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Mesoscale Discussion 1640
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   Mesoscale Discussion 1640
   NWS Storm Prediction Center Norman OK
   0358 AM CDT Mon Sep 11 2017

   Areas affected...northeast Florida...southwest Georgia...and
   adjacent coastal waters

   Concerning...Tornado Watch 484...

   Valid 110858Z - 111100Z

   The severe weather threat for Tornado Watch 484 continues.

   SUMMARY...Tornadoes associated with Hurricane Irma will remain
   possible through the overnight and into the morning.

   DISCUSSION...Hurricane Irma continues to move generally northward
   through the Florida Peninsula this morning. The low-level wind field
   remains quite strong across northeast Florida, where Jacksonville's
   VAD indicates 0-3 kilometer storm-relative helicity values near 900
   m2/s2. This strong low-level wind field should continue through the
   overnight and into the morning, lifting northward along and
   northeast of Hurricane Irma's center.

   Within this strong low-level flow field, cellular convection was
   embedded within a primary Hurricane Irma feeder band extending
   eastward from near the Florida-Georgia state line into the Atlantic
   at 0845Z. To the north of this primary feeder band, additional
   cellular convection was organizing into another feeder band from
   Glynn County, GA, east-northeast into the Atlantic. Within both of
   these larger feeder bands, convection has periodically exhibited
   transient, mini-supercellular characteristics. The strength of the
   low-level kinematic fields will continue to support updraft rotation
   and the potential for a couple tornadoes through the overnight,
   especially along and near coastal areas within
   discrete/semi-discrete convection. 

   Farther north, a surface boundary off the Georgia-South Carolina
   coast demarks the more unstable airmass to the east from the more
   stable airmass to the west. Here, cellular convection over the
   Atlantic may continue to exhibit periodic rotation; however as the
   convection moves west across the boundary into the more stable
   airmass, weakening of the convection will likely occur, limiting the
   tornado potential. As Hurricane Irma lifts northward with time, the
   increasing easterly component of the low-level flow should allow the
   surface boundary to move inland, increasing the threat for tornadoes
   along coastal areas.

   ..Marsh.. 09/11/2017

   ...Please see for graphic product...


   LAT...LON   29458242 29538265 29838267 30138247 30708239 31238267
               31428262 31498214 31828207 32168171 32228143 32198112
               31567989 31138027 30668042 30128029 29497998 29448112
               29278117 29278142 29408142 29358166 28968167 28958226
               29068256 29208254 29258239 29458242 

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Page last modified: September 11, 2017
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