Alert archive / Pennsylvania / Mesoscale Discussions in 2021
Mesoscale Discussion 1882
The Storm Prediction Center issued this mesoscale discussion at Oct 21, 2021, 7:56 PM EDT for 12 counties in Pennsylvania; Marshall County, Ohio County, West Virginia. It was in force for 2 hr.
The area
The outline as issued.
The facts
- Issued
- Oct 21, 2021, 7:56 PM EDT (23:56 UTC)
- Expired
- Oct 21, 2021, 10:00 PM EDT
- In force
- 2 hr
- Issued by
- the Storm Prediction Center
- Concerning
- Severe potential...watch unlikely
- Chance of a watch
- 20%
Counties Allegheny County, PA, Armstrong County, PA, Butler County, PA, Cambria County, PA, Clarion County, PA, Clearfield County, PA, Fayette County, PA, Greene County, PA, Indiana County, PA, Jefferson County, PA, Washington County, PA, Westmoreland County, PA, Marshall County, WV, Ohio County, WV
The radar
The Pittsburgh radar (KPBZ), 67 km from the middle of the area, began a volume at 23:55 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
Pittsburgh, PA (KPIT), launched 12:00Z, 11.9 hours before the alert went out and 56 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
- 0 J/kg
- Mixed-layer CIN
- No buoyant parcel
- Mixed-layer LCL
- 1175 m above ground
- 0-6 km bulk shear
- 48 kt
- 0-1 km storm-relative helicity
- 105 m²/s²
- 0-3 km storm-relative helicity
- 188 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 0.0
The text
As transmitted.
116
ACUS11 KWNS 212357
SWOMCD
SPC MCD 212356
PAZ000-WVZ000-OHZ000-220200-
Mesoscale Discussion 1882
NWS Storm Prediction Center Norman OK
0656 PM CDT Thu Oct 21 2021
Areas affected...Portions of western Pennsylvania
Concerning...Severe potential...Watch unlikely
Valid 212356Z - 220200Z
Probability of Watch Issuance...20 percent
SUMMARY...The threat for a couple of tornadoes is expected to
persist for a couple more hours before storms weaken and the severe
threat gradually diminishes. Given the expected short duration
tornado threat, a Tornado Watch issuance remains unlikely.
DISCUSSION...Multiple discrete/semi-discrete supercells continue to
exhibit instances of strong low-level rotation, from Butler County
PA to Ohio County WV. Multiple TDS signatures were noted with some
of these storms in the past few minutes via KPBZ dual-polarimetric
radar data. These storms continue to rotate within an environment
characterized by weak buoyancy (MLCAPE barely reaching 500 J/kg
MLCAPE), but with strong low-level shear. Specifically, PBZ VAD
profiler data have shown consistent elongated hodographs with 200+
0-1km SRH, with METAR observations also showing some localized
backing of the surface winds across southwest PA. As such, storm
organization and periods of low-level rotation/brief tornadoes will
likely continue for a couple more hours, before boundary-layer
stabilization results in a gradual waning of the severe threat. A
Tornado Watch issuance is not expected.
..Squitieri/Dial.. 10/21/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...CTP...PBZ...
LAT...LON 39718099 40388039 41097992 41307939 41307862 41137820
40797816 40637825 40247871 39977907 39817933 39727973
39628048 39718099SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Oct 21, 2:39 PM EDT | Mesoscale Discussion | SPC | Allegheny, Beaver, Butler, Clarion, Crawford, Erie and 6 more | 3 hr |
| Oct 21, 5:25 PM EDT | Mesoscale Discussion | SPC | Lawrence, Mercer | 2 hr |
Nearby
Mesoscale Discussions in Pennsylvania, 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.