SPC AC 280100

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0800 PM CDT Thu Aug 27 2020

   Valid 280100Z - 281200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE NORTHERN PLAINS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE LOWER TO MID MISSISSIPPI VALLEY AND FROM PARTS OF THE GREAT
   LAKES TO THE NORTHEAST...

   ...SUMMARY...
   Significant severe wind, large hail and a couple tornadoes will be
   possible this evening in parts of the northern Plains. A severe
   threat will also be possible in parts of the southern Great Lakes,
   central Appalachians and mid-Atlantic. A few brief tornadoes may
   also occur in association with Tropical Storm Laura across the Lower
   Mississippi Valley.

   ...Northern Plains/Upper Mississippi Valley...
   The latest water vapor imagery shows a zonal flow pattern in place
   across the north-central states. A well-defined shortwave trough and
   associated vorticity max is evident in the flow across eastern
   Montana. At the surface, a most airmass is located in much of the
   northern Plains where dewpoints are mostly in the 60s and lower 70s
   F. Surface winds backed to the east across much of South Dakota and
   a surface low is present across far northwest Nebraska. The RAP is
   analyzing a corridor of moderate to strong instability to the
   northeast of the surface low, extending from the Black hills
   east-southeastward across southern South Dakota. MLCAPE along this
   corridor is estimated from 2500 to 3500 J/kg.

   As large-scale ascent associated with the approaching shortwave
   trough increases, thunderstorms will gradually expand in coverage
   across the northern High Plains. The strong instability combined
   with 0-6 km shear in the 45 to 55 kt and steep low to mid-level
   lapse rates, will be favorable for supercells with large hail.
   Hailstones of greater than 2 inches in diameter may occur with the
   more dominant supercells. A couple tornadoes will be possible in
   areas where low-level shear is strongest. As convective coverage
   expands this evening, an MCS is expected to organize in western
   South Dakota. This MCS will move eastward across central and eastern
   South Dakota during the mid to late evening. Wind damage will be
   likely with the faster moving line segments embedded in the MCS.
   Damaging wind gusts greater than 65 kt will be possible with the
   more intense part of the MCS.

   ...Central Appalachians/Mid Atlantic...
   The latest water vapor imagery shows west-northwest mid-level flow
   is located across the Northeast. At the surface, a boundary is
   analyzed across Pennsylvania extending eastward into northern New
   Jersey. Thunderstorms are ongoing along parts of this boundary. To
   the south, surface dewpoints are in the lower 70s F which is
   contributing to MLCAPE in the 1500 to 2500 J/kg range. This combined
   with steep low-level lapse rates will be favorable for damaging wind
   gusts for a few more hours as cells move east-southeastward across
   the region. The severe threat could eventually reach eastern
   Maryland and Delaware later this evening.

   ...Lower to Mid Mississippi Valley/Central Gulf Coast States...
   The center of Tropical Storm Laura is currently located in central
   Arkansas. Bands of heavy rainfall extend from north of the center in
   northern Arkansas south-southeastward into northern and central
   Mississippi. The environment ahead of this band is characterized by
   surface dewpoints in the mid 70s F, moderate instability (MLCAPE
   near 1500 J/kg) and strong low-level shear. For this reason,
   rotating cells embedded in this rain band will have a tornado
   threat. The tornado threat is expected to persist trough the evening
   into the early overnight period as Laura tracks to the northeast
   into the mid Mississippi Valley.

   ..Broyles.. 08/28/2020

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