Alert archive / Virginia / Mesoscale Discussions in 2017

Mesoscale Discussion 539

April 21, 20175 states

The Storm Prediction Center issued this mesoscale discussion at Apr 21, 2017, 4:19 PM EDT for 5 states. It was in force for 56 min.

The area

The outline as issued.

The facts

Issued: Apr 21, 2017, 4:19 PM EDT (20:19 UTC)
Expired: Apr 21, 2017, 5:15 PM EDT
In force: 56 min
Issued by: the Storm Prediction Center
Chance of a watch: 20%

All 173 counties

Anne Arundel County, MD, Calvert County, MD, Charles County, MD, Howard County, MD, Montgomery County, MD, Prince George's County, MD, Saint Mary's County, MD, Alamance County, NC, Alexander County, NC, Alleghany County, NC, Ashe County, NC, Avery County, NC, Buncombe County, NC, Burke County, NC, Caldwell County, NC, Caswell County, NC, Catawba County, NC, Cleveland County, NC, Davidson County, NC, Davie County, NC, Durham County, NC, Forsyth County, NC, Franklin County, NC, Gates County, NC, Granville County, NC, Guilford County, NC, Halifax County, NC, Haywood County, NC, Henderson County, NC, Hertford County, NC, Iredell County, NC, Lincoln County, NC, Madison County, NC, McDowell County, NC, Mitchell County, NC, Northampton County, NC, Orange County, NC, Person County, NC, Polk County, NC, Randolph County, NC, Rockingham County, NC, Rowan County, NC, Rutherford County, NC, Stokes County, NC, Surry County, NC, Vance County, NC, Warren County, NC, Watauga County, NC, Wilkes County, NC, Yadkin County, NC, Yancey County, NC, Carter County, TN, Johnson County, TN, Sullivan County, TN, Unicoi County, TN, Washington County, TN, 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, Bland County, VA, Botetourt County, VA, Brunswick County, VA, Buckingham County, VA, Campbell County, VA, Caroline County, VA, Carroll County, VA, Charles City County, VA, Charlotte County, VA, Chesterfield County, VA, City of Alexandria, VA, City of Bristol, VA, City of Buena Vista, VA, City of Charlottesville, VA, City of Colonial Heights, VA, City of Covington, VA, City of Danville, VA, City of Emporia, VA, City of Fairfax, VA, City of Falls Church, VA, City of Franklin, VA, City of Fredericksburg, VA, City of Galax, VA, City of Harrisonburg, VA, City of Hopewell, VA, City of Lexington, VA, City of Lynchburg, VA, City of Manassas, VA, City of Manassas Park, VA, City of Martinsville, VA, City of Newport News, VA, City of Petersburg, VA, City of Radford, VA, City of Richmond, VA, City of Roanoke, VA, City of Salem, VA, City of Staunton, VA, City of Suffolk, VA, City of Waynesboro, VA, City of Williamsburg, VA, Craig County, VA, Culpeper County, VA, Cumberland County, VA, Dinwiddie County, VA, Essex County, VA, Fairfax County, VA, Fauquier County, VA, Floyd County, VA, Fluvanna County, VA, Franklin County, VA, Giles County, VA, Gloucester County, VA, Goochland County, VA, Grayson County, VA, Greene County, VA, Greensville County, VA, Halifax County, VA, Hanover County, VA, Henrico County, VA, Henry County, VA, Isle of Wight County, VA, James City County, VA, King and Queen County, VA, King George County, VA, King William County, VA, Lancaster County, VA, Loudoun County, VA, Louisa County, VA, Lunenburg County, VA, Madison County, VA, Mecklenburg County, VA, Middlesex County, VA, Montgomery County, VA, Nelson County, VA, New Kent County, VA, Northumberland County, VA, Nottoway County, VA, Orange County, VA, Page County, VA, Patrick County, VA, Pittsylvania County, VA, Powhatan County, VA, Prince Edward County, VA, Prince George County, VA, Prince William County, VA, Pulaski County, VA, Rappahannock County, VA, Richmond County, VA, Roanoke County, VA, Rockbridge County, VA, Rockingham County, VA, Russell County, VA, Smyth County, VA, Southampton County, VA, Spotsylvania County, VA, Stafford County, VA, Surry County, VA, Sussex County, VA, Tazewell County, VA, Washington County, VA, Westmoreland County, VA, Wythe County, VA, York County, VA, Greenbrier County, WV, McDowell County, WV, Mercer County, WV, Monroe County, WV, Summers County, WV

The radar

The Roanoke radar (KFCX), 118 km from the middle of the area, began a volume at 20:22 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

Roanoke, VA (KRNK), launched 12:00Z, 8.3 hours before the alert went out and 46 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.

Skew-T log-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.

Mixed-layer CAPE: 17 J/kg
Mixed-layer CIN: -95 J/kg
Mixed-layer LCL: 1040 m above ground
0-6 km bulk shear: 23 kt
0-1 km storm-relative helicity: 88 m²/s²
0-3 km storm-relative helicity: 35 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.0

Open this sounding in Storm Lab

The text

As transmitted.

656 
ACUS11 KWNS 212019
SWOMCD
SPC MCD 212019 
VAZ000-MDZ000-NCZ000-DCZ000-WVZ000-TNZ000-SCZ000-212115-

Mesoscale Discussion 0539
NWS Storm Prediction Center Norman OK
0319 PM CDT Fri Apr 21 2017

Areas affected...Northern and western North Carolina...far
northeastern Tennessee...far southern West Virginia...much of
Virginia...portions of southern Maryland...and the District of
Columbia

Concerning...Severe potential...Watch unlikely 

Valid 212019Z - 212115Z

Probability of Watch Issuance...20 percent

SUMMARY...Isolated to scattered convection will persist in the
region through the early evening, with a downward trend in intensity
expected after dark.  A WW is not anticipated for this activity.

DISCUSSION...Isolated thunderstorms continue to move eastward at
approximately 25-30 knots across the region.  These storms were in
an environment characterized by relatively modest deep shear (around
25-35 knots) and relatively weak instability (500-1250 J/kg MUCAPE).
The deep shear is relatively marginal for well organized storms,
although a few instances of damaging wind gusts may occur with the
strongest activity owing to steep low-level lapse rates.  Hail
approaching 1" in diameter may also occur.  Low-level boundaries
near northern Virginia/southern Maryland may locally enhance updraft
intensity and attendant severe threat, although this threat is
expected to be brief.  Storms should diminish in intensity after
dark with loss of solar heating. 

Given the marginal and isolated nature of the severe threat, a WW
issuance is not anticipated for this activity.

..Cook/Goss.. 04/21/2017

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...AKQ...LWX...RAH...RNK...RLX...GSP...MRX...

LAT...LON   39347672 39297727 39027798 38567885 38307976 37938090
            37448186 36608255 36108284 35578305 35288287 35148219
            35538034 35917841 36147723 36277670 36787647 37317638
            38107630 38787641 39067648 39347672

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Apr 21, 2:13 PM EDT	Mesoscale Discussion	SPC	Alexander, Alleghany, Ashe, Avery, Buncombe, Burke and 19 more	2 hr
Apr 21, 2:12 PM EDT	Mesoscale Discussion	SPC	Chester, Decatur, Fayette, Hardeman, Hardin, Haywood and 4 more	93 min

Nearby

Mesoscale Discussions in Virginia, 2017 Mesoscale Discussions in North Carolina, 2017 Mesoscale Discussions in Maryland, 2017 Every mesoscale discussion in 2017

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.