September 28, 2011 Early Morning Severe Weather Event

An interesting severe weather event unfolded across central North Carolina during the early morning hours of September 28, 2011.

Synoptic Overview:
Prior to and during the severe weather event, a stalled upper level low was centered in vicinity of southern Lake Michigan, with southwest flow aloft present downstream of the upper low over the Carolinas and Mid-Atlantic. A shortwave moved from Alabama into Georgia by 21Z on the 27th, progressing into north-central SC via southwest flow aloft by 04Z on the 28th. Thereafter, the shortwave progressed through central NC (crossing through the Triangle) between 05-10Z. In the lower levels, a very weak surface trough and stalled front was present over the western Piedmont. Dewpoints varied from the upper 50s to lower 60s behind the front along/west of I-77, while dewpoints ahead of the front east of I-77 were in the lower 70s.

Thermodynamics:
An unusually unstable airmass was present over central NC in advance of the approaching MCV and shortwave trough during the early morning hours of September 28. SPC mesoanalysis indicated 1500-2000 J/kg of MLCAPE was present over central NC between 04-06Z on the 28th, owing to surface temps in the mid 70s (73-76F), dewpoints in the lower 70s and mid-level lapse rates on the order of ~6C/km.

Mesoscale Overview:
Convection developed well upstream over AR/LA/MS during the early morning hours of September 27th. This convection spawned an MCV, which subsequently tracked E/NE through central AL into upstate GA.  Based on radar reflectivity loops from KBMX, KFFC and KGSP, this is not the MCV that tracked into central NC late in the evening on September 27th.  Furthermore, given the track of the MCV, it appears unlikely that this feature played much of a role in forcing convection near Atlanta between 20-21Z September 27th. (Click below images to see the loops)

The convection that developed in vicinity of Atlanta between 20-21Z on September 27th appears to have developed along an outflow boundary in advance of a shortwave tracking from AL into GA. This area of convection subsequently tracked into upstate SC after 00Z on the 28th. Between 00-03Z on the 28th, reflectivity imagery showed a noticable ‘swirl’ associated with this area of convection as it tracked through SC, and velocity data showed a broad inbound/outbound couplet. Based on radar imagery, it appears likely that an MCV developed in association with this persistent area of convection as it tracked through central SC toward central NC. Between 04-08Z, the MCV tracked northeast from Union/Anson counties through Montgomery, Randolph, Guilford, Alamance, and Caswell counties, moving into south-central VA by 0830Z.

Note an additional MCV that moves into Wilmington’s CWA, southeast of the MCV that tracked into central NC. This MCV is not included in the MCV track analysis below.

Discussion:
As the MCV tracked into NC, it helped spawn the development of additional convection in vicinity of the Charlotte metro area. This convection, in turn, began to build eastward along outflow toward the Triangle where the most unstable airmass was located. A pronounced bow-echo developed in vicinity of Randolph County around 0430Z. The bow surged into Chatham county while the northern book-end vortex tracked almost due north into eastern Guilford and western Alamance counties. After the bow surged into Chatham county, the line of convection weakened as it approached the Triangle. However, between 0630-0730Z, convection began to re-intensify over central Wake county, possibly due to additional forcing in the form of DPVA associated with the shortwave tracking directly through central NC between 05-10Z.

During the evening shift on September 27th, we noted that the combination of the approaching MCV, shortwave, and the presence of moderate instability would result in at least an isolated severe threat across central NC during the overnight hours. We chatted with SPC on 12planet to see if they would extend the 5% wind/hail probabilities into central NC (they did), updated the AFD/HWO to reflect a mention of severe weather overnight, and updated the grids to account for higher precipitation chances than previously expected. This is a good example of a situation where close analysis of conditions over and upstream of the area resulted in stronger situational awareness with regard to the possibility of severe weather later that night.

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE RALEIGH NC
1027 PM EDT TUE SEP 27 2011

NCZ007>011-021>028-038>043-073>078-083>086-088-089-290230-
PERSON-GRANVILLE-VANCE-WARREN-HALIFAX-FORSYTH-GUILFORD-ALAMANCE-
ORANGE-DURHAM-FRANKLIN-NASH-EDGECOMBE-DAVIDSON-RANDOLPH-CHATHAM-WAKE-
JOHNSTON-WILSON-STANLY-MONTGOMERY-MOORE-LEE-HARNETT-WAYNE-ANSON-
RICHMOND-SCOTLAND-HOKE-CUMBERLAND-SAMPSON-
1027 PM EDT TUE SEP 27 2011

THIS HAZARDOUS WEATHER OUTLOOK IS FOR CENTRAL NORTH CAROLINA.

