Alert archive / Virginia / Tornado Watches in 2018

Tornado Watch 382

September 17, 20185 counties in Maryland; 28 counties in Virginia

The Storm Prediction Center issued this tornado watch at Sep 17, 2018, 3:42 PM EDT for 5 counties in Maryland; 28 counties in Virginia. It was in force for 4 hr.

The area

The outline as issued.

The facts

Issued: Sep 17, 2018, 3:42 PM EDT (19:42 UTC)
Expired: Sep 17, 2018, 8:04 PM EDT
In force: 4 hr
Issued by: the Storm Prediction Center
Largest hail expected: 0.5 in
Strongest gust expected: 60 kt

All 33 counties

Anne Arundel County, MD, Calvert County, MD, Charles County, MD, Prince George's County, MD, Saint Mary's County, MD, Amelia County, VA, Arlington County, VA, Caroline County, VA, Chesterfield 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, City of Richmond, VA, Cumberland County, VA, Essex County, VA, Fairfax County, VA, Goochland County, VA, Hanover County, VA, Henrico County, VA, King and Queen County, VA, King George County, VA, King William County, VA, Louisa County, VA, New Kent County, VA, Powhatan County, VA, Prince William County, VA, Richmond County, VA, Spotsylvania County, VA, Stafford County, VA, Westmoreland County, VA

The radar

The Sterling radar (KLWX), 110 km from the middle of the area, began a volume at 19:45 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.

What SPC expected

The probabilities SPC published with the watch, for the watch area over its life.

Two or more tornadoes: 30%
One or more EF2 to EF5 tornadoes: 10%
Ten or more severe wind reports: 5%
One or more gusts of 65 knots or more: 5%
Ten or more severe hail reports: 5%
One or more hailstones of 2 inches or more: 5%
Six or more combined severe hail and wind reports: 20%

Warnings issued inside it

7 tornado, severe thunderstorm and flash flood warnings for its counties while it was in force.

Issued	Product	Office	Counties	Lasted
Sep 17, 3:48 PM EDT	Tornado Warning	AKQ	Caroline, Hanover, Henrico, King William	42 min
Sep 17, 3:48 PM EDT	Tornado Warning	AKQ	Amelia	27 min
Sep 17, 3:54 PM EDT	Tornado Warning	AKQ	Chesterfield, Powhatan	36 min
Sep 17, 4:14 PM EDT	Tornado Warning	AKQ	Chesterfield, Hanover, Henrico, City of Richmond	31 min
Sep 17, 4:44 PM EDT	Tornado Warning	AKQ	Chesterfield, Goochland, Hanover, Henrico, Powhatan, City of Richmond	31 min
Sep 17, 5:13 PM EDT	Tornado Warning	AKQ	Hanover, Henrico, King William	32 min
Sep 17, 6:29 PM EDT	Severe Thunderstorm Warning	AKQ	Amelia, Chesterfield, Henrico, Powhatan, City of Richmond	46 min

Discussions around it

SPC mesoscale discussions over the same counties from four hours before the watch to its end.

The nearest sounding before it

Washington/Dulle, VA (KIAD), launched 12:00Z, 7.7 hours before the alert went out and 107 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: 117 J/kg
Mixed-layer CIN: -10 J/kg
Mixed-layer LCL: 632 m above ground
0-6 km bulk shear: 18 kt
0-1 km storm-relative helicity: 119 m²/s²
0-3 km storm-relative helicity: 176 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.1

Open this sounding in Storm Lab

The text

As transmitted.

206 
WWUS20 KWNS 171939
SEL2  
SPC WW 171939
DCZ000-MDZ000-VAZ000-CWZ000-180200-

URGENT - IMMEDIATE BROADCAST REQUESTED
Tornado Watch Number 382
NWS Storm Prediction Center Norman OK
340 PM EDT Mon Sep 17 2018

The NWS Storm Prediction Center has issued a

* Tornado Watch for portions of 
  District Of Columbia
  Central Maryland
  Eastern Virginia
  Coastal Waters

* Effective this Monday afternoon and evening from 340 PM until
  1000 PM EDT.

* Primary threats include...
  A couple tornadoes possible

SUMMARY...Isolated thunderstorms with small-scale embedded
circulations will continue to affect the watch area this afternoon,
with a few brief tornadoes possible.

The tornado watch area is approximately along and 30 statute miles
east and west of a line from 35 miles west southwest of Richmond VA
to 35 miles east northeast of Washington DC. For a complete
depiction of the watch see the associated watch outline update
(WOUS64 KWNS WOU2).

PRECAUTIONARY/PREPAREDNESS ACTIONS...

REMEMBER...A Tornado Watch means conditions are favorable for
tornadoes and severe thunderstorms in and close to the watch
area. Persons in these areas should be on the lookout for
threatening weather conditions and listen for later statements
and possible warnings.

&&

AVIATION...Tornadoes and a few severe thunderstorms with hail
surface and aloft to 0.5 inches. Extreme turbulence and surface wind
gusts to 60 knots. A few cumulonimbi with maximum tops to 500. Mean
storm motion vector 21025.

...Hart

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Sep 17, 3:25 PM EDT	Mesoscale Discussion	SPC	Calvert, Charles, Saint Mary's	2 hr
Sep 17, 7:09 PM EDT	Mesoscale Discussion	SPC	Anne Arundel, Calvert, Charles, Prince George's, Saint Mary's, Talbot and 1 more	2 hr
Sep 17, 10:28 AM EDT	Mesoscale Discussion	SPC	Amelia, Brunswick, Buckingham, Charlotte, Chesterfield, Cumberland and 18 more	2 hr

Nearby

Tornado Watches in Virginia, 2018 Tornado Watches in Maryland, 2018 Every tornado watch in 2018

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.