Alert archive / Pennsylvania / Mesoscale Discussions in 2021
Mesoscale Discussion 1391
The Storm Prediction Center issued this mesoscale discussion at Jul 29, 2021, 4:26 PM EDT for 4 states. It was in force for 94 min.
The area
The outline as issued.
The facts
- Issued
- Jul 29, 2021, 4:26 PM EDT (20:26 UTC)
- Expired
- Jul 29, 2021, 6:00 PM EDT
- In force
- 94 min
- Issued by
- the Storm Prediction Center
- Concerning
- Tornado watch 401
All 35 counties
Kent County, DE, New Castle County, DE, Anne Arundel County, MD, Baltimore County, MD, Carroll County, MD, Cecil County, MD, City of Baltimore, MD, Frederick County, MD, Harford County, MD, Howard County, MD, Kent County, MD, Montgomery County, MD, Queen Anne's County, MD, Burlington County, NJ, Camden County, NJ, Cumberland County, NJ, Gloucester County, NJ, Hunterdon County, NJ, Mercer County, NJ, Salem County, NJ, Somerset County, NJ, Warren County, NJ, Berks County, PA, Bucks County, PA, Carbon County, PA, Chester County, PA, Delaware County, PA, Lancaster County, PA, Lebanon County, PA, Lehigh County, PA, Monroe County, PA, Montgomery County, PA, Northampton County, PA, Philadelphia County, PA, York County, PA
The radar
The Philadelphia radar (KDIX), 122 km from the middle of the area, began a volume at 20:26 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 12:00Z, 8.4 hours before the alert went out and 183 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
- 269 J/kg
- Mixed-layer CIN
- -91 J/kg
- Mixed-layer LCL
- 893 m above ground
- 0-6 km bulk shear
- 39 kt
- 0-1 km storm-relative helicity
- 62 m²/s²
- 0-3 km storm-relative helicity
- 62 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 0.2
The text
As transmitted.
607
ACUS11 KWNS 292027
SWOMCD
SPC MCD 292026
NJZ000-PAZ000-DEZ000-MDZ000-292200-
Mesoscale Discussion 1391
NWS Storm Prediction Center Norman OK
0326 PM CDT Thu Jul 29 2021
Areas affected...Southeastern PA...NJ...Northern VA...Central
MD...Delmarva
Concerning...Tornado Watch 401...
Valid 292026Z - 292200Z
The severe weather threat for Tornado Watch 401 continues.
SUMMARY...Threat for damaging wind gusts and/or brief tornadoes will
continue for the next few hours.
DISCUSSION...A large area of showers and thunderstorms continues to
move eastward across the northern Mid-Atlantic. Much of this
activity has remained shallow and transient. However, a few more
robust updrafts have been able to develop, including two which are
currently ongoing over MD, one over Carroll County MD and the other
over Howard County MD, and another over Lehigh County PA. Low to
mid-level rotation has been noted in each one of these storms,
suggesting the slightly stronger flow associated with this shortwave
trough (sampled well by recent LWX VAD profiles) coupled with the
southerly surface winds is enough to promote updraft rotation. This
current trend of transient updrafts capable of damaging wind gusts
and/or brief tornadoes will likely continue for the next few hours.
..Mosier.. 07/29/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...OKX...PHI...CTP...LWX...
LAT...LON 40877596 41147542 40787442 39667467 38997507 38787661
39347758 40167678 40877596SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Jul 29, 2:23 PM EDT | Tornado Watch | SPC | Kent, New Castle, Sussex | 8 hr |
| Jul 29, 11:58 AM EDT | Mesoscale Discussion | SPC | Allegany, Garrett | 2 hr |
| Jul 29, 1:21 PM EDT | Mesoscale Discussion | SPC | Carroll, Frederick, Washington | 114 min |
| Jul 29, 3:00 PM EDT | Severe Thunderstorm Watch | SPC | Allegany, Calvert, Charles, Dorchester, Garrett, Montgomery and 5 more | 6 hr |
| Jul 29, 7:09 PM EDT | Mesoscale Discussion | SPC | Atlantic, Burlington, Camden, Gloucester, Mercer, Middlesex and 2 more | 96 min |
| Jul 29, 3:13 PM EDT | Severe Thunderstorm Watch | SPC | Allegheny, Armstrong, Beaver, Butler, Cambria, Clearfield and 6 more | 7 hr |
| Jul 29, 6:34 PM EDT | Mesoscale Discussion | SPC | Allegheny, Beaver, Fayette, Greene, Washington | 116 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.