Alert archive / Pennsylvania / Mesoscale Discussions in 2021

Mesoscale Discussion 1388

July 29, 20214 states

The Storm Prediction Center issued this mesoscale discussion at Jul 29, 2021, 1:21 PM EDT for 4 states. It was in force for 114 min.

The area

The outline as issued.

The facts

Issued: Jul 29, 2021, 1:21 PM EDT (17:21 UTC)
Expired: Jul 29, 2021, 3:15 PM EDT
In force: 114 min
Issued by: the Storm Prediction Center
Concerning: Severe potential...watch possible
Chance of a watch: 40%

Counties Carroll County, MD, Frederick County, MD, Washington County, MD, Adams County, PA, Bedford County, PA, Blair County, PA, Cumberland County, PA, Dauphin County, PA, Franklin County, PA, Fulton County, PA, Huntingdon County, PA, Juniata County, PA, Mifflin County, PA, Perry County, PA, Snyder County, PA, York County, PA, Loudoun County, VA, Berkeley County, WV, Jefferson County, WV, Morgan County, WV

The radar

The State College radar (KCCX), 86 km from the middle of the area, began a volume at 17:22 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

Washington/Dulle, VA (KIAD), launched 12:00Z, 5.4 hours before the alert went out and 145 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: 269 J/kg
Mixed-layer CIN: -91 J/kg
Mixed-layer LCL: 893 m above ground
0-6 km bulk shear: 39 kt
0-1 km storm-relative helicity: 62 m²/s²
0-3 km storm-relative helicity: 62 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.2

Open this sounding in Storm Lab

The text

As transmitted.

395 
ACUS11 KWNS 291722
SWOMCD
SPC MCD 291721 
PAZ000-MDZ000-VAZ000-WVZ000-291915-

Mesoscale Discussion 1388
NWS Storm Prediction Center Norman OK
1221 PM CDT Thu Jul 29 2021

Areas affected...Central/South-Central PA...Central MD...Extreme
Northern VA

Concerning...Severe potential...Watch possible 

Valid 291721Z - 291915Z

Probability of Watch Issuance...40 percent

SUMMARY...Increase in storm coverage and intensity possible, with a
corresponding threat for damaging wind gusts.

DISCUSSION...Convectively induced shortwave trough continues to
progress quickly eastward across the upper OH Valley. Strongest
forcing for ascent within this shortwave appears to be moving
through central and southwest PA now, demarcated by a line of
slightly higher reflectivity from roughly Clearfield County
southwestward through Greene County. Estimated motion with the
northern/central portion of the line is eastward at around 45 kt.
Southern portion of this line appears to be a bit slower, with storm
motion estimated to be east-southeastward at 35 kt. In both cases,
this area of stronger ascent will continue to move eastward across
PA and adjacent northern WV/western MD as the downstream air mass
destabilizes.

Abundant mid to upper level clouds are obscuring the low-level cloud
field as well as tempering heating. Even so, some cumulus can still
be seen across eastern areas of the region and temperatures have
risen into the upper 70s/low 80s. Based on modified RAP forecast
soundings, any remaining convective inhibition is minimal.
Consequently, an increase in storm coverage and intensity seems
possible as this shortwave continues eastward into this region over
the next hour or two. The overall environment supports bowing line
segments capable of damaging wind gusts with any more robust,
long-lived storms.

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

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

ATTN...WFO...CTP...LWX...

LAT...LON   40877782 40887711 40747663 40237645 39397690 39017767
            39737881 40547830 40877782

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Jul 29, 11:58 AM EDT	Mesoscale Discussion	SPC	Allegany, Garrett	2 hr
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	Allegheny, Armstrong, Beaver, Butler, Cambria, Clearfield and 6 more	7 hr
Jul 29, 6:34 PM EDT	Mesoscale Discussion	SPC	Allegheny, Beaver, Fayette, Greene, Washington	116 min
Jul 29, 7:09 PM EDT	Mesoscale Discussion	SPC	Philadelphia	96 min

Nearby

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