Alert archive / Pennsylvania / Mesoscale Discussions in 2021
Mesoscale Discussion 1721
The Storm Prediction Center issued this mesoscale discussion at Sep 8, 2021, 10:42 PM EDT for 4 states. It was in force for 2 hr.
The area
The outline as issued.
The facts
- Issued
- Sep 8, 2021, 10:42 PM EDT (02:42 UTC)
- Expired
- Sep 9, 2021, 12:45 AM EDT
- In force
- 2 hr
- Issued by
- the Storm Prediction Center
- Concerning
- Severe thunderstorm watch 491...492
All 25 counties
Baltimore County, MD, Carroll County, MD, Cecil County, MD, City of Baltimore, MD, Harford County, MD, Howard County, MD, Montgomery County, MD, Essex County, NJ, Hunterdon County, NJ, Morris County, NJ, Passaic County, NJ, Somerset County, NJ, Union County, NJ, Warren County, NJ, Berks County, PA, Bucks County, PA, Chester County, PA, Delaware County, PA, Lancaster County, PA, Lehigh County, PA, Montgomery County, PA, Northampton County, PA, Arlington County, VA, City of Falls Church, VA, Loudoun County, VA
The radar
The Dover AFB radar (KDOX), 134 km from the middle of the area, began a volume at 02:44 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 nearest sounding before it
Washington/Dulle, VA (KIAD), launched 00:00Z, 2.7 hours before the alert went out and 148 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
- 1652 J/kg
- Mixed-layer CIN
- -18 J/kg
- Mixed-layer LCL
- 1146 m above ground
- 0-6 km bulk shear
- 43 kt
- 0-1 km storm-relative helicity
- 115 m²/s²
- 0-3 km storm-relative helicity
- 136 m²/s²
- Significant tornado parameter
- 1.0
- Supercell composite
- 7.0
The text
As transmitted.
279
ACUS11 KWNS 090242
SWOMCD
SPC MCD 090242
NJZ000-NYZ000-PAZ000-DEZ000-MDZ000-DCZ000-VAZ000-090445-
Mesoscale Discussion 1721
NWS Storm Prediction Center Norman OK
0942 PM CDT Wed Sep 08 2021
Areas affected...Northern Middle Atlantic
Concerning...Severe Thunderstorm Watch 491...492...
Valid 090242Z - 090445Z
The severe weather threat for Severe Thunderstorm Watch 491, 492
continues.
SUMMARY...Damaging winds are possible with squall line over the next
couple hours.
DISCUSSION...Well-defined squall line has matured and currently
extends from northern NJ-eastern PA-western MD-northern VA. Over the
last hour or so, a substantial north-south line segment evolved over
southern PA and this line has advanced into Berks County PA, with
some noted acceleration in the last half hour promoting a more
bow-shaped structure. It appears the greatest short-term wind threat
will be ahead of this convection as it advances across Berks County
toward northwestern NJ.
..Darrow.. 09/09/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...OKX...PHI...CTP...LWX...
LAT...LON 38967802 39777677 40597601 41227424 40757408 40007517
38837709 38967802SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Sep 8, 8:29 PM EDT | Severe Thunderstorm Watch | SPC | Anne Arundel, Baltimore, Carroll, Cecil, Frederick, Harford and 5 more | 4 hr |
| Sep 8, 9:25 PM EDT | Mesoscale Discussion | SPC | Hunterdon, Morris, Passaic, Somerset, Sussex, Warren | 95 min |
Nearby
Mesoscale Discussions in Pennsylvania, 2021Mesoscale Discussions in Maryland, 2021Mesoscale Discussions in New Jersey, 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.