Alert archive / Virginia / Mesoscale Discussions in 2021

Mesoscale Discussion 1781

September 23, 20214 states

The Storm Prediction Center issued this mesoscale discussion at Sep 23, 2021, 3:13 AM EDT for 4 states. It was in force for 5 hr.

The area

The outline as issued.

The facts

Issued: Sep 23, 2021, 3:13 AM EDT (07:13 UTC)
Expired: Sep 23, 2021, 8:15 AM EDT
In force: 5 hr
Issued by: the Storm Prediction Center
Concerning: Severe potential...watch unlikely
Chance of a watch: 5%

All 64 counties

Kent County, DE, New Castle County, DE, 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, 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, Wicomico County, MD, Adams County, PA, Berks County, PA, Chester County, PA, Cumberland County, PA, Dauphin County, PA, Lancaster County, PA, Lebanon County, PA, Perry County, PA, York County, PA, Accomack County, VA, Arlington County, VA, Caroline County, VA, Charles City County, VA, City of Alexandria, VA, City of Colonial Heights, VA, City of Fairfax, VA, City of Falls Church, VA, City of Hopewell, VA, City of Petersburg, VA, City of Poquoson, VA, City of Richmond, VA, City of Williamsburg, VA, Essex County, VA, Fairfax County, VA, Gloucester County, VA, Hanover County, VA, Henrico 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, Surry County, VA, Westmoreland County, VA, York County, VA

The radar

The Sterling radar (KLWX), 79 km from the middle of the area, began a volume at 07:16 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 00:00Z, 7.2 hours before the alert went out and 70 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: 100 J/kg
Mixed-layer CIN: -17 J/kg
Mixed-layer LCL: 652 m above ground
0-6 km bulk shear: 21 kt
0-1 km storm-relative helicity: 161 m²/s²
0-3 km storm-relative helicity: 199 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.1

Open this sounding in Storm Lab

The text

As transmitted.

019 
ACUS11 KWNS 230714
SWOMCD
SPC MCD 230713 
PAZ000-MDZ000-DEZ000-NJZ000-VAZ000-DCZ000-231215-

Mesoscale Discussion 1781
NWS Storm Prediction Center Norman OK
0213 AM CDT Thu Sep 23 2021

Areas affected...eastern Virginia to the Delmarva...southeast
Pennsylvania

Concerning...Severe potential...Watch unlikely 

Valid 230713Z - 231215Z

Probability of Watch Issuance...5 percent

SUMMARY...Convection along a cold front could yield isolated severe
over the next several hours, but chances appear too low for any
watch consideration. A brief/weak tornado or strong wind gusts would
be primary concerns.

DISCUSSION...Weak convection persists along a cold front, with
sporadic lightning noted from southern PA across VA and into
northeast NC. A warm, moist air mass remains ahead of the
slow-moving front, with 70s F dewpoints contributing to 500-1000
J/kg MUCAPE.

VWP observations as well as model-derived soundings show favorable
low-level shear ahead of the front where surface winds are backed,
with effective SRH over 200 m2/s2. Otherwise, winds aloft remain
nearly unidirectional out of the south/southwest, limiting effective
shear.

Little if any destabilization is anticipated due to theta-e
advection, and lift along the front will remain rather steady state
as it moves slowly eastward. Convective elements that can maximize
residence time along the front may exhibit rotation at times, with
any wind or weak tornado threat likely short lived/localized.

..Jewell/Edwards.. 09/23/2021

...Please see www.spc.noaa.gov for graphic product...

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

LAT...LON   37297585 37097633 37067707 37207746 37387758 37847748
            39047743 39717747 40427741 40757695 40757645 40597595
            40177558 39417547 38357558 37297585

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Sep 23, 10:30 AM EDT	Mesoscale Discussion	SPC	Kent, New Castle, Sussex	2 hr
Sep 23, 1:29 PM EDT	Mesoscale Discussion	SPC	Bucks, Northampton, Philadelphia	61 min

Nearby

Mesoscale Discussions in Virginia, 2021 Mesoscale Discussions in Maryland, 2021 Mesoscale Discussions in Pennsylvania, 2021 Every mesoscale discussion in 2021

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.