Alert archive / Pennsylvania / Mesoscale Discussions in 2021

Mesoscale Discussion 1256

July 11, 2021Allegany County, Garrett County, Maryland; 21 counties in Pennsylvania; 19 counties in West Virginia

The Storm Prediction Center issued this mesoscale discussion at Jul 11, 2021, 2:05 PM EDT for Allegany County, Garrett County, Maryland; 21 counties in Pennsylvania; 19 counties in West Virginia. It was in force for 55 min.

The area

The outline as issued.

The facts

Issued: Jul 11, 2021, 2:05 PM EDT (18:05 UTC)
Expired: Jul 11, 2021, 3:00 PM EDT
In force: 55 min
Issued by: the Storm Prediction Center
Concerning: Severe potential...watch likely
Chance of a watch: 80%

All 42 counties

Allegany County, MD, Garrett County, MD, Allegheny County, PA, Armstrong County, PA, Beaver County, PA, Bedford County, PA, Blair County, PA, Butler County, PA, Cambria County, PA, Centre County, PA, Clarion County, PA, Clearfield County, PA, Fayette County, PA, Franklin County, PA, Fulton County, PA, Greene County, PA, Huntingdon County, PA, Indiana County, PA, Jefferson County, PA, Mifflin County, PA, Somerset County, PA, Washington County, PA, Westmoreland County, PA, Barbour County, WV, Brooke County, WV, Doddridge County, WV, Grant County, WV, Hampshire County, WV, Hancock County, WV, Harrison County, WV, Lewis County, WV, Marion County, WV, Marshall County, WV, Mineral County, WV, Monongalia County, WV, Morgan County, WV, Ohio County, WV, Preston County, WV, Taylor County, WV, Tucker County, WV, Tyler County, WV, Wetzel County, WV

The radar

The Pittsburgh radar (KPBZ), 88 km from the middle of the area, began a volume at 18:08 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.

Severe Thunderstorm Watch 372Jul 11, 2:40 PM EDT

The nearest sounding before it

Pittsburgh, PA (KPIT), launched 12:00Z, 6.1 hours before the alert went out and 73 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: 5 J/kg
Mixed-layer CIN: -180 J/kg
Mixed-layer LCL: 979 m above ground
0-6 km bulk shear: 36 kt
0-1 km storm-relative helicity: 90 m²/s²
0-3 km storm-relative helicity: 135 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.0

Open this sounding in Storm Lab

The text

As transmitted.

985 
ACUS11 KWNS 111806
SWOMCD
SPC MCD 111805 
PAZ000-MDZ000-WVZ000-VAZ000-OHZ000-111900-

Mesoscale Discussion 1256
NWS Storm Prediction Center Norman OK
0105 PM CDT Sun Jul 11 2021

Areas affected...Far Southeastern Ohio...Western
Pennsylvania...Northern West Virginia...and Western Maryland

Concerning...Severe potential...Watch likely 

Valid 111805Z - 111900Z

Probability of Watch Issuance...80 percent

SUMMARY...Thunderstorms are expected to increase in coverage and
intensity this afternoon across the area posing a threat for
damaging winds and a brief tornado or two. A watch is likely.

DISCUSSION...Strong heating has occurred across the area this
morning/early afternoon ahead of minor perturbations moving across
Ohio in southwesterly flow aloft.  Consequently, modest
destabilization is occurring in the presence of upper 60s F
dewpoints, resulting in around 1000 J/kg of MLCAPE per the latest
mesoanalysis.  The VAD profile at PBZ indicates that midlevel flow
is around 40 knots, which should support at least brief updraft
organization/rotation.  

Thunderstorms associated with midlvel perturbations are beginning to
intensify across southern Ohio and should continue to organize they
move into a more favorable environment to the northeast.  In
addition, more isolated/discrete storms may also form out ahead in
the warm sector, as the warm front continues to retreat northward. 
These storms will primarily pose a threat of damaging winds, but the
low-level shear is also sufficient to support the potential for a
brief tornado or two.

..Jirak/Guyer.. 07/11/2021

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

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

LAT...LON   39268109 39798094 40278077 40738057 40968015 41217947
            41217847 41117781 40777745 40507741 39977764 39547801
            39277843 38937932 38958077 39268109

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Jul 11, 2:40 PM EDT	Severe Thunderstorm Watch	SPC	Allegany, Garrett	8 hr
Jul 11, 5:41 PM EDT	Mesoscale Discussion	SPC	Bedford, Blair, Cambria, Centre, Clearfield, Clinton and 12 more	94 min

Nearby

Mesoscale Discussions in Pennsylvania, 2021 Mesoscale Discussions in West 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.