Mesoscale Discussion 1715
NWS Storm Prediction Center Norman OK
0315 PM CDT Wed Sep 08 2021
Areas affected...Eastern Pennsylvania...northwest New
Jersey...southeastern New York
Concerning...Severe potential...Watch possible=20
Valid 082015Z - 082215Z
Probability of Watch Issuance...40 percent
SUMMARY...The threat for severe weather will gradually increase into
early evening across parts of the Mid-Atlantic. Damaging wind gusts
and isolated large hail are the main concerns. A conditional tornado
threat exists in eastern Pennsylvania and southeastern New York.
Timing and coverage of severe storms is uncertain. Trends will be
monitored for a possible watch.
DISCUSSION...The 18Z observed Albany sounding showed relatively weak
capping, but signs of subsidence aloft. This general profile seems
to capture what is being observed on visible satellite across the
Mid-Atlantic. Areas of cumulus, particularly within eastern
Pennsylvania where temperatures are in the mid 80s F, have shown a
gradual increase in depth. The subsidence aloft is likely limiting
vertical development. With time, and additional mid-level cooling,
storms should be able to initiate in the warm sector. Farther west,
warm conveyor belt precipitation has generally been weak with only
small embedded convective elements. A gradual increase in convective
character may occur as this activity encounters more buoyant air.
The greatest severe potential likely exists with any warm sector
storms that can develop. Damaging winds, isolated large hail, and
perhaps a tornado could occur. Eastern Pennsylvania into
southeastern New York would be the favored zone for low-level
rotation given slightly backed surface winds. Storms within the warm
conveyor belt should exhibit a more linear storm mode and pose
primarily a damaging wind gust risk.
There is some uncertainty as to when storms will intensify this
afternoon. Much of the guidance suggest it may not be until around=20
or after 8 PM EDT. With potentially unfavorable timing diurnally,
trends will have to continue to be monitored for a possible watch
later this afternoon.
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