Late-Season HSLC event on Wednesday December 7, 2011

A late-season HSLC severe weather event occurred on Wednesday December 7, 2011.        (Click on the below radar image to see a ~10 hour loop)


A robust (albeit deamplifying) shortwave trough lifted northeast from upstate GA/SC through western NC into central/western VA during the late afternoon and evening hours of December 7.  This shortwave helped a surface low deepen along a baroclinic zone that had slowly moved into the Mid-Atlantic and Carolinas during the prior 24 hours. The surface low began to deepen over the NC Piedmont Wednesday afternoon and continued to deepen as it tracked northeast along/offshore the Mid-Atlantic and New England coast Wednesday night and Thursday morning.

Instability throughout the event was very marginal at best, with SBCAPE values in the 100-250 J/kg range and surface dewpoints in the upper 50s to lower 60s.  Wind shear was extreme, with 0-6 km bulk shear values between 70-100 knots and 0-1 km SRH values in the 300-400 m2/s2 range.


A broken line of low-topped convection developed along/ahead of the cold front in the western Carolinas and progressed through central NC during the late afternoon and evening.  The primary threat with the line of convection that developed was damaging winds, with little in the way of rotation noted aside from periodic weak circulations along the line of low-topped convection. An impressive LLJ was noted just ahead of the front, with 50-60 knot southerly/southwesterly flow at 925-850 mb. This was not a big event.  Most of the damage reports in central NC were isolated/scattered at best and confined to a few downed trees and powerlines. However, many ASOS’s/AWOS’s in central NC did report wind gusts between 45-60 mph.  Interestingly, some of the stronger wind gusts actually occurred 10-20 minutes after the leading line of convection passed through. My initial thought was that this occurred due to strong pressure rises and continued shower activity in the immediate wake of the front/leading line of convection, but I’m not sure.

This event is interesting in that the instability and shear were not too different from the 11/28/2011 non-event (which was a close analog to the 11/16/2006 event).  However, both lightning and severe weather were observed on December 7. What was the difference?  I think the big difference this time around was the fact that we had a very robust/compact shortwave progressing northeast through the Carolinas into VA. This provided excellent forcing for ascent coincident with the cold front progressing through the area and may also have played a role thermodynamically – allowing weak instability to develop in association with strong layer-lifting in the presence of a somewhat dry mid-level airmass and an abnormally warm/moist boundary layer (for December).

On 11/28/2011, the warm sector was very similar to that on 12/07/2011.  Although there were good synoptic height falls coincident with the frontal passage on November 28, the most focused DPVA (associated with an upper low tracking northeast along the southern Appalachians) occurred well after (about 12 hours)  the cold frontal passage.  Additionally, more mid and upper level moisture was present on 11/28 versus 12/07.

All else being equal, my anecdotal experience suggests that late season HSLC events are more likely to produce severe weather when there is a robust shortwave producing strong/focused DPVA coincident with the surface frontal passage. Not only does this enhance the overall forcing and kinematics (in this case via deepening of the surface low along the front and strengthening of the low level wind fields), it can also enhance thermodynamics when there is at least some dry mid-level air in the presence of a warm/moist boundary layer.

Anyone else have any thoughts on this event? I’d be interested in hearing what others have to think. A summary LSR from NWS RAH for December 7 is posted below.

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE RALEIGH NC
828 PM EST WED DEC 07 2011

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0427 PM     TSTM WND GST     PIEDMONT TRIAD APT      36.10N  79.94W
12/07/2011  M47 MPH          GUILFORD           NC   ASOS

            GSO ASOS GUSTED TO 47 MPH AT 427 PM.

0440 PM     TSTM WND GST     LAURINBURG MAXTON APT   34.79N  79.37W
12/07/2011  M49 MPH          SCOTLAND           NC   ASOS

            LAURINBURG ASOS GUSTED TO 49 MPH AT 440 PM.

0450 PM     TSTM WND GST     MACKALL ARMY AIRFIELD   35.04N  79.50W
12/07/2011  M58 MPH          RICHMOND           NC   AWOS

            KHFF GUSTED TO 58 MPH AT 450 PM.

0505 PM     TSTM WND DMG     1 SSE HOPE MILLS        34.96N  78.94W
12/07/2011                   CUMBERLAND         NC   911 CALL CENTER

            TREE DOWN BLOCKING THE LIGHT AT PARKTON AND S. MAIN ST.

0510 PM     TSTM WND GST     FT BRAGG SIMMONS AAF    35.14N  78.93W
12/07/2011  M51 MPH          CUMBERLAND         NC   ASOS

            KFBG GUSTED TO 51 MPH AT 510 PM.

0518 PM     TSTM WND GST     POPE AFB                35.18N  79.03W
12/07/2011  M53 MPH          CUMBERLAND         NC   ASOS

            KPOB GUSTED TO 53 MPH AT 518 PM.

0518 PM     TSTM WND DMG     6 WSW LINDEN            35.22N  78.84W
12/07/2011                   CUMBERLAND         NC   911 CALL CENTER

            TREE AND POWERLINES DOWN ON RAMSY ST IN FAYETTEVILLE.

0520 PM     TSTM WND DMG     2 SW OAK HILL           36.41N  78.74W
12/07/2011                   GRANVILLE          NC   911 CALL CENTER

            TREE DOWN AT SATTERWHITE AND GOSHEN RD.

0520 PM     TSTM WND DMG     3 NNW OAK HILL          36.47N  78.73W
12/07/2011                   GRANVILLE          NC   911 CALL CENTER

            TREE DOWN NEAR 8200 BLOCK OF HWY 96 NORTH.

