textproduct: Charleston

WHAT HAS CHANGED

The Aviation Section has been updated for the 00Z TAF issuance.

KEY MESSAGES

- 1) There is the potential for localized heavy rainfall early Saturday morning across the forecast area, especially along the coastal counties.

- 2) The typical summertime pattern will continue this weekend into next week. The threat for isolated strong to severe thunderstorms and localized heavy rainfall will exist each day.

- 3) Minor coastal flooding possible along the South Carolina coast with evening high tide cycles next week.

DISCUSSION

KEY MESSAGE 1: There is the potential for localized heavy rainfall early Saturday morning across the forecast area, especially along the coastal counties.

An inverted surface trough is forecast to meander closer towards the SE coastline tonight and into early Saturday morning. This feature will serve as the main driver of potential showers and tstorms early Saturday morning, as convection over the marine waters pushes onshore. The HREF is the most aggressive of the CAMs, and features the earliest arrival timing with the heavy rain potential, with the 12Z HREF from today (8/7) showing the probability of >2" along the coastal counties as roughly 20-30%. The 12Z HRRR is slightly slower than the HREF with bringing the convection onshore, but still features the threat of heavy rainfall. The Weather Prediction Center has highlighted most of the local forecast area in a Slight Risk for Excessive Rainfall tomorrow (Saturday). Given the uncertainty regarding the timing the forecast features the raw NBM, which seemed to be a good average of the guidance, with showers/tstorms moving onshore roughly in the 6-8AM timeframe. Rainfall totals are forecast to be 0.5-1", with locally higher amounts (highest along the coast).

KEY MESSAGE 2: The typical summertime pattern will continue this weekend into next week. The threat for isolated strong to severe thunderstorms and localized heavy rainfall will exist each day.

For Sunday and Monday, high pressure ridging in from offshore and very weak troughing inland will bring mainly scattered, diurnally driven convection. A few stronger to marginally severe storms are possible each afternoon, but the overall severe risk is low. Additionally, with very light flow throughout the cloud layer, slow storm motions will be the norm, and locally heavy rainfall will occur under the strongest storms. Expect heat index values to reach 100-105 each afternoon, with some locations potentially threatening the 108 Heat Advisory Criteria along the Hwy 17/sea breeze corridor each afternoon.

Upper subsidence strengthens by the middle of next week, resulting in a modest reduction in afternoon thunderstorm coverage, but warmer temps and greater heat index values. Tuesday through Thursday look to bring more coverage of 105-110 heat index values, especially closer to the coast. Continue to monitor forecast updates for this potential period of dangerous heat.

KEY MESSAGE 3: Minor coastal flooding possible along the South Carolina coast with evening high tide cycles next week.

Perigean new moon will bring elevated tidal ranges next week. Even modest tidal departures during the evening high tide cycles Monday through Thursday would bring tidal levels approaching or exceeding minor coastal flooding thresholds. Continue to monitor forecast updates through the weekend as the total water level forecast gets refined.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

VFR conditions should prevail at CHS/JZI/SAV terminals through much of tonight, generally through 09-10Z Saturday. Chances for showers and thunderstorms increase approaching daybreak at all terminals, likely impacting JZI initially (~11Z), then at SAV/CHS during the following 1-2 hours. Rounds of showers/thunderstorms are then possible at all terminals mid-morning into early afternoon hours, which should produce TEMPO MVFR conditions (possibly IFR during heavy rainfall) at times, which is included in TEMPO and PROB30 groups at all terminals. Activity should then shift inland returning conditions to VFR by around 20Z Saturday at all terminals.

Extended Aviation Outlook: Periodic flight restrictions are expected in showers and thunderstorms this week and into this weekend.

MARINE

Through Tonight: Quiet conditions are expected into tonight. As an inverted trough, meanders towards the SE coast overnight, showers and thunderstorms are forecast to develop over the marine waters. These showers/tstorms will persist across the marine waters into Saturday morning.

Saturday through Tuesday: Atlantic high pressure will be the dominant synoptic feature through the weekend. Light to moderate winds prevail with gusty winds along the land/sea interface each afternoon associated with the sea breeze. Be alert for the potential for nocturnal jetting to set up Sunday into early next week, keeping breezy conditions in place along the beaches overnight. This feature is typically poorly resolved by models, but the combination of the weak surface trough shifting inland with persistent strong high pressure offshore and very warm water makes for a favorable setup.

CHS WATCHES/WARNINGS/ADVISORIES

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