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
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.
...Please see www.spc.noaa.gov 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