textproduct: Charleston

WHAT HAS CHANGED

All Key Messages and sections have been updated.

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 Friday and Saturday evening high tides 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 risk for scattered, light rain will linger through the rest of the afternoon as as weak isentropic ascent persists. The core of the strongest lift is forecast gradually shift offshore so the area of enhanced rainfall impacting parts of Beaufort, Jasper and Chatham Counties should diminish over the coming hours.

The forecast philosophy for tonight into Saturday as 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 Friday and Saturday evening high tides along the lower South Carolina coast.

Astronomical tide levels will peak tonight with the full moon. Continued NE winds will maintain a notable positive tidal anomaly for the next 24-36 hrs. Last evening's high tide anomaly was +1.2 ft, so if we project that into this evening, we could touch 7.2 ft MLLW in Charleston Harbor, which has prompted a Coastal Flood Advisory for Charleston and coastal Colleton counties from 7PM to 11 PM. It 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 /14Z FRIDAY THROUGH TUESDAY/

01/18z TAF Discussion: KCHS/KJZI/KSAV: Widespread IFR conditions are expected to overspread the area tonight as low pressure begins to organize offshore. The heaviest rains are poised to impact all three terminals between sunrise and late morning where cigs could approach airfield minimums. There will be a chance for some elevated tstms, especially in the sunrise to mid-morning hours. PROB30 from 13-16z was included at all sites to highlight this potential.

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...Coastal Flood Advisory from 7 PM to 11 PM EDT this evening for SCZ149-150. MARINE...None.