Alert archive / Virginia / Mesoscale Discussions in 2021
Mesoscale Discussion 1797
The Storm Prediction Center issued this mesoscale discussion at Oct 4, 2021, 4:44 PM EDT for 6 states. It was in force for 2 hr.
The area
The outline as issued.
The facts
- Issued
- Oct 4, 2021, 4:44 PM EDT (20:44 UTC)
- Expired
- Oct 4, 2021, 6:45 PM EDT
- In force
- 2 hr
- Issued by
- the Storm Prediction Center
- Concerning
- Severe potential...watch unlikely
- Chance of a watch
- 5%
All 49 counties
Kent County, DE, New Castle County, DE, Anne Arundel County, MD, Baltimore 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, Atlantic County, NJ, Burlington County, NJ, Camden County, NJ, Cape May County, NJ, Cumberland County, NJ, Gloucester County, NJ, Salem County, NJ, Bucks County, PA, Chester County, PA, Delaware County, PA, Lancaster County, PA, Montgomery County, PA, Philadelphia 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, Clarke County, VA, Culpeper County, VA, Fairfax County, VA, Fauquier County, VA, Loudoun County, VA, Madison County, VA, Prince William County, VA, Rappahannock County, VA, Stafford County, VA, Warren County, VA, Jefferson County, WV
The radar
The Sterling radar (KLWX), 88 km from the middle of the area, began a volume at 20:43 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, 8.7 hours before the alert went out and 81 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
- 461 J/kg
- Mixed-layer CIN
- -12 J/kg
- Mixed-layer LCL
- 643 m above ground
- 0-6 km bulk shear
- 20 kt
- 0-1 km storm-relative helicity
- -47 m²/s²
- 0-3 km storm-relative helicity
- 14 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- -0.1
The text
As transmitted.
498
ACUS11 KWNS 042044
SWOMCD
SPC MCD 042044
NJZ000-PAZ000-DEZ000-MDZ000-DCZ000-VAZ000-WVZ000-042245-
Mesoscale Discussion 1797
NWS Storm Prediction Center Norman OK
0344 PM CDT Mon Oct 04 2021
Areas affected...portions of the Mid Atlantic
Concerning...Severe potential...Watch unlikely
Valid 042044Z - 042245Z
Probability of Watch Issuance...5 percent
SUMMARY...A few strong thunderstorms near the stationary front may
pose an isolated risk for strong/damaging outflow winds this
afternoon and evening. Expected storm coverage should be limited
suggesting a watch is not needed.
DISCUSSION...As of 2030 UTC, regional radar and satellite imagery
showed isolated thunderstorms developing along the eastern crest of
the Blue Ridge Mountains in western VA and eastern WV. Aided by weak
low-level convergence south of a stalled front and along a surface
pressure trough, these storms are ongoing in a warm, and weak to
moderately unstable environment. SPC mesoanalysis shows 1000-1500
J/kg of MLCAPE has developed, which should continue to support
updrafts as they track northeastward across portions of the Mid
Atlantic this afternoon and evening. Observed vertical shear remains
weak, with 20-25 kts of 0-6km shear from local VADs. This should
result in loosely organized multicells/clusters, with the potential
for strong outflow winds into this evening.
Latest hi-res guidance and observational trends suggest the ongoing
storms will likely continue northeastward with time into northern
VA, MD and southern PA. Clearing ahead of the convection suggests
potential for storms to linger into the evening. Thus, a low-end
risk for a few damaging wind gusts may exist across portions of he
Mid Atlantic with any stronger storms/weakly organized storm
clusters. Uncertainty on the nature of the severe threat and limited
storm coverage suggests a weather watch is unlikely.
..Lyons/Guyer.. 10/04/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...PHI...AKQ...CTP...LWX...
LAT...LON 38337833 39297815 40327614 40407499 39787426 39197458
38877538 38597635 38307716 38187750 38337833Nearby
Mesoscale Discussions in Virginia, 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.