Mesoscale Discussion 1304

Issued

Jul 17, 2021, 12:00 PM EDT (16:00 UTC)

Expired

Jul 17, 2021, 2:30 PM EDT

In force

3 hr

Issued by

the Storm Prediction Center

Concerning

Severe potential...severe thunderstorm watch likely

Chance of a watch

80%

The Philadelphia radar (KDIX), 108 km from the middle of the area, began a volume at 16:03 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 382Jul 17, 1:10 PM EDT

118 
ACUS11 KWNS 171601
SWOMCD
SPC MCD 171600 
NYZ000-CTZ000-MAZ000-NJZ000-PAZ000-DEZ000-MDZ000-DCZ000-VAZ000-WVZ000-171830-

Mesoscale Discussion 1304
NWS Storm Prediction Center Norman OK
1100 AM CDT Sat Jul 17 2021

Areas affected...Portions of northern VA...far eastern
WV...MD...DC...DE...NJ...central/eastern PA...southern NY...and
western CT/MA

Concerning...Severe potential...Severe Thunderstorm Watch likely 

Valid 171600Z - 171830Z

Probability of Watch Issuance...80 percent

SUMMARY...The threat for scattered damaging wind gusts should
increase this afternoon. Severe Thunderstorm Watch issuance is
likely early this afternoon.

DISCUSSION...A weak, positively tilted upper trough extends across
parts of the lower Great Lakes and OH Valley as of 16Z. A belt of
25-40 kt of mid-level southwesterly flow was observed with the 12Z
sounding from PIT ahead of this feature. Recent visible satellite
imagery shows mostly clear conditions generally along/east of the
Appalachian and Blue Ridge Mountains. Strong diurnal heating will
likely allow temperatures to continue to climb into the 80s and 90s
across the much of the Mid-Atlantic and into parts of southern New
England this afternoon. A rather moist low-level airmass, with
surface dewpoints in the upper 60s to mid 70s, exists across this
region. Even with mid-level lapse rates remaining poor, MLCAPE is
expected to reach 1500-3000 J/kg, stronger across northern VA into
MD, eastern PA, and NJ. The better mid-level flow may tend to remain
displaced slightly to the west and north of the strongest
instability. Still, around 25-35 kt of effective bulk shear should
prove sufficient for mainly multicell clusters. 

Steep low-level lapse rates owing to the robust heating and a very
well mixed boundary layer suggest a threat for scattered damaging
wind gusts from convective downdrafts as scattered to numerous
storms develop over the higher terrain and then move eastward. The
northern extent of the severe threat will likely be constrained by a
warm front extending across southern NY into southern New England.
Slightly stronger low-level and deep-layer shear in the vicinity of
this warm front may support a couple of marginal supercell
structures, and a brief tornado or two appears possible. Latest
mesoanalysis estimates suggest weak convective inhibition remains
across the Mid-Atlantic. But, continued heating will erode this
inhibition over the next couple of hours, and storms are expected to
develop during the early afternoon. Even though some uncertainty
still exists regarding the timing of Severe Thunderstorm Watch
issuance, a Watch will likely be needed early this afternoon as the
severe threat increases across much of the Mid-Atlantic into parts
of southern New England.

..Gleason/Dial.. 07/17/2021

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

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

LAT...LON   38777495 38447637 38487774 39027872 40297851 41457792
            42097657 42337524 42417406 42627334 42647255 42037245
            41437290 40677335 39587411 38777495

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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.