0525 PM     TSTM WND GST     FAYETTEVILLE REGIONAL A 34.99N  78.88W
12/07/2011  M48 MPH          CUMBERLAND         NC   ASOS

            KFAY GUSTED TO 48 MPH AT 525 PM.

0525 PM     TSTM WND DMG     1 SE CARTHAGE           35.34N  79.41W
12/07/2011                   MOORE              NC   911 CALL CENTER

            *** 1 INJ *** TREE FELL ON MOBILE HOME ON BLACK STREET
            IN CARTHAGE AT 525 PM. A WOMAN WAS TRANSPORTED TO THE
            HOSPITAL. EXTENT OF INJURIES UNKNOWN AT THIS TIME.

0540 PM     TSTM WND DMG     5 ESE STEM              36.18N  78.64W
12/07/2011                   GRANVILLE          NC   911 CALL CENTER

            TREE DOWN ON SAM MOSS HAYES RD.

0542 PM     TSTM WND DMG     6 NE SPIVEY`S CORNER    35.32N  78.46W
12/07/2011                   JOHNSTON           NC   911 CALL CENTER

            MULTIPLE TREES DOWN IN MEADOW... NC.

0544 PM     TSTM WND GST     RDU INTERNATIONAL AIRPO 35.88N  78.79W
12/07/2011  M41 MPH          WAKE               NC   ASOS

            KRDU GUSTED TO 41 MPH AT 544 PM.

0545 PM     TSTM WND DMG     2 WNW KITTRELL          36.23N  78.47W
12/07/2011                   VANCE              NC   911 CALL CENTER

            TREE DOWN BLOCKING RD ON LYNN BANK ESTATES RD.

0552 PM     TSTM WND DMG     2 NE SALEMBURG          35.03N  78.48W
12/07/2011                   SAMPSON            NC   911 CALL CENTER

            ELECTRICAL LINES DOWN ON BEARSKIN RD.

0605 PM     TSTM WND DMG     7 NE GARLAND            34.86N  78.30W
12/07/2011                   SAMPSON            NC   911 CALL CENTER

            TREE DOWN ON EBENEZER FOREST RD. TIME AND LOCATION
            APPROXIMATED.

0618 PM     TSTM WND DMG     2 SW SPRING HOPE        35.92N  78.14W
12/07/2011                   NASH               NC   911 CALL CENTER

            TREE DOWN BLOCKING ROAD ON S. BIG WOODS RD.

0625 PM     TSTM WND DMG     5 W ARCOLA              36.27N  78.07W
12/07/2011                   WARREN             NC   911 CALL CENTER

            TWO TREES DOWN IN THE SOUTHERN PART OF THE COUNTY... NO
            FURTHER DETAILS.

0629 PM     TSTM WND GST     SEYMOUR JOHNSON AFB     35.34N  77.97W
12/07/2011  M45 MPH          WAYNE              NC   AWOS

            KGSB GUSTED TO 45 MPH AT 629 PM.

0635 PM     TSTM WND GST     GOLDSBORO WAYNE APT     35.46N  77.97W
12/07/2011  M45 MPH          WAYNE              NC   AWOS

            KGWW GUSTED TO 45 MPH AT 635 PM.

0655 PM     TSTM WND GST     ROCKY MOUNT WILSON APT  35.86N  77.88W
12/07/2011  M46 MPH          NASH               NC   ASOS

            KRWI GUSTED TO 46 MPH AT 655 PM.

0705 PM     TSTM WND DMG     8 WSW SCOTLAND NECK     36.11N  77.55W
12/07/2011                   HALIFAX            NC   911 CALL CENTER

            MULTIPLE TREES DOWN REPORTED NEAR THE INTERSECTION OF
            BARNHILL ROAD AND DOUGLAS HILL FARM ROAD. TIME ESTIMATED
            VIA RADAR.

0715 PM     TSTM WND GST     HALIFAX COUNTY APT      36.33N  77.64W
12/07/2011  M41 MPH          HALIFAX            NC   AWOS

            KIXA GUSTED TO 41 MPH AT 715 PM.

0747 PM     NON-TSTM WND GST LAURINBURG MAXTON APT   34.79N  79.37W
12/07/2011  M41 MPH          SCOTLAND           NC   ASOS

            KMEB GUSTED TO 41 MPH AT 747 PM.

0748 PM     NON-TSTM WND GST FAYETTEVILLE REGIONAL A 34.99N  78.88W
12/07/2011  M44 MPH          CUMBERLAND         NC   ASOS

            KFAY GUSTED TO 44 MPH AT 748 PM.
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About bvincentnws

Meteorologist National Weather Service 2003 - Current
This entry was posted in High Shear Low Cape Severe Wx. Bookmark the permalink.

2 Responses to Late-Season HSLC event on Wednesday December 7, 2011

  1. Matt Parker says:

    Hi Brandon,

    Nice summary. My impression was also that the peak gusts in my N. Raleigh neighborhood occurred toward the *end* of the heavy precip. Given the large ground-relative winds aloft (GSO 00Z raob had a 30 kt W wind at 900 m AGL) I suspect that downward momentum transport was also an important player in the severe surface winds as well. Based on my subjective assessment of the gusts in N. Raleigh (I estimate 40-50 kts) I am guessing that, had there still been leaves on the trees, we would have had a lot more widespread wind damage.

    All the best,
    Matt

  2. nws-pat moore says:

    We had a very interesting thunderstorm develop in the colder air across Caldwell County well to the west of the primary convective band in the early evening. The conclusion was that the damage produced by the storm was from straight-line wind, but there are other factors that suggest this storm would be worth a closer look. Perhaps I will be able to post an update some time in the future.

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