Irma 8 a.m. Friday: NWS-Charleston forecast through Monday | News | The Press and Standard

by | September 8, 2017 8:21 am

National Weather Service, Charleston:
SHORT TERM /SATURDAY THROUGH MONDAY/...
Saturday: A sharp moisture gradient will exist across the area
from northwest to southeast. Dry, cool high pressure will remain
anchored over the Great Lakes region, maintaining a northeast
flow across the area. Meanwhile, as Hurricane Irma approaches
southern Florida, moisture will steadily increase off the
Atlantic above the boundary layer. Isentropic ascent along the
stalled front will produce increasing coverage of showers over
the coastal waters, eventually spreading into coastal areas and
points farther inland late in the afternoon into the night. A
steadily tightening pressure gradient across the area will
result in breezy conditions overnight.

Sunday: The tropical airmass off the Atlantic will overspread
most of the forecast area, pushing the front farther to the
northwest. The latest NHC forecast shows Irma moving through
south-central Florida Sunday evening. This will spread 2"+ PWATs
across much of southeast GA and portions of southern SC by late
Sunday. There is the potential for some upper jet divergence
across coastal southeast GA late Sunday afternoon which could
increase the potential for heavy rainfall and localized flooding
well in advance of the hurricane. Sunday night we expect
torrential rainfall to gradually overspread much of southeast GA
and far southern SC as Irma lifts north. Weak surface-based
instability will touch coastal areas late Sunday night while
helicity increases substantially. Isolated tornadoes may be
possible over the waters and immediate coastal areas, especially
late.
The brunt of Irma`s impacts will occur Monday as it is forecast
to lift north into southern Georgia. The current forecast takes
Irma over much of Florida as it tracks north, weakening it
considerably prior to its closest approach to our inland Georgia
zones. This would lower the threat for catastrophic wind damage
across the area, though we still show a prolonged period of
30-40 kt winds with 45-50 kt gusts on Monday. Any eastward shift
in the track would bring stronger winds to the area than
currently forecast, especially if Irma were to move back over
the Atlantic at some point.

A decent threat for short-track tornadoes exists on Monday,
especially in southern SC and coastal GA where models show a
tongue of low-level instability coinciding with substantial
low-level helicity values. Widespread heavy rainfall also
expected, with the greatest totals and thus best flooding
potential over southeast Georgia.

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