Possible HSLC Event Monday 28 February

Near the end of the conference call this past Thursday, Matt wondered (to paraphrase) if something could be done to help forecasters right before or during an HSLC event. I am still of the opinion that situational awareness is half the battle during one of these events. Having a realistic set of expectations about what sort of severe weather is most likely, and a corresponding set of conceptual models and radar signatures to look for, is critical to success. Anything we can do to increase situational awareness about what to expect and what to look for in the days or hours leading up to an event would be beneficial. Clearly, there is more that we can do to help forecasters prepare. For example, at my office, during briefings, some forecasters will continue to concentrate on storm relative helicity as an important parameter in HSLC environments, when bulk shear is probably a better indicator of the potential for damaging winds and brief tornadoes.

One of the things we can do for forecasters, prior to an event, is give them something to increase their situational awareness for the potential of a HSLC environment. A set of important parameters could be put together in a “bundle” that could be incorporated into a procedure for viewing model data in the AWIPs D2D. Here’s an example of a bundle that I have developed for that purpose (Figs. 1 and 2). The vectors and color fill is the 0-2.5 km bulk shear (after the Trapp and Weisman modelling study) and the yellow contours are the surface based CAPE.

Figure 1. Model forecast of surface based CAPE (yellow contours; J kg-1) and bulk shear in the 0-2.5 km layer (kts; vectors and color fill) from the 40-km GFS 0000 UTC 27 February 2011 model cycle and valid at 1800 UTC 28 February.

Figure 2. As in Fig. 1, except for 0000 UTC 1 March.

An AWIPS bundle such as this would be refined based on the outcome of the CSTAR research.

The upcoming event looks compelling. It is notable that nearly the entire HSLC study area is included in the Day 2 slight risk (Fig. 3) as of this writing.

Figure 3. Storm Prediction Center convective outlook for Day 2 issued at 0644 UTC on 27 February 2011 and valid 1200 UTC 28 February to 1200 UTC 1 March 2011.

Pat Moore, WFO Greenville-Spartanburg

About nws-pat moore

B.S. Meteorology, State Univ. of New York - College at Oneonta (1987) M.S. Meteorology, The Florida State University (1996) National Weather Service (12/3/90 to present), stationed at GSP since 8/16/98.
This entry was posted in CSTAR, High Shear Low Cape Severe Wx. Bookmark the permalink.

4 Responses to Possible HSLC Event Monday 28 February

  1. Pat,
    This does indeed look interesting, and this event appears to have greater potential for some instability as compared to Friday’s event, as both the 12Z/27th NAM and GFS show MLCAPE values of 200-400 J/kg and 0-3 km shear of 50-60 kts Monday afternoon (although this projected instability is a bit less than what is expected over the Tennessee Valley). Even heading into central and eastern NC/VA during the early/mid afternoon, SREF probabilities of effective bulk shear greater than 50 kts are over 70%, as are the probabilities of SCP over 3 and sig tor parameter over 1.


  2. Matt Parker says:

    Wow, SPC D2 18Z update to MDT… at least for GSP’s and RNK’s area of responsibility. I will be monitoring with interest. Best to all who are on shift for this.

  3. nws-pat moore says:

    All systems are “Go” right now for a severe weather event across the HSLC study area for today. The SPC has shrinked the Moderate Risk for the time being, that is until the QLCS cranks up across the southeast and they have to start adjusting it.

    The evolution of this event should be very interesting. Over the Tennessee Valley, the models are developing enough surface based CAPE such that it should be above our somewhat arbitrary threshold for HSLC storms. The shear vector is not exactly parallel to the front either. Will we see embedded supercells within the line? I suspect the answer will be ‘yes’ over Tennessee, northern Alabama, and northwestern Georgia, where CAPE will be highest. By the time it makes it to the western Carolinas and northeast Georgia, the 03Z SREF gives a relatively low probability of exceeding 500 J/kg of CAPE, so I suspect we will see more of an embedded QLCS with many bowing/breaking segments.

    Wish us luck.


  4. Matt Parker says:

    SPC mesoanalysis not showing much CAPE at this time, but QLCS appears to be forming some discrete cells/breaks as it crosses into VA.

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