Alert archive / Virginia / Mesoscale Discussions in 2012

Mesoscale Discussion 1968

September 18, 201213 counties in Maryland; 8 counties in North Carolina; 45 counties in Virginia

The Storm Prediction Center issued this mesoscale discussion at Sep 18, 2012, 3:52 PM EDT for 13 counties in Maryland; 8 counties in North Carolina; 45 counties in Virginia. It was in force for 113 min.

The area

The outline as issued.

The facts

Issued
Sep 18, 2012, 3:52 PM EDT (19:52 UTC)
Expired
Sep 18, 2012, 5:45 PM EDT
In force
113 min
Issued by
the Storm Prediction Center
All 66 counties

Anne Arundel County, MD, Baltimore County, MD, Calvert County, MD, Charles County, MD, City of Baltimore, MD, Dorchester County, MD, Howard County, MD, Prince George's County, MD, Saint Mary's County, MD, Somerset County, MD, Talbot County, MD, Wicomico County, MD, Worcester County, MD, Bertie County, NC, Camden County, NC, Chowan County, NC, Gates County, NC, Hertford County, NC, Northampton County, NC, Pasquotank County, NC, Perquimans County, NC, Accomack County, VA, Brunswick County, VA, Caroline County, VA, Charles City County, VA, Chesterfield County, VA, City of Alexandria, VA, City of Chesapeake, VA, City of Colonial Heights, VA, City of Emporia, VA, City of Franklin, VA, City of Hampton, VA, City of Hopewell, VA, City of Newport News, VA, City of Norfolk, VA, City of Petersburg, VA, City of Poquoson, VA, City of Portsmouth, VA, City of Richmond, VA, City of Suffolk, VA, City of Virginia Beach, VA, City of Williamsburg, VA, Dinwiddie County, VA, Essex County, VA, Gloucester County, VA, Greensville County, VA, Hanover County, VA, Henrico County, VA, Isle of Wight County, VA, James City County, VA, King and Queen County, VA, King George County, VA, King William County, VA, Lancaster County, VA, Mathews County, VA, Middlesex County, VA, New Kent County, VA, Northampton County, VA, Northumberland County, VA, Prince George County, VA, Richmond County, VA, Southampton County, VA, Surry County, VA, Sussex County, VA, Westmoreland County, VA, York County, VA

The radar

The Norfolk radar (KAKQ), 97 km from the middle of the area, began a volume at 19:54 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

The nearest sounding before it

Wallops Is Stn, VA (KWAL), launched 12:00Z, 7.9 hours before the alert went out and 101 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.

Skew-T log-pSkew-T log-p diagram, Wallops Is Stn, VA at 12:00Z on 2012-09-18Temperature and dew point through the depth of the atmosphere, with the mixed-layer parcel. CAPE 205 joules per kilogram.LCLLFCEL10008507005003002001001 km2 km3 km6 km9 km12 km15 km-30°-20°-10°0°10°20°30°40°
Temperature in red, dew point in green, the mixed-layer parcel dashed.
HodographHodograph, Wallops Is Stn, VA at 12:00Z on 2012-09-18The wind through the lowest 10 kilometers. Zero to 1 km storm-relative helicity 134 square meters per square second.19395878Sfc1 km3 km6 km9 kmRMLM
The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.
Mixed-layer CAPE
205 J/kg
Mixed-layer CIN
-9 J/kg
Mixed-layer LCL
778 m above ground
0-6 km bulk shear
33 kt
0-1 km storm-relative helicity
134 m²/s²
0-3 km storm-relative helicity
150 m²/s²
Significant tornado parameter
0.1
Supercell composite
0.9

Open this sounding in Storm Lab

The text

As transmitted, in the capitals the NWS used until 2016.

999 
ACUS11 KWNS 181953
SWOMCD
SPC MCD 181952
DEZ000-MDZ000-VAZ000-NCZ000-DCZ000-182145-

MESOSCALE DISCUSSION 1968
NWS STORM PREDICTION CENTER NORMAN OK
0252 PM CDT TUE SEP 18 2012

AREAS AFFECTED...PARTS OF CNTRL/ERN MD...WASHINGTON DC...ERN
VA...NERN NC

CONCERNING...TORNADO WATCH 642...

VALID 181952Z - 182145Z

THE SEVERE WEATHER THREAT FOR TORNADO WATCH 642 CONTINUES.

SUMMARY...THE THREAT FOR SVR WINDS AND TORNADOES CONTINUES ACROSS WW
642.

