textproduct: Charleston

WHAT HAS CHANGED

The Aviation section has been updated for the 00z TAF issuance. Added a Key Message for coastal flooding potential mid to late week.

KEY MESSAGES

- 1) Rain chances increase on Tuesday.

- 2) A cold front could bring a threat of strong thunderstorms Wednesday afternoon, and will be followed by cooler weather and gusty winds.

- 3) The threat for coastal flooding will increase mid to late week.

DISCUSSION

KEY MESSAGE 1: Rain chances increase on Tuesday.

Tuesday: Rain chances look to increase today as our stubborn upper level low that has been parked over Florida for the past five days will lift northeast across the western Atlantic. Moisture will continue to stream in from the east off the Atlantic with PWATS continuing to hover in and around 2 inches. Ongoing showers and storms across the marine waters this morning should persist into the early afternoon likely staying just offshore; however, an isolated shower could come ashore. By this afternoon the seabreeze should begin to push inland resulting in scattered showers and thunderstorms to develop. Given increase cloud coverage and rather deep moist profiles, instability should be meager at best around 1000 J/KG which while should support some isolated thunderstorms should keep any strong to severe storms at a minimum. While PWATS remain high for this time of year, the overall poor convective environment may not support heavy rainfall either; however, if a stronger storm sits over an urban area long enough there is enough environmental support for heavy rain rate driven urban nuisance flooding. Storm should subside by the early evening.

Wednesday: By late Tuesday night into early Wednesday morning, the upper low will have lifted north of the area and a backdoor surface cold front will drop south. Scattered showers and storms will likely accompany the dropping front. Any strong storm threat looks to be centered around eastern GA during the afternoon depending on how quickly the front moves through the area. Given overall weak forcing and the general speed of the front; would not be surprised if we see POPs lower over the next 48 hours and result in a cooler and drier Wednesday than currently advertised.

KEY MESSAGE 2: A cold front will bring gusty winds Wednesday and Thursday, followed by cooler weather through the end of the week.

Behind the cold front, an intensifying pressure gradient will produce breezy to gusty conditions across southeast South Carolina and southeast Georgia. Winds will increase from north to south Wednesday, following the passage of the front, initially across SE South Carolina before spreading into SE Georgia. Post-frontal wind gusts on Wednesday look to peak at 20-25 mph, and a little stronger on Thursday with widespread gusts of 25-30 mph, and up to 30-35 mph over elevated bridges and overpasses. While winds are expected to remain below Wind Advisory thresholds, gusts over Lake Moultrie could approach 25 knots at times, mainly Thursday. Gusty conditions are expected to subside Thursday night.

Along with the breezy conditions, the surface high pressure wedge building south across the Carolinas and Georgia will establish cool, dry air for the latter portion of the week. High temperatures Thursday and Friday are forecast to top out in the mid to upper 70s and lower 80s across Southeast South Carolina, and the lower 80s for Southeast Georgia, which is several degrees below normal for late September. Temperatures will rebound closer to seasonal normals for the weekend.

KEY MESSAGE 3: The threat for coastal flooding will increase mid to late week.

As a full moon approaches on 9/26, breezy northeast winds Wednesday evening into Friday may result in some coastal flooding issues as tidal departures rise. We will need to keep an eye on the potential starting as early as Wednesday afternoon for coastal Charleston and Colleton counties, expanding southwards to include the remainder of the SE SC coast into SE GA Thursday afternoon/evening.

AVIATION /06Z TUESDAY THROUGH SATURDAY/

VFR conditions continue across most of the area; however, guidance hints that another round of stratus and fog will be possible by the early morning hours. The greater chance for occurrence is at KSAV where MVFR has been maintained, though there is potential for IFR or lower. VFR should return mid morning. Brief restrictions in showers/thunderstorms will be possible in the afternoon, which is currently highlighted with PROB30 groups. Winds will remain light out of the northeast throughout the period.

Extended Aviation Outlook:Flight restrictions are possible Wednesday morning as low ceilings develop ahead of a cold front. Additional brief restrictions remain possible Wednesday afternoon and evening due to scattered showers and thunderstorms. Behind the front, gusty north to northeast winds will affect the terminals Wednesday through Thursday.

MARINE

Through Tuesday: A decaying surface high pressure ahead of a slowly approaching cold front will continue the benign marine weather conditions across our local waters. Periodic showers and thunderstorms during this time may be capable of gusty winds and frequent lightning.

Wednesday through Sunday: In the wake of a cold front, a building inland wedge coupled with a deepening surface low off the Mid- Atlantic coast will drive a strong northeasterly wind surge. Marine conditions will deteriorate Wednesday evening, with wind gusts peaking Thursday evening. Small Craft Advisories will likely be needed across all waters, with Gale Watches/Warnings potentially required for the offshore waters beyond 20 nm. Seas will build significantly with the wind surge, peaking at 5 to 9 feet Thursday night. While winds and seas will gradually diminish over the weekend as the pressure gradient relaxes, seas will remain elevated, with 6 ft waves persisting across the outer offshore zones potentially into Sunday morning.

CHS WATCHES/WARNINGS/ADVISORIES

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