textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Heavy rainfall could produce localized flooding over southeast Georgia today.

- 2) Localized flash flooding possible across portions of Southeast Georgia and the South Carolina Lowcountry on Saturday.

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

DISCUSSION

KEY MESSAGE 1: Heavy rainfall could produce localized flooding over southeast Georgia today.

A stalled front will linger in the vicinity today. Aloft, a mid level trough will extend over the mid-Atlantic states with shortwave energy progged to lift across the local region through tonight. Shower and thunderstorm coverage will be highest across southeast Georgia this afternoon into this evening where high moisture content (noted by PWats of over 2 inches/95th percentile of climo) and various boundaries will be in play. Slow storm motions could result in torrential downpours and localized flooding. 12z HREF indicates a 10-30% chance of 3"/3 hr, with a 70-90% of 1"/3 hr, in these locations. Flood Advisories, or perhaps even a Flash Flood Warning, are possible. Convective coverage will be more isolated to scattered further north past the Savannah River through the daytime.

Showers and thunderstorms will likely persist to some extent overnight as the aforementioned front lifts back northward and shortwave energy provides extra lift. Higher rain chances should transition to southeast South Carolina by daybreak. While locally heavy rainfall will be possible, the overall flooding threat should be lesser during the overnight hours than earlier in the day.

KEY MESSAGE 2: Localized flash flooding possible across portions of Southeast Georgia and the South Carolina Lowcountry on Saturday.

The mid-levels will consist of a shortwave approaching our region from the west in the morning, then moving overhead during the afternoon. At the surface, a stationary front will meander across our area. There will be decent forcing at the surface and aloft, along with deep moisture. PWATs should be around 2", which is about 2 standard deviations per NAEFS, and above the 90% mark for CHS Sounding Climatology per SPC. This will support efficient warm-rain processes and high rainfall rates.

The main forecast challenge remains the exact placement of the front and the associated heaviest rainfall band. If it's further south, then the heavier rainfall will fall across southeast Georgia and the Savannah Metro. If it's further north, then the heavier rainfall will fall across the SC Lowcountry and the Charleston Metro.

Given all of the moisture, the overall severe thunderstorm threat is low. But there should be enough instability to fire off some embedded thunderstorms with locally very heavy rainfall. Areas with back-building and/or training will have the highest rainfall amounts. Localized rainfall totals of 1-3" could be possible. The Weather Prediction Center (WPC) still has our area under a Marginal Risk (Level 1 of 4) for Excessive Rainfall on Saturday. Showers and thunderstorms will begin to dissipate late Saturday night as High pressure starts to build in from the north.

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

A period of moderate to breezy northeast winds today in combination with the approaching full moon will bring elevated high tide levels this evening (occurring around 745 PM). Latest guidance indicates tide levels should remain shy of minor coastal flood criteria, and luckily the threat for heavy rainfall around the time of high tide tonight is low. No Coastal Flood Advisory is anticipated.

Elevated tidal departures may persist through this weekend. Minor inundation of very low lying areas of Charleston and Colleton Counties is possible Saturday and Sunday evenings. However, showers and thunderstorms may track across the City of Charleston during high tide Saturday evening. The combination of moderate to heavy downpours coinciding with high tide may increase the potential for flooding.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

30/00z TAF Discussion: KCHS/KJZI: There are no near term concerns. Risk for some light rain will increase Saturday morning as a wave of isentropic lift pushes through. The rain should remain fairly light with a low risk for tstms. Rain chances look to increase late morning and especially in the afternoon at both terminals. TEMPO groups for TSRA were introduced from 18-21z to trend. There is a low end risk for some brief MVFR or IFR cigs just before daybreak, but confidence was not high enough to include with cycle. Certainly something to watch, but guidance is mixed on its potential.

KSAV: The near term tstm risk has ended. Some light rain could spread back into the terminal early Saturday as a wave of isentropic lift pushes through, but the better chances look to hold west and north of the terminal. There is a modest potential for a brief period of IFR cigs just before daybreak. A TEMPO group for 6SM BKN007 was introduced for now. Risk for tstms will increase by late morning/early afternoon, but better chances look to hold north across the lower South Carolina coast. PROB30s 16-19z will hold for now. Any tstm risk should end by 19-20z.

Extended Aviation Outlook: Periodic flight restrictions are expected due to numerous showers and thunderstorms, along with patchy overnight fog and/or stratus.

MARINE

Through tonight: Gusty east to northeast winds this afternoon will settle overnight as the pressure gradient weakens. Conditions should remain below Small Craft Advisory criteria.

Saturday through Wednesday: A cold front will move south through our coastal waters Saturday night into Sunday morning, followed by High pressure passing to our north. The interaction between these two synoptic features will cause a surge in NE winds, with gusts around 30 kt late Saturday night through Sunday evening. This will also cause short-period seas to build. Therefore, Small Craft Advisories will be needed for all of our waters on Sunday, including the Charleston Harbor. Conditions will improve Sunday night into Monday.

Rip Currents: Low rip current risk for Saturday. For Sunday, our internal calculator has a borderline moderate/high risk, while RCMOS has a High Risk. We opted to start out with a high risk for all of our beaches.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...None.