Alert archive / Pennsylvania / Mesoscale Discussions in 2021
Mesoscale Discussion 1555
The Storm Prediction Center issued this mesoscale discussion at Aug 18, 2021, 3:29 PM EDT for 4 states. It was in force for 91 min.
The area
The outline as issued.
The facts
- Issued
- Aug 18, 2021, 3:29 PM EDT (19:29 UTC)
- Expired
- Aug 18, 2021, 5:00 PM EDT
- In force
- 91 min
- Issued by
- the Storm Prediction Center
- Concerning
- Tornado watch 447
All 55 counties
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, Washington County, MD, Adams County, PA, Berks County, PA, Blair County, PA, Carbon County, PA, Centre County, PA, Chester County, PA, Clinton County, PA, Columbia County, PA, Cumberland County, PA, Dauphin County, PA, Franklin County, PA, Fulton County, PA, Huntingdon County, PA, Juniata County, PA, Lackawanna County, PA, Lancaster County, PA, Lebanon County, PA, Lehigh County, PA, Luzerne County, PA, Lycoming County, PA, Mifflin County, PA, Monroe County, PA, Montour County, PA, Northampton County, PA, Northumberland County, PA, Perry County, PA, Schuylkill County, PA, Snyder County, PA, Sullivan County, PA, Union County, PA, Wyoming County, PA, York County, PA, Arlington County, VA, City of Fairfax, VA, City of Falls Church, VA, City of Manassas Park, VA, City of Winchester, VA, Clarke County, VA, Fairfax County, VA, Frederick County, VA, Loudoun County, VA, Berkeley County, WV, Jefferson County, WV, Morgan County, WV
The radar
The State College radar (KCCX), 99 km from the middle of the area, began a volume at 19:33 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, 7.5 hours before the alert went out and 155 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
- 1011 J/kg
- Mixed-layer CIN
- -1 J/kg
- Mixed-layer LCL
- 592 m above ground
- 0-6 km bulk shear
- 30 kt
- 0-1 km storm-relative helicity
- 151 m²/s²
- 0-3 km storm-relative helicity
- 175 m²/s²
- Significant tornado parameter
- 0.6
- Supercell composite
- 4.6
The text
As transmitted.
189
ACUS11 KWNS 181930
SWOMCD
SPC MCD 181929
PAZ000-MDZ000-DCZ000-VAZ000-WVZ000-182100-
Mesoscale Discussion 1555
NWS Storm Prediction Center Norman OK
0229 PM CDT Wed Aug 18 2021
Areas affected...Central/eastern PA...northern
VA...MD...DC...eastern WV Panhandle
Concerning...Tornado Watch 447...
Valid 181929Z - 182100Z
The severe weather threat for Tornado Watch 447 continues.
SUMMARY...The tornado and damaging-wind threat will continue to
gradually increase this afternoon.
DISCUSSION...While cells with occasional weak rotation have been
noted recently across central PA and northern MD, supercell
structures have this far struggled to be sustained across WW 447.
Earlier discrete cells may have struggled to mature in the midlevel
dry slot (noted in WV imagery), but renewed development may be more
vigorous this afternoon as the dry slot shifts eastward and ascent
associated with the primary midlevel vorticity maximum spreads over
the region.
Low-level shear/SRH remains sufficient to support some tornado
threat with any sustained supercell. The greatest relative tornado
risk may reside across central PA in the vicinity of the
increasingly diffuse warm front, as convection deepens to the south
and moves through the baroclinic zone this afternoon. However, any
sustained cell within the warm sector will also pose some threat.
Locally damaging wind gusts will also remain possible, especially in
areas where some steepening of low-level lapse rates has occurred.
..Dean.. 08/18/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...PHI...BGM...CTP...LWX...
LAT...LON 39697854 40127844 40867847 41357793 41757700 41707616
41537530 41327509 40817517 39737580 39067618 38737742
39137851 39457848 39697854SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Aug 18, 8:46 AM EDT | Mesoscale Discussion | SPC | Frederick, Washington | 119 min |
| Aug 18, 10:58 AM EDT | Mesoscale Discussion | SPC | Anne Arundel, Baltimore, Calvert, Carroll, Charles, Frederick and 9 more | 92 min |
| Aug 18, 11:37 AM EDT | Tornado Watch | SPC | Allegany, Anne Arundel, Baltimore, Calvert, Carroll, Cecil and 12 more | 11 hr |
| Aug 18, 4:57 PM EDT | Mesoscale Discussion | SPC | Anne Arundel, Baltimore, Carroll, Cecil, Frederick, Harford and 6 more | 93 min |
| Aug 17, 10:04 PM EDT | Mesoscale Discussion | SPC | Bedford, Bland, Botetourt, Carroll, Craig, Floyd and 17 more | 116 min |
| Aug 18, 2:38 PM EDT | Mesoscale Discussion | SPC | Charles City, Gloucester, Isle of Wight, James City, King and Queen, Lancaster and 20 more | 97 min |
Nearby
Mesoscale Discussions in Pennsylvania, 2021Mesoscale Discussions in Maryland, 2021Mesoscale Discussions in 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.