Mesoscale Discussion 1621

Issued

Jul 31, 2019, 12:39 PM EDT (16:39 UTC)

Expired

Jul 31, 2019, 1:45 PM EDT

In force

66 min

Issued by

the Storm Prediction Center

Chance of a watch

60%

The Boston radar (KBOX), 97 km from the middle of the area, began a volume at 16:41 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.

• Severe Thunderstorm Watch 547Jul 31, 1:15 PM EDT

657 
ACUS11 KWNS 311639
SWOMCD
SPC MCD 311639 
MEZ000-MAZ000-NHZ000-RIZ000-CTZ000-NYZ000-VTZ000-NJZ000-311745-

Mesoscale Discussion 1621
NWS Storm Prediction Center Norman OK
1139 AM CDT Wed Jul 31 2019

Areas affected...Portions of the northern NJ...southern NY and much
of New Enlgand

Concerning...Severe potential...Watch possible 

Valid 311639Z - 311745Z

Probability of Watch Issuance...60 percent

SUMMARY...Thunderstorms are expected to increase in coverage over
the next couple of hours and shift northeast across the Northeast
U.S. These storms will pose a threat from strong, locally damaging
winds, and a watch may be needed in the next hour or so.

DISCUSSION...Strong heating ahead of a surface cold front was
apparent in 16z observations from the mid-Atlantic into New England
where temperatures in the mid 80s to low 90s were already being
reported. Southwesterly low level flow is maintaining a very moist
boundary layer, with surface dewpoints in the upper 60s to low 70s.
The net result is a corridor of 1000 J/kg MLCAPE across the region.
An MCV was noted in radar loops, approaching Albany NY as of 1620z.
Thunderstorms were beginning to intensify on the southern flank of
this feature across Ulster and Orange counties in NY, and Sussex
county in NJ. Given the aforementioned downstream environment,
continued intensification of these storms is expected. While
effective shear remains modest (around 25 kt) some localized
enhancement aided by the MCV is possible, and organized line
segments capable of mainly strong, locally damaging wind gusts is
possible as storms track east/northeast through the afternoon. 

Additional CU development was noted in visible satellite imagery
across portions of CT into RI and MA where the strongest heating has
occurred. A surface trough also was apparent in late surface
analysis stretching from the Chesapeake Bay vicinity northeast into
central ME. Convective initiation could eventually occur from the CU
field, which appears to be co-located with the surface trough.
Modest deep layer flow and midlevel lapse rates will temper more
intense cells. However, high PW values and sufficient instability,
combined with steepening low level lapse rates will support multiple
bands of storms capable of strong, locally damaging wind gusts. A
watch may be needed in the next hour or so across portions of the
Northeast.

..Leitman/Hart.. 07/31/2019

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

ATTN...WFO...GYX...BOX...BTV...OKX...ALY...PHI...BGM...

LAT...LON   40587281 40427378 40717447 41007472 41497464 41907449
            42517378 43397256 44007163 44277105 44317052 44147025
            43917020 43347037 42547070 41867098 41477132 41097165
            40667253 40587281

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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.