textproduct: Charleston

WHAT HAS CHANGED

The Aviation Section has been updated for the 12Z TAF issuance.

KEY MESSAGES

- 1) Unsettled weather with a risk for minor/flash flooding today into Monday.

- 2) Cooler temperatures along with breezy to windy conditions are expected into the middle of the week.

- 3) Coastal flooding and significant coastal hazards are expected this week.

DISCUSSION

KEY MESSAGE 1: Unsettled weather with a risk for minor/flash flooding today into Monday.

Our unsettled and soggy pattern continues on throughout the next 24 hours as deep upper troughing swings through the eastern US. Scattered showers and light rain have persisted across a chunk of the area this morning as our first subtle shortwave lifted out of the area. The next wave should be moving into the area later this afternoon and evening and the influences of this feature can be noted with precipitation starting to blossom across central Georgia the last few hours. With PWATS in and around 2.3 to 2.4 inches, widespread showers and embedded storms should continue to develop across Georgia into South Carolina. Rainfall should continue to move through the area through much of the late afternoon and into the overnight hours. Precip totals of 1 to 3 inches look possible across much of South Carolina this evening and overnight with 2 to 4 inches possible across eastern Georgia. Given the abnormally high PWAT values, some storms will be capable of locally heavier rates that could result in some minor flood. Especially where storms are able to train and locations where rain has already fallen the last 24 hours. Rain should persist through the overnight and into Monday morning before the trailing cold front slowly pushes through the area. Rain chances should diminish from north to south Monday afternoon with maybe some heavier rain lingering across coastal Georgia into the evening.

KEY MESSAGE 2: Cooler temperatures along with breezy to windy conditions are expected into the middle of the week

Models are still exhibiting a consistent signal for strong cold air damming (CAD) to rapidly develop Tuesday as a cold front drops farther to the south across Florida and 1025 hPa high pressure wedges south out of the central Appalachians/Mid-Atlantic region. The pressure gradient is forecast to quickly tighten over the Lowcountry and Southeast Georgia Tuesday afternoon into Tuesday night as cyclogenesis initiates offshore of the Georgia and Northeast Florida coasts. Where the surface low eventually develops will have a direct impact on the local wind forecast with the latest blended data suggesting the gradient may not quiet as tight as what previous runs indicated. Whether this temporary or the beginning of a more definitive shift is to be determined; however, the various guidance packages tend to under forecast wind/wind gusts and over forecast high temperatures in strong CAD events. The forecast continues to make local adjustments to reflect these biases.

Winds: Breezy to windy conditions appear likely with the highest winds occurring across the coastal corridor, especially at the beaches. Expect wind gusts of 20-30 mph inland with 30-40 mph at the coast. Some exposed beach locations such as Tybee Island, Hunting Island, parts of Hilton Head and Sapelo Island could see gusts approach 45 mph at times. Winds could also end up being a bit higher on some of the taller bridges around the Charleston and Savannah Metro Areas (i.e. the Ravenel, Don Holt and Talmadge bridges) and over the elevated, exposed bridges and highways heading out to the various barrier islands (i.e. US-80 heading to Tybee Island, the Isle of Palms Connector, Harbor River Bridge and the McKinley Washington Jr. Bridge). A Wind Advisory (minimum criteria: sustained 30 mph or wind gusts 40 mph) could be needed for parts of the coast, mainly for the Tuesday afternoon/evening time period when the gradient is forecast to be at its tightest and NBM probabilities for wind gusts >40 mph are at their highest. Breezy to locally windy conditions will linger at the coast into Wednesday, but should remain below Wind Advisory levels. For Lake Moultrie, hazardous conditions are possible by early Tuesday morning with northeast winds around 20 kt with gusts 25-30 kt. Waves on the open lake waters could reach as high as 2-3 ft at times, especially over the central and southwest portions of the lake. A Lake Wind Advisory may be needed.

