Alert archive / Pennsylvania / Mesoscale Discussions in 2021
Mesoscale Discussion 1136
The Storm Prediction Center issued this mesoscale discussion at Jun 30, 2021, 4:41 PM EDT for 4 states. It was in force for 2 hr.
The area
The outline as issued.
The facts
- Issued
- Jun 30, 2021, 4:41 PM EDT (20:41 UTC)
- Expired
- Jun 30, 2021, 6:45 PM EDT
- In force
- 2 hr
- Issued by
- the Storm Prediction Center
- Concerning
- Severe thunderstorm watch 328
All 41 counties
Allegany County, MD, Baltimore County, MD, Carroll County, MD, Frederick County, MD, Garrett County, MD, Howard County, MD, Montgomery County, MD, Washington County, MD, Adams County, PA, Bedford County, PA, Berks County, PA, Blair County, PA, Columbia County, PA, Cumberland County, PA, Dauphin County, PA, Fayette County, PA, Franklin County, PA, Fulton County, PA, Huntingdon County, PA, Juniata County, PA, Lancaster County, PA, Lebanon County, PA, Mifflin County, PA, Montour County, PA, Northumberland County, PA, Perry County, PA, Schuylkill County, PA, Snyder County, PA, Somerset County, PA, Union County, PA, York County, PA, City of Winchester, VA, Clarke County, VA, Frederick County, VA, Berkeley County, WV, Hampshire County, WV, Jefferson County, WV, Mineral County, WV, Monongalia County, WV, Morgan County, WV, Preston County, WV
The radar
The Sterling radar (KLWX), 108 km from the middle of the area, began a volume at 20:44 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, 8.7 hours before the alert went out and 113 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.
- Mixed-layer CAPE
- 40 J/kg
- Mixed-layer CIN
- -217 J/kg
- Mixed-layer LCL
- 1553 m above ground
- 0-6 km bulk shear
- 14 kt
- 0-1 km storm-relative helicity
- -26 m²/s²
- 0-3 km storm-relative helicity
- 76 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 0.0
The text
As transmitted.
816
ACUS11 KWNS 302041
SWOMCD
SPC MCD 302041
PAZ000-MDZ000-VAZ000-WVZ000-302245-
Mesoscale Discussion 1136
NWS Storm Prediction Center Norman OK
0341 PM CDT Wed Jun 30 2021
Areas affected...Southern Pennsylvania and the West
Virginia...Maryland...Virginia border
Concerning...Severe Thunderstorm Watch 328...
Valid 302041Z - 302245Z
The severe weather threat for Severe Thunderstorm Watch 328
continues.
SUMMARY...An isolated severe wind threat will continue into the
early evening hours across parts of WW 328, as well as across
southern/southeast Pennsylvania and the broader northeast WV,
northern VA, and western MD region. A downstream watch will be
issued to address this potential.
DISCUSSION...Recent regional radar and IR imagery shows convection
gradually decreasing across southwest PA as a cold pool becomes
established (as noted in recent surface observations). Convective
development across PA has largely become confined to outflow
boundaries on the periphery of this air mass where the thermodynamic
conditions remain favorable for a supporting strong to severe
storms. The severe threat will be greatest on the southern and
eastern edges of the cold pool where temperatures in the mid/upper
90s are maintaining steep low-level lapse rates that will support
damaging winds with any stronger downbursts.
Storms developing off the terrain near the WV/MD/VA border should
move into southern to southeastern PA over the next few hours and
may produce a few instances of damaging winds. The extent of the
threat may largely be determined by convective coverage, which
appears localized at this point. However, one or two organized
clusters could evolve in this environment and pose a more
substantial threat. Despite the conditional nature of the threat, a
watch will be issued to address this potential.
..Moore.. 06/30/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...PHI...BGM...CTP...LWX...PBZ...RLX...
LAT...LON 39728018 40127916 40657800 41117714 41127623 40777573
40377554 39897601 39437653 39087716 39097788 39077864
39187924 39337990 39728018SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Jun 30, 4:51 PM EDT | Severe Thunderstorm Watch | SPC | Allegany, Carroll, Frederick, Washington | 3 hr |
| Jun 30, 6:58 PM EDT | Mesoscale Discussion | SPC | Carroll, Frederick, Montgomery | 92 min |
| Jun 30, 11:30 AM EDT | Mesoscale Discussion | SPC | Allegheny, Armstrong, Beaver, Blair, Butler, Cambria and 23 more | 2 hr |
| Jun 30, 11:47 AM EDT | Severe Thunderstorm Watch | SPC | Allegheny, Armstrong, Beaver, Butler, Clarion, Fayette and 9 more | 7 hr |
| Jun 30, 12:46 PM EDT | Severe Thunderstorm Watch | SPC | Bedford, Blair, Cambria, Cameron, Centre, Clearfield and 20 more | 7 hr |
| Jun 30, 2:16 PM EDT | Mesoscale Discussion | SPC | Carbon, Lackawanna, Luzerne, Monroe, Pike, Wayne and 1 more | 119 min |
| Jun 30, 3:21 PM EDT | Severe Thunderstorm Watch | SPC | Berks, Bradford, Carbon, Lackawanna, Lehigh, Luzerne and 6 more | 7 hr |
| Jun 30, 3:29 PM EDT | Mesoscale Discussion | SPC | Allegheny, Armstrong, Bedford, Blair, Cambria, Centre and 16 more | 2 hr |
| Jun 30, 4:48 PM EDT | Mesoscale Discussion | SPC | Lackawanna, Susquehanna, Wayne | 87 min |
Nearby
Mesoscale Discussions in Pennsylvania, 2021Mesoscale Discussions in Maryland, 2021Mesoscale Discussions in West Virginia, 2021Every 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.