Alert archive / Virginia / Mesoscale Discussions in 2021
Mesoscale Discussion 1479
The Storm Prediction Center issued this mesoscale discussion at Aug 11, 2021, 1:01 PM EDT for 7 counties in Maryland; 65 counties in Virginia; 9 counties in West Virginia. It was in force for 89 min.
The area
The outline as issued.
The facts
- Issued
- Aug 11, 2021, 1:01 PM EDT (17:01 UTC)
- Expired
- Aug 11, 2021, 2:30 PM EDT
- In force
- 89 min
- Issued by
- the Storm Prediction Center
- Concerning
- Severe potential...watch possible
- Chance of a watch
- 60%
All 81 counties
Carroll County, MD, Charles County, MD, Frederick County, MD, Howard County, MD, Montgomery County, MD, Prince George's County, MD, Washington County, MD, Albemarle County, VA, Alleghany County, VA, Amelia County, VA, Amherst County, VA, Appomattox County, VA, Arlington County, VA, Augusta County, VA, Bath County, VA, Bedford County, VA, Botetourt County, VA, Buckingham County, VA, Campbell County, VA, Caroline County, VA, Charlotte County, VA, Chesterfield County, VA, City of Alexandria, VA, City of Buena Vista, VA, City of Charlottesville, VA, City of Covington, VA, City of Fairfax, VA, City of Falls Church, VA, City of Fredericksburg, VA, City of Harrisonburg, VA, City of Lexington, VA, City of Lynchburg, VA, City of Manassas, VA, City of Manassas Park, VA, City of Richmond, VA, City of Staunton, VA, City of Waynesboro, VA, City of Winchester, VA, Clarke County, VA, Culpeper County, VA, Cumberland County, VA, Essex County, VA, Fairfax County, VA, Fauquier County, VA, Fluvanna County, VA, Frederick County, VA, Goochland County, VA, Greene County, VA, Hanover County, VA, Henrico County, VA, Highland County, VA, King and Queen County, VA, King George County, VA, King William County, VA, Loudoun County, VA, Louisa County, VA, Madison County, VA, Nelson County, VA, Nottoway County, VA, Orange County, VA, Page County, VA, Powhatan County, VA, Prince Edward County, VA, Prince William County, VA, Rappahannock County, VA, Rockbridge County, VA, Rockingham County, VA, Shenandoah County, VA, Spotsylvania County, VA, Stafford County, VA, Warren County, VA, Westmoreland County, VA, Berkeley County, WV, Grant County, WV, Hampshire County, WV, Hardy County, WV, Jefferson County, WV, Mineral County, WV, Morgan County, WV, Pendleton County, WV, Pocahontas County, WV
The radar
The Sterling radar (KLWX), 106 km from the middle of the area, began a volume at 17:02 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, 5.0 hours before the alert went out and 110 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
- 553 J/kg
- Mixed-layer CIN
- -190 J/kg
- Mixed-layer LCL
- 1301 m above ground
- 0-6 km bulk shear
- 13 kt
- 0-1 km storm-relative helicity
- 20 m²/s²
- 0-3 km storm-relative helicity
- 77 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 0.0
The text
As transmitted.
499
ACUS11 KWNS 111701
SWOMCD
SPC MCD 111701
MDZ000-VAZ000-DCZ000-WVZ000-111830-
Mesoscale Discussion 1479
NWS Storm Prediction Center Norman OK
1201 PM CDT Wed Aug 11 2021
Areas affected...Far eastern West Virginia...northern and central
Virginia...and portions of Maryland.
Concerning...Severe potential...Watch possible
Valid 111701Z - 111830Z
Probability of Watch Issuance...60 percent
SUMMARY...Storms are expected to move off the higher terrain of the
Appalachians and pose a threat for damaging wind gusts this
afternoon/evening.
DISCUSSION...Convective initiation has begun in the higher terrain
in Randolph and Pendleton Counties and vicinity. The airmass is
still capped in the lower elevations according to SPC Mesoanalysis,
but with rapid surface heating, expect the remaining inhibition to
be eroded within the next hour. Visible satellite gives further
support for this as cumulus have started to form in the lower
elevations across northern Virginia. The 12Z LWX RAOB showed
moderately steep mid-level lapse rates (~7 C/km) but very weak
effective shear (6 kts). These steep mid level lapse rates combined
with temperatures in the low 90s and dewpoints in the upper 60s has
combined for MLCAPE around 2000 J/kg per SPC mesoanalysis. Some
additional heating may bring greater instability and MLCAPE around
2500 J/kg within the next hour. In addition, the airmass will be
very supportive for strong downdrafts with DCAPE already around 1200
J/kg.
Storm organization is not anticipated given the shear profile is not
forecast to improve much from the LWX 12Z RAOB. However, the
favorable thermodynamic environment and storm density, which should
lead to clustering, should be sufficient for a damaging wind threat.
A watch may be needed in the next 1 to 2 hours.
..Bentley/Thompson.. 08/11/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...AKQ...LWX...RNK...PBZ...RLX...
LAT...LON 38657994 39327924 39687826 39707725 39517665 38617656
37657686 37087780 36997884 37147975 37678024 38657994SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Aug 11, 2:02 PM EDT | Severe Thunderstorm Watch | SPC | Allegany, Anne Arundel, Baltimore, Calvert, Carroll, Cecil and 9 more | 6 hr |
| Aug 11, 3:02 PM EDT | Severe Thunderstorm Watch | SPC | Caroline, Kent, Queen Anne's, Talbot | 6 hr |
| Aug 11, 3:47 PM EDT | Mesoscale Discussion | SPC | Anne Arundel, Baltimore, Calvert, Carroll, Cecil, Charles and 9 more | 88 min |
| Aug 11, 6:44 PM EDT | Mesoscale Discussion | SPC | Cecil | 91 min |
| Aug 11, 11:50 AM EDT | Mesoscale Discussion | SPC | Brooke, Hancock, Ohio | 115 min |
Nearby
Mesoscale Discussions in Virginia, 2021Mesoscale Discussions in West 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.