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