Alert archive / Pennsylvania / Mesoscale Discussions in 2015

Mesoscale Discussion 404

April 20, 201515 counties in Pennsylvania

The Storm Prediction Center issued this mesoscale discussion at Apr 20, 2015, 5:57 PM EDT for 15 counties in Pennsylvania. It was in force for 2 hr.

The area

The outline as issued.

The facts

Issued: Apr 20, 2015, 5:57 PM EDT (21:57 UTC)
Expired: Apr 20, 2015, 8:00 PM EDT
In force: 2 hr
Issued by: the Storm Prediction Center

Counties Bradford County, PA, Cameron County, PA, Centre County, PA, Clinton County, PA, Columbia County, PA, Luzerne County, PA, Lycoming County, PA, Montour County, PA, Northumberland County, PA, Potter County, PA, Snyder County, PA, Sullivan County, PA, Tioga County, PA, Union County, PA, Wyoming County, PA

The radar

The State College radar (KCCX), 73 km from the middle of the area, began a volume at 21:56 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 92Apr 20, 7:55 PM EDT

The nearest sounding before it

Buffalo Intl Airport, NY (KBUF), launched 12:00Z, 10.0 hours before the alert went out and 211 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: 0 J/kg
Mixed-layer CIN: No buoyant parcel
Mixed-layer LCL: 982 m above ground
0-6 km bulk shear: 28 kt
0-1 km storm-relative helicity: 429 m²/s²
0-3 km storm-relative helicity: 437 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.0

Open this sounding in Storm Lab

The text

As transmitted, in the capitals the NWS used until 2016.

117 
ACUS11 KWNS 202158
SWOMCD
SPC MCD 202157 
PAZ000-NYZ000-210000-

MESOSCALE DISCUSSION 0404
NWS STORM PREDICTION CENTER NORMAN OK
0457 PM CDT MON APR 20 2015

AREAS AFFECTED...PORTIONS OF N-CNTRL PA AND S-CNTRL NY

CONCERNING...TORNADO WATCH 90...

VALID 202157Z - 210000Z

THE SEVERE WEATHER THREAT FOR TORNADO WATCH 90 CONTINUES.

SUMMARY...ISOLATED TO SCATTERED TSTMS WILL CONTINUE TO DEVELOP NEWD
ACROSS N-CNTRL PA INTO S-CNTRL NY WITH SOME SVR RISK.

DISCUSSION...THE CCX VWP IS SAMPLING 35-50 KT OF MID-LEVEL
FLOW...WHICH IS ENCOURAGING THE ONGOING NEWD SPREAD OF CONVECTION
FROM CNTRL PA WHILE OFFERING MODERATE DEEP SHEAR AND SOME DMGG
WIND/BRIEF TORNADO POTENTIAL. THE TORNADO WATCH HAS BEEN LOCALLY
EXTENDED NWD ACROSS PARTS OF N-CNTRL PA WHERE MARGINAL INSTABILITY
EXISTS.

AREAS FARTHER N INTO FAR N-CNTRL PA AND S-CNTRL NY WILL CONTINUE TO
BE MONITORED FOR POSSIBLE WW ISSUANCE THROUGH THE EVENING. THIS WILL
OCCUR AS MID-LEVEL HEIGHT FALLS CONTINUE OVERSPREADING THE AREA
ATTENDANT TO MIGRATORY IMPULSES ON THE ERN RIM OF A CNTRL/ERN CONUS
CYCLONE AND COLD MID-LEVEL TEMPERATURES CONTINUE TO OVERLIE THE
REGION. HOWEVER...WITH STABLE WAVE-CLOUD STRUCTURES OBSERVED IN VIS
IMAGERY AND SFC OBS SUGGESTING ONLY MODEST THETA-E...AND EVENTUAL
NOCTURNAL STABILIZATION...THE OVERALL MAGNITUDE OF THE SVR POTENTIAL
IS UNCERTAIN N OF THE EXPANDED OUTLINE OF WW 90.

..COHEN.. 04/20/2015

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...PHI...BGM...BUF...CTP...

LAT...LON   40897827 41517870 42227784 42117620 41027539 40667655
            40897827

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Apr 20, 1:54 PM EDT	Mesoscale Discussion	SPC	Adams, Bedford, Cumberland, Franklin, Fulton, Lancaster and 1 more	3 hr
Apr 20, 3:08 PM EDT	Tornado Watch	SPC	Adams, Bedford, Berks, Blair, Bucks, Cambria and 32 more	9 hr
Apr 20, 3:23 PM EDT	Mesoscale Discussion	SPC	Bradford, Cameron, Centre, Clearfield, Clinton, Elk and 8 more	2 hr
Apr 20, 5:01 PM EDT	Mesoscale Discussion	SPC	Adams, Bedford, Berks, Blair, Chester, Cumberland and 10 more	119 min
Apr 20, 7:31 PM EDT	Mesoscale Discussion	SPC	Lackawanna, Susquehanna, Wayne	2 hr
Apr 20, 7:56 PM EDT	Severe Thunderstorm Watch	SPC	Bradford, Carbon, Lackawanna, Luzerne, Monroe, Northampton and 4 more	4 hr

Nearby

Mesoscale Discussions in Pennsylvania, 2015 Every mesoscale discussion in 2015

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.