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.

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

AVIATION /01Z MONDAY THROUGH FRIDAY/

01/00z TAF Discussion: KCHS/KJZI: Showers will linger at both terminals this evening before picking up in coverage and intensity overnight as weak low pressure develop and passes just offshore. The coastal front may move close enough to KJZI to allow for a few tstms to develop, so TSRA was introduced roughly 05-08z to account for this. No tstms were included at KCHS for now. Activity should gradually wind down after daybreak as low pressure moves farther offshore, although some wrap around shower activity may linger through mid-morning. IFR cigs are expected to develop overnight, but bases should remain just above alternate minimums, although it could be close at times. Clouds should lift and scatter out during the day with VFR returning by early afternoon.

KSAV: Shower activity should diminish by mid-evening. Widespread IFR and LIFR cigs are expected to expand across much of the region tonight, impacting the terminal for much of the overnight period. Cigs are expected to bottom out below alternate minimums, but stay above airfield minimums 07-11z. Some of the stratus could build-down to product some light fog, but vsbys were limited to 5SM for now. Clouds should lift and scatter out during the day with VFR returning by early afternoon.

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: The Small Craft Advisories for the South Carolina nearshore waters will be allowed to expire.

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...None.