textproduct: Charleston

WHAT HAS CHANGED

Key Message 1 was updated.

KEY MESSAGES

- 1) Locally heavy rainfall and isolated flash flooding remains possible through this evening, especially across southeast Georgia. The risk for heavy rainfall and flooding could spread along parts of the lower South Carolina coast, including the Charleston Metro Area, early Monday morning.

- 2) A low-end threat for localized minor flooding continues through Tuesday as a stalled front and an approaching cold front support daily showers and thunderstorms.

- 3) Minor coastal flooding is possible along the Charleston and Colleton County coastlines this evening and Tuesday evening.

DISCUSSION

KEY MESSAGE 1: Locally heavy rainfall and isolated flash flooding remains possible through this evening, especially across southeast Georgia. The risk for heavy rainfall and flooding could spread along parts of the lower South Carolina coast, including the Charleston Metro Area, early Monday morning.

31/21z surface analysis showed cold air damming has now spread south of the Altamaha River, being reinforced by ongoing light to moderate rains falling across much of the South Carolina Lowcountry and Southeast Georgia. There remains some weak instability across Southeast Georgia into far southern South Carolina per the latest instability meso plots, but the risk for deep convection has mostly ended. There remains a heavy rainfall threat through the night with warm cloud processes likely dominating where pockets of deeper moisture and some instability could help to locally enhanced some rainfall rates, but any risk for flash flooding should remain fairly isolated.

Cyclogenesis is expected to initiate near the Georgia and far southern South Carolina coast later tonight. The developing low is forecast to pass east of the Charleston County coast by daybreak Monday. The coastal front offshore and its associated surface-based instability could make a brief run for parts of the lower South Carolina coast during the early morning hours as the low begins to organize, but should not make too much progression inland (likely no farther inland than US-17 at best). Convection is expected to blossom over parts of the South Carolina coast, including the parts of the Charleston Tri- County region, late tonight as the low organizes and low-level convergence near the coastal front increases. A heavy rain/flooding threat could develop in this region for a few hours with possibly a somewhat elevated risk for Downtown Charleston if hourly rainfall rates are high enough.

Grids and text products were updated to reflect near term trends and better resolve the coastal front and track of the low overnight.

KEY MESSAGE 2: A low-end threat for localized minor flooding continues through Tuesday as a stalled front and an approaching cold front support daily showers and thunderstorms.

A stalled front will linger across the forecast area through Monday, interacting with a moist airmass characterized by PWAT values of 1.6-1.9 inches. This will maintain an unsettled pattern with scattered to numerous showers and a few embedded thunderstorms. The meandering front draped along the SC coast and southern Georgia along with a weak area of low pressure developing off the SC coast will focus the greatest rainfall potential across coastal areas of southeast South Carolina. Tuesday, an approaching cold front from the north will slowly drop through the area, providing continued support for scattered convection. As a result, shower and thunderstorm coverage should begin to decrease from north to south Tuesday evening, with the greatest rainfall potential shifting toward southeast Georgia and areas along and south of I-16 ahead of the front.

Although convection will remain somewhat disorganized, slow storm motions and localized training could result in pockets of repeated rainfall. Given the modest rainfall amounts, the overall flood threat remains on the lower end. For Monday, the latest NBM indicates a 30-60% chance of exceeding 1 inch with the greater chances occurring across coastal South Carolina. Additionally, there is a 15-30% chance of rainfall exceeding 1 inch across portions of the area for Tuesday. Urban and low- lying areas, and those locations that received 5+ inches of rain over the weekend, may be more susceptible to minor flooding if additional showers set up over the area.

KEY MESSAGE 3: Minor coastal flooding is possible along the Charleston and Colleton County coastlines this evening and Tuesday evening.

Gusty east-northeasterly winds are causing elevated tidal departures for this evening's high tide (9:05 PM). The TWL forecast is 7.0 ft MLLW, which would result in brief, minor coastal flooding in the usual flood prone areas across Charleston and Coastal Colleton Counties. Also, if showers and thunderstorms track over the region around the time of high tide, then flooding could be exacerbated. Therefore, the issuance of a Coastal Flood Advisory is possible for this evening.

Another surge of northeasterly winds could generate minor coastal flooding for these same locations Tuesday evening.

AVIATION /21Z SUNDAY THROUGH FRIDAY/

18Z TAFs: KCHS and KJZI: Gusty NE winds are maintaining MVFR ceilings. Radar imagery indicates some showers nearby. However, the probabilities of direct impacts to either site are too low to mention in either TAF this afternoon. The higher probabilities of direct impacts should occur after midnight. This is also when ceilings are expected to lower to IFR. Conditions should start to improve late in the TAF time period.

KSAV: Gusty NE winds have kept BKN ceilings in the MVFR to VFR categories. Radar imagery indicates numerous showers with isolated thunderstorms near the airport. CAMs continue to point to direct impacts to the airport, so we maintained the TEMPO group from 18-22Z. This may need to be amended based on future radar trends. Otherwise, this activity along with the winds will trend lower this evening. Ceilings will also lower, first to MVFR and then to IFR overnight. Conditions should start to improve late in the TAF time period.

Extended Aviation Outlook: Periodic flight restrictions are possible through Tuesday due to scattered showers and isolated thunderstorms. Patchy overnight fog and/or low stratus are also possible.

MARINE

Tonight: Gusty E winds from late this afternoon will ease this evening and overnight, with seas trending lower as well. The Small Craft Advisory for the Charleston Harbor has expired, and the remaining Advisories for the SC waters within 20 nm are scheduled to end this evening.

Monday through Friday: Transient benign marine conditions on Monday will begin to deteriorate Tuesday morning. A weak area of low pressure could develop offshore on Monday before moving farther into the Atlantic. Meanwhile, strengthening high pressure to the north will cause northeasterly winds to surge Tuesday through Wednesday night. Winds are expected to range from 20 to 25 kt with gusts around 30 kt across the waters. A brief period of gusts up to 35 kt is possible Wednesday morning across the waters 20 to 60 nm offshore. Small Craft Advisory conditions appear likely, with a low- end chance for gale conditions. Strong east-northeasterly swell will also build into the local waters Tuesday evening, with seas peaking Wednesday at 5 to 7 ft in the nearshore waters and 7 to 9 ft in the offshore waters.

Marine conditions are expected to gradually improve Wednesday night into Thursday as the low moves farther out into the Atlantic. No additional marine concerns are anticipated through the remainder of the period.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for AMZ360- 362.