Alert archive / Pennsylvania / Mesoscale Discussions in 2021
Mesoscale Discussion 1265
The Storm Prediction Center issued this mesoscale discussion at Jul 12, 2021, 1:32 PM EDT for 7 states. It was in force for 88 min.
The area
The outline as issued.
The facts
- Issued
- Jul 12, 2021, 1:32 PM EDT (17:32 UTC)
- Expired
- Jul 12, 2021, 3:00 PM EDT
- In force
- 88 min
- Issued by
- the Storm Prediction Center
- Concerning
- Severe potential...watch possible
- Chance of a watch
- 60%
All 105 counties
Kent County, DE, New Castle County, DE, Sussex County, DE, Allegany County, MD, Anne Arundel County, MD, Baltimore County, MD, Calvert County, MD, Caroline County, MD, Carroll County, MD, Cecil 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, Queen Anne's County, MD, Talbot County, MD, Washington County, MD, Atlantic County, NJ, Bergen County, NJ, Burlington County, NJ, Camden County, NJ, Cape May County, NJ, Cumberland County, NJ, Essex County, NJ, Gloucester County, NJ, Hudson County, NJ, Hunterdon County, NJ, Mercer County, NJ, Middlesex County, NJ, Monmouth County, NJ, Morris County, NJ, Ocean County, NJ, Passaic County, NJ, Salem County, NJ, Somerset County, NJ, Sussex County, NJ, Union County, NJ, Warren County, NJ, Bronx County, NY, Kings County, NY, Nassau County, NY, New York County, NY, Queens County, NY, Richmond County, NY, Adams County, PA, Bedford County, PA, Berks County, PA, Blair County, PA, Bucks County, PA, Carbon County, PA, Chester County, PA, Columbia County, PA, Cumberland County, PA, Dauphin County, PA, Delaware County, PA, Franklin County, PA, Fulton County, PA, Huntingdon County, PA, Juniata County, PA, Lancaster County, PA, Lebanon County, PA, Lehigh County, PA, Luzerne County, PA, Mifflin County, PA, Monroe County, PA, Montgomery County, PA, Montour County, PA, Northampton County, PA, Northumberland County, PA, Perry County, PA, Philadelphia County, PA, Pike 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, 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, Loudoun County, VA, Madison County, VA, Page County, VA, Prince William County, VA, Rappahannock County, VA, Shenandoah County, VA, Stafford County, VA, Warren County, VA, Berkeley County, WV, Hampshire County, WV, Hardy County, WV, Jefferson County, WV, Morgan County, WV
The radar
The Dover AFB radar (KDOX), 117 km from the middle of the area, began a volume at 17:35 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, 5.5 hours before the alert went out and 119 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
- 974 J/kg
- Mixed-layer CIN
- -83 J/kg
- Mixed-layer LCL
- 724 m above ground
- 0-6 km bulk shear
- 21 kt
- 0-1 km storm-relative helicity
- 10 m²/s²
- 0-3 km storm-relative helicity
- 53 m²/s²
- Significant tornado parameter
- 0.1
- Supercell composite
- 0.8
The text
As transmitted.
479
ACUS11 KWNS 121733
SWOMCD
SPC MCD 121732
NYZ000-CTZ000-NJZ000-PAZ000-DEZ000-MDZ000-DCZ000-VAZ000-WVZ000-121900-
Mesoscale Discussion 1265
NWS Storm Prediction Center Norman OK
1232 PM CDT Mon Jul 12 2021
Areas affected...Portions of the Northeast
Concerning...Severe potential...Watch possible
Valid 121732Z - 121900Z
Probability of Watch Issuance...60 percent
SUMMARY...A few multicell clusters of storms should emerge by mid to
late afternoon, posing a threat for damaging winds. Trends will
continue to be monitored for a possible severe thunderstorm watch.
DISCUSSION...Visible satellite shows morning clearing over the
region has allowed surface temperatures to warm into the mid to
upper 80s F amid dew point temperatures in the low 70s F. This has
led to somewhat rapid destabilization, with moderate instability
(MLCAPE 2000-2500 J/kg) being analyzed on surface mesoanalysis.
Scattered convection is already developing across portions of
southern Pennsylvania, western Maryland, and northern Virginia, as
convective temperatures are being eclipsed amid a weak cap. Despite
these thermodynamic indices, mid-level lapse rates and deep-layer
shear remains rather weak (20-25 kt effective bulk shear). Thus,
storms may struggle to organize in the short-term.
Additional storms should develop across the region as
isolation/destabilization continues. This should lead to the
development of a few multicell clusters of storms this afternoon.
Should this occur, strong low-level lapse rates should support a
damaging wind potential. Thus, convective trends will continue to be
monitored for possible watch issuance this afternoon.
..Karstens/Dial.. 07/12/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...OKX...PHI...BGM...AKQ...CTP...LWX...
LAT...LON 38357674 38277825 38917898 40467848 41157717 41387507
40907296 39787391 38697498 38357674SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Jul 12, 3:34 PM EDT | Severe Thunderstorm Watch | SPC | New Castle | 5 hr |
| Jul 12, 7:25 PM EDT | Mesoscale Discussion | SPC | New Castle | 2 hr |
| Jul 11, 8:58 PM EDT | Mesoscale Discussion | SPC | Carroll, Washington | 62 min |
Nearby
Mesoscale Discussions in Pennsylvania, 2021Mesoscale Discussions in New Jersey, 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.