Area Forecast Discussion
Issued by NWS Charleston, SC

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441 FXUS62 KCHS 181110 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 710 AM EDT Tue Jun 18 2024 .SYNOPSIS... High pressure will extend across the region early this week. A trough of low pressure could approach the Southeast U.S coast by late week. && .NEAR TERM /THROUGH TONIGHT/... The lowest temperatures this morning were as follows: Walterboro (KRBW)...63F Ravenel.............63F Near Huger..........63F For today: A high amplitude mid and upper ridge will cover much of the eastern states into the northeast Gulf of Mexico, with the center near the Mason-Dixon Line with 500 mb heights that are near 5950-5960 meters. This occurs in tandem with a strong and humongous sized surface ridge that covers the western Atlantic and eastern third of the country. there are also hints that a very subtle inverted trough could form offshore. The large scale subsidence associated with the ridge aloft, a pronounced cap around 850 mb, and poor thermodynamics suggest that convection will be hard to come by. However, a deep easterly flow, convergence from the sea breeze, some low level Theta-E ridging, and maybe very weak isentropic ascent will produce isolated to scattered showers. The best chances will be over the Georgia coastal counties where the low level convergence is the best, and PWat is higher than further north. We did maintain a mention of slight chance thunder since it is mid June, and the NBM does show isolated coverage this afternoon. But given weak buoyancy and the HREF indicative of less than 10% chance of t-storms, we would not be surprised if none actually occur. There exists a tight gradient around the surf ace high through the day, and given deep mixing and a boost from the sea breeze, easterly winds will reach 10-20 mph, and even a little higher along and near the coast. Gusts will reach as much as 25 or maybe 30 mph, especially over the coastal counties. Temperatures will be close to climo, but dew points will be mainly in the 60s, which equates to afternoon RH`s mainly below 50%. Certainly these are not summerlike values. Tonight: Little to no change in the pattern both at the surface and aloft, with even some increase in the strength of both ridges. A continued deep easterly flow and sufficient moisture will result in at least isolated showers coming in off the ocean, especially impacting coastal Georgia. Although winds diminish inland, they do continue rather gusty near and at the coast. This will result in a fairly large range of temperatures; lows 64-68F degrees well inland (where the lowest dew points are found, and middle 70s on the barrier islands, and over the Charleston and Savannah metro regions. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY/... he local forecast area will remain within the southern periphery of upper level high pressure centered over New England on Wednesday. A similar pattern will be present at the surface. The high pressure at the surface will shift further eastward towards Bermuda on Thursday and Friday. The high pressure aloft is forecast to remain over the East Coast through the period. Unseasonably quiet weather is forecast Wednesday and Thursday as subsidence persists aloft, with only slight chance to chance PoPs featured in the forecast. Additionally, thunder chances are rather low as instability is negligible across the forecast area. Thunder chances have been capped at slight chance for Wednesday and Thursday. Friday the pattern turns more active as the subsidence begins to break down as well as an influx of moisture arrives, originating from a low pressure disturbance off the east coast of Florida. The development of the low pressure into anything other than a broad area of low pressure/open trough is still very low confidence. Most ensemble guidance continues to suggest little formation. Showers and thunderstorms are forecast to come ashore Friday afternoon, increasing in coverage in the afternoon hours. The greatest chances remain across southeast GA where the greatest moisture profile is. Temperatures through the period are forecast to be right around normal, with highs in the upper 80s to around 90 and overnight lows in the upper 60s to low 70s. && .LONG TERM /FRIDAY NIGHT THROUGH MONDAY/... A broad upper level trough will begin to trek across the southeast U.S. on Sunday as high pressure aloft shifts further south. At the surface a cold front will approach the region late this weekend. Between forcing from the approaching cold front and the local afternoon sea breeze, precipitation chances are forecast to increase through the weekend heading into next week. Temperatures will warm through the period, with low to mid 90s on Saturday warming to upper 90s on Monday. && .AVIATION /11Z TUESDAY THROUGH SATURDAY/...
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Prevailing VFR conditions are expected at CHS/JZI/SAV terminals through 12Z Wednesday. However, brief MVFR ceilings are possible this morning with onshore flow off the Atlantic, particularly at the SAV terminal. There is enough risk of showers at KSAV this afternoon that we included VCSH after 16Z. Otherwise, gusty easterly winds will peak at least near 20 or 25 kt at the terminals from about 16Z through sunset. Extended Aviation Outlook: Prevailing VFR. Brief flight restrictions are possible in afternoon showers/thunderstorms Wednesday, especially at KSAV. Chances of flight restrictions increase at CHS/JZI/SAV terminals later in the week with showers and thunderstorms moving onshore.
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&& .MARINE... Today and tonight: A large and strong ridge of high pressure will encompass the coastal waters, with maybe a subtle inverted trough to form nearby. There will be as much as a 2-3 millibar pressure gradient in place locally, resulting in rather elevated easterly winds of at least 15 or 20 kt and gusty. The favorable long duration onshore fetch will allow for seas to reach 4-5 feet within 20 nm today, and 5-6 feet on the outer Georgia waters. It is there where we have a Small Craft Advisory beginning early this evening. Seas will build another foot or so tonight all waters, and we might need to expand the coverage of the Small Craft Advisories. Wednesday through Saturday: Generally, high pressure will prevail across the local marine waters. Northeasterly winds 15 to 20 knots will persist across the local waters Wednesday into Friday morning. Gusts may approach 25 knots at times. Winds Friday will remain out of the NE, however easing slightly to 10 to 15 knots. Conditions will continue to improve into Saturday, with winds shifting to the SE around 10 knots by Saturday night. Seas are forecast to build across the local marine zones, peaking Thursday with 5 to 6 ft across the nearshore waters and 7 to 8 ft across the 20-60 nm offshore GA waters. Small Craft Advisories will be needed for all waters outside the Charleston Harbor through Thursday night. Seas will decrease Friday, with 3 to 4 ft forecast Saturday morning. Rip Currents: Although there still isn`t much swell that will impact the beaches today, given the modest onshore winds and numerous rip currents at Tybee yesterday, and enhanced risk of rip currents will occur. In combination with the Rip Current MOS and a locally developed Rip Current Calculator, we are showing a Moderate Risk of Rip Currents for the South Carolina beaches, and High Risk for the Georgia beaches today. A High Risk for rip currents has been hoisted for Wednesday across all area beaches. An enhanced risk of rip currents will likely persist through the end of the week as long period swell impacts the region. High Surf: Breaking wave heights could reach 5 ft or greater Wednesday night into Thursday, especially along the Charleston County coast. A High Surf Advisory may be required. && .TIDES/COASTAL FLOODING... With the approaching full moon (6/22) tides in the Charleston Harbor could reach minor flood stage with the evening high tides both Wednesday and Thursday. Coastal Flood Advisories could be required for Charleston and Coastal Colleton Counties. Tides at Fort Pulaski are forecast to remain below minor flood stage. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...High Risk for Rip Currents through this evening for GAZ117-119-139- 141. SC...None. MARINE...Small Craft Advisory from 6 PM this evening to 6 AM EDT Saturday for AMZ374. && $$ NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE...