High Helicity / Low CAPE Severe Wx Environments?

[NEXRAD]

Hi all,

Some technical research flavored questions to ponder...

While reading this morning's NWS Melbourne AFD I noted that the overnight forecast team's AFD mentions Friday Dec. 18th's set-up as exhibiting "a classic high helicity/low cape tstm environment." I've not seen this exact phrasing used before and was wondering what is meant by "classic" in this case?

Two additional questions:

1) Have there been any systematic studies of Florida severe weather environments with a specific focus on the relationship between CAPE and other factors? In my recollection, it seems that many events exhibit marginal instability and yet still produce significant severe weather (e.g. tornado outbreaks of 12/25/06 and 02/02/07).

2) While searching for information on the high helicity/low CAPE environment, I noted a website by storm chaser Jon Davies who worked with some NWS forecasters to examine such an environment. Davies makes the conclusion that forecasting low CAPE tornado environments might be enhanced by "development of CAPE/shear products that might highlight 'small CAPE/large shear' environments favorably distributed in the vertical for tornadoes (e.g., parameters combining low-level shear and CAPE below 3-5 km above ground?)." -- Has such a product been yet developed? Davies case study is from 2004.

Many thanks to any who can offer some insights or answer any of these questions!

- Jay
South Florida

[brunota2003]

One of the problems I can see dealing with forecasting for the Florida peninsula is the fact you are surrounded by warmish waters on both sides.

I know back home in eastern NC, we tend to have issues regarding low CAPE situations, especially with storms coming off the Atlantic. Those storms wouldn't last too long onshore (for us), but they would last long enough to produce a tornado threat for the coastal counties and the counties inland of the coastal counties. Granted, the storms would never drop a tornado that was too big (mainly waterspouts that would come ashore...so F0, maybe low end F1 at most *usually*).

Being down in Florida though, you guys also usually have more boundaries floating around for storms to fire up on (I dunno, about during the winter though). Whether it be the sea breeze during the day, or the land breeze at night or outflow from storms that push onshore.

Not that any of that answers your questions, sorry.

[NEXRAD]

Thanks for the reply, and no worries about not having an answer.

I'm considering sending these questions off in an email to a forecaster (or two) I know at NWS offices. I understand that a high helicity-low CAPE environment is more prone to inducing low-topped convection and perhaps shallow, more tropical-like supercells, but I'm curious to know whether there's been deeper research into this topic in conjunction with forecasting Florida (and from what you mention - North Carolina) tornado events. One note I have from watching radars tonight is that some of the cells offshore Broward County are exhibiting quite impressive couplets despite the helicity values being thoroughly unimpressive at the moment. Another note is that I've seen several events (the 2006 and 2007 events I mention in the original post and the February 22-23, 1998 tornado outbreak) where the storm cells are merely strong to marginally severe as they come onshore the Florida Gulf Coast. After reaching inland 50 to 75 miles, the cells kick-up and start rotating. It's a strange phenomenon. While I suspect boundaries might be to blame, the fact that the 1998, 2006, and 2007 event all transpired over a similar section of Florida with similar storm cell evolution just seems unusual to me.

- Jay
South Florida

[brunota2003]

I know of a few case studies that have been done on low instability cases that resulted in tornadoes, but no real outbreaks or anything. These were all in NC, still interesting reading (as are most case studies).

Most recent one (this was an inland event, no strong convection was expected):
http://www.erh.noaa.gov/mhx/EventReview ... 090327.php

Event from 2004:
http://www.erh.noaa.gov/mhx/EventReview ... 040607.php

Heavy shower produced a tornado in 2005:
http://www.erh.noaa.gov/mhx/EventReview ... 050625.php

Another heavy shower, turned into t-storm, tornado in 2004:
http://www.erh.noaa.gov/mhx/EventReview ... 040629.php