textproduct: Charleston

SYNOPSIS

High pressure will weaken and shift out of the area on Thursday ahead of an approaching cold front. The cold front is expected to move through the area Friday morning and high pressure will return through the weekend. Another cold front will move through from the north on Monday, followed by high pressure across the region into the middle of next week.

NEAR TERM /THROUGH THURSDAY/

Satellite and marine observations show a weak coastal trough is becoming better defined just offshore as high pressure shifts farther east out in the Atlantic. Models are similar in showing isentropic ascent developing through the night as the coastal trough sharpens offshore. Impulses ejecting out ahead of the pronounced shortwave digging across Texas into northern Mexico will begin to traverse the region early Thursday. The combination of strengthening isentropic ascent and increasing UVVs aloft ahead of these impulses will support a slow, but steady increase in shower coverage as net moisture values climb, likely starting over the Atlantic near the coastal trough itself with bands slowly expanding/developing onshore as daybreak approaches. The strongest 295-300K isentropic ascent will impact the upper Georgia and far southern South Carolina coasts, directed into the lower Savannah River Valley region through daybreak. The highest pops, 30-50%, were placed in this region consistent with the much of the near term guidance and the 17/12z HREF. Lows tonight will range from the mid-upper 40s inland to the mid- upper 50s at the beaches, much warmer compared to the past several nights.

SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/

Thursday and Thursday night: Aloft, the day will start with southwest flow ahead of a southern stream shortwave riding along the Gulf Coast. This shortwave will lift toward the forecast area through the day then should gradually phase with a stronger northern stream trough across the Midwest and the Great Lakes region. At the surface, high pressure will start the day just off the coast of New England and will shift off to the northeast through the day. Across our area, the inland high will steadily weaken and a coastal trough will shift inland and northward much like a warm front feature. The forecast area will then be placed solidly in the warm sector ahead of an advancing cold front that will begin to move through from west to east late Thursday night. Rain chances will be underway starting Thursday morning as the coastal trough starts to move inland and we get into a feed of moisture and convergence along the coast stretching inland. Among the hi-res model solutions, the consensus favors the feed of showers to be somewhere in the corridor between Savannah and Beaufort along the GA coast and the lower SC coast. This feed of showers is then expected to migrate up the coast through the afternoon hours, impacting the Charleston coast and the Tri-County region during the afternoon and into the early evening. Precipitation associated with the trough aloft and the arriving cold front should then spread in from the west during the evening and should wrap up just before sunrise Friday morning. There remains some uncertainty regarding how widespread the rainfall will become Thursday evening into early Friday morning, primarily due to some model solutions handling of convection across north FL and the potential for it to pass just to the south and remain offshore. The potential for isolated thunder continues, but the instability is expected to be weak and preclude any severe weather threat. Rainfall amounts are forecast to generally be in the 0.25-0.75" range with the event. Temperatures should be mild, with highs surging into the upper 60s on Thursday followed by lows only in the mid to upper 50s before the cold front passes later Friday morning.

Friday through Saturday: The trough axis will shift offshore Friday morning followed by zonal flow through Saturday. Current model consensus favors the cold front shifting offshore by mid to late Friday morning and then dry high pressure will build in through the first half of the weekend. There could be a band of weakening showers along the cold front Friday morning, but most model guidance suggests this shower activity will dissipate and not produce any notable rainfall. Look for low to mid 60s for highs each day, with chilly temperatures in the low to mid 30s inland Friday night.

Lake Winds: Southwest winds are forecast to surge ahead of a front early Friday morning, becoming more westerly behind the front through the mid Friday morning hours. There could be a short window of time where gusts to around 25 knots could occur across Lake Moultrie, and a Lake Wind Advisory could be needed.

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/

Overall, the long term period is expected to feature quiet weather for the latter half of the weekend through early next week. Aloft, zonal flow will prevail on the north side of a large building ridge that extends across the Caribbean, the Gulf, and across Central America. A backdoor front is expected to push through on Monday, resulting in below normal temperatures. Thereafter, we should see temperatures quickly rebound and head towards above normal values as we move into the middle of next week.

AVIATION /00Z THURSDAY THROUGH MONDAY/

0Z TAFs: High resolution guidance indicates that a coastal trough or warm front will develop off of GA/SC this evening. The trough should push inland tonight, supporting isolated to scattered showers, beginning near the mouth of the Savannah River around midnight and then spreading northward through the rest of the night. Conditions should remain VFR through Thursday morning. HREF indicates that rainfall rates may increase enough to result in MVFR conditions, possibly associated with MVFR ceilings. Southeast winds over KCHS and KJZI may becoming gusty during the afternoon, with gusts near 20 kts.

Extended Aviation Outlook: Periods of sub-VFR conditions will be possible Thursday and Thursday night ahead of an approaching cold front. The front will move through the area by mid to late Friday morning and prevailing VFR conditions will return. There remains a low end chance of LLWS Thursday night as a strong winds just above the surface surge across the region.

MARINE

Through Tonight: The local marine area will become increasingly influenced by a sharpening coastal trough just offshore. Northeast winds will generally prevail, except becoming east to southeast over the eastern portions of the Georgia offshore waters overnight. Speeds will average 10 kt or less, except increase to 10-15 kt in the far eastern portions of the Georgia offshore waters. Seas will remain 4 ft or less.

Thursday through Monday: A coastal trough will lift northward and inland Thursday morning, followed by increasing southerly flow Thursday evening and Thursday night ahead of an approaching cold front. Wind speeds should increase into the 20-25 knot range with gusts up to 30 knots late Thursday night. Small Craft Advisories will likely be needed for all of the waters, potentially even Charleston Harbor. The cold front will pass through Friday morning and winds will turn westerly. Winds will remain elevated in the westerly flow. Overall, winds and seas will through Friday and SCA conditions should come to an end. Conditions should then be quiet through the weekend. The next surge of winds and seas will come Monday as a backdoor front passes through from north to south. Another round of SCA's could be needed for at least portions of the waters on Monday.

One other thing to keep an eye on is that there are some hints in model guidance that a period of marine fog will be possible Thursday evening. The best chances will be across the nearshore waters, but the overall duration should be short-lived as a cold front will pass through Friday morning.

CHS WATCHES/WARNINGS/ADVISORIES

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