textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Unsettled weather expected through Monday.

- 2) Cooler temperatures along with breezy to windy conditions are expected next week.

- 3) Coastal flooding is likely next week.

DISCUSSION

KEY MESSAGE 1: Unsettled weather expected through Monday.

A rather soggy pattern is shaping up the next 48 hours as high pressure moves out of the way and rather deep upper troughing pivots over the eastern US. Broad southwesterly flow has already begun to overspread the area with a plume of rich Gulf moisture streaming northward into the Carolinas. To our north a slowly advancing backdoor front has made its way to near the SC/NC border providing a focus for heavy rainfall north of our area this afternoon. With PWAT values ranging around 2.3 inches this afternoon, our 12z sounding broke the daily record high at Charleston with a PWAT of 2.32 inches, scattered to numerous showers and embedded thunderstorms have developed across the area. These slow moving storms will be capable of producing locally heavy rainfall with rainfall amounts around 1 inch or so. Storms should continue to slowly push northward throughout the afternoon and into the early evening before diminishing overnight.

The big rains look to come on Sunday into Monday as our upper trough deepens across the eastern US and eventually pushes the cold front through our area sometime on Monday. By Sunday the slowing backdoor front should stall somewhere north of our area as a weak surface disturbance lifts north out of the northern Gulf Coast. PWATS will continue to climb to around 2.3 to 2.4 inches which is well above the 98h percentile for PWATS and nearing daily record highs for PWATS. Scattered to numerous showers and storms are expected to develop by midday along the subtle seabreeze and spread inland during the afternoon. However, this is just the beginning as the heavier rain arrives during the early evening to overnight hours Sunday into early Monday morning as the surface disturbance lifts across the area and the cold front moves into the area. Widespread moderate rain should overspread the area from southwest to northeast with pockets of heavier rain embedded. Rainfall amounts look to range in the 1 to 3 maybe 2 to 4 inch range and given high PWAT values some localized higher totals of 5 to 6 inches cannot be ruled out. Given the rather spread out nature of the rain, this event is looking more like a good soaking rain that helps chip away at our long standing drought conditions rather than a widespread flooding rain threat. However, given some areas along the coast have already picked up 1 to 2 inches today and the potential for locally higher pockets in and around the major metro areas a flash flood threat does exist if storms are able to sit over the poor drainage areas long enough. As a result we are under a slight risk (level 2 of 5) for excessive rainfall. Rain will likely linger along the coast through Monday morning before the front eventually pushes offshore leading to drier conditions late Monday afternoon and evening. Light rain could linger even longer depending on the speed of the front but all risks for heavy rain should end by Monday afternoon.

KEY MESSAGE 2: Cooler temperatures with breezy to windy conditions are expected next week.

Confidence is steadily increasing that a strong cold air damming (CAD) event will impact the Carolinas into Georgia next week as high pressure wedges south across much of the Southeast States. The wedge front looks to clear the Altamaha River Monday night as drier air advects to the south in its wake. The pressure pattern will evolve into a classic CAD by daybreak Tuesday as some degree of surface cyclogenesis initiates offshore of the Georgia or Northeast Florida coast. The strength, timing and position of this low is still a bit in question, but its overall propagation looks slow and will only help to further tighten the pressure gradient across the area. Breezy to windy conditions are likely to occur Tuesday into Wednesday as a result with wind gusts approaching 30 mph inland with 35-45 mph across the coastal corridor. The strongest gusts look to occur along the beaches where the latest NBM 24-hr probabilities for wind gusts >40 mph are running about 35-65%, highest from roughly Hilton Head south to Sapelo Island, including Tybee Island. A Wind Advisory (minimum criteria: sustained 30 mph or wind gusts 40 mph) will likely needed for portions of the coast Tuesday into Wednesday with a Lake Wind Advisory for Lake Moultrie. Winds could end up being a bit higher on some of the elevated bridges around the Charleston and Savannah Metro Areas and over the elevated, exposed bridges and highways heading out to the various barrier islands.

In addition to the winds, it will be noticeably cooler mid-week with the strong CAD in place. Complicating matters is the potential for shower activity to push inland from off the Atlantic, especially across portions of Southeast Georgia. With drier air advecting south, the opens up the possibility for some diabatic cooling to occur, which models never handle well in these scenarios. This is typically why the NBM tends to exhibit such a warm bias in cold air damming events. The colder side of the latest NBM guidance as favored given this bias, but even this may prove too warm in places, especially if winds end up being more backed with time, particularly well inland. The best rain chances Tuesday will be directed into coastal Georgia and over the coastal waters with a very tight net moisture gradient expected to occur. PWATs look to range from 0.60- 0.70" across interior Southeast South Carolina to around 2" across McIntosh County, GA. Drier conditions are expected for Wednesday into Thursday, although some degree of shower activity may linger along parts of coastal Georgia.

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 next 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 favorable for strong and dangerous rip currents by Tuesday with stiff northeast winds expected. In addition, there will be an increased risk for high surf along the beaches with the latest NWPS guidance showing a potential for 4-7 ft breakers 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 /00Z SUNDAY THROUGH THURSDAY/

00Z TAFs: Radar indicates numerous showers across our area, with flight restrictions associated with these showers. Areas with no showers are generally VFR. Instead of getting fancy and trying to time out the showers impacting the TAF sites with flight restrictions versus no showers at the TAF sites and VFR, opted to simplify the TAFs and have prevailing showers with MVFR. Though, brief IFR is possible in the heaviest showers overnight. The showers will increase in intensity Sunday afternoon.

Extended Aviation Outlook: Flight restrictions are possible with elevated rain chances Sunday into Monday. Elevated rain chances will linger near KSAV through Tuesday. Gusty winds are likely Monday night into Wednesday.

MARINE

Tonight through Monday: Wind/seas are expected to remain well below Small Craft Advisory levels as high pressure lingers across the western Atlantic this weekend. In general, onshore winds should top out near 10-15 kt, with modest surging each afternoon along the land/sea interface. Seas will generally range between 1-3 ft. However, increasing chances of showers/thunderstorms are expected through Sunday, which could lead to gusty winds and reduced vsbys at times, especially late Sunday. A cold front will be pushing through the region Monday morning, pushing the heaviest rainfall south of the area by the evening.

Monday Night through Wednesday: The main forecast concern still centers on the elevated risk for sustained gales Tuesday and Tuesday night, possibly lingering into Wednesday as a tight pressure gradient develops between inland high pressure and offshore low pressure. There is even a growing potential for wind gusts to approach storm-force, mainly over the offshore waters from Edisto Beach to Altamaha Sound out 20-60 NM. NBM 24-hr max wind gust probabilities for >48 kt in this area are running 20-35%, but it is not exactly clear how frequent these gusts will become should they occur. Much will depend on how tight the pressure gradient eventually becomes and where the greatest pinching sets up. Gale Watches/Warnings will eventually be needed for all nearshore and offshore waters with some potential for Storm Watches/Warnings, again, mainly over the offshore waters. For Charleston Harbor, there is a potential for gale-force gusts Tuesday into Tuesday night and a Gale Watch/Warning may eventually be needed there as well. At the very least, a Small Craft Advisory will be issued. Maximum seas look to occur Tuesday night, ranging from 5-9 ft nearshore waters and 9- 12 ft over the offshore waters.

CLIMATE

Record High Minimum Temperatures:

October 3: KCHS: 75/1986

October 4: KCHS: 75/1995 KSAV: 76/1995

Record Precipitation:

October 5: KCXM: 1.17/1915

CHS WATCHES/WARNINGS/ADVISORIES

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