Alert archive / West Virginia / Mesoscale Discussions in 2021

Mesoscale Discussion 1387

July 29, 2021Allegany County, Garrett County, Maryland; 7 counties in Virginia; 21 counties in West Virginia

The Storm Prediction Center issued this mesoscale discussion at Jul 29, 2021, 11:58 AM EDT for Allegany County, Garrett County, Maryland; 7 counties in Virginia; 21 counties in West Virginia. It was in force for 2 hr.

The area

The outline as issued.

The facts

Issued: Jul 29, 2021, 11:58 AM EDT (15:58 UTC)
Expired: Jul 29, 2021, 2:00 PM EDT
In force: 2 hr
Issued by: the Storm Prediction Center
Concerning: Severe potential...watch possible
Chance of a watch: 40%

All 30 counties

Allegany County, MD, Garrett County, MD, Augusta County, VA, Bath County, VA, City of Harrisonburg, VA, City of Staunton, VA, Highland County, VA, Rockingham County, VA, Shenandoah County, VA, Barbour County, WV, Braxton County, WV, Gilmer County, WV, Grant County, WV, Greenbrier County, WV, Hampshire County, WV, Hardy County, WV, Harrison County, WV, Lewis County, WV, Marion County, WV, Mineral County, WV, Monongalia County, WV, Nicholas County, WV, Pendleton County, WV, Pocahontas County, WV, Preston County, WV, Randolph County, WV, Taylor County, WV, Tucker County, WV, Upshur County, WV, Webster County, WV

The radar

The Pittsburgh radar (KPBZ), 176 km from the middle of the area, began a volume at 16:00 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 watches that followed

Watches SPC issued over any of the same counties within four hours of this discussion.

The nearest sounding before it

Pittsburgh, PA (KPIT), launched 12:00Z, 4.0 hours before the alert went out and 161 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: 136 J/kg
Mixed-layer CIN: -76 J/kg
Mixed-layer LCL: 1222 m above ground
0-6 km bulk shear: 43 kt
0-1 km storm-relative helicity: 175 m²/s²
0-3 km storm-relative helicity: 213 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.3

Open this sounding in Storm Lab

The text

As transmitted.

223 
ACUS11 KWNS 291559
SWOMCD
SPC MCD 291558 
VAZ000-MDZ000-WVZ000-PAZ000-291800-

Mesoscale Discussion 1387
NWS Storm Prediction Center Norman OK
1058 AM CDT Thu Jul 29 2021

Areas affected...Central/Eastern WV...Far Western MD...Far Northwest
VA

Concerning...Severe potential...Watch possible 

Valid 291558Z - 291800Z

Probability of Watch Issuance...40 percent

SUMMARY...Increasing thunderstorm coverage is possible over the next
few hours, with a resulting increase in the threat for damaging wind
gusts. Convective trends are being monitored for possible watch
issuance.

DISCUSSION...Recent radar and satellite imagery depict a
well-defined mesoscale convective vortex over southeast OH.
Estimated motion of this MCV is southeast at around 42-45 kt, which
takes it into central WV near 17Z and to the northern WV/VA border
by 18Z. Air mass downstream of this MCV is only weakly buoyant as
this time, but limited cloud cover should result in further diurnal
heating and resulting destabilization. Ascent attendant to the MCV
coupled with the anticipated air mass destabilization will likely
result in thunderstorm development. Overall thunderstorm coverage is
still uncertain, particularly given the potential for some mixing
out of the low-level moisture currently in place. Nevertheless, this
region is on the southern edge of the stronger mid-level flow and
some localized enhancement of the low-level flow is possible in the
immediate vicinity of the MCV. As a result, the presence of at least
modest vertical shear suggests a few stronger, more robust updrafts
are possible, with an attendant threat for damaging wind gusts. As
mentioned, storm coverage is still uncertain but convective trends
will be monitored over the next few hours for potential watch
issuance downstream across eastern WV, far western MD, and far
northwest VA.

..Mosier/Hart.. 07/29/2021

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

ATTN...WFO...CTP...LWX...RNK...PBZ...RLX...

LAT...LON   39518056 39907984 39817897 39507845 38697855 37837935
            37838043 38328114 39518056

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Jul 29, 1:21 PM EDT	Mesoscale Discussion	SPC	Carroll, Frederick, Washington	114 min
Jul 29, 2:23 PM EDT	Tornado Watch	SPC	Anne Arundel, Baltimore, Caroline, Carroll, Cecil, Frederick and 7 more	8 hr
Jul 29, 3:00 PM EDT	Severe Thunderstorm Watch	SPC	Allegany, Calvert, Charles, Dorchester, Garrett, Montgomery and 5 more	6 hr
Jul 29, 4:26 PM EDT	Mesoscale Discussion	SPC	Anne Arundel, Baltimore, Carroll, Cecil, Frederick, Harford and 5 more	94 min
Jul 29, 3:13 PM EDT	Severe Thunderstorm Watch	SPC	Barbour, Brooke, Calhoun, Doddridge, Gilmer, Hancock and 19 more	7 hr
Jul 29, 6:34 PM EDT	Mesoscale Discussion	SPC	Brooke, Doddridge, Hancock, Harrison, Marion, Marshall and 8 more	116 min

Nearby

Mesoscale Discussions in West Virginia, 2021 Mesoscale Discussions in Virginia, 2021 Mesoscale Discussions in Maryland, 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.