textproduct: Charleston

WHAT HAS CHANGED

Tide levels have dropped below minor flood criteria in the Charleston Harbor, so the Coastal Flood Advisory was allowed to expire.

KEY MESSAGES

- 1) A widespread, significant rainfall event will occur across the South Carolina Lowcountry and Southeast Georgia through Saturday.

- 2) Minor coastal flooding possible with the Saturday evening high tide along the lower South Carolina coast.

DISCUSSION

KEY MESSAGE 1: A widespread, significant rainfall event will occur across the South Carolina Lowcountry and Southeast Georgia through Saturday.

The forecast philosophy for tonight into Saturday has not changed significantly. Cyclogenesis is expected to initiate off the South Carolina/Georgia coast overnight as a potent mid- level disturbance over Texas propagates to the east. Isentropic ascent is forecast to steadily strengthen overnight as a well- defined 850 hPa front located over southern portions of Alabama and Georgia jumps north and a southerly low-level jet nudges in from the southwest. This should result in a steady increase in showers/tstms overnight with the greatest coverage poised to setup across the interior prior to daybreak closer to the 850 hPa front and axis of strongest 850-700 hPa frontogenesis. Rain will continue to expand and spread east through the morning hours as the strongest deep-layered forcing for ascent swings through with the approaching disturbance and upper difluence associated with a dual-jet structure between the polar and subtropical jets peaks. There could be a period of enhanced elevated instability where some negative EPV aligns with at least two bands of strong mid-level frontogenesis, so isolated to scattered embedded tstms are still possible (especially Southeast Georgia into south coastal South Carolina) despite the lack of any meaningful surface-based instability. Rain chances will begin to quickly diminish from west-east by mid-afternoon as the upper disturbance passes offshore and low pressure quickly exists the area. Dry conditions should prevail Sunday evening and linger into the overnight hours. Pops tonight range from 50-100% (lowest along the middle Georgia coast) with 100% pops everywhere for Saturday.

Storm total rainfall amounts of 1-2" through Saturday evening still look on track. 01/13z NBM probabilities are as follows: * Prob for >1": 85-95% (highest inland) * Prob for >2": 25-55% (highest far interior Southeast Georgia) * Prob for >3": 5-20% (highest far interior Southeast Georgia)

This represents a solid much needed rainfall given the current drought situation, but it also looks like rainfall rates will be low enough to keep the threat of flash flooding very low. There could still be some minor flooding issues in low-lying and poor drainage areas across the Charleston and Savannah metro areas as pockets of periodic moderate to locally heavy rain move through.

KEY MESSAGE 2: Minor coastal flooding possible with the Saturday evening high tide along the lower South Carolina coast.

Astronomical tide levels will peak tonight with the full moon. Coastal flooding threat is a bit more marginal for Saturday evening since the winds will tip to a more NNW direction which should bring down the anomalies. We would need +1.1 ft to reach the 7.0 ft minor coastal flood threshold, which may be a stretch based on that wind forecast.

AVIATION /02Z SATURDAY THROUGH WEDNESDAY/

KCHS/KJZI/KSAV: Widespread IFR conditions are expected to overspread the area tonight as low pressure begins to organize offshore. Cigs could even approach airfield minimums at times overnight into Saturday morning. The heaviest rains are poised to impact all three terminals between sunrise and late morning, with a chance for some elevated tstms. PROB30 from 13-17z was maintained at all sites to highlight this potential. While rainfall should begin to taper off by mid-afternoon, low clouds look to hang around through the end of the valid TAF cycle (00z Sunday).

Extended Aviation Outlook: Expect improving conditions Saturday evening; otherwise, there are no concerns through Tuesday.

MARINE

A brief period of marginal Small Craft Advisory conditions is possible over the SC nearshore and GA offshore waters Saturday afternoon into Saturday night as surface low pressure develops along the SC coast. At the moment, we only have a few hrs of 25 kt wind gusts, and seas possibly reaching 6 ft over far outer portions, so an advisory would be very marginal.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...None.