textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) A front will sag southward this afternoon, with isolated to scattered showers and thunderstorms possible this afternoon.

- 2) Much needed rain will fall across the South Carolina Lowcountry and Southeast Georgia Friday into Saturday.

- 3) Coastal Flood Advisory issued for Charleston and coastal Colleton county for tonight's high tide cycle. Minor coastal flooding remains possible with the evening high tide cycle again Friday and Saturday.

DISCUSSION

KEY MESSAGE 1: A front will sag southward this afternoon, with isolated to scattered showers and thunderstorms possible this afternoon.

As of 2 PM this afternoon a surface front could be analyzed extending from Hilton Head, SC westward roughly along I-16. While guidance is a little mixed on additional shower development this afternoon, the 12Z HREF and the last several runs of the HRRR have all been supportive of some isolated shower development east of I- 95. The best chance for precipitation will likely be in southeast Georgia where the front hasn't reached yet and some sunshine is allowing the atmosphere to remain unstable. PoPs this afternoon remain in the 40 to 50% range across this area. While a grumble of thunder is possible the overall risk for severe thunderstorms is very low.

KEY MESSAGE 2: Much needed rain will fall across the South Carolina Lowcountry and Southeast Georgia Friday into Saturday.

Friday: Rain chances will begin to increase Friday as weak isentropic ascent develops atop of the stalled front over Florida. There are indications that a more consolidate area of mostly light rain could develop by mid-morning mainly between the I-16 an I-26 corridors before shifting offshore. Guidance is essentially split on this possibility with some members showing a much wetter trend while others are remaining a bit drier. An in-the-middle approach was taken to construct pops, nudging them up to 50-70% in this area; however, confidence on how this will play out is only moderate at best. Better rain chances will likely hold off until Friday night (especially late) as the 850 hPa warm front lifts north through the area and a stronger low-level jet induces even stronger isentropic ascent. There will also be increasing contributions from deep- layered quasi-geostrophic forcing ahead of an approaching mid-level disturbance which will only further aid in widespread rain development as daybreak Saturday approaches. Overnight pops were increased to 60-90%, highest across the interior and across parts of the lower South Carolina coast.

Saturday: Confidence is high that beneficial rains will fall across the drought-stricken Lowcountry and Southeast Georgia Saturday as cyclogenesis consolidates just offshore. As forcing for ascent increases ahead of a potent southern stream disturbance, widespread rains will steadily fill in and impact the entire area Saturday. Rain will gradually end from west-east late Saturday afternoon into the evening hours as the developing surface low moves away, passing just south of Cape Hatteras and eventually out into the open Atlantic Saturday night.

There continues to be consistent signals that a corridor of heavier rainfall could set up across the interior within a region of enhanced lift induced by a dual-jet structure between the polar jet to the north and a weaker subtropical jet segment to the south. Current model guidance consensus focuses this potential over the southern Midlands and CSRA into far interior Southeast South Carolina and Southeast Georgia; however, the exact placement of the axis of heavier rain is still bit uncertain. The 30/13z NBM guidance shows a 70-85% chance of rainfall totals >1", 40-65% chance for >1.5", and a 20-40% chance for >2" for the entire area. The risk for flash flooding is very low given that soils are parched with extreme to exceptional drought conditions in place and rain rates should be low enough to minimize the risk of rapid downpours. There could still be some issues in low-lying and poor drainage areas across the Charleston and Savannah Metro areas if pockets of intense rainfall with enhanced rainfall rates where clusters of elevated tstms occur.

Surface-based instability is virtually nonexistent with the region likely remaining well embedded in the cold sector, but a few elevated tstms are certainly possible given the degree of forcing noted aloft.

KEY MESSAGE 3: Coastal Flood Advisory issued for Charleston and coastal Colleton county for tonight's high tide cycle. Minor coastal flooding remains possible with the evening high tide cycle again Friday and Saturday.

Astronomical tide levels will continue to climb as we approach the full moon on Friday (5/1). A Coastal Flood Advisory has been issued for Charleston and coastal Colleton counties for tonight, with tide levels expected between 7.1-7.3 ft MLLW, resulting in minor coastal flooding impacts. Winds may become more supportive of increasing tidal departures later today and continuing into Saturday. While this isn't expected to lead to any coastal flooding along the southeast Georgia coast, the tide level at Charleston is expected to peak around 7-7.2 ft MLLW each evening. Additional Coastal Flood Advisories could be needed for the Charleston and coastal Colleton coast.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Prevailing VFR through the 18Z TAF period. There is a risk of an isolated shower/tstorm this afternoon at KCHS/KJZI/KSAV, however confidence is very low in any direct impacts to the terminals. The best chances would be at KSAV where VCSH has been included this evening. Rain chances increase again in the morning hours on Friday. VCSH has been included at all terminals Friday morning as there is low confidence on the timing of the showers.

Extended Aviation Outlook: Flight restrictions and low ceilings are likely from showers and tstms Friday into Saturday.

MARINE

There is a risk for winds to reach marginal Small Craft Advisory levels Saturday as low pressure develops offshore and pulls away from the region. Gusts to 25 kt may linger over parts of the offshore waters through Saturday night into early Sunday morning. Seas look to peak 2-4 ft nearshore waters 4-6 ft offshore waters late Saturday afternoon into the early evening hours.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...Coastal Flood Advisory from 6 PM to 10 PM EDT this evening for SCZ149-150. MARINE...None.