textproduct: Charleston

SYNOPSIS

High pressure will reside inland today, while a coastal trough lingers offshore. The trough could shift inland Friday into Friday night. A cold front is expected to slowly push across the region this weekend into early next week. Another cold front could impact the area towards the middle of next week.

NEAR TERM /THROUGH TONIGHT/

Today: Aloft, the forecast area will remain within nearly zonal flow as an embedded shortwave dampens as it shifts eastward. At the surface, high pressure will remain centered across eastern Canada and the Northeast as it ridges down the Eastern Seaboard and across GA and the Carolinas. Just offshore, a coastal trough is expected to sharpen through the day. Within the trough, there will be sufficient low-level convergence and moisture to continue generating shower activity much like we are already seeing on radar imagery this morning. One of the main forecast issues will be determining if these showers will be able to push onshore and move inland. Most hi- res model guidance suggests that while a few light returns could move inland, any resulting rainfall amounts will be very light and mostly not measurable. Therefore, we have kept the forecast dry for land areas, other than a few time periods where 20 percent chances clip the southeast GA coast. We should see plentiful coverage of clouds today in multiple layers. The lack of prolonged sun and persistent northeast flow will keep temperatures on the cool side. Forecast highs range in the low to mid 50s for much of the area, with mid to upper 50s possible along and south of I-16. Overall, such temperatures would be on the order of 5-7 degrees below normal for late December.

Tonight: Beginning in the evening, the nearby coastal trough is expected to begin shifting closer to the coast and the feed of mostly low-level moisture becomes aligned more inland especially into southeast SC and the Tri-County region. The forecast features expanding rain chances, with the highest values in the 30-40 percent range into Colleton County and the Tri-County region. Overall rainfall amounts should be on the light side, with most places receiving less than a tenth of an inch. Lows are forecast to fall into the low to mid 40s inland, and the upper 40s along the coast.

SHORT TERM /FRIDAY THROUGH SUNDAY/

High pressure will be in place inland to start the day Friday, while a coastal trough/front lingers offshore. The trough is expected to shift inland as a warm front and erode the wedge later in the day or Friday night. Some shower activity will push onshore and penetrate further inland than previously forecast, so rain chances, while still only in the 20-30% range, were expanded. Temperatures will be tricky and dependent on the inland progression of the coastal front. There could end up bring a notable gradient. Current forecast has temperatures ranging from the upper 50s over the far interior to the upper 60s over coastal southeast Georgia. Lows will span the 50s.

Model consensus has the wedge/warm front inland of the area Saturday morning with high pressure at the surface and aloft now offshore. While moisture will be increasing, the best forcing stays to the west so precip coverage should be limited. PoPs are no higher than 20-30%. Rain chances will then be on the increase late Saturday night as a cold front approaches the region. Temperatures will be seasonally warm with highs in the low to mid 70s and lows Saturday night in the upper 50s to around 60.

Sunday will be the most active day of the set. A stronger shortwave will pivot into the eastern U.S., bringing a cold front across the region. The front will likely exit offshore Sunday night. A deep moisture feed will bring PWats well above climatological normals. With larger scale forcing for ascent also present, numerous to widespread showers are expected, moving into the inland areas during the morning then progressing towards the coast through the afternoon and into the evening. While instability progs are weak, there looks to be enough to justify mention of thunder in the forecast, with the better potential being over southeast Georgia. Instability would be the limiting factor for any severe weather as wind fields are strong. High temperatures will peak in the 70-75 F range.

LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/

High pressure will return on Monday, but it will be short-lived as another quick-moving shortwave and associated surface low shifts east from the Central Plains. The surface low will track across the Ohio Valley or Great Lakes region and eventually into the Northeast, bringing the next cold front through the area, likely Tuesday night. Deeper moisture seems to be lacking with this one, so shower potential is more limited. High pressure will build in on Wednesday following fropa.

AVIATION /09Z THURSDAY THROUGH MONDAY/

KCHS/KJZI: The 06z TAF period begins with VFR conditions which should prevail for much of, if not all, of the forecast period. VFR ceilings should lower through the day, and the potential for MVFR conditions will increase this evening. Also, rain showers are expected to start streaming onshore this evening, and that has been reflected with the introduction of VCSH around 02-03z.

KSAV: The 06z TAF period begins with VFR conditions, but there are already numerous sites across southeast GA reporting MVFR ceilings. Model guidance is mixed, but generally support MVFR conditions beginning around mid morning and then prevailing through the rest of the period. The arrival of MVFR ceilings could be uneven, with periods of ceilings alternating from VFR to MVFR. Model guidance even suggests that IFR ceilings will be possible later this evening, but confidence in that scenario isn't high enough to introduce at this time. An occasional light rain shower will be possible as well, but no significant impacts are expected at this time.

Extended Aviation Outlook: Flight restrictions in low clouds could linger into Friday. Additional flight restrictions are possible in early morning fog/stratus over the weekend, also in showers/isolated thunderstorms on Sunday. VFR is expected to return Monday.

MARINE

Today through tonight: The combination of high pressure inland and a nearby trough will produce a tight pressure gradient across the local waters. This will drive northeast flow of 20-25 knots and gusts as high as 30 knots through the day. Overnight, the gradient is expected to gradually relax as the trough shifts closer to the coast. Winds will remain northeasterly, but will decrease to become 15-20 knots at the highest. Seas are expected to average 4-6 feet across the nearshore waters, with a few 7 footers near the 20 nm line at times. In the outer waters, seas should average 5-7 feet. Seas will peak during the day, then slowly begin to decrease overnight. Small Craft Advisories remain in effect for all waters outside Charleston Harbor through the period.

Friday through Tuesday: Marine conditions will improve on Friday, but 6 ft seas will take a little longer to subside over the outer Georgia waters so the ongoing Small Craft Advisory will continue through 06z Saturday. Flow turns southeast and eventually south over the weekend ahead of an approaching cold front. Winds and seas are currently forecast to stay below advisory levels, with the exception of the outer Georgia water where 6 ft seas could once again return. Weak high pressure and relatively benign conditions are expected for Monday behind the front. Another cold front will approach on Tuesday.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory until 6 AM EST Friday for AMZ350-352-354. Small Craft Advisory until 1 AM EST Saturday for AMZ374.