Alert archive / Pennsylvania / Mesoscale Discussions in 2021
Mesoscale Discussion 1557
The Storm Prediction Center issued this mesoscale discussion at Aug 18, 2021, 4:57 PM EDT for 4 states. It was in force for 93 min.
The area
The outline as issued.
The facts
- Issued
- Aug 18, 2021, 4:57 PM EDT (20:57 UTC)
- Expired
- Aug 18, 2021, 6:30 PM EDT
- In force
- 93 min
- Issued by
- the Storm Prediction Center
- Concerning
- Tornado watch 447
All 38 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, Prince George's County, MD, Washington County, MD, Adams County, PA, Berks County, PA, Chester County, PA, Columbia County, PA, Cumberland County, PA, Dauphin County, PA, Franklin County, PA, Juniata County, PA, Lancaster County, PA, Lebanon County, PA, Luzerne County, PA, Mifflin County, PA, Montour County, PA, Northumberland County, PA, Perry County, PA, Schuylkill County, PA, Snyder County, PA, Union County, PA, York County, PA, Arlington County, VA, City of Alexandria, VA, City of Fairfax, VA, City of Falls Church, VA, Fairfax County, VA, Loudoun County, VA, Jefferson County, WV
The radar
The Sterling radar (KLWX), 105 km from the middle of the area, began a volume at 20:58 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, 9.0 hours before the alert went out and 104 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.
247
ACUS11 KWNS 182057
SWOMCD
SPC MCD 182057
PAZ000-MDZ000-DCZ000-VAZ000-WVZ000-182230-
Mesoscale Discussion 1557
NWS Storm Prediction Center Norman OK
0357 PM CDT Wed Aug 18 2021
Areas affected...MD...DC...northern VA...central/eastern PA
Concerning...Tornado Watch 447...
Valid 182057Z - 182230Z
The severe weather threat for Tornado Watch 447 continues.
SUMMARY...The threat for a couple tornadoes and isolated damaging
wind gusts will continue into the early evening.
DISCUSSION...At 2045Z, robust convection has recently developed
across south-central PA, in conjunction with the primary midlevel
vorticity maximum associated with post-tropical cyclone Fred.
Further east, convection continues to occasionally flare up across
southeast PA and northern MD. The westernmost convective band now
moving across south-central PA should be maintained as it moves
eastward this evening. While the strongest low-level shear/SRH may
remain east of this band, occasional rotating cells will be possible
into this evening with the band, posing a threat of isolated
damaging wind gusts and a couple tornadoes.
Additional rotating cells will remain possible further east,
potentially evolving out of showers ongoing across southeast PA, or
along the southern edge of a cluster moving across central PA, where
a diffuse baroclinic zone remains in place. Some tornado threat may
still evolve with any sustained cells in this region, where
low-level shear/SRH remain somewhat enhanced.
..Dean.. 08/18/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...PHI...BGM...CTP...LWX...
LAT...LON 40857774 41267737 41237584 40317567 39767580 39457595
38977633 38797668 38797715 38857772 39127789 39467797
39957787 40857774SPC 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, 3:29 PM EDT | Mesoscale Discussion | SPC | Anne Arundel, Baltimore, Carroll, Cecil, Frederick, Harford and 5 more | 91 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.