Alert archive / Virginia / Mesoscale Discussions in 2007

Mesoscale Discussion 1821

August 20, 20075 states

The Storm Prediction Center issued this mesoscale discussion at Aug 20, 2007, 7:11 PM EDT for 5 states. It was in force for 94 min.

The area

The outline as issued.

The facts

Issued: Aug 20, 2007, 7:11 PM EDT (23:11 UTC)
Expired: Aug 20, 2007, 8:45 PM EDT
In force: 94 min
Issued by: the Storm Prediction Center

All 71 counties

Kent County, DE, New Castle County, DE, Sussex County, DE, Allegany County, MD, Anne Arundel County, MD, Baltimore County, MD, Calvert County, MD, Caroline County, MD, Carroll County, MD, Cecil County, MD, Charles County, MD, City of Baltimore, MD, Dorchester County, MD, Frederick County, MD, Garrett County, MD, Harford County, MD, Howard County, MD, Kent County, MD, Montgomery County, MD, Prince George's County, MD, Queen Anne's County, MD, Saint Mary's County, MD, Somerset County, MD, Talbot County, MD, Washington County, MD, Wicomico County, MD, Adams County, PA, Bedford County, PA, Franklin County, PA, Fulton County, PA, Somerset County, PA, York County, PA, Arlington County, VA, Augusta County, VA, City of Alexandria, VA, City of Fairfax, VA, City of Falls Church, VA, City of Fredericksburg, VA, City of Harrisonburg, VA, City of Manassas, VA, City of Manassas Park, VA, City of Staunton, VA, City of Winchester, VA, Clarke County, VA, Culpeper County, VA, Fairfax County, VA, Fauquier County, VA, Frederick County, VA, Greene County, VA, Highland County, VA, King George County, VA, Loudoun County, VA, Madison County, VA, Orange County, VA, Page County, VA, Prince William County, VA, Rappahannock County, VA, Rockingham County, VA, Shenandoah County, VA, Spotsylvania County, VA, Stafford County, VA, Warren County, VA, Berkeley County, WV, Grant County, WV, Hampshire County, WV, Hardy County, WV, Jefferson County, WV, Mineral County, WV, Morgan County, WV, Pendleton County, WV, Tucker County, WV

The radar

The Sterling radar (KLWX), 28 km from the middle of the area, began a volume at 23:13 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

Washington/Dulle, VA (KIAD), launched 12:00Z, 11.2 hours before the alert went out and 36 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: 2 J/kg
Mixed-layer CIN: 0 J/kg
Mixed-layer LCL: 491 m above ground
0-6 km bulk shear: 38 kt
0-1 km storm-relative helicity: 91 m²/s²
0-3 km storm-relative helicity: 145 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.4

Open this sounding in Storm Lab

The text

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

000 
ACUS11 KWNS 202312
SWOMCD
SPC MCD 202311 
DEZ000-MDZ000-NJZ000-PAZ000-VAZ000-DCZ000-WVZ000-210045-

MESOSCALE DISCUSSION 1821
NWS STORM PREDICTION CENTER NORMAN OK
0611 PM CDT MON AUG 20 2007

AREAS AFFECTED...SRN PA...NRN WV INTO MD...NRN VA AND THE DELMARVA

CONCERNING...SEVERE THUNDERSTORM WATCH 621...

VALID 202311Z - 210045Z

THE SEVERE WEATHER THREAT FOR SEVERE THUNDERSTORM WATCH 621
CONTINUES.

TRENDS WILL CONTINUE TO BE MONITORED.  BUT...THE NEED FOR A NEW WW
IS NOT CURRENTLY ANTICIPATED.  AND...THE REMAINING VALID PORTIONS OF
WW 621 MAY BE ALLOWED TO EXPIRE AS SCHEDULED AT 01Z.

WEAKENING REMNANTS OF PERHAPS THE MOST SIGNIFICANT EARLY STORM
CLUSTER IS NOW SPREADING INTO THE NORTH CENTRAL APPALACHIANS. 
AND...THE UPPER IMPULSE SUPPORTING ACTIVITY LIKELY IS WEAKENING IN
CONFLUENT REGIME EXTENDING EAST OF THE UPPER OHIO VALLEY INTO THE
NORTHERN MID ATLANTIC STATES.  HOWEVER...FAIRLY VIGOROUS NEW
CONVECTIVE DEVELOPMENT HAS TAKEN PLACE ACROSS CENTRAL WEST VIRGINIA
DURING THE PAST COUPLE OF HOURS...APPARENTLY IN A ZONE OF ENHANCED
LOW-LEVEL WARM ADVECTION...WHICH IS EXPECTED TO SPREAD TO THE LEE OF
THE HIGHER TERRAIN...ACROSS MARYLAND/NORTHERN VIRGINIA...BY 01-02Z. 
WEST VIRGINIA STORMS ARE PROBABLY ROOTED IN A BOUNDARY LAYER WITH
CAPE OF 1500-2000 J/KG.  BUT...AS ACTIVITY DEVELOPS TO THE LEE OF
THE CENTRAL APPALACHIANS...IT WILL BECOME ELEVATED ABOVE A FRONTAL
ZONE...WHICH...AT THE SURFACE...HAS STALLED ACROSS SOUTHERN
VIRGINIA.  STILL...SHEAR IN THE CLOUD BEARING LAYER ABOVE THE
FRONTAL INVERSION COULD SUPPORT SOME MARGINALLY SEVERE HAIL IN
ISOLATED STRONGER STORMS DURING THE EVENING HOURS.

..KERR.. 08/20/2007

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

ATTN...WFO...PHI...AKQ...CTP...LWX...PBZ...RLX...

39777940 40187883 40237769 40067660 39707572 38897512
38277526 37977586 38197674 38127781 38117933 38527971
38967968 39537952

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Aug 19, 8:14 PM EDT	Mesoscale Discussion	SPC	Kent, Sussex	91 min
Aug 19, 11:58 PM EDT	Mesoscale Discussion	SPC	Dorchester, Saint Mary's, Somerset, Wicomico, Worcester	92 min
Aug 20, 12:18 PM EDT	Mesoscale Discussion	SPC	Garrett	117 min
Aug 20, 1:19 PM EDT	Severe Thunderstorm Watch	SPC	Garrett	7 hr
Aug 20, 4:24 PM EDT	Mesoscale Discussion	SPC	Allegheny, Beaver, Fayette, Greene, Washington, Westmoreland	66 min
Aug 19, 8:39 PM EDT	Severe Thunderstorm Watch	SPC	Accomack, Amelia, Caroline, Charles City, Chesterfield, Cumberland and 40 more	5 hr

Nearby

Mesoscale Discussions in Virginia, 2007 Mesoscale Discussions in Maryland, 2007 Mesoscale Discussions in West Virginia, 2007 Every mesoscale discussion in 2007

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.