Marine Weather and Tide Forecast for Charleston, SC

KEY MESSAGES
- 1) Locally heavy rainfall and isolated flash flooding possible across portions of southeast Georgia and southeast South Carolina today.

- 2) Unsettled weather conditions persist across portions of Southeast Georgia and the South Carolina Lowcountry on Sunday and Monday.

- 3) Evening high tides could bring minor inundation of very low-lying areas along the Charleston and Colleton County coastlines through the weekend.

DISCUSSION
KEY MESSAGE 1: Locally heavy rainfall and isolated flash flooding possible across portions of southeast Georgia and southeast South Carolina today.

Aloft, the pattern will transition to become more zonal with embedded shortwave shortwave energy poised to pass through the forecast area during the day. At the surface, analysis shows a subtle stationary boundary currently stretching west to east across the forecast area will slowly meander back to the north for the daytime hours. Also of note, the atmosphere will remain quite moist with precipitable water values expected to be in the 1.8-2.0 range which would be above the 90 percentile and approaching the daily max according to the SPC Sounding Climatology. All of this to say that with some shortwave energy aloft, a boundary nearby at the surface, all within a very moist atmosphere, we expect to see another convectively active day.

One main difference from the last few days will be the steering flow in the mid-levels. Models consistently show ~20-25 knots of west-northwest mid-level flow which will produce a much different storm motion. Though there is a surface boundary across the area, it is quite subtle and the general consensus among hi-res models and the HRRR is that convection will not tend to focus on one particular area. Therefore, the threat for intense rainfall will come mainly from clusters of storms training over the same areas within the relatively progressive flow. So while the HRRR highlights much of the area in 30-50% probabilities for >1" in 3 hours, the probabilities for >3" in 3 hours is quite low for any one particular area. So, it seems reasonable to say that there is certainly the potential for a few isolated pockets of 1-3" rainfall amounts in 1-3 hour time periods. This thinking follows with the entire forecast area being highlighted in the WPC Day 1 Excessive Rainfall Outlook Marginal Risk area. Convection will likely initiate by around midday and the threat for locally heavy rainfall will continue into the evening hours.

KEY MESSAGE 2: Unsettled weather conditions persist across portions of Southeast Georgia and the South Carolina Lowcountry on Sunday and Monday.

Numerous showers and thunderstorms possible on Sunday and Monday as the stalled frontal boundary lingers across the Southeast and a very moist airmass characterized by anomalously high PWAT values continues to stream northward into the region.
While large-scale forcing appears relatively weak, several subtle disturbances embedded within southwesterly flow aloft should be sufficient to support unsettled weather conditions throughout Sunday and Monday. There is a Marginal Risk (Level 1/4) for Excessive Rainfall on Sunday to highlight the risk of localized flooding. Although activity should remain somewhat disorganized, localized training and repeated rounds of rainfall could be possible where outflow boundaries become established.
As a result, localized flooding could be possible on Sunday and Monday, particularly in urban areas and low-lying locations.

KEY MESSAGE 3: Evening high tides could bring minor inundation of very low-lying areas along the Charleston and Colleton County coastlines through the weekend.

Elevated tidal departures may persist through the weekend for this evening's and Sunday evening's high tide. The majority of guidance suggests that tidal levels should remain shy of minor coastal flood criteria for this evening, however minor inundation of very low-lying areas could be possible along Charleston and Coastal Colleton Counties especially if showers and thunderstorms track over the region around the time of high tide (8:27 EDT). The better chance for minor coastal flood criteria being met will be with Sunday's evening high tide (9:05 EDT).

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/
While the 06z TAF period begins with VFR conditions at KCHS, KJZI, and KSAV, there are a few low confidence forecast issues to discuss. First, model guidance suggests there could be a period of MVFR or IFR ceilings closer to sunrise. The best chance for IFR appears to be at KSAV, where we are advertising IFR ceilings and light fog between 09-12z. Further east, chances appear less for IFR and MVFR seems more likely. KCHS and KJZI both contain MVFR ceilings around or just after sunrise for a few hours through mid morning. The next issue will be thunderstorm potential in the afternoon and evening. Storm motion will be different today, with storms generally tracking from west-northwest to east-southeast. Current timing is for storms to begin around KCHS and KJZI first, generally in the 18-22z time period. Current thinking is that KSAV will be a bit later, more into the late afternoon and early evening time period.

Extended Aviation Outlook: Expect periodic flight restrictions through early next week as unsettled weather conditions will persist. Patchy overnight fog and/or stratus possible as well.

MARINE
Today: Light northwesterly winds will be apparent this morning, and then veer out of southwest by the afternoon into the overnight as a stalled frontal boundary lingers across the local waters. Seas will range from 2 to 3 ft.

Sunday: Breezy east-northeasterly winds at 15 to 20 kt with gusts near 30 kt possible across the nearshore South Carolina waters as the region becomes sandwiched between a stalled frontal boundary and a surface high pressure system. Seas will also increase as a short-lived east-southeasterly swell surges into the local waters. Expect the nearshore and offshore southeast South Carolina waters to peak to 5 to 7 ft, while the nearshore and offshore waters southeast Georgia waters to peak 4-6 ft. Therefore, Small Craft Advisories (SCAs) have been issued for the nearshore South Carolina waters on Sunday, and then also the outer Georgia waters late Sunday into Monday. It's possible to see the Charleston Habor gust near 25 kt early Sunday morning through mid-day, however opted out of a SCA as conditions remain borderline.

Monday and Tuesday: Expect moderate north-northeasterly flow to persist through mid-week as several subtle disturbances embedded within southwesterly flow aloft traverses across the local waters. Seas will range from 2 to 4 ft on Monday, and then become 2 to 3 ft on Tuesday.

Rip Currents: There is a Low Risk of rip currents at all beaches today. For Sunday, the internal calculator has a borderline moderate/high risk, while RCMOS has a High Risk.
Thus, opted to keep the High Risk for all beaches on Sunday.

CHS WATCHES/WARNINGS/ADVISORIES
GA...None.
SC...None.
MARINE...Small Craft Advisory from 2 AM to 10 PM EDT Sunday for AMZ360.
Small Craft Advisory from 6 AM to 10 PM EDT Sunday for AMZ362.
Small Craft Advisory from 6 PM Sunday to 8 AM EDT Monday for AMZ384.