Area Forecast Discussion
Issued by NWS Columbia, SC
Issued by NWS Columbia, SC
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320 FXUS62 KCAE 250732 AFDCAE Area Forecast Discussion National Weather Service Columbia SC 332 AM EDT Wed Sep 25 2024 .SYNOPSIS... Warm and mainly dry conditions are expected today as the region remains under the influence of upper ridging to the east. Rain chances increase tonight as tropical moisture is advected into the forecast area. Tropical Storm Helene is forecast to become a hurricane today as it moves north into the Gulf of Mexico. Confidence in impacts from Helene Thursday into Friday across our area continues to increase, but uncertainty remains in the exact track and impacts of the system. && .NEAR TERM /THROUGH TONIGHT/... Key Message(s): - Areas of low clouds and patchy fog develop tonight. - Partly to mostly cloudy and warm today. - A few afternoon and evening showers or thunderstorms possible, mainly across the Western Midlands and CSRA. - Rain chances increase tonight, especially in the west. Latest satellite imagery shows an area of mid to high level cloudiness streaming into the CSRA and southwestern Midlands with low clouds developing in the Columbia area and points to the north and east. Areas of low clouds and patchy fog are expected to develop tonight but the rain chance remains low, mainly restricted to our northwestern areas. Lows tonight should fall into the lower 70s. Upper ridging will remain anchored off the eastern seaboard today while an upper low sets up over Southern Illinois. This will result in a southerly flow across the Southeastern US with increasing PWATs across the forecast area. Having said that, it appears that rain chances will be low through the daytime hours for the majority of the forecast area. A few showers or thunderstorms may develop, especially across the Western Midlands and CSRA. It`ll be partly to mostly cloudy and warm with forecast highs generally in the mid-80s. Tropical Storm Helene, currently centered east of Cancun, is forecast to be become a hurricane today and move north into the GOMEX. Moisture will stream into the region ahead of the cyclone resulting in an increase chance for rain tonight. Once again, the highest probability of precipitation will be across our western counties. The rain may be heavy at times towards daybreak in parts of the the CSRA and far western Midlands. Low temperatures will be in the lower 70s. && .SHORT TERM /THURSDAY THROUGH FRIDAY/... Key Message(s): - Impacts from Helene begin to affect the region Thursday. - Heavy rain, nocturnal tornadoes, and gusty winds are the expected threats from Helene. - Biggest impacts overnight Thursday into early Friday morning. Confidence continues to increase that the area will be impacted by Helene, but there remains some uncertainty in the track and exact impacts. Continue to monitor the latest forecasts for updates. Helene is forecast to continue moving north northeastward, likely making landfall in the big bend of Florida Thursday evening, then passing just to our west Friday morning. Latest guidance continues to indicate that it moves quickly north and begins to phase with an upper low in the Lower Mississippi Valley. These factors will likely aid in keeping Helene`s structure fairly well in tact as it approaches the area, which is unusual to have a tropical system remain in tact this far inland from the Gulf. As Helene approaches and phases with the upper low, it is forecast to bend northwestward. Impacts from Helene are expected to move into the CSRA Thursday morning with rainfall and continue north and westward through the day. The biggest impacts are expected overnight Thursday into the pre- dawn hours on Friday. With the fast movement, Helene should be mostly out of our area by Friday evening. While the center of Helene is forecast to pass to our west, we`ll still feel some impacts. Heavy rain: Moisture from Helene will increase through the day on Thursday, bringing PWATs to around 2.5 inches or greater. This will lead to the heavy rain threat, especially overnight as the center nears the forecast area. The heaviest rain threat is currently over the CSRA into the western Midlands, with an even greater threat in the Upstate. As with many tropical systems, a sharp rainfall gradient is likely to set up, which looks to be along the I-26 corridor as of the latest forecast. The location of the gradient is hard to pin down at this point, so it could change. Total rainfall amounts range roughly 2-5", with locally higher amounts possible. This could lead to localized flash or urban flooding, especially in the CSRA. However, don`t think a flash flood watch is warranted at this time due to the isolated nature of the threat. With the heaviest rains expected in the Upstate, eventually that water has to go through the rivers in the days after Helene, so river flooding is becoming increasingly likely. Nocturnal tornadoes: There is a high probability (80-90%) that we will be in the favored northeast quadrant of Helene for tornadoes. Ensemble means for surface based CAPE remain in the 250-500 J/kg, but some deterministic model solutions are showing higher values than that. Storm relative helicity are pretty high, especially given that is during the pre-dawn hours. In general, storm helicity values max out around 500-700 m2s2 with the different model solutions. Thus, the tornado threat continues to increase for the early pre-dawn morning on Friday, unfortunately. The greatest threat for tornadoes is along and east of the I-20 corridor. Be sure to have multiple ways to receive warnings! Gusty winds: As Helene approaches, models suggest 850 mb winds increase to 50+ knots, which will likely result in wind gusts in excess of 40 mph at times. LREF and NBM continue to indicate a great than 80% chance of such winds for most of the forecast area. I think the highest likelihood of these winds will be in the western portions of the forecast area, closer to the center. As with the other threats, the main timing appears to be late Thursday/early Friday morning for the strongest gusts. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... Key Message(s): - Helene becomes absorbed by upper level low on Friday night and Saturday - Cutoff low hangs around through early next week Helene is expected to get absorbed by the upper low. Then the low meanders around the area for a few days. As a result, daily chances of precipitation continue for much of the period. Temperatures are expected to be seasonal through the time period. && .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/... Ceiling Restrictions Possible at the Terminals this Morning.... Latest satellite imagery shows an area of mid to high level cloudiness streaming into the CSRA and southwestern Midlands with low clouds developing in the Columbia area and points to the north and east. Areas of low clouds and patchy fog are expected to develop at the terminals tonight, likely resulting in low MVFR/IFR conditions, especially towards daybreak with conditions improving by mid-morning. Light and variable winds tonight turn southerly and may be gusty at times this afternoon. The chance of rain remains low at the terminals through the end of the TAF period. EXTENDED AVIATION OUTLOOK... Restrictions are possible Thursday into Friday as Tropical Storm Helene passes west of the forecast area. Low ceilings, heavy rainfall, and strong winds are possible. && .CAE WATCHES/WARNINGS/ADVISORIES... SC...None. GA...None. && $$