textproduct: Charleston

WHAT HAS CHANGED

The Aviation section has been updated for the 12z TAF issuance.

KEY MESSAGES

- 1) Locally heavy rainfall will remain a threat for portions of southeast Georgia and southeast South Carolina through Tuesday.

- 2) Minor coastal flooding is possible along the Charleston and Colleton County coastline with the Tuesday evening high tide.

DISCUSSION

KEY MESSAGE 1: Locally heavy rainfall will remain a threat for portions of southeast Georgia and southeast South Carolina through Tuesday.

Through tonight: The main concern for locally heavy rainfall and perhaps an isolated flash flood threat will come primarily during the early morning hours. Surface analysis shows a surface low taking shape just off the GA/SC coast which will then gradually track to the east through daybreak. As it does, bands of convergence will feed back into the coast within a very moist environment defined by precipitable water values in excess of 2 inches. Hi-res model guidance has displayed some run-to-run inconsistency, but still seems to favor the SC coast and specifically the Charleston County coast for the heaviest rain through sunrise. Storm motion is quite slow and when combined with warm cloud processes, rainfall efficiency is quite high. The main concern will be for urbanized areas along the SC coast, specifically Beaufort and Charleston. The threat for locally heavy rainfall and flooding should persist through daybreak and then come to an end as the surface low pulls away to the east. For the rest of the day, guidance would suggest that there won't be too much convective coverage as the flow turns offshore. The next best chance of convection will come during the late evening hours as upstream thunderstorms along the cold front approaching from the north shift into the area. These storms should have enough motion to prevent much of a flood threat.

Tuesday: The cold front should start the day having cleared southeast SC and continue to sink southward. This should put the focus for afternoon convective coverage across southeast Georgia. Fortunately this keeps most of the forecast area dry, but we will have to watch the progression of the front with time and continue to assess the heavy rain potential across southeast Georgia.

KEY MESSAGE 2: Minor coastal flooding is possible along the Charleston and Colleton County coastline with the Tuesday evening high tide.

A strong surge of northeast flow will occur along the southeast SC coast on Tuesday behind a passing cold front. This northeast flow will produce a surge in tide levels, especially around the time of the Tuesday evening high tide. The astronomical tide level is 5.5 ft MLLW, so it would require a rather anomalous 1.5 ft departure to reach the minor coastal flood threshold of 7 ft MLLW. For context, historical analysis shows that a departure of 1.5 ft or higher has only occurred with ~7% of all minor coastal flood events on record. Still, minor coastal flooding is possible and a Coastal Flood Advisory could be needed.

AVIATION /12Z MONDAY THROUGH FRIDAY/

KCHS and KJZI: IFR ceilings have already started to scatter out but we could still see at least brief periods of IFR through about mid morning. We should see VFR conditions return by around midday. Thunderstorm coverage this afternoon is expected to be isolated to scattered and confidence in direct impacts is low. Instead there is a better chance for storms to pass through from the north late this evening as a cold front approaches the area. For now, we have just included VCSH for this evening period. Finally, there is some potential for MVFR or IFR stratus to develop in the early Tuesday morning hours. Confidence is low in this potential, but certainly something to consider with subsequent TAF packages.

KSAV: IFR ceilings and fog have settled in this morning and will likely take until mid morning to scatter out and improve. VFR conditions aren't expected to return until around midday. Thunderstorm coverage this afternoon is expected to be isolated to scattered and confidence in direct impacts is low. There is again potential for MVFR or IFR stratus to develop in the early Tuesday morning hours, though confidence is too low to include in the forecast at this point.

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

Today through Friday: No significant concerns through Monday night. Then on Tuesday a surge of northeast flow will spread across the local waters, peaking Tuesday night. This will bring solid Small Craft Advisory conditions to all waters (outside Charleston Harbor) with frequent gusts. Furthermore, guidance suggests there is a window of time early Wednesday morning when gale force gusts will be possible as well. Northeast winds will remain elevated Wednesday before tapering off into Thursday. Overall, Small Craft Advisories will likely be needed for a time period from Tuesday morning through Thursday morning for both winds and seas.

CHS WATCHES/WARNINGS/ADVISORIES

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