Temperatures/Rain Chances: There looks to be a large moisture gradient that sets up across the area Tuesday with the strong onshore flow likely keeping a modest degree of low-level moisture in place across parts of Southeast Georgia (especially at the coast) while drier air from deeper within the CAD region advects to the south/southwest. PWATs will range from as low as 0.60-0.80" well inland for areas adjacent to the CSRA/Southern Midlands and as high as 1.85" across the far south near Darien, GA. As rainfall spreads inland across the far south from off the Atlantic, there will likely be a sharp cut off along the northern periphery of any shower activity as it encounters increasingly deeper/drier air. Rainfall falling into the drier air will also likely help reinforce the CAD due to diabatic effects. The colder side of the NBM guidance envelop is still favored over the NBM 50%, especially inland. In fact, the coolest temperatures Tuesday afternoon may be observed in the area from far interior Southeast Georgia into the far southern portions of the Lowcountry where the potential for diabatic cooling along the northern fringe of the area of greatest rainfall could be maximized. Wedge conditions will linger into Wednesday with lower rain chances before moderating Thursday into Friday, although how quickly this occurs is in question.

KEY MESSAGE 3: Coastal flooding and significant coastal hazards are expected next week.

Coastal Flooding: Strong northeast winds associated with a wedge of high pressure inland and low pressure offshore in combination with high astronomical tide levels will likely support coastal flooding issues this week. The pattern suggests a minor/moderate saltwater inundation setup by mid-week, but even major coastal flooding cannot be ruled out during the peak of the event given persistent/strong northeasterly winds and building seas along the Southeast U.S. Coast. Coastal Flood Watches/Warnings and/or Advisories could be needed.

Rip Currents/High Surf/Beach Erosion: The setup looks highly favorable for strong and dangerous rip currents Tuesday into Wednesday with breezy to windy northeast winds expected. In addition, there will be an increased risk for high surf along the beaches with the latest NWPS guidance still showing a potential for 4-7 ft breakers (highest coastal Georgia) developing in the surf zone by Tuesday afternoon and lingering into Wednesday. A High Surf Advisory may eventually be needed for some beaches. Significant beach erosion is also likely, particularly around times of high tide. NWPS beach erosion probabilities are highest around the Tuesday afternoon high tide.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Mostly MVFR to VFR conditions this afternoon will slowly deteriorate to MVFR to IFR tonight as widespread rain and storms develop across the area. IFR vsbys and cigs will be possible after 00z tonight for majority of the area with some LIFR ceilings possible around midnight. Low ceilings and rain should persist through most of the night as light and variable winds should turn more northerly with time on Monday as a cold front drops down from the north. IFR ceilings should continue to prevail through most of the day tomorrow.

Extended Aviation Outlook: Gusty winds and low clouds are possible Tuesday all terminals. Shower impacts are possible, mainly at KSAV. Gusty winds will linger into Wednesday.

MARINE

Today and Monday: Wind/seas are expected to remain well below Small Craft Advisory levels as Atlantic high pressure continues to shift further offshore in an advance of a cold front entering the area from the north and stalling nearby this afternoon into the evening. The greatest impact across local waters should occur with gusty showers and thunderstorms south of the front, which could lead to Special Marine Warnings and/or Statements. Outside of this activity, south/southeast winds around 10 kt or less today are anticipated to turn more northerly near/behind the front tonight into the day on Monday. Seas will generally range between 1-3 ft, largest across offshore waters today.

Monday Night through Thursday: The pressure gradient will rapidly tighten late Monday night into the day Tuesday as cold air damming sets up across the Carolinas and Georgia and low pressure develops well offshore of the Georgia and Northeast Florida coast. The risk for gales remains elevated with little change in net probabilities for frequent gusts to 34 kt. The gradient looks tight, but may not be as tight as some previous runs had suggested, likely due to a weaker/slightly farther offshore progression of the surface low. Whether this is a one cycle artifact, or the beginning of a more definitive trend is yet to be determined as models do to tend to under forecast pinched gradient regimes associated with strong CAD events. At this point, confidence is high enough to hoist a Gale Watch for all nearshore and offshore marine zones from Tuesday morning through late Tuesday night/early Wednesday morning with frequent gusts to 35-40 kt likely. Probabilities for storm-force gusts have diminished and look unlikely at this time, but this will be watched carefully. For Charleston Harbor, the risk for gales is not quite high enough for a Gale Watch this far out, but one may eventually be needed pending further data. At the very least, A Small Craft Advisory will be required. Seas look to peak Tuesday night, ranging from 6-10 ft nearshore waters with 9-12 ft offshore waters. Both winds and seas will remain well into Small Craft Advisory levels Wednesday with 6 ft seas lingering into Thursday.

CLIMATE

Record Precipitation:

October 5: KCXM: 1.17/1915

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Gale Watch from late Monday night through late Tuesday night for AMZ360-362-380. Gale Watch from late Monday night through Wednesday morning for AMZ364-382-384.