Alert archive / Pennsylvania / Mesoscale Discussions in 2018

Mesoscale Discussion 1336

August 21, 20184 states

The Storm Prediction Center issued this mesoscale discussion at Aug 21, 2018, 4:56 PM EDT for 4 states. It was in force for 2 hr.

The area

The outline as issued.

The facts

Issued: Aug 21, 2018, 4:56 PM EDT (20:56 UTC)
Expired: Aug 21, 2018, 7:00 PM EDT
In force: 2 hr
Issued by: the Storm Prediction Center

All 52 counties

Allegany County, MD, Carroll County, MD, Frederick County, MD, Garrett County, MD, Montgomery County, MD, Washington County, MD, Adams County, PA, Armstrong County, PA, Bedford County, PA, Blair County, PA, Cambria County, PA, Centre County, PA, Clearfield County, PA, Cumberland County, PA, Dauphin County, PA, Fayette County, PA, Franklin County, PA, Fulton County, PA, Huntingdon County, PA, Indiana County, PA, Jefferson County, PA, Juniata County, PA, Mifflin County, PA, Perry County, PA, Snyder County, PA, Somerset County, PA, Westmoreland County, PA, York County, PA, City of Harrisonburg, VA, City of Winchester, VA, Clarke County, VA, Culpeper County, VA, Fauquier County, VA, Frederick County, VA, Greene County, VA, Loudoun County, VA, Madison County, VA, Page County, VA, Rappahannock County, VA, Rockingham County, VA, Shenandoah County, VA, Warren 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, Preston County, WV, Tucker County, WV

The radar

The State College radar (KCCX), 129 km from the middle of the area, began a volume at 21:00 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 nearest sounding before it

Washington/Dulle, VA (KIAD), launched 12:00Z, 8.9 hours before the alert went out and 138 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: 187 J/kg
Mixed-layer CIN: -22 J/kg
Mixed-layer LCL: 597 m above ground
0-6 km bulk shear: 32 kt
0-1 km storm-relative helicity: 122 m²/s²
0-3 km storm-relative helicity: 183 m²/s²
Significant tornado parameter: 0.1
Supercell composite: 0.6

Open this sounding in Storm Lab

The text

As transmitted.

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SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Aug 21, 2:16 PM EDT	Mesoscale Discussion	SPC	Allegany, Garrett	119 min
Aug 21, 2:41 PM EDT	Severe Thunderstorm Watch	SPC	Allegany, Carroll, Frederick, Garrett, Washington	7 hr
Aug 21, 5:03 PM EDT	Mesoscale Discussion	SPC	Charles, Saint Mary's	117 min
Aug 21, 7:06 PM EDT	Mesoscale Discussion	SPC	Anne Arundel, Baltimore, Carroll, Cecil, Frederick, Harford and 7 more	84 min
Aug 21, 12:45 PM EDT	Mesoscale Discussion	SPC	Fayette, Greene, Washington, Westmoreland	3 hr

Nearby

Mesoscale Discussions in Pennsylvania, 2018 Mesoscale Discussions in Virginia, 2018 Mesoscale Discussions in West Virginia, 2018 Every mesoscale discussion 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.