Alert archive / Pennsylvania / Mesoscale Discussions in 2021
Mesoscale Discussion 1551
The Storm Prediction Center issued this mesoscale discussion at Aug 18, 2021, 10:58 AM EDT for 4 states. It was in force for 92 min.
The area
The outline as issued.
The facts
- Issued
- Aug 18, 2021, 10:58 AM EDT (14:58 UTC)
- Expired
- Aug 18, 2021, 12:30 PM EDT
- In force
- 92 min
- Issued by
- the Storm Prediction Center
- Concerning
- Severe potential...tornado watch likely
- Chance of a watch
- 80%
All 68 counties
Anne Arundel County, MD, Baltimore County, MD, Calvert County, MD, Carroll County, MD, Charles 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, Saint Mary's County, MD, Talbot County, MD, Washington County, MD, Adams County, PA, Armstrong County, PA, Bedford County, PA, Blair County, PA, Cambria County, PA, Cameron County, PA, Centre County, PA, Clearfield County, PA, Clinton County, PA, Columbia County, PA, Cumberland County, PA, Dauphin County, PA, Franklin County, PA, Fulton County, PA, Huntingdon County, PA, Indiana County, PA, Jefferson County, PA, Juniata County, PA, Lancaster County, PA, Lebanon County, PA, Lycoming County, PA, Mifflin County, PA, Montour County, PA, Northumberland County, PA, Perry County, PA, Snyder County, PA, Sullivan County, PA, Union County, PA, York County, PA, Arlington County, VA, City of Alexandria, VA, City of Fairfax, VA, City of Falls Church, VA, City of Fredericksburg, VA, City of Manassas, VA, City of Manassas Park, VA, City of Winchester, VA, Clarke County, VA, Culpeper County, VA, Fairfax County, VA, Fauquier County, VA, Frederick County, VA, King George County, VA, Loudoun County, VA, Prince William County, VA, Rappahannock County, VA, Spotsylvania County, VA, Stafford County, VA, Warren County, VA, Westmoreland County, VA, Berkeley County, WV, Jefferson County, WV, Morgan County, WV
The radar
The Sterling radar (KLWX), 73 km from the middle of the area, began a volume at 14:59 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, 3.0 hours before the alert went out and 76 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.
321
ACUS11 KWNS 181458
SWOMCD
SPC MCD 181458
PAZ000-MDZ000-VAZ000-DCZ000-WVZ000-181630-
Mesoscale Discussion 1551
NWS Storm Prediction Center Norman OK
0958 AM CDT Wed Aug 18 2021
Areas affected...Parts of northern VA...DC...MD...Eastern WV
Panhandle...Central/eastern PA
Concerning...Severe potential...Tornado Watch likely
Valid 181458Z - 181630Z
Probability of Watch Issuance...80 percent
SUMMARY...The tornado threat is expected to gradually increase into
this afternoon. Tornado Watch issuance is likely by 16Z.
DISCUSSION...At 1445Z, T.D. Fred is still moving northeastward near
the OH/WV border northeast of Parkersburg, WV. A warm front extends
east-northeast from the cyclone across southern PA, while a surface
confluence axis is noted extending from southwest PA into central
VA. Rich low-level moisture will continue to stream northward across
PA in conjunction with the warm front this afternoon, with some
breaks in the clouds allowing for some modest destabilization from
central PA southward into MD/northern VA.
Convection will likely increase in coverage with time near/north of
the warm front, with more scattered convection possible along/east
of the surface confluence axis into MD/northern VA. This afternoon,
0-1 km SRH will likely be maximized over central PA (potentially in
excess of 200 m2/s2), though somewhat enhanced SRH will also extend
southward into MD/VA where surface winds remain backed to a
southeasterly direction. Scattered supercells are likely to evolve
with time as convection matures, with an attendant threat of a few
tornadoes and isolated/scattered damaging wind gusts. Tornado Watch
issuance is likely by 16Z in response to these threats.
..Dean/Dial.. 08/18/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...PHI...BGM...AKQ...CTP...LWX...PBZ...
LAT...LON 39917870 40117924 40987955 41557793 41697687 41517648
39577607 38807613 38177625 38017683 38027722 38147775
38387826 39917870SPC 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, 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 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 Virginia, 2021Mesoscale Discussions in Maryland, 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.