Alert archive / Virginia / Mesoscale Discussions in 2017

Mesoscale Discussion 545

April 22, 201712 counties in North Carolina; 24 counties in Virginia

The Storm Prediction Center issued this mesoscale discussion at Apr 22, 2017, 11:57 AM EDT for 12 counties in North Carolina; 24 counties in Virginia. It was in force for 2 hr.

The area

The outline as issued.

The facts

Issued: Apr 22, 2017, 11:57 AM EDT (15:57 UTC)
Expired: Apr 22, 2017, 2:00 PM EDT
In force: 2 hr
Issued by: the Storm Prediction Center
Chance of a watch: 40%

All 36 counties

Alleghany County, NC, Ashe County, NC, Caswell County, NC, Granville County, NC, Person County, NC, Rockingham County, NC, Stokes County, NC, Surry County, NC, Vance County, NC, Warren County, NC, Wilkes County, NC, Yadkin County, NC, Bedford County, VA, Brunswick County, VA, Campbell County, VA, Carroll County, VA, Charlotte County, VA, City of Danville, VA, City of Galax, VA, City of Martinsville, VA, City of Radford, VA, City of Roanoke, VA, City of Salem, VA, Floyd County, VA, Franklin County, VA, Grayson County, VA, Halifax County, VA, Henry County, VA, Lunenburg County, VA, Mecklenburg County, VA, Montgomery County, VA, Nottoway County, VA, Patrick County, VA, Pittsylvania County, VA, Pulaski County, VA, Wythe County, VA

The radar

The Roanoke radar (KFCX), 84 km from the middle of the area, began a volume at 16:00 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.

Severe Thunderstorm Watch 157Apr 22, 3:05 PM EDT

The nearest sounding before it

Roanoke, VA (KRNK), launched 12:00Z, 4.0 hours before the alert went out and 64 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: 28 J/kg
Mixed-layer CIN: -103 J/kg
Mixed-layer LCL: 755 m above ground
0-6 km bulk shear: 47 kt
0-1 km storm-relative helicity: 106 m²/s²
0-3 km storm-relative helicity: 131 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.1

Open this sounding in Storm Lab

The text

As transmitted.

800 
ACUS11 KWNS 221558
SWOMCD
SPC MCD 221557 
NCZ000-VAZ000-221800-

Mesoscale Discussion 0545
NWS Storm Prediction Center Norman OK
1057 AM CDT Sat Apr 22 2017

Areas affected...Southern Virginia and northern North Carolina

Concerning...Severe potential...Watch possible 

Valid 221557Z - 221800Z

Probability of Watch Issuance...40 percent

SUMMARY...Scattered thunderstorms are expected to develop early this
afternoon, and a few of the storms could produce damaging gusts and
marginally severe hail.  This area will continue to be monitored
through the afternoon, though the need for a watch is unclear.

DISCUSSION...Differential surface heating and a slow-moving front
across southern VA will likely focus thunderstorm initiation early
this afternoon.  Visible satellite imagery shows cumulus development
near the NC/VA border in a zone of differential heating, where
surface observations show temperatures warming into the mid-upper
70s with dewpoints in the mid 60s.  12z soundings revealed rather
modest midlevel lapse rates over this area, but continued surface
heating will boost MLCAPE into the 1000-1500 J/kg range, and a lack
of convective inhibition should allow scattered thunderstorm
development early this afternoon.  Some increase in storm coverage
may also be related to a band of ascent spreading east of the
southern Appalachians, in association with a weak convectively
enhanced midlevel trough.  

There is some enhancement to low-level shear on the immediate cool
side of the front sagging southward across southern VA, but wind
profiles will remain largely unidirectional with effective bulk
shear of 35-40 kt near and south of the front.  Thus, a few
organized storms/clusters, potentially with some supercell
structures, will pose a primary risk for damaging winds this
afternoon.  The scenario appears marginal for a severe thunderstorm
watch at this time, though the area will continue to be monitored.

..Thompson/Weiss.. 04/22/2017

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

ATTN...WFO...AKQ...RAH...RNK...

LAT...LON   36257840 36237946 36228031 36128082 36248132 36428162
            36758157 37228063 37387985 37237872 37207786 36937762
            36607762 36367784 36257840

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Apr 22, 3:17 PM EDT	Severe Thunderstorm Watch	SPC	Alamance, Caswell, Durham, Edgecombe, Forsyth, Franklin and 12 more	4 hr
Apr 22, 4:12 PM EDT	Mesoscale Discussion	SPC	Alamance, Caswell, Durham, Edgecombe, Forsyth, Franklin and 12 more	2 hr
Apr 22, 7:40 PM EDT	Mesoscale Discussion	SPC	Cherokee	65 min

Nearby

Mesoscale Discussions in Virginia, 2017 Mesoscale Discussions in North Carolina, 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.