textproduct: Charleston

WHAT HAS CHANGED

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

KEY MESSAGES

- 1) Unsettled weather expected today 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 today through Monday.

Today: Weak high pressure meandering across the western Atlantic will gradually give way/drift east further offshore with the approach of large-scale troughing across the Midwest to Northeast regions. Deep moisture return (PWATs ~2.25 inches) occurring along the western edge of the Atlantic high along with vort energy traversing inland should favor greater rain/shower coverage, initially starting out as light rain activity that drifts north across Southeast Georgia into Southeast South Carolina late this morning, then increasing in coverage heading into peak heating hours this afternoon. Although instability is meager, deep moisture and modest forcing could support moderate showers with a few rumbles of thunder this afternoon into early evening, mainly in the form of beneficial rains with a rather limited concern for minor flooding across inland zones.

Sunday and Monday: Shower/thunderstorm coverage is likely to increase as the digging mid-upper trough advances toward the Mid- Atlantic and Northeast, eventually forcing a cold front toward the local late weekend into early next week. This backdoor cold front could stall near the area on Sunday while low pressure shifts across the local area, then offshore. PWATs and forcing peak late Sunday, especially during overnight hours with a fair amount of isentropic lift in place, setting the stage for widespread rains with embedded thunderstorms during much of the night and into early morning Monday. Rainfall totals in the 1-3 inch range remain reasonable across the area with locally higher amounts possible while the front slowly shifts south and offshore as low pressure develops over the western Atlantic and high pressure builds in from the north Monday afternoon. There should be a decent gradient of precip Monday afternoon, depending on the position of the sfc front, but a decreasing trend from north to south is the anticipated outcome late day and/or evening. Given deep moisture, ample forcing, and the slower progression of the front, some spots have the potential for seeing minor and/or perhaps flash flooding issues, especially if activity trains over same areas than have seen rainfall during the 48 hour period. The latest WPC Excessive Rainfall Outlook includes Southeast South Carolina and Southeast Georgia in a Slight Risk for excessive rainfall on Sunday, and Marginal Risk for excessive rainfall on Monday.

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

Ahead of an approaching cold front, daily highs will remain in the lower to mid 80s over the weekend. As a surface low pressure moves off the northeast coast on Monday, the cold front will slide across the area ushering in cooler temperatures with highs in the mid to upper 70s. Surface high pressure moves in from the Great Lakes region onto the northeast coast overnight into Tuesday starting a cold air damming (CAD) event, sending additional cooler air down the eastern seaboard. High temperatures will be limited to the lower to mid 70s Tuesday and Wednesday, though isolated instances of highs in the upper 60s can't be ruled out for inland areas. Overnight lows in the lower 50 to lower 60s are expected Wednesday through Friday morning, coolest for inland areas.

The CAD event will also feature breezy/windy conditions as the surface pressure gradient gets pinched, with northeast winds developing Monday with gusts expected to remain below 20 mph. As the strong surface high pressure moves to the east coast, the pressure gradient is further increased, with additional increases from a coastal surface low pressure that may develop along the cold front. As a result, Tuesday will be breezy/windy with areas furthest away from the coast potentially seeing wind gusts to 30 mph, while those along the coast may see gusts to 40 mph. Wednesday will see winds a touch lower as the surface high pressure begins to weaken, with areas along the coast likely seeing gusts to 35 mph, while inland areas will be closer to 20 mph. A Lake Wind Advisory for Lake Moultrie may be warranted on Tuesday and again on Wednesday.

KEY MESSAGE 3: Coastal flooding is likely next week.

Breezy northeasterly winds associated with strong cold air damming in combination with high astronomical tide levels will likely support coastal flooding issues next week. The pattern suggests a minor/moderate saltwater inundation setup mid week next 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 Coast. Coastal Flood Watches/Warnings and/or Advisories could eventually be needed along the Southeast SC/GA coasts.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

CHS/JZI: TEMPO MVFR cigs between 12-14Z have been introduced at JZI early this morning. VFR conditions are then likely to prevail, prior to light rain/showers drifting north and eventually impacting the terminals late morning and afternoon. TEMPO MVFR conditions remain between 17-21Z Saturday at JZI and a PROB30 group for MVFR conditions between 17-23Z at CHS. Guidance suggests a short break of precip activity prior to MVFR conditions developing/spreading across the terminals Saturday evening. PROB30 groups between 02Z-08Z Sunday, followed by VFR and light rain through 12Z Sunday.

SAV: Patchy dense fog continues to impact the terminal early this morning, but should lift/erode within the next 1-2 hours while light rains develop near the area. Additional rounds of showers could lead to MVFR conditions this afternoon into early evening, which remains under a PROB30 group between 17-23Z. Conditions should then remain VFR between 00Z-06Z Sunday, followed by PROB30 MVFR conditions between 08Z-12Z Sunday.

Extended Aviation Outlook: Flight restrictions are possible with increasing showers and thunderstorms on Sunday and into early next week. Gusty winds are likely on Tuesday and Wednesday.

MARINE

Today and Sunday: 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 this weekend, which could lead to gusty winds and reduced vsbys at times, especially late Sunday.

Monday - Thursday: A cold front will be pushing through the region Monday morning, pushing the heaviest rainfall south of the area by the evening. Greater impacts are expected from the switch to northerly winds that will be strengthening through the day as surface high pressure moves in, causing marine conditions to worsen quickly. Expect winds to increase Monday evening into overnight Tuesday into the 25 to 34 knot gusts range, further strengthening into Gales (35 to 45 knots) Tuesday morning through Wednesday. 6 foot waves are expected to quickly move in overnight into Tuesday morning, pushing up along the coast by Tuesday afternoon as 10 to 11 ft waves build 20 to 60 nm offshore, with wave heights beginning to very slowly subside on Wednesday. Conditions are expected to improve on Thursday, though rough seas will likely persist into at least Friday especially for the 20 to 60nm offshore zones. Small Craft Advisories, Gale Watches, and Gale Warnings will likely be needed. The hazardous marine conditions will likely result in hazardous beach conditions, including dangerous rip currents, high surf, and possibly beach erosion.

CHS WATCHES/WARNINGS/ADVISORIES

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