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.