Alert archive / Virginia / Mesoscale Discussions in 2012

Mesoscale Discussion 1809

August 25, 201211 counties in North Carolina; 21 counties in Virginia

The Storm Prediction Center issued this mesoscale discussion at Aug 25, 2012, 6:12 AM EDT for 11 counties in North Carolina; 21 counties in Virginia. It was in force for 3 hr.

The area

The outline as issued.

The facts

Issued: Aug 25, 2012, 6:12 AM EDT (10:12 UTC)
Expired: Aug 25, 2012, 8:45 AM EDT
In force: 3 hr
Issued by: the Storm Prediction Center
Chance of a watch: 20%

All 32 counties

Camden County, NC, Chowan County, NC, Currituck County, NC, Dare County, NC, Gates County, NC, Hertford County, NC, Hyde County, NC, Pasquotank County, NC, Perquimans County, NC, Tyrrell County, NC, Washington County, NC, Accomack County, VA, City of Chesapeake, VA, City of Franklin, VA, City of Hampton, VA, City of Newport News, VA, City of Norfolk, VA, City of Poquoson, VA, City of Portsmouth, VA, City of Suffolk, VA, City of Virginia Beach, VA, City of Williamsburg, VA, Gloucester County, VA, Isle of Wight County, VA, James City County, VA, Mathews County, VA, Middlesex County, VA, Northampton County, VA, Southampton County, VA, Surry County, VA, Sussex County, VA, York County, VA

The radar

The Norfolk radar (KAKQ), 119 km from the middle of the area, began a volume at 10:15 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

Newport, NC (KMHX), launched 00:00Z, 10.2 hours before the alert went out and 171 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-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

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: 952 J/kg
Mixed-layer CIN: -3 J/kg
Mixed-layer LCL: 483 m above ground
0-6 km bulk shear: 41 kt
0-1 km storm-relative helicity: 70 m²/s²
0-3 km storm-relative helicity: 191 m²/s²
Significant tornado parameter: 0.6
Supercell composite: 4.0

Open this sounding in Storm Lab

The text

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

035 
ACUS11 KWNS 251013
SWOMCD
SPC MCD 251012
MDZ000-VAZ000-NCZ000-251245-

MESOSCALE DISCUSSION 1809
NWS STORM PREDICTION CENTER NORMAN OK
0512 AM CDT SAT AUG 25 2012

AREAS AFFECTED...PORTIONS SERN VA...EXTREME ERN NC.

CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY

VALID 251012Z - 251245Z

PROBABILITY OF WATCH ISSUANCE...20 PERCENT

SUMMARY...THOUGH MRGL...POTENTIAL IS INCREASING FOR SMALL SUPERCELLS
OVER THIS REGION...AND A SHORT-LIVED TORNADO OR BRIEF DAMAGING GUSTS
CANNOT BE RULED OUT.

DISCUSSION...MOISTURE CHANNEL IMAGERY SHOWS WEAK BUT STILL
WELL-DEFINED MID-UPPER CYCLONE MOVING SLOWLY NEWD ACROSS NRN
NC...AND FCST TO COVER MORE OF ERN VA THROUGH MIDDAY.  CONVECTIVE
ENHANCEMENT TO RELATED VORTICITY FIELD IS LIKELY GIVEN TRENDS FROM
PREVIOUS DAY...AND ONGOING PATTERN OF INCREASING RADAR REFLECTIVITY
MAGNITUDES...CONVECTIVE COVERAGE...AND COOLING CLOUD TOPS IN IR
IMAGERY.  SFC MESOANALYSIS SHOWS MAIN LOW OVER COASTAL NC BETWEEN
MHX-HSE...ATTACHED TO SFC BAROCLINIC ZONE THAT EXTENDS SWD OVER
ATLC.  NRN STRETCH OF THAT BOUNDARY...AS OF 10Z...ARCHED FROM LOW
NNWWD NEAR ASJ THEN NEWD TO EWD ACROSS SERN VA AND MOUTH OF
CHESAPEAKE BAY.  THIS CORRESPONDS CLOSELY TO SRN PORTION OF BKN LINE
OF TSTMS THAT HAS PRODUCED TRANSIENT SIGNATURES OF LOW-MIDDLE LEVEL
CYCLONIC SHEAR.  BOUNDARY SHOULD MOVE NWD 5-10 KT OVER SERN VA.
THOUGH SUBTLE...BAROCLINICITY WITH VA SECTION OF THIS BOUNDARY IN
PARTICULAR SHOULD BE REINFORCED BY THAT CONVECTION.  AUTOMATED
MESOANALYSES ALSO INDICATE FIELD OF ENHANCED LOW-LEVEL VORTICITY
NEAR BOUNDARY...N THROUGH NE OF FKN THEN EWD OFF COAST.  RELATED
BACKED FLOW WILL ENHANCED LOW-LEVEL HODOGRAPHS...WHILE CONVERGENCE
IN WEAKLY CAPPED ENVIRONMENT MAINTAINS SUITABLE LIFT FOR TSTMS.

MODIFIED RAOBS AND FCST SOUNDINGS SUGGEST VERY MEAGER BUOYANCY
ACROSS THIS REGION...BECAUSE OF WEAK DEEP-LAYER LAPSE RATES COMBINED
WITH TEMPORAL/NOCTURNAL THERMAL MIN INLAND.  NONETHELESS...SFC DEW
POINTS 70S F SUPPORT SUFFICIENTLY RICH LOW-LEVEL THETAE TO YIELD
SFC-BASED EFFECTIVE INFLOW PARCELS...AND MLCAPE RANGING FROM AROUND
500 J/KG NEAR NTU TO LESS THAN 100 J/KG ALONG I-95 S RIC.  MAIN
LIMITING FACTORS WILL REMAIN LACK OF MORE ROBUST BUOYANCY...AND
MESSY CONVECTIVE MODE...THOUGH EPISODIC/EMBEDDED MESOCYCLONES REMAIN
POSSIBLE.  FARTHER SE ACROSS OUTER BANKS REGION...LOW-LEVEL
SHEAR/VORTICITY APPEAR WEAKER UNDER GREATER CAPE...BUT WITH MORE
DISCRETE CONVECTIVE MODE.  TORNADO/WIND POTENTIAL THERE IS EVEN MORE
CONDITIONAL/ISOLATED...BUT NONZERO.

..EDWARDS/THOMPSON.. 08/25/2012

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

ATTN...WFO...AKQ...MHX...

LAT...LON   34727641 35867670 36397721 36827742 37537689 38057516
            37857542 37137582 37127596 36947599 36077570 35657545
            35157552 35227556 35077601 34837634 34727641

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Aug 24, 8:46 PM EDT	Mesoscale Discussion	SPC	Carteret, Pamlico	59 min
Aug 25, 11:22 AM EDT	Mesoscale Discussion	SPC	Accomack, Gloucester, Isle of Wight, Lancaster, Mathews, Middlesex and 8 more	113 min
Aug 25, 4:46 PM EDT	Mesoscale Discussion	SPC	Accomack	89 min

Nearby

Mesoscale Discussions in Virginia, 2012 Mesoscale Discussions in North Carolina, 2012 Every 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.