textproduct: Charleston

WHAT HAS CHANGED

All sections and Key Messages were updated.

KEY MESSAGES

- 1) Isolated severe thunderstorms are possible Thursday.

- 2) Unsettled weather late this week lingers into this weekend.

DISCUSSION

KEY MESSAGE 1: Isolated severe thunderstorms are possible Thursday.

The region will remain well embedded within a pre-frontal warm sector Thursday as a cold front approaches from the northwest and waves of vorticity pass through aloft. There remains much uncertainty on how convection will evolve through the day, but trends suggest there could be two waves of convection; one during the morning hours with a secondary area possibly moving through along and/or just ahead of cold front itself later in the afternoon/evening. 0-6km bulk shear in excess of 40 kt will be more than supportive of organized convective clusters, but the degree of available instability will be heavily modulated by how much convection occurs during the morning hours and will it end quick enough to support some recovery prior to the cold front pushing offshore. The various CAMs remains mixed on this potential which leads to lower confidence on the potential for severe weather. There are signals that a narrow corridor of higher surface-based instability could materialize roughly along/east of a Glennville-Savannah Metro-Beaufort line which may support a local severe weather maxima. This idea is supported by a blend of the various machine learning models, although the risk for more organized strong/severe convection still looks to remain south and southwest of the forecast area where the chances for better instability to be realized is the highest.

Damaging winds look to be the primary hazard with any loosely organized, multi-celluar clusters that can form; however, a low- end tornado threat could develop if enough instability can be realized. The Storm Prediction Center (SPC) continues to highlight the entire area in a marginal risk (level 1/5) in the Day 2 WFO guidance outlook which looks reasonable. If there is a more definitive trend for higher instability to realized, then a slight risk (level 2/5) may be introduced in later periods, most likely in parts of Southeast Georgia.

KEY MESSAGE 2: Unsettled weather late this week lingers into this weekend.

Thursday is looking increasingly unsettled for the South Carolina Lowcountry and Southeast Georgia as a cold front approaches during the day and pushes offshore during early nighttime hours. The combination of increasing lift with deepening moisture and possibly some surface-based instability will support numerous to widespread showers/tstms. There is still much uncertainty on the the exact timing and coverage at any particular hour, but confidence is increasing that many areas will see some some much needed rainfall albeit light. Storm total amounts through afternoon are not impressive, only running between 0.10-0.50", but some localized areas could see as much as 1-2" given the expected nature of the convection with training possible. Flooding is unlikely given the lack of strong instability, ongoing severe to exceptional drought conditions and the expected progressive nature of the strongest tstm cores, but some brief, minor street flooding can not be ruled out in some of the more urbanized areas.

Thursday night into Friday continues to trend drier with the cold front poised to push to the south and stall across northern and central Florida as high pressure nudges into the Southeast States. Unsettled conditions will spread back into the region this weekend into early next week as the front moves back to the north and waves of low pressure ride up along it and just offshore. This is in response to the approach of a damping southern stream shortwave/upper low that is forecast to eject out of northern Mexico. Significant impacts are not expected, but more beneficial rainfall is likely.

AVIATION /00Z THURSDAY THROUGH MONDAY/

0Z TAFs: A cold front will approach the region tonight and Thursday, then pushing across the terminals during the late afternoon hours. A convective complex, sourced from the Deep South, is expected to reach KSAV/KCHS/KJZI during the early daylight hours. HREF indicates a solid band of 40-50 probs of 1 hr thunder Thursday morning/afternoon. Based on this timing, the thunder will be highlighted with PROB30s between 13-18Z, with TS at KSAV between 17-20Z. Showers are possible preceding and after the thunderstorm band. Gusty SW winds may develop across the terminals around daybreak, with gusts near 20 kts. Winds are forecast to veer from the west by late afternoon as the cold front passes over the terminals.

Extended Aviation Outlook: Additional flight restrictions are possible late weekend into early next week with showers/tstms developing ahead of and/or occurring with another front approaching the region early next week.

MARINE

Conditions remain very close to Small Craft Advisory thresholds in the Charleston Harbor today, but speeds have generally settled into the 15-20 kt range with gusts occasionally surpassing 25 kt. It is still possible winds may surge a bit more, so trends are being monitored. It would not take much of a surge to force the issuance of the advisory.

Southerly winds will increase ahead of an approaching cold front tonight. Probabilities for frequent gusts to 25 kt have increased to >70% across both South Carolina nearshore legs and the Georgia offshore leg, so Small Craft Advisories have been issued from noon through 10 AM Thursday. It is unclear how widespread convection over the waters will be Thursday morning, so it is possible that convection could disrupt the southerly wind field ahead of the front. If this does not occur, then the advisories may very well need to be extended into Thursday afternoon.

Rip Currents: Increasing winds along the lower South Carolina coast coupled with building 2-4 ft swell will support a moderate risk for rip currents along the Charleston County beaches for Thursday.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory from midnight tonight to 10 AM EDT Thursday for AMZ360-362-384.