Area Forecast Discussion
Issued by NWS Columbia, SC
Issued by NWS Columbia, SC
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823 FXUS62 KCAE 260755 AFDCAE Area Forecast Discussion National Weather Service Columbia SC 355 AM EDT Thu Sep 26 2024 .SYNOPSIS... Rain chances increase tonight as tropical moisture is advected into the forecast area. Hurricane Helene is forecast strengthen into a major hurricane in the eastern Gulf of Mexico as it approaches the Florida Panhandle region. Widespread rain and severe weather will become increasingly likely across the area by Thursday night. Uncertainty remains in the exact track and impacts of the system. However, confidence in impacts is increasing with tropical storm force winds the main threat. A few tornadoes will be possible Thursday night along with flash flooding. The storm is expected to move west and north of the area Friday with diminishing winds. && .NEAR TERM /THROUGH TONIGHT/... Key Messages through 7am Thursday: - Rain chances increase from southwest to northeast. - A thunderstorm or two cannot be ruled out though daybreak, mainly in the CSRA. It`s a balmy night across the forecast area as tropical moisture continues to stream northward out of the Gulf of Mexico ahead of an approaching cold front. Much of the region has been dry so far tonight with the majority of the rain passing west of the CWA. However, this heavier rainfall is moving slowly eastward and is starting to fall across the CSRA with a few showers further east in the southwestern Midlands. Expect rain chances to rise from southwest to northwest through the overnight period. Will need to monitor for the potential of a few embedded thunderstorms tonight, primarily in the CSRA. The latest SPC Mesoanalysis shows MLCAPE of at least 500 J/kg across the entire FA with an area in excess of 1000 J/kg over much of the Midlands. This, combined with 0-6km wind shear in excess of 40 knots, could promote thunderstorm development despite the time of day. Lows tonight will be in the upper 60s to lower 70s. Key Messages Today into Tonight: - Overall messaging in the near term is unchanged. - Conditions deteriorate through the daytime hours as Hurricane Helene moves north, passing to our west tonight. - Tropical Storm Warning in effect for the entire CWA. Potential gusty winds in excess of 40 mph, especially tonight. - Tornado threat increases, peaking this evening into tonight. - Flood Watch in effect this morning through Friday afternoon for the CSRA and southwestern Midlands. Areas of heavy rainfall could result in flash flooding, especially south and west of I-26. As of the 2am NHC advisory, Hurricane Helene has maximum sustained winds of 85 mph and is located in the southeastern Gulf of Mexico. The current forecast track brings the hurricane ashore this evening in the Florida Panhandle, rapidly moving northward through Western Georgia tonight. Rain chances will continue to rise ahead of the cyclone as tropical moisture and isentropic lift increase across the region. The most significant impacts are expected this evening continuing through the overnight hours. Potential hazards from Helene include strong wind gusts, isolated tornadoes, heavy rainfall, and localized flooding. Wind: A Tropical Storm Warning is in effect for the entire CWA. The hurricane has an unusually large wind field for a tropical cyclone in the lower latitudes and will be transitioning to an extratropical cyclone after landfall resulting in a tricky wind forecast for the Midlands and CSRA. What we can say with higher confidence is that the strongest winds are likely across the CSRA and Southwestern Midlands which will be closer to the cyclone center. NBM wind gust probabilities greater than of 40 mph are in excess of 60 percent across the FA and 90 percent south and west of I-26. Winds of this strength have the potential to produce localized wind damage including isolated to scattered power outages. Tornadoes: With Helene forecast to pass to our west, this places the FA in the right-front quadrant of the cyclone which is a favorable location for tropical tornadoes. The Day 1 SPC Outlook places the majority of the CWA in a Slight (2/5) risk for severe weather and a 5 percent probability of a tornado. An Enhanced (3/5) risk, with a 10 percent tornado probability, is located to the southeast over the Charleston CWA. The combination of strong 0-6 km wind shear and increasing helicity will support tornado development in any thunderstorms that develop. The greatest risk will come tonight in rainbands associated with Helene. It is important to note that many, but not all, tropical tornadoes develop quickly and are short lived resulting in limited lead time. This threat is compounded by the timing of greatest risk which is during the overnight hours. Heavy Rainfall and Flash Flooding: A Flood Watch in effect this morning through Friday afternoon for the CSRA and Southwestern Midlands. The latest WPC Excessive Rainfall Outlook has the western half of the FA in a Moderate (3/4) risk and a Slight (2/4) risk in the east. Rainfall amounts are likely to be highest closest to the track. NBM rainfall probabilities in excess of 2 inches line up well with the existing