Day 1 Severe Weather Outlook

Valid 232000Z – 241200Z


Thunderstorms with large hail and damaging winds will be possible
today across parts of southwest and central Arizona. Storms with a
few strong wind gusts will be possible in parts of the Northeast.

…20Z Outlook Update…
Some adjustments to the categorical (10 percent probability)
thunderstorm line have been made, largely to account for current
trends in observational data and the latest model output. With
regard to severe probabilities, adjustments have been confined to
the Northeast, where destabilization has remained quite weak.

Cloud cover has inhibited appreciable surface heating today near and
to the immediate west of the Allegheny Mountains. Although there is
some increase in showers ongoing across parts of western
Pennsylvania into southeastern Ohio, on the eastern edge of stronger
heating to the west of the Appalachians, there appears little
potential for substantive intensification as this activity spreads
into the cooler and more stable air, particularly given the general
mid-level drying/subsidence evident in water vapor imagery.

Even within the corridor remaining in 5 percent severe
probabilities, from the Williamsport and Wilkes-Barre/Scranton area
of Pennsylvania northeastward into portions of the Hudson/Champlain
Valleys, boundary layer destabilization has been rather weak.
However, models suggest that strengthening large-scale ascent will
become focused along the frontal zone across this region during the
next few hours, aided by mid/upper forcing downstream of the
approaching mid-level trough. This could provide support for
increasing/strengthening thunderstorm development in the presence
of 30-50 kt westerly mean ambient flow, which may pose some risk for
localized potentially damaging wind gusts late this afternoon into
early evening.

..Kerr.. 09/23/2019

.PREV DISCUSSION… /ISSUED 1111 AM CDT Mon Sep 23 2019/

Morning water vapor loop shows an intense upper trough digging into
CA/NV. Midlevel height falls, cooling temperatures aloft, and large
scale forcing are overspreading AZ in advance of this system. This
should lead to multiple rounds of thunderstorms today and tonight
over the region. Morning convection and clouds are reinforcing a
baroclinic zone that extends across central AZ. This boundary
should be the focus of thunderstorms though the day, with ample CAPE
and weak CIN over southern AZ. Vertical shear profiles are
sufficient for a few supercell storms capable of very large hail and
damaging wind gusts. An isolated tornado cannot be ruled out.

A low-amplitude and progressive shortwave trough is moving across
the Great Lakes region today, with an associated cold front
approaching western NY/PA. Widespread clouds are present today
across much of NY into VT, which should limit daytime
heating/destabilization in the area of strongest forcing and
convective potential. This is expected to keep the overall severe
risk in this area MRGL. Have extended the MRGL risk area farther
south in parts of central PA where stronger heating is occurring and
at least isolated afternoon thunderstorms are possible. Despite the
weak instability throughout the risk area, strong winds aloft and
large-scale forcing suggest a risk of gusty/damaging wind gusts in
the strongest storms.


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