.DAY ONE...THROUGH TONIGHT.

SCATTERED SHOWERS AND THUNDERSTORMS ARE EXPECTED OVERNIGHT IN
ASSOCIATION WITH AN UPPER LEVEL DISTURBANCE MOVING ACROSS THE AREA.
A FEW ISOLATED STORMS COULD BECOME SEVERE...WITH A PRIMARY THREAT
OF DAMAGING WINDS AND PENNY SIZED HAIL.

.DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY.

HAZARDOUS WEATHER IS NOT EXPECTED AT THIS TIME.
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
947 PM EDT TUE SEP 27 2011

.NEAR TERM /THROUGH TONIGHT/...
AS OF 947 PM TUESDAY...

THE WATER VAPOR SATELLITE IMAGERY INDICATED A VORT MAX TRACKING
SLOWLY ENE ACROSS UPSTATE SOUTH CAROLINA AND SOUTHWESTERN NORTH
CAROLINA. ISOLATED SEVERE THUNDERSTORMS HAVE OCCURRED IN THE PAST FEW
HOURS IN SC IN ASSOCIATION WITH A CLUSTER OF ACTIVITY CURRENTLY IN
THE CHARLOTTE AND KANNAPOLIS AREA. CONVECTIVE PARAMETERS SUGGEST
THAT WE WILL CONTINUE TO HAVE A CHANCE OF THUNDERSTORMS WELL INTO
THE NIGHT AS THE CURRENT VORT MAX TRACKS OVER OUR REGION. THE
SURFACE DEW POINTS WERE IN THE LOWER 70S ACROSS MUCH OF THE AREA
WITH A BACKED (SE) SURFACE FLOW (ALBEIT LIGHT). THE MAIN THREAT FOR
SEVERE SHOULD REMAIN SE OF THE GREENSBORO AREA... WITH THE FORECAST
STORM MOTION FROM AROUND 250 AT 25KT. A COMBINATION OF WIND AND HAIL
HAS BEEN REPORTED FROM CAE AND GSP THIS EVENING... WITH THE LATEST
STORM REPORT FROM THE CHARLOTTE AREA WHERE TREES WERE DOWNED.
THUS... OUR MAIN THREATS WILL BE THAT COMBINATION.

WE WILL INCREASE POP INTO THE 30-40 PERCENT CHANCE RAIN AND KEEP
LOWS WARM WITH INCREASED CLOUDS AND SE-S FLOW.
PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE RALEIGH NC
908 PM EDT WED SEP 28 2011

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

1230 AM     HAIL             1 S GREENSBORO          36.06N  79.83W
09/28/2011  E1.00 INCH       GUILFORD           NC   PUBLIC

            ONE INCH HAIL VICINITY OF I-40 I-85 JUNCTION

1230 AM     TSTM WND DMG     4 W SEAGROVE            35.55N  79.85W
09/28/2011                   RANDOLPH           NC   DEPT OF HIGHWAYS

            TWO 15 TO 24 INCH DIAMETER TREES DOWN ON NC 134 BETWEEN
            STATE ROAD 127 AND 1118

1235 AM     TSTM WND DMG     RANDLEMAN               35.82N  79.80W
09/28/2011                   RANDOLPH           NC   PUBLIC

            TWO TREES DOWN...ONE ON POWER LINES 200 BLOCK POPULAR
            STREET

0253 AM     TSTM WND DMG     2 S RALEIGH             35.78N  78.64W
09/28/2011                   WAKE               NC   BROADCAST MEDIA

            TREE DOWN BLOCKING MCDOWELL STREET IN DOWNTOWN RALEIGH

0255 AM     TSTM WND DMG     1 SE RALEIGH            35.80N  78.63W
09/28/2011                   WAKE               NC   BROADCAST MEDIA

            TREE ON A HOUSE ON PERSHING ROAD IN DOWNTOWN RALEIGH

0315 AM     TSTM WND DMG     1 SSE RALEIGH           35.80N  78.63W
09/28/2011                   WAKE               NC   TRAINED SPOTTER

            A TREE COLLAPSED ONTO A ROOF AT 1335 COURTLAND

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