textproduct: Charleston

WHAT HAS CHANGED

All section have been updated.

KEY MESSAGES

- 1) Locally heavy rainfall and isolated flash flooding possible again today, especially across southeast Georgia.

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

- 3) Elevated tidal departures could yield minor coastal flooding to Charleston and Colleton County coastlines this evening.

DISCUSSION

KEY MESSAGE 1: Locally heavy rainfall and isolated flash flooding possible again today, especially across southeast Georgia.

Another round of locally heavy rainfall will yield an isolated flash flooding threat this afternoon and evening, especially for the southeast Georgia area. Aloft, west or west-northwest flow will prevail. At the surface, high pressure sourced back to the Great Lakes region will begin pushing in across the Carolinas early this morning. As the high builds in, it will push the very subtle stationary boundary that has lingered across the area the last few days back to the south and southwest. Most guidance favors the high building in as far south as the Savannah River, aligning the boundary across south and southeast GA for the afternoon and evening hours. This will have notable implications for the potential focusing of convection later today.

With the boundary expected to be aligned across southeast GA within the best convergence zone on the periphery of the high, all coincident with impressive precipitable water values in the 2.0-2.2" range, conditions will be supportive of very intense rainfall rates. But perhaps the biggest concern for heavy rainfall will be the likelihood of minimal storm motion, anchoring/back building of convection as model soundings show ~20 kts of mid-level flow atop easterly flow in the sub-850 mb layer. Therefore it follows that the HREF contains 3-hour probabilities of >1" well into the 50-70 percent range and probabilities of >3" of 30-50 percent. The main area of concern is southeast GA mainly along and south of I-16 and especially as you get closer to the Altamaha. This region is located within the WPC Day 1 Excessive Rainfall Outlook Slight Risk area. Convection should initiate around midday or the early afternoon, and then persist into the evening with the focus gradually shifting further to the south with time. Given the exceedingly active and wet conditions in this area over the last week or so, isolated flash flooding is certainly a concern.

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

A stalled frontal boundary will continue to linger across the Southeast combined with a very moist airmass characterized by PWAT values ranging from 1.5 to 2.0 inches. This will keep the pattern rather unsettled on Monday and Tuesday.

Monday: Expect shower and thunderstorm coverage to increase through the late morning and afternoon esp. across the South Carolina Lowcountry. While activity remains somewhat disorganized, slow cell motions, boundary interactions, and localized training could still produce pockets of heavy rainfall. This will yield localized flooding in urban areas and low-lying areas through the late morning and into the evening. However, convection should gradually diminish overnight with some lingering showers persisting near the coastline.

Tuesday: As a low pressure system tries to develop offshore of the South Carolina coastline, the aforementioned frontal boundary will settle farther southward across northern Florida. This set-up will shift the focus to southeast Georgia on Tuesday, and expect showers and thunderstorm coverage to increase through the late morning and afternoon esp. near and south of I-16. The overall flooding threat appears limited, however localized training and repeated rounds of rainfall could be still possible where outflow boundaries become established. Coverage seems to be little less across the South Carolina Lowcountry compared to Monday as dry, northeasterly flow becomes established in the northern zones of the CWA. Similar to Monday, convection should gradually diminish overnight with some lingering showers possible near the coastline.

KEY MESSAGE 3: Elevated tidal departures could yield minor coastal flooding to Charleston and Colleton County coastlines this evening.

Breezy east-northeasterly flow will yield elevated tidal departures for this evening's high tide (9:05PM). The recent guidance from PETSS10, PETSS50, and ETSS suggests this evening's high tide will likely range from 6.9 to 7.2 ft MLLW at the Charleston Harbor. Thus, the forecast notes a 7.0 ft MLLW for this evening's high tide and this will likely result in brief, shallow coastal flooding in the usual prone flood 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.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

KCHS and KJZI: MVFR ceilings have arrived along with the surge of northeast flow this morning. There is potential for ceilings to lower into the IFR range for a short time period this morning, before returning to MVFR and lingering through the day. Regarding afternoon thunderstorm chances, the main concern will be for areas further to the south and southwest so we have kept the afternoon clear of storms for now. A better chance for showers and storms will come late tonight, along with ceilings lowering to IFR. This will primarily happen around or just after 06z and continue through the end of the TAF period.

KSAV: IFR ceilings have reached the terminal and will likely stick around for at least a few hours. Thunderstorm chances are expected to ramp up around and just after midday. We have maintained a TEMPO group for the 18-22z time period to account for at least MVFR conditions, though if the airport were to take a direct hit IFR would be likely within heavy rainfall reducing visibilities. The risk of storms will diminish by the early evening. For the rest of the period, there is good model agreement that ceilings will lower into the IFR range late in the evening and continue through the rest of the night.

Extended Aviation Outlook: Expect periodic flight restrictions early next week due to unsettled weather conditions. Patchy overnight fog and/or stratus are also possible.

MARINE

Today: 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 surface high pressure shifting overhead. Seas will also increase as a short-lived east-southeasterly swell surges into the local waters. Expect the nearshore South Carolina waters to peak to 4 to 6 ft in the afternoon, while the nearshore and offshore waters southeast Georgia waters to peak 3 to 4 ft in the evening. Therefore, Small Craft Advisories (SCAs) have been maintained for the nearshore South Carolina waters on Sunday considering the elevated winds and seas. Also, a SCA has been issued for the Charleston Harbor as recent guidance has shown gusts of 25-26 kts early Sunday morning through the early afternoon. Opted to cancel the SCA across the outer Georgia waters as the latest wave guidance has backed off quite a bit with the strength of the swell.

Monday through Wednesday: Moderate north-northeasterly flow will persist on Monday through Tuesday afternoon 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 becoming 3 to 5 ft on Tuesday afternoon.

Shifting the focus to Tuesday evening, the forecast becomes considerable more interesting. At the base of an East Coast upper-lvl trough, a closed-off low appears to spin up offshore of the South Carolina coastline on Tuesday evening before slowly tracking out into the Mid-Atlantic Ocean. Thus, this will cause for northeasterly flow to surge on Tuesday evening into Wednesday night. Expect winds to range from 20 to 25 kt with gusts up 30 kt across the nearshore and offshore South Carolina and Georgia waters. It's possible to see gusts up 34-35 kt across the offshore waters on Wednesday morning. Therefore, Small Craft Advisories deem likely across the local waters on Tuesday evening though Wednesday night, with the potential for Gale Warnings being needed for the offshore waters. Strong east- northeasterly swell will also surge into the local waters Tuesday evening and allow for seas to peak to 5 to 7 ft in the nearshore waters and 7 to 9 ft in the offshore waters on Wednesday afternoon. Thereafter, as the closed-off low tracks further offshore, marine conditions should gradually subside overnight on Wednesday.

Rip Currents: A Moderate Risk for rip currents has been maintained at all beaches for today. And then, a Low Risk is expected for Monday.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory until 2 PM EDT this afternoon for AMZ340. Small Craft Advisory until 8 PM EDT this evening for AMZ360. Small Craft Advisory until 8 PM EDT this evening for AMZ362.