SPC AC 230509

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1209 AM CDT Wed Sep 23 2020

   Valid 231200Z - 241200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM LOUISIANA
   INTO SOUTHWEST MISSISSIPPI...FROM NORTHERN MINNESOTA INTO
   WISCONSIN...AND OVER COASTAL WASHINGTON...

   ...SUMMARY...
   A few strong to severe storms are possible Wednesday evening from
   northern Minnesota into Wisconsin with hail and gusty winds. A
   tornado cannot be ruled out over parts of Louisiana and southwest
   Mississippi with the remnants of Beta. A few strong storms may
   affect coastal Washington Wednesday evening and overnight.

   ...Synopsis...
   An upper ridge over the Great Basin will flatten as a powerful upper
   trough impacts the Pacific Northwest. Substantial height falls and
   increasing winds aloft will impact the state of Washington late in
   the day and overnight, with a few strong storms expected near the
   coast.

   To the east, a weaker progressive feature will move across the
   northern Plains and upper Mississippi Valley, providing increasing
   shear and lift. A plume of 50s to near 60 F dewpoints will aid
   destabilization, favoring isolated strong to severe storms.

   To the south, the remnants of Beta will eject northeast across
   Louisiana and Mississippi as a positive-tilt upper trough moves
   toward the middle and lower Mississippi Valley. A more substantial
   advection of moisture and pressure falls is anticipated compared to
   the previous day, supporting the addition of low tornado
   probabilities.

   ...Louisiana into southwest Mississippi...
   The surface low is forecast to track from the upper Texas Coast this
   morning into central Louisiana by evening, with pressure falls
   extending east across Mississippi. Veering surface winds and
   increasing low-level flow overall will exist as a warm front lifts
   north, oriented roughly from central Louisiana into southwest
   Mississippi at 00Z. An increasing height gradient aloft will enhance
   midlevel winds, with 30 kt expected at 500 mb. Meanwhile, 850 mb
   winds will increase to around 30 kt, with higher values during the
   evening.

   A small area of tornado potential may develop after about midday
   over Louisiana as the 1000-1500 J/kg SBCAPE develops coincident with
   200 m2/s2 effective SRH. This will support isolated severe storms,
   possibly a couple supercells with a tornado risk. Beyond 00Z, the
   better lift from warm advection may transition the storms to
   elevated to the north.

   ...Northern Minnesota into northwest Wisconsin...
   Good heating will occur during the day, prior to the shortwave
   trough arriving. Though lift will be maximized after 00Z, steep
   lapse rates aloft and increasing deep-layer shear will favor mainly
   elevated storms forming over northern Minnesota, and moving rapidly
   southeast into Wisconsin. Mid to high-level winds of 30 to 50 kt
   respectively will lengthen hodographs, favoring both hail and
   perhaps small forward-propagating storms with gusty winds. The time
   of day may mitigate the overall wind risk.

   ...Coastal Washington...
   Intense height falls will occur late in the day with the strongest
   cooling aloft and destabilization after 00Z across western
   Washington. A prominent leading band of precipitation associated
   with warm advection in the lower levels will give way to storm cells
   moving rapidly east off the ocean. While only a few hundred J/kg
   MLCAPE will exist, strong large-scale lift as well as long
   hodographs may favor a few long-lived cells, possibly with rotation
   which will enhanced wind gust threat. Small hail is also possible
   given cold temperatures aloft.

   ..Jewell/Wendt.. 09/23/2020

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