some response--nice guy, so may update later.

 

[Michael Carlson]

I didn't call anything in because I did not see any strong rotation in the wall cloud feature. I did see some strong rotation in the upper part of the base but it quickly dissipated. As the storm got closer the wall cloud feature turned into more of a shelf cloud as the storm went outflow dominate fast.

I feel GR2 and GR3 over estimate hail sizes because I have been in numerous hail cores when the marker shows 3.00 and it turns out to be a large swath of pea size hail.

I think NWS did the right thing in not putting out a warning because as soon as that storm matured it got ingested into the squall line. To me I just feel it was severe enough to issue a warning.

 

[Jason Harris]

Ok, thanks for the additional perspective: so the rotation wasn't persistent enough nor in the wall cloud--got it. In that case, were you using the term "supercell" somewhat loosely? Would we call this storm having supercellular characteristics rather than having enough persistent rotation and depth of rotation to qualify? I can't help but see the definition of a supercell as somewhat ambiguous, even when attempts are made to be more exact like here:
--

I have already stated my definition of a supercell in several publications (notably, Doswell and Burgess 1993). That definition is that a supercell is a convective storm that possesses a deep, persistent mesocyclone. By "deep" I mean that the circulation meeting mesocyclone criteria is present and vertically connected through a significant (say, 1/3) fraction of the depth of the convective storm. By "persistent" I mean in comparison to a convective time scale defined by the time it takes a parcel to rise from the base of the updraft to its top (on the order of 10-20 min). A "mesocyclone" can be defined in many ways; I prefer to use the vorticity magnitude, where a "mesocyclonic vorticity unit" is 10-2 s-1.

http://www.cimms.ou.edu/~doswell/Conference_papers/SELS96/Supercell.html

 

[Jason Harris]

Here's what NWS at Melbourne had to say on the subject:

I spoke with our Radar Warning Meteorologist who worked the event you had questions about.

He explained that this particular cell was the most suspicious one of the day and the only one actually evaluated for possible (Severe Thunderstorms) Warning issuance.

The decision not to warn was based in part on the radar-based Vertically Integrated Liquid (VIL) parameter which had increased to 50-55 for one scan near or just north of the western Brevard/Indian River County line, but then collapsed on the very next scan (indicating that the updraft had strengthened, but then weakened substantially). The cell was one of many that developed in linear clusters in the higher warm/humid air ahead of the main squall line. A couple of these cells in Brevard and Indian River Counties briefly tried to "spin up" (aloft) as approached the coast, developing brief reflectivity pendants and brief moderate-strong low level Storm-Relative Motion (SRM) couplets, before becoming (rear-flank) downdraft (RFD) dominant as they moved offshore. The spin-ups all happened aloft (above 0.5 degree scan and never at the 0.5 degree elevation). The attempts at (brief, aloft) "spin-up"are a rather common occurrence and in this case (as with many others) occurred because there was enhanced surface inflow near the coast (stronger SE winds along the space/treasure coasts all afternoon along with a pseudo east coast sea breeze feature).

Concerning the hail algorithm, while we certainly do look at the output, it almost always indicates much larger values than are actually occurring. Values below 2 inches are typically of little concern, whereas the larger values cause it to analyze the vertical radar data within the storm closely. We don't have any warning "trigger" based on the hail size indicators alone. Normally when the probability of severe hail algorithm indicates over 80% and the hail size is over 2", there is a fair correlation with penny or larger hail (of course now our warning threshold is for quarter sized hail). I'd say that in most cases, a reduction of at least 50% (often more) in the hail size is necessary in the central FL environment (especially during the warm season).

The key is to look at the actual radar data (especially if you have GR2 or GR3) and don't over-rely on the algorithm output (whether hail, wind or TVS).

 

[Michael Carlson]

Ok, thanks for the additional perspective: so the rotation wasn't persistent enough nor in the wall cloud--got it. In that case, were you using the term "supercell" somewhat loosely? Would we call this storm having supercellular characteristics rather than having enough persistent rotation and depth of rotation to qualify? I can't help but see the definition of a supercell as somewhat ambiguous, even when attempts are made to be more exact like here:]

If I had to 'term/categorize' this particular storm I would call it a short lived strong thunderstorm. It exhibited supercell like structure and characteristics but wasn't the definition type of supercell.