Alert archive / Pennsylvania / Mesoscale Discussions in 2021
Mesoscale Discussion 1481
The Storm Prediction Center issued this mesoscale discussion at Aug 11, 2021, 2:03 PM EDT for 48 counties in Pennsylvania. It was in force for 87 min.
The area
The outline as issued.
The facts
- Issued
- Aug 11, 2021, 2:03 PM EDT (18:03 UTC)
- Expired
- Aug 11, 2021, 3:30 PM EDT
- In force
- 87 min
- Issued by
- the Storm Prediction Center
- Concerning
- Severe potential...watch possible
- Chance of a watch
- 60%
All 48 counties
Adams County, PA, Armstrong County, PA, Bedford County, PA, Berks County, PA, Blair County, PA, Bucks County, PA, Cambria County, PA, Cameron County, PA, Carbon County, PA, Centre County, PA, Chester County, PA, Clarion County, PA, Clearfield County, PA, Clinton County, PA, Columbia County, PA, Cumberland County, PA, Dauphin County, PA, Delaware County, PA, Elk County, PA, Fayette County, PA, Forest County, PA, Franklin County, PA, Fulton County, PA, Greene County, PA, Huntingdon County, PA, Indiana County, PA, Jefferson County, PA, Juniata County, PA, Lancaster County, PA, Lebanon County, PA, Lehigh County, PA, Luzerne County, PA, Lycoming County, PA, Mifflin County, PA, Monroe County, PA, Montgomery County, PA, Montour County, PA, Northampton County, PA, Northumberland County, PA, Perry County, PA, Schuylkill County, PA, Snyder County, PA, Somerset County, PA, Sullivan County, PA, Union County, PA, Washington County, PA, Westmoreland County, PA, York County, PA
The radar
The State College radar (KCCX), 76 km from the middle of the area, began a volume at 18:07 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, 6.1 hours before the alert went out and 169 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
- 553 J/kg
- Mixed-layer CIN
- -190 J/kg
- Mixed-layer LCL
- 1301 m above ground
- 0-6 km bulk shear
- 13 kt
- 0-1 km storm-relative helicity
- 20 m²/s²
- 0-3 km storm-relative helicity
- 77 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 0.0
The text
As transmitted.
851
ACUS11 KWNS 111804
SWOMCD
SPC MCD 111803
NJZ000-PAZ000-DEZ000-MDZ000-WVZ000-111930-
Mesoscale Discussion 1481
NWS Storm Prediction Center Norman OK
0103 PM CDT Wed Aug 11 2021
Areas affected...Portions of Pennsylvania
Concerning...Severe potential...Watch possible
Valid 111803Z - 111930Z
Probability of Watch Issuance...60 percent
SUMMARY...A damaging wind threat is increasing across portions of
southern Pennsylvania.
DISCUSSION...A cluster of storms has had intermittently stronger
intensity at times as it has moved from northern Ohio into western
Pennsylvania. However, over the past 30 minutes, it appears storm
activity on the leading edge has started to realize the increasing
surface based instability ahead of this line of storms as the line
has filled in and reflectivity has become more intense. There is a
well established cold pool associated with this activity with
temperatures in the upper 60s to near 70 in its wake. Given this
well-established cold pool, combined with MLCAPE around 2500 J/kg,
DCAPE around 1000 J/kg, and effective shear around 25 knots per SPC
mesoanalysis, downstream persistence/potential strengthening is
expected.
In addition, thunderstorm activity is anticipated ahead of this
line. While this activity is not expected to be overly organized, a
favorable thermodynamic environment will support strong
updraft/downdraft pairs and the potential for waterloaded downdrafts
capable of isolated damaging wind gusts.
Therefore, given these factors, a severe thunderstorm watch may be
needed across this region.
..Bentley/Thompson.. 08/11/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...PHI...BGM...CTP...LWX...PBZ...
LAT...LON 40157507 40007514 39867538 39837559 39697606 39737824
39738050 40088053 40477988 40837967 41057960 41507935
41697796 41427622 41147534 40637513 40157507SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Aug 11, 9:31 AM EDT | Mesoscale Discussion | SPC | Lawrence, Mercer | 89 min |
| Aug 11, 11:50 AM EDT | Mesoscale Discussion | SPC | Allegheny, Armstrong, Beaver, Butler, Cambria, Clarion and 12 more | 115 min |
| Aug 11, 3:02 PM EDT | Severe Thunderstorm Watch | SPC | Adams, Bedford, Berks, Blair, Bucks, Carbon and 26 more | 6 hr |
| Aug 11, 4:57 PM EDT | Severe Thunderstorm Watch | SPC | Crawford, Erie, Lawrence, Mercer, Venango | 3 hr |
| Aug 11, 6:44 PM EDT | Mesoscale Discussion | SPC | Berks, Bucks, Carbon, Chester, Delaware, Lancaster and 6 more | 91 min |
Nearby
Mesoscale Discussions in Pennsylvania, 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.