Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 272041 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 441 PM EDT Tue Oct 27 2020 .SYNOPSIS... High pressure will linger through the middle of the week. A storm system will bring impacts to the region Thursday, which will help push a cold front across the area Thursday night. High pressure will then return for the weekend. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... As of 4 PM: Latest SPC mesoanalysis indicated that MLCAPE was limited to portions of the Deep South, including southern GA. This northern edge of the MLCAPE is the likely position of a stationary front. The front should slide north as a warm front tonight as TC Zeta moves north over the Gulf of Mexico and sfc high pressure pivots over the western Atlantic. By daybreak Tuesday, the warm front is expected to push north of the SE US I-20 corridor, leaving a wavy trough across the forecast area. At H85, GFS indicates that temperatures will warm from 14C across the inland SC/GA counties this evening to 16C by daybreak. Forecast soundings show a strong inversion between 850-750 mb tonight. The combination of the increasing moisture and weak isentropic lift between 300-310k should yield increasing cloud cover across the CWA late tonight. However, partly to mostly clear conditions and light winds may allow patchy fog to develop late tonight inland, lingering as clouds arrive during the pre-dawn hours. Min temperatures should range around 5 degrees warmer than values observed this morning, from the low 60s across CHS Tri-county to near 70 along the Altamaha River. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/... Wednesday: Aloft, ridging persists over the area while a deep low resides over the Southern Plains. At the surface, the region is sandwiched between high pressure offshore to the northeast and Hurricane Zeta to the southwest. Zeta is expected to make landfall along the LS/MS coast where it will then track northeast. Its mid/upper-level counterpart looks to merge with the stronger upper- level low, which will cause Zeta to undergo an extra-tropical transition. Surface winds shift primarily to the southwest in the afternoon advecting offshore moisture; however, most of the deeper, substantial moisture associated with the remnants of Zeta is expected remain to our west. Isolated showers are possible in the far inland SC/GA counties, with PoPs capped at slight chance/low-end chance in those areas. Max and min temps are forecast above-normal for this time of year; low to mid 80s during the day with overnight temps in the upper 60s and lower 70s across the area. By Thursday, the low pressure system will be situated just to our west and shall continue to track to our northeast throughout the day. The strong upper-level system tilting to the west with height will provide an abundance of DPVA/forcing, but its target area maintains just west of the forecast area. Again, isolated showers are possible in the far inland SC/GA counties during the day. By Thursday night, a cold front trailing the system will approach the region. Models are struggling to analyze the amount of rainfall that will be accompanied by the front. While surface forcing and a decent amount of moisture trailing along the eastern seaboard (PWATS ranging from 1.5" to 2.5" across model guidance) are present, substantial upper level forcing and instability are lacking. A slight chance of thunder is possible with any isolated showers that form over the forecast area just prior to the arrival of the front. QPF is minimal at this point, yet some models show the possibility for a half-inch of rain. On the other hand, the bigger concern is for gusty winds ahead of, with, and just behind the front. If there is intense mixing, we could need a Lake Wind Advisory for Moultrie. Beyond that, drier air and clearing conditions are expected to move in from the west after midnight. This will be the last day of 80 degree temperatures for our area this week, which could put some record high temperatures at risk of being broken. Friday: A mid-level trough over the East Coast in the morning will shift offshore with time. Meanwhile the cold front will have moved offshore by the morning. High pressure will build to the north and a significantly drier air mass will move in. Additionally, subsidence will result in clear skies. Cold air advection within northwest to north surface winds will lead to much colder temperatures. Highs will return to near-normal; only reaching the low to mid 70s. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... Zonal flow is expected to prevail in the mid-levels Saturday, followed by a long-wave trough over the East Coast Sunday into Monday. At the surface, high pressure passing to our north should bring dry conditions and below normal temperatures. && .AVIATION /20Z TUESDAY THROUGH SUNDAY/... VFR conditions at KCHS and KSAV to start the 18Z TAF period. Skies are forecast to clear overnight at KCHS leading to prime conditions for radiational cooling to occur, resulting in the formation of ground fog. This could result in MVFR or even IFR conditions, however no restrictions have been introduced as of this TAF issuance. Broken to overcast skies along and ahead of a warm front across northern Florida will remain overnight at KSAV. This will limit the amount of radiational cooling and lower the chances for fog formation. Overnight this warm front will move northward into southern GA and eventually southern SC. After 14Z prevailing VFR at both KCHS and KSAV, however cannot rule out a brief moment of MVFR at KSAV associated with some lingering lower clouds ahead of the northward moving warm front. Extended Aviation Outlook: A storm system will bring flight restrictions Thursday and Thursday night. Additionally, gusty winds are expected Thursday into Friday. && .MARINE... Tonight: The center of broad high pressure will shift over the western Atlantic late tonight. Steady ENE winds should continue across the marine zones overnight, speeds between 5-10 kts. Seas are forecast to range between 2-3 ft tonight. Extended Marine: Weak high pressure located to our northeast Wednesday morning will eventually shift into the Central Atlantic. Additionally, Hurricane Zeta is expected to make landfall along the Lower MS Valley Wednesday night, then combine with a separate storm system. This new storm system will pass to our north on Thursday, with its trailing front moving offshore late Thursday night. The pressure gradient will rapidly increase Thursday and will result in increasing winds and building seas. Small Craft Advisories will likely be needed for all of the waters, including the Charleston Harbor, with gusts increasing to 25 kt and seas building to 4-6 ft across the majority of the marine zones. The worst conditions are expected during the frontal passage Thursday evening. Gusts could reach Gale force across portions of the waters during that time period, along with steep, wind-driven seas building up to 7-8 ft in portions of the waters. The front is expected to move offshore by Friday and high pressure builds in from the west, which will lead to a brief window of lower winds and seas in the afternoon hours. However, conditions should deteriorate again Friday night as cold air advection and the pressure gradient steepens again. These near gale conditions should persist through the weekend. && .CLIMATE... Record warmth possible Thursday, October 29: KCHS: 85/1946 and 72/1984 KCXM: 82/1984 and 72/1984 KSAV: 87/1996 and 71/2019 && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...NED/RJB SHORT TERM...BRM LONG TERM... AVIATION...CPM/NED MARINE...BRM CLIMATE...

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