SPC AC 151711

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1211 PM CDT Sat Sep 15 2018

   Valid 161200Z - 171200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF THE CAROLINAS...

   ...SUMMARY...
   Severe risk appears minimal across most of the U.S. Sunday, though a
   couple of brief tornadoes may occur in conjunction with Florence.

   ...Synopsis...
   A broad anticyclone will dominate the synoptic pattern across the
   central and eastern U.S. while troughing persists across the West. 
   Meanwhile, Florence will continue to migrate slowly northward
   along/near the southern Appalachians during the day , with
   persistent bands of precipitation occurring to the east and
   southeast of the circulation center.  A cold front will migrate
   southward across the northern Plains late in the period and reach
   the South Dakota/Nebraska border area by 12Z Monday - with lee
   troughing persisting south of this trough across the High Plains
   throughout the forecast period.

   ...Carolinas and Virginia...
   Enhanced low-level southeasterly flow will continue on the eastern
   periphery of the circulation of Florence, with bands of convection
   originating from a weakly to moderately unstable pre-convective
   airmass (buoyed by mid/upper 70s F dewpoints) along coastal areas
   and near the Gulf Stream.  Models suggest that enough low-level
   shear will persist to support updraft rotation and perhaps a brief
   tornado or two - especially early in the forecast period before
   Florence migrates away from the region.  Shear profiles will support
   updraft rotation farther north into western Virginia and vicinity,
   although any resultant tornado threat will likely be tied to whether
   areas of insolation can boost surface-based instability.  Marginal
   probabilities have been introduced for this outlook where
   shear/instability will support some tornado risk.

   ...Eastern Montana...
   Isolated/elevated thunderstorm activity is expected early in the
   forecast period downstream of a mid-level trough centered over the
   Pacific Northwest.  Convective coverage may be highest right at the
   beginning of the forecast period, with storms migrating
   northeastward into Canada relatively quickly through 15Z or so.  

   ...South Dakota and vicinity...
   Weak convergence in the 850-700mb layer will combine with steep
   mid-level lapse rates and sufficient deep shear to foster a marginal
   hail threat toward 12Z Monday.  However, the timing of convective
   development is in question - and concentration of the storms may be
   1) isolated at best and 2) hold off into the early D3 period.  Given
   uncertainties, severe probabilities are withheld although any
   convection will pose a marginal hail risk.

   ...MAXIMUM RISK BY HAZARD...
   Tornado:   2%     - Marginal
   Wind:      5%     - Marginal
   Hail:     <5%     - None

   ..Cook.. 09/15/2018

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