DFW: Severe thunderstorms possible Tuesday Evening 4/26

I typically do not share model data because they can be very complex. I do want to show one of the state-of-the-art programs that I use in my operations to analyze model-simulated soundings. The models do not automatically compute these data; the additional software turns the raw output into meaningful information.

At least one of the midday models is indicating that convective available potential energy (CAPE) will exceed 4,000 J/kg by 7 pm tomorrow evening (Tuesday 4/26). This is astounding, and certainly considered “extreme” instability. However, extreme instability doesn’t always create the destruction that we build up in our minds. Many people focus too much on the magnitudes of one or two parameters, and miss the big picture.

Over the past two years, I’ve customized this software to display the parameters that I use most often when forecasting (vertical list on the right side). I have several custom lists that I use seasonally (my winter list is much different).

I’ve drawn a red box around four of these: A & B) storm-relative helicity, C) precipitable water content, and D) total CAPE…

Sounding Parameter Magnitudes and Expectations

A & B: Storm-relative helicity (Marginal)
C: Precipitable Water > 99th Percentile (Excessive)
D: Total CAPE > 4,000 J/kg (Extreme)

● If I have seen this setup once, I’ve seen it (and chased it) a hundred times.

● Marginal storm-relative helicity argues strongly against large/violent tornadoes, especially with slightly veered surface winds ahead of the dry line, which I discussed last week.

● Excessive CAPE certainly can cause very large hail, and I do expect to receive reports of up to baseball sized hail tomorrow to the west/northwest of D/FW.  CAPE is a measure of buoyancy; the larger the CAPE, the more buoyant air parcels will be… and the more buoyant they are, the more rapidly they will rise.

● Excessive CAPE can also create an atmosphere in which there are so many updrafts (thunderstorms) that form over a very short period of time that they eventually meld into a conglomerate (multi-cellular convective system)…. especially when storm-relative helicity is marginal.

● The severe weather algorithm in this software also identifies the “favored” storm characteristic as “multicells” (red box in the upper left).

● The final clue is the excessive precipitable water content that is well above the 99th percentile for April (based on a 65-year database).

Forecast sounding for Fort Worth valid 7pm CDT April 26, 2016
Forecast sounding for Fort Worth valid 7pm CDT April 26, 2016

Climatological Distribution of Tornado Intensity vs. SRH/CAPE

The following are probability distributions of tornado intensity as a function of storm-relative helicity & CAPE  (from Kerr and Darkow, 1996).

Mean storm-relative helicity values (m 2 s 0 2 ) for the F-scale subsets in the (a) 0 – 1-km layer, (b) 0 – 2-km layer, and (c) 0 – 3-km layer. (d) Mean CAPE values (J kg 0 1 ) for the F-scale subsets

Significant Thresholds for Forecasting Severe Thunderstorms

Forecast parameters for severe convection
Forecast parameters for severe convection

The Forecast for Tuesday Evening 4/26

● The supercells that form out west will tend to be high-precipitation (HP).

● As the thunderstorms congeal into a quasi-linear convective complex, and move into North Texas (between 6 pm and 10 pm), rainfall rates of up to 2 inches per hour can be expected. Large hail will be possible, especially with the leading edge and with lead cells that form ahead of the advancing line. The chance of rain is near 100%.

● I am forecasting 1 to 3 inches of rain (locally 4+ inches). Localized flooding will be possible.

● Damaging winds are possible with any convective system, particularly one that is utilizing 4,000 Joules/kg of convective energy. In fact, I have been considering the potential for bowing segments that can enhance the potential for sudden/intense downbursts.

● For North Texas, I still believe the tornado threat is low, but non-zero. It’s quite possible, that a Tornado Watch will be issued at SPC discretion, and I certainly can’t rule out an isolated tornado or two.

● In summary, my main forecast concerns are: destructive winds from bowing segments and downbursts, areas of large hail (but I do expect hail sizes to come down as the large convective system organizes — we will have to monitor lead storms that form in advance).

Next Update…

Source:  Kerr, B. W., and G. L. Darkow, 1996: Storm-relative winds and helicity in the tornadic thunderstorm environment. Wea. Forecasting,11, 489 – 505.

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