Flood Watch, therefore no changes are being made with this forecast package. Forecast rainfall amounts vary greatly from east to west with the potential for 2-5" inches of rainfall with locally higher amounts in the watch area. && .SHORT TERM /FRIDAY through Saturday Night/... Key Message(s): - Very strong and gusty winds along with heavy rain continuing Friday morning - Conditions improving with winds subsiding through the day Friday - Remnants of Helene becomes absorbed by upper level low on Friday night and Saturday Helene will continue marching northward through GA Friday morning and then turn northwestward during the late morning and afternoon hours. By Friday evening Helene is expected to be along the central TN/KY border. At this point the system will be caught up with a cutoff upper level low and become quasi- stationary Friday night and Saturday before beginning to return eastward Saturday night. Wind: The time of concern will be through midday Friday. Winds at sunrise Friday are expected to continue in the 25 to 30 mph range with gusts 40 to 45 mph with the highest gusts across the CSRA and central Midlands. Through mid morning winds will begin slowly subsiding and by 11 am sustained winds will be 20 to 25 mph with gusts 30 to 35 mph. The slowing trend will continue through the midday and afternoon hours as Helene pushes NW of the forecast area with winds by mid afternoon around 15 mph with gusts around 25 mph. With sunset Friday evening winds will further diminish with around 10 mph and gusts to 15 mph Friday night. Rainfall: Heavy rainfall will also continue Friday morning mainly across the across the central Midlands through the Pee Dee. With Helene moving rapidly northwest of the area Friday morning expect a short window for heavy rainfall with most of the heavy rain moving north of the forecast area by midday. The next concern with the rainfall will be rising rivers due to the rainfall across the forecast area as well as heavier amounts across the Upstate which will be moving through area rivers and lakes. Tornadoes: The tornado threat will also continue Friday morning mainly for the Pee Dee area and a slightly lower degree in the northern and central Midlands. Most of the central and northern Midlands are in a marginal risk of severe weather from SPC which has a 2 percent chance of a tornado and for the Pee Dee region a slight risk which is a 5 percent chance of a tornado. Expect the tornado threat to rapidly push northward and exit the forecast area by midday. Saturday and Saturday night: Helene will be stalled well northwest of the area through the day Saturday then begin moving eastward Saturday night. This will keep the area under partly cloudy skies with drier air over the forecast area. Although the weather concern Saturday and Saturday night will be minimal the threat of flooding as water from the Upstate moves through the hydrologic system will need to be monitored. High temperatures Saturday afternoon will be in the low to mid 80s with overnight lows in the mid to upper 60s. && .LONG TERM /Sunday THROUGH WEDNESDAY/... Key Message(s): - Cutoff low hangs around through early next week The remnants of Helene are likely to be completely phased with an upper low centered over the Mid-Atlantic. This upper low along with an attendant surface low are forecast to drift eastward through the long-term period. The latest guidance is trending toward a more drier few days with just a slight chance for afternoon showers in the northern portions of the forecast area through Tuesday. The trend in temperatures remains to be around or slightly cooler than normal. && .AVIATION /07Z THURSDAY THROUGH MONDAY/... Conditions Deteriorate at the Terminals Today.... An area of rain and embedded thunderstorms is starting to move into the CSRA early this morning with a few lighter returns elsewhere. Rain chances will increase from southwest to northwest this morning while ceilings continue to fall. MVFR ceilings are currently being reported at CUB and the other terminals are likely to follow suit before daybreak. Low ceilings and periodic visibility restrictions will then persist for the remainder of the TAF period, especially in areas of heavier rainfall. There is a thunderstorm risk as well but confidence in timing is not high enough to warrant inclusion in the TAFs at this time. East to southeast winds will generally be between 5 and 10 knots for much of the day, increasing this evening as Hurricane Helene approaches. Restrictions are expected to be widespread and persistent tonight with low ceilings, isolated to scattered thunderstorms, heavy rainfall, and strong winds both aloft and at the surface. EXTENDED AVIATION OUTLOOK... Restrictions are possible into Friday morning as Helene passes west of the forecast area. Low ceilings, heavy rainfall, and strong winds are possible. && .CAE WATCHES/WARNINGS/ADVISORIES... SC...Tropical Storm Warning for SCZ016-018-020>022-025>031-035-037- 038-041-115-116-135>137. Flood Watch from 8 AM EDT this morning through Friday afternoon for SCZ018-025-030-035. GA...Tropical Storm Warning for GAZ040-063>065-077. Flood Watch from 8 AM EDT this morning through Friday afternoon for GAZ040-063>065-077. && $$