SPC AC 250542

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1242 AM CDT Tue Aug 25 2020

   Valid 251200Z - 261200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE MID-ATLANTIC...

   ...SUMMARY...
   Thunderstorms associated with damaging wind gusts will be possible
   today in the central and northern Appalachians eastward to the
   Atlantic Seaboard. The greatest wind-damage threat is forecast
   across the Mid-Atlantic states.

   ...Northeast...
   An upper-level trough will move southeastward from Quebec into the
   Northeast today. At the surface, a cold front will advance quickly
   southeastward across the central and northern Appalachians and then
   to the Atlantic Seaboard. Ahead of the front, surface dewpoints from
   the mid 60s to the lower 70s F will result in a southwest to
   northeast corridor of moderate instability by afternoon.
   Thunderstorms will likely develop along the front and move quickly
   southeastward onto the Atlantic coastal plains by afternoon.
   Forecast soundings along the instability axis at 18Z show very steep
   low-level lapse rates near 8.0 C/km with westerly low-level flow.
   Winds in the low to mid-levels veer to the west-northwest with
   height. This profile would be favorable for damaging gusts with
   multicells that organize into short line segments. 

   Two areas will have potential for more widespread damaging wind
   gusts. The first area will be in the Mid-Atlantic where model
   forecasts suggest the greatest amount of instability will develop.
   The Enhanced Risk area is maintained for parts of the Mid-Atlantic
   and expanded slightly westward where convection is expected to
   organize along the front. The second area would be across southern
   New England where deep-layer shear is forecast to be stronger than
   in the Mid-Atlantic. If it appears that an organized line segment
   will move across southern New England, then an upgrade to Enhanced
   Risk could need to be considered. A third area along the front,
   including much of Ohio and western Pennsylvania, looks to have
   potential for damaging gusts as well.

   ...Northern Plains/Upper Mississippi Valley/Western Great Lakes...
   An upper-level ridge will move eastward across the northern Plains
   today as a cold front advances southeastward across the western
   Dakotas. A moist airmass will be in place ahead of the front where
   surface dewpoints will be in the mid to upper 60s F. This should
   allow for a corridor of moderate instability to develop by
   afternoon. In response to a weakening capping inversion, isolated
   thunderstorm development will be possible during the late afternoon.
   Thunderstorm coverage is expected to increase across parts of the
   upper Mississippi Valley during the evening as low-level flow
   strengthens across the region. Although this activity may be
   elevated in nature, hail and strong gusty winds may occur with the
   stronger cells.

   ...Desert Southwest...
   At the surface, a low will deepen today across far southwestern
   Arizona. To the east of the low, a corridor of low-level moisture
   will be in place from southwestern Arizona extending northeastward
   into central Arizona. Moderate instability will develop along this
   corridor by afternoon. Thunderstorms that form in the stronger
   instability west of the Tuscon and Phoenix areas will have a
   potential for marginally severe wind gusts. A few strong wind gusts
   may also occur with convection that develops along the Mogollon Rim.

   ...Northwestern States...
   Southwest mid-level flow will be in place across much of the
   northwestern U.S. today. At the surface, a moist axis will be
   located from parts of northern Nevada into western Idaho, where
   destabilization will take place during the day. Thunderstorms are
   expected to form along the western edge of the stronger instability
   and then move east-northeastward across the region. The instability
   combined with moderate deep-layer shear and steep low-level lapse
   rates may be enough for isolated damaging gusts.

   ..Broyles/Moore.. 08/25/2020

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