HSLC environment across the Carolinas and Mid-Atlantic Tuesday and Tuesday Night

It’s that time of year again when we grow suspicious of low pressure systems organizing over the northwest Gulf of Mexico and lifting northeast across the southeastern United States.  We’ve been watching how the models handle the system expected to move through Tuesday and Tuesday night for a few days now.  Recent runs are coming into better agreement that part of our region will have a good chance at having a combination of high shear and low CAPE after about 1800 UTC on 16 November.  The 0000 UTC 14 Nov cycle of the GFS and 0300 UTC 14 Nov cycle of the SREF show the main surface low pressure center moving west of the Appalachians, and now the 0000 UTC run of the ECMWF has moved in that direction as well.

Forecasters at  GSP are painfully aware of how difficult it can be to issue an effective warning for severe weather associated with a QLCS in a high shear low CAPE environment.  Situational awareness of being in the proper environment for “Broken-S” radar reflectivity signatures is thought to be half the battle.  The modelling studies of Trapp and Weisman (2003) suggest that shear in the 0-2.5 km AGL layer of 20 m/s (~40 kt) is sufficient for the development of low level mesovortices through the downward tilting of environmental crosswise vorticity.  Local case studies suggest that a CAPE of as little as 200 J/kg is enough buoyancy to support a QLCS capable of producing brief tornadoes.  Thus, forecasters look for the combination of 0-2.5 km shear of 40 kt or greater and CAPE of 200 J/kg to highlight a broken-S threat if a QLCS were to develop.  I have created an AWIPS bundle to show this, which I have included below…

Surface-based CAPE (J/kg; yellow contours) and 0-2.5 km bulk shear magnitude (kt; color fill) and shear vector (kt; barbs) from the 0600 UTC 14 November 2010 cycle of the GFS40 model, 66-hour forecast valid at 0000 UTC on 17 November.

Same as above, except 0-2.5 km bulk shear shown as dark blue contours and surface based CAPE shown as a color fill image.

 

As the images show, the GFS40 has the southern part of the GSP forecast area and the western part of the CAE forecast area with (what we think are) the proper ingredients for “broken-S” events late Tuesday.  One of the useful outcomes of the CSTAR project will hopefully be a better understanding of the important parameters for “Broken-S” and other HSLC severe weather that can be incorporated into an AWIPS bundle that we can all use to increase our situational awareness.

-pat

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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.

3 Responses to HSLC environment across the Carolinas and Mid-Atlantic Tuesday and Tuesday Night

  1. Great heads-up on this potential event, Pat, and I like your AWIPS procedures. Your post prompted me to look back at a couple of our local event summaries for HSLC events, and some striking similarities are evident. The March 27, 2009, multiple tornado event in our eastern counties (http://www4.ncsu.edu/~nwsfo/storage/cases/20090327/), for example, also exhibited small lapse rates and low CAPE (MLCAPE under 200 J/kg) with high values of both deep layer shear (50-60 kts) and low level shear (over 100 m2/s2), and a surface boundary (with a weak surface wind field) running southwest to northeast through central NC. Upper air maps (http://www.spc.noaa.gov/cgi-bin-spc/getuadata.spc?MyDate1=090328&Time1=00&MyDate2=&Time2=12&align=V&Levels=All) also show the mid level trough displaced to our west, although the GFS/ECMWF/Canadian for the upcoming case take the low closer, just west of the Appalachians. As you noted, with this Tuesday/Tuesday night event, low level shear is impressive with 35-40 kts of bulk shear and a well curved hodograph in the 0-1 km layer. This system also has the potential for strong upper divergence with an approaching 130 kt southwesterly jet over the southeast states at 250 mb, which could help stretch low level vorticity into the vertical, helping to make up for what may be lacking in the thermodynamic department. Will be interesting to see what transpires.

    -Gail

  2. Hunter Coleman @ WFO CAE says:

    Pat,

    We also noticed the possibility of a HSLC environment setting up for Tuesday and Tuesday night as it appears we will fall within the warm sector as models continue to show the surface low tracking west of the Appalachians through the eastern Tennessee Valley. It is interesting to note that the 00z/15 NAM is not nearly as strong with the surface low compared to the GFS/ECMWF/SREF and also hints at strong convection developing along the Gulf Coast states (which if verifies may cut off moisture transport northward over our areas). However…the NAM at this point looks to be somewhat of an outlier so it will be interested to see how this event unfolds. The anomaly display is showing some decent anomalies as well, http://www.wrh.noaa.gov/slc/projects/anomaly/frames_sector.php?dom=se&name=SE&heading=Southeast.

    • Barrett Smith says:

      This definitely turned out to be an event, though not a tornadic event. I wasn’t working so I don’t know what radar signatures looked like, or if there were signs of rotation early in the event but no touchdowns, but increasingly unidirectional wind profiles turned it into a straight line wind event despite tremendous shear.

      On a semi related note, there was discussion at our office in the 6-12 hours prior to the event about the possibility of increased CAPE through the release of convective instability. Early in the afternoon if became apparent that dry air would increase over Upstate SC and Western NC prior to the passage of the mid-upper level trough. With dry air atop a moist layer from ~800mb down, layer lifting from the trough passage could steepen lapse rates from differential cooling and increase instability over the area. It’s difficult to discern how much this may have augmented CAPE values as cooling aloft was also taking place, but it seems every plausible that it did indeed help. I don’t know how this affected the tornado threat, if it even did, but I find it interesting nonetheless.

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