Alert archive / Pennsylvania / Mesoscale Discussions in 2021
Mesoscale Discussion 1739
The Storm Prediction Center issued this mesoscale discussion at Sep 13, 2021, 7:07 PM EDT for 11 counties in Pennsylvania. It was in force for 113 min.
The area
The outline as issued.
The facts
- Issued
- Sep 13, 2021, 7:07 PM EDT (23:07 UTC)
- Expired
- Sep 13, 2021, 9:00 PM EDT
- In force
- 113 min
- Issued by
- the Storm Prediction Center
- Concerning
- Severe thunderstorm watch 496
Counties Carbon County, PA, Centre County, PA, Clearfield County, PA, Columbia County, PA, Jefferson County, PA, Luzerne County, PA, Montour County, PA, Northumberland County, PA, Schuylkill County, PA, Snyder County, PA, Union County, PA
The radar
The State College radar (KCCX), 72 km from the middle of the area, began a volume at 23:10 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, 11.1 hours before the alert went out and 235 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
- 217 J/kg
- Mixed-layer CIN
- -593 J/kg
- Mixed-layer LCL
- 1400 m above ground
- 0-6 km bulk shear
- 33 kt
- 0-1 km storm-relative helicity
- 116 m²/s²
- 0-3 km storm-relative helicity
- 107 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 0.0
The text
As transmitted.
909
ACUS11 KWNS 132307
SWOMCD
SPC MCD 132307
PAZ000-140100-
Mesoscale Discussion 1739
NWS Storm Prediction Center Norman OK
0607 PM CDT Mon Sep 13 2021
Areas affected...Parts of central and eastern Pennsylvania
Concerning...Severe Thunderstorm Watch 496...
Valid 132307Z - 140100Z
The severe weather threat for Severe Thunderstorm Watch 496
continues.
SUMMARY...The risk for damaging wind gusts may gradually increase
with an evolving and organizing cluster of storms near the
Williamsport vicinity into areas between Wilkes-Barre and Allentown
by 9-10 PM EDT. It is possible a new severe weather watch may
become needed east of WW 496.
DISCUSSION...Thunderstorm development continues to increase aided by
low-level confluence along a weak frontal zone. Forcing for ascent
accompanying a remnant MCV and associated mid-level speed maximum
emerging from the lower Great Lakes region is also probably
contributing to development, and the environment appears favorable
for upscale growth and the evolution of an organizing convective
system.
A moist boundary layer along the southeastern slopes of the
Allegheny Front appears characterized by sizable CAPE up to around
2000 J/kg, beneath steep lower/mid tropospheric lapse rates on the
northern periphery of a remnant plume of elevated mixed-layer air.
Deep-layer shear is strong beneath a belt of 40-50 kt
west-northwesterly flow around 500 mb.
Although large hail and localized strong gusts currently appear the
most prominent severe hazards in isolated stronger cells, as
activity grows upscale, heavy precipitation loading may gradually
contribute to the downward transport of momentum on a broader scale,
with increasing risk for damaging surface gusts. The best potential
for this appears to be with an ongoing clustering of intensifying
convection currently still north of Williamsport, as it develops
east-southeastward into areas west through north of the Allentown
vicinity by 01-02Z.
..Kerr.. 09/13/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...PHI...BGM...CTP...PBZ...
LAT...LON 41217782 41287735 41327672 41227615 41217550 40817563
40597669 40677743 40867838 41197927 41217782SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Sep 12, 10:25 PM EDT | Severe Thunderstorm Watch | SPC | Erie | 4 hr |
| Sep 13, 12:26 AM EDT | Mesoscale Discussion | SPC | Susquehanna | 94 min |
| Sep 13, 4:11 PM EDT | Mesoscale Discussion | SPC | Cameron, Clinton, Elk, Erie, Forest, McKean and 2 more | 2 hr |
| Sep 13, 5:53 PM EDT | Severe Thunderstorm Watch | SPC | Cameron, Centre, Clarion, Clearfield, Clinton, Columbia and 14 more | 3 hr |
| Sep 13, 7:44 PM EDT | Severe Thunderstorm Watch | SPC | Bucks, Carbon, Lehigh, Luzerne, Monroe, Northampton | 4 hr |
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.