Area Forecast Discussion

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Daily thunderstorms are expected across southeast Georgia and southeast South Carolina this weekend and into early next week. An increase in rain chances is expected by the middle of next week, with cooler temperatures thereafter.

DISCUSSION

KEY MESSAGE 1: Daily thunderstorms are expected across southeast Georgia and southeast South Carolina this weekend and into early next week. An increase in rain chances is expected by the middle of next week, with cooler temperatures thereafter.

Through tonight: Aloft, the forecast area will sit within the eastern extent of ridging centered near the ArkLaTex and on the northern periphery of a closed low over the FL peninsula. At the surface, the setup is pretty diffuse, but generally features weak high pressure just off the coast. Diurnal shower and thunderstorm development will be largely dependent on surface heating and modest convergence along a weak sea breeze this afternoon and evening. Model guidance is in decent agreement that convective initiation should take place along the SC coast, especially north of the Santee along the Grand Strand where satellite imagery shows some differential heating boundaries. Thunderstorm activity should then gradually progress to the southwest along the sea breeze and the southeast SC coast. The best coverage is anticipated to be centered across an area from Beaufort/Jasper/Colleton counties and to the southwest into southeast GA into the evening hours. The environment isn't particularly interesting with MLCAPE values in the 1,500-2,000 J/kg range and DCAPE values topping out around 800 J/kg or so. Flow through the column is 10-15 knots at best. While we can't completely rule out 1 or 2 strong to marginally severe storms, the overall severe threat is low. With precipitable water values over 2 inches, there will be potential for locally heavy rainfall. Thunderstorm coverage should steadily diminish through the evening and also continue to shift to the southwest.

Sunday through the middle of the week: Sunday should be similar to Saturday with diurnal convection driven by the sea breeze and coverage in the isolated to scattered range. Monday should also bring about rather modest coverage mainly in the form of diurnal convection. Better convective coverage and higher rain chances are anticipated Tuesday into Wednesday as a front approaches from the north. There could be a threat of locally heavy rainfall during this mid week time period as coverage increases in the presence of precipitable water values in excess of 2 inches.

Behind the mid week cold front, noticeably cooler air is expected for the latter part of the week. Highs in the upper 70s to low 80s for southeast SC and low to mid 80s for southeast GA look probable at this time, which would be a few degrees below normal for late September.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

The 18z TAF period begins with VFR conditions at KCHS, KJZI, and KSAV. The first forecast concern is for afternoon and evening thunderstorm impacts. Thunderstorms are expected to develop along the sea breeze this afternoon, likely occurring near KCHS and KJZI first and PROB30 groups are in place in the 21-23z time period. KSAV should come a bit later and a PROB30 group is in effect from 22-00z. Thunderstorms should move to the southwest and away from the terminals through the late evening hours. The next potential forecast issue is for low stratus and fog to spread into the terminals from inland around sunrise Sunday. Model guidance is still mixed regarding the extent of fog/stratus and how low flight categories could be. For now we maintained MVFR ceilings starting around 10-11z, but there is definitely potential for IFR conditions. Whatever fog/stratus does develop should mix out and dissipate by mid morning Sunday.

Extended Aviation Outlook: The main potential for sub-VFR conditions will come from afternoon and evening showers and thunderstorms each day. The best chance of showers and thunderstorms is expected to come Tuesday into Wednesday.

MARINE

Through tonight: Very quiet conditions will continue across the local waters. Light onshore flow this evening will remain light and turn more northeasterly overnight. Seas are expected to average around 2 ft. A few thunderstorms will be possible along the coast this evening, potentially producing gusty winds and frequent lightning.

Sunday through Thursday: Conditions across the local waters will remain quiet through Monday with northeast or east flow generally no more than 10 knots. However, this will change as we get into the middle of the week and a cold front pushes through from the north. This will result in a strong surge of northeast flow beginning Wednesday and continuing through Thursday. There will be increasing probabilities of gusts into the 25-30 knot range with seas building upwards of 6 ft as well. Small Craft Advisories will likely be needed for some, if not all, of the local waters including Charleston Harbor. Also of note, rounds of thunderstorms will be possible at times ahead of an along the front which could bring the potential for strong wind gusts and frequent lightning.

CHS WATCHES / WARNINGS / ADVISORIES

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