DISCUSSION...A WELL-DEVELOPED SQUALL LINE WITH EMBEDDED LEWP
STRUCTURES EXTENDING FROM THE BALTIMORE AREA SSWWD INTO S-CNTRL VA
AND ADJACENT NRN NC WILL CONTINUE ADVANCING EWD. THE SQUALL LINE
WILL LIKELY BE MAINTAINED AS IT APPROACHES THE COASTAL PLAIN AS DEEP
SHEAR VECTORS REMAIN ORIENTED NEARLY PARALLEL TO THE LEADING EDGE OF
THE COLD POOL WHILE INFLOW AIR IS CHARACTERIZED BY 500-1000 J/KG OF
MLCAPE PER MODIFIED 12Z RAOBS. LOCALIZED POCKETS OF
INSOLATION/DIABATIC HEATING...ESPECIALLY OVER SERN VA AND NERN
NC...ARE BOOSTING BOUNDARY-LAYER INSTABILITY. AND...WITH A STRONG
LOW-LEVEL JET STREAM SUPPORTING 35-45 KT OF 0-1-KM BULK SHEAR...SVR
WIND GUSTS WILL BE LIKELY...AND THE TORNADO THREAT WILL BE ENHANCED
IN CONJUNCTION WITH EMBEDDED MESOVORTICES. IN THE WAKE OF THE
CONVECTIVE LINE...VEERED LOW-LEVEL FLOW AND STABLE THERMODYNAMIC
PROFILES WILL MITIGATE THE SVR THREAT...AND REMAINING VALID PORTIONS
OF WW 642 MAY BE CLEARED FOLLOWING THE PASSAGE OF THE LINE.

..COHEN.. 09/18/2012

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...PHI...AKQ...LWX...

LAT...LON   36047625 35927695 36387751 36767810 37367766 38617717
            39477689 39647663 39577633 39057610 38717593 38357526
            37547540 37067560 36047625

SPC that day

Everything else SPC issued on the same UTC day.

IssuedProductOfficeCountiesLasted
Sep 18, 8:49 AM EDTMesoscale DiscussionSPCAnne Arundel, Baltimore, Calvert, Carroll, Charles, Frederick and 8 more116 min
Sep 18, 10:54 AM EDTTornado WatchSPCAnne Arundel, Baltimore, Calvert, Carroll, Charles, Dorchester and 11 more9 hr
Sep 18, 11:14 AM EDTMesoscale DiscussionSPCCecil91 min
Sep 18, 11:19 AM EDTMesoscale DiscussionSPCAnne Arundel, Baltimore, Calvert, Caroline, Carroll, Cecil and 13 more86 min
Sep 18, 12:10 PM EDTTornado WatchSPCCaroline, Cecil, Kent, Queen Anne's, Talbot9 hr
Sep 18, 1:27 PM EDTMesoscale DiscussionSPCAnne Arundel, Baltimore, Calvert, Carroll, Charles, Dorchester and 10 more63 min
Sep 18, 1:56 PM EDTMesoscale DiscussionSPCCecil, Harford64 min
Sep 18, 4:21 PM EDTMesoscale DiscussionSPCCaroline, Cecil114 min
Sep 18, 6:15 PM EDTMesoscale DiscussionSPCSomerset, Wicomico, Worcester30 min
Sep 17, 8:14 PM EDTMesoscale DiscussionSPCCherokee, Clay, Graham, Haywood, Henderson, Jackson and 4 more2 hr
Sep 18, 12:23 AM EDTMesoscale DiscussionSPCAlexander, Buncombe, Burke, Cabarrus, Caldwell, Catawba and 14 more97 min
Sep 18, 9:45 AM EDTTornado WatchSPCAlamance, Anson, Chatham, Cumberland, Davidson, Durham and 25 more8 hr
Sep 18, 11:35 AM EDTMesoscale DiscussionSPCUnion85 min
Sep 18, 1:00 PM EDTTornado WatchSPCBeaufort, Carteret, Craven, Dare, Duplin, Greene and 9 more5 hr
Sep 18, 1:21 PM EDTMesoscale DiscussionSPCAlamance, Anson, Chatham, Cumberland, Davidson, Durham and 25 more84 min
Sep 18, 4:11 PM EDTMesoscale DiscussionSPCAlamance, Anson, Beaufort, Carteret, Chatham, Craven and 40 more94 min

Nearby

Mesoscale Discussions in Virginia, 2012Mesoscale Discussions in Maryland, 2012Mesoscale Discussions in North Carolina, 2012Every mesoscale discussion in 2012

Sources

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.