Alert archive / Pennsylvania / Mesoscale Discussions in 2021

Mesoscale Discussion 1758

September 15, 20214 states

The Storm Prediction Center issued this mesoscale discussion at Sep 15, 2021, 8:11 PM EDT for 4 states. It was in force for 2 hr.

The area

The outline as issued.

The facts

Issued: Sep 15, 2021, 8:11 PM EDT (00:11 UTC)
Expired: Sep 15, 2021, 10:15 PM EDT
In force: 2 hr
Issued by: the Storm Prediction Center
Concerning: Severe thunderstorm watch 503

Counties Franklin County, MA, Hampden County, MA, Hampshire County, MA, Worcester County, MA, Sussex County, NJ, Warren County, NJ, Dutchess County, NY, Orange County, NY, Putnam County, NY, Adams County, PA, Berks County, PA, Carbon County, PA, Lancaster County, PA, Lebanon County, PA, Lehigh County, PA, Monroe County, PA, Northampton County, PA, York County, PA

The radar

The Brookhaven radar (KOKX), 135 km from the middle of the area, began a volume at 00:13 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

Brookhaven, NY (KOKX), launched 00:00Z, 0.2 hours before the alert went out and 135 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: 664 J/kg
Mixed-layer CIN: -93 J/kg
Mixed-layer LCL: 1000 m above ground
0-6 km bulk shear: 38 kt
0-1 km storm-relative helicity: 172 m²/s²
0-3 km storm-relative helicity: 188 m²/s²
Significant tornado parameter: 0.3
Supercell composite: 3.4

Open this sounding in Storm Lab

The text

As transmitted.

693 
ACUS11 KWNS 160011
SWOMCD
SPC MCD 160011 
MAZ000-NHZ000-RIZ000-CTZ000-VTZ000-NYZ000-NJZ000-PAZ000-MDZ000-160215-

Mesoscale Discussion 1758
NWS Storm Prediction Center Norman OK
0711 PM CDT Wed Sep 15 2021

Areas affected...Eastern Pennsylvania...Southeast New
York...Southern New England

Concerning...Severe Thunderstorm Watch 503...

Valid 160011Z - 160215Z

The severe weather threat for Severe Thunderstorm Watch 503
continues.

SUMMARY...An isolated wind-damage threat will likely continue for a
few more hours across parts of WW 503. The threat should become
increasingly marginal with time. Issuance of a new weather watch
appears very unlikely.

DISCUSSION...The latest radar imagery near Philadelphia shows a
broken line of strong thunderstorms located from west-central
Maryland to eastern Pennsylvania. The line is located ahead of a
cold front in a moist airmass with surface dewpoints generally from
65 to 70 F. This was contributing to moderate instability with the
RAP showing MLCAPE across the watch generally in the 1000 to 1500
J/kg range. In addition, regional WSR-88D VWPs have 0-6 km shear in
the 30 to 40 kt range with west to southwest flow in the low to
mid-levels. This combined with the instability will continue to
support short line segments capable of isolated wind damage. As
instability drops and outflow spreads across the region this
evening, the wind-damage threat should become increasingly isolated
and marginal.

..Broyles.. 09/16/2021

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

ATTN...WFO...GYX...BOX...OKX...ALY...PHI...BGM...CTP...LWX...

LAT...LON   39687606 39737745 40007742 40267686 40127670 40247659
            40547664 40417639 40557617 40527603 40747581 40937601
            41177543 41107501 41327477 41437460 41607437 41617400
            42037394 41987353 42057350 41997304 42707290 42747218
            42737186 42547153 42037150 41977212 41737216 41577235
            41667256 41567275 41577292 41617296 41507324 41497335
            41627351 41367353 41307392 41177414 41157418 41207430
            40947468 40557525 40347564 40137592 40047591 39687606

Nearby

Mesoscale Discussions in Pennsylvania, 2021 Mesoscale Discussions in Massachusetts, 2021 Mesoscale Discussions in New York, 2021 Every mesoscale discussion in 2021

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.