Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
960 FOUS30 KWBC 260053 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 853 PM EDT Wed Sep 25 2024 Day 1 Valid 01Z Thu Sep 26 2024 - 12Z Thu Sep 26 2024 ...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHEAST GEORGIA THROUGH FAR WESTERN SOUTH CAROLINA AND WESTERN NORTH CAROLINA... ..LIFE-THREATENING FLASH FLOODING INCREASINGLY LIKELY OVER THE SOUTHERN APPALACHIANS.. Only minor adjustments were made to the ongoing risk areas based primarily on recent radar and HRRR trends. A significant rainfall event will continue to evolve through the overnight hours into Thursday across portions of the FL Panhandle into the southern Appalachians. Trends support a slightly further east rainfall axis through 12z Thursday, and thus the risk areas were nudged in this direction. Recent trends also suggest the rain axis will only push a a bit further east before stalling, and then even coming back a bit west by late tonight as strong south southeasterly flow increases. The heaviest rainfall rates will likely occur along the eastern edge of the rain shield where better instability will be present, supporting localized 2-3" per hour rainfall. Even as instability erodes overnight enough weak instability combined with the upslope flow into the terrain will support hourly rainfall periodically exceeding 1 to 1.5". Additional rainfall through 12z within the MDT and High risk areas will generally be within the 2-5" range...however localized amounts of 5-7" are possible. Flash flood coverage and severity is expected to increase across the MDT and High risk areas (GA into the southern Appalachians) through the overnight hours as conditions continue to saturate. As Helene moves northward across the Gulf of Mexico outer rain bands will increasingly impact the west coast of FL and eventually the FL Panhandle. While these bands will initially be quick moving, we may see some brief training later tonight, with 3-6" rainfall amounts possible. Still a bit unclear whether these totals stay just offshore or get into the western FL coast, but enough of a risk to maintain the Slight risk area. The areas near Apalachicola in the Fl panhandle will also likely become a focus for increasing convective coverage overnight. Expect the flash flood risk to increase here as well. Chenard Day 2 Valid 12Z Thu Sep 26 2024 - 12Z Fri Sep 27 2024 ...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL IN PORTIONS OF THE SOUTHERN APPALACHIANS AND NORTHEAST GEORGIA... ..LIFE-THREATENING FLASH FLOODING LIKELY OVER THE SOUTHERN APPALACHIANS AND PORTIONS OF THE SOUTHEAST.. The extensive period of heavy rainfall during the D1 period will continue through D2 across portions of the interior Southeast U.S with a primary focus across the Southern Appalachians where the back end of our predecessor rainfall event (PRE) will still be occurring. Further to the south, all eyes turn to Hurricane Helene as the storm continues to move quickly through the Eastern Gulf within the weakness created by the trough to the north. The northern extent of Helene`s tropical moisture will begin nosing into the Gulf coast with PWAT anomalies steadily rising from 1-2 deviations above normal to as high as 4 deviations above normal by the time it makes landfall on Thursday evening. Despite a fast forward propagation, copious amounts of rainfall will impact the Florida Panhandle over to the Big Bend with the feeder bands on the eastern flank of the cyclone impacting the coastal areas of the Florida Peninsula. Rainfall amounts between Panama City to Tallahassee will reach between 8-12" with local maxima of up to 15" possible within the axis of Helene`s landfall as the storm moves almost due north, perpendicular to the coastal plain. For this reason, a High Risk was added to portions of the Florida Panhandle/Big Bend, including the Tallahassee area, as well as across Apalachicola. By Thursday night into Friday morning, Helene will continue making steady progress northward with a large rain shield capable of dropping 3-6" of rainfall within the core of the cyclone in short periods of time. A swath of 6+ inch totals will progress northward, eventually cascading into the Atlanta metro and surrounding locales generating a higher threat of flash flood potential within the urban corridor. Areas in the path of Helene will undoubtedly see significant flood potential with small rivers and streams likely becoming inundated very quickly to exacerbate the flood threat within the rural areas of Georgia. The recent HREF blended mean QPF footprint is robust for the period with a large areal extent of 5+ inches with the secondary maxima positioned across Northeast GA where the next round of significant rains will approach with Helene`s core making an approach. The situation across the Southern Appalachians is becoming increasingly severe with regards to the ongoing flash flood threats from the PRE and eventually with the approach of Helene`s core as the cyclone moves up through Western GA. This will place the eastern quadrant of the cyclone across Central and Northern GA with a strong, prevailing east southeast flow expected to bank against the terrain situated over North GA through the Western Carolinas, a mechanism for maximizing ascent within a deep tropical moisture plume that will enhance rainfall rates for several hours moving through late Thursday night into Friday morning. The 12z HREF mean QPF output continued to signal for prospects of 3-6+ inches within a short time frame as the core of higher PWATs advect into the region and douse the Southern Appalachians in another round of significant rainfall with rates generally steady between 1-2"/hr with some small probabilities (10-15%) for hourly rates pushing close to 3"/hr in the stronger convective cores embedded in the precip shield. The HREF neighborhood probability for >5" has peaked at 90+% for the Southern Appalachian front, a signal reserved for only the most impactful events, correlating well with the EAS signal of >90% as well for at least 3". The combination of multiple periods of heavy rainfall with elevated rates likely thanks to Helene will produce significant, life- threatening flash flooding across the area over Northeastern GA through the Blue Ridge of SC and NC, including areas in and around Asheville and to the west of Greenville- Spartanburg in Upstate SC. Several landslides due to the prolific rainfall are very likely with a history of these types of events producing catastrophic damage from both flooding and the prospects of land and mudslides within the complex terrain. For these purposes, and in coordination with the GSP and FFC WFO`s, the High Risk was maintained and expanded further southwest to just northeast of the Atlanta metro to account for the most significant signals for flash flood prospects during the period. Considering the scope of Helene`s size, the flash flood threat will not be relegated to a small corridor like we see in some tropical events. The trough to the west will play a pivotal role in the steering mechanism for Helene as the storm approaches the latitude of North GA, eventually pulling the cyclone more northwest towards the back end of the forecast period. Due to the progression, there is a growing consensus within the deterministic of the heavy rain threat also shifting into Northern AL with the area in and around Huntsville to the TN state line becoming more favorable for flash flooding potential within the western flank of the cyclone. The signal is not as prolific as the Southern Apps or where Helene makes landfall, but the probability and QPF fields certainly depict a significant amount of rain (3-5") within a time frame generally between 4-8 hrs prior to the end of the forecast period. This swath will extend across Eastern AL and Western GA up through Northern AL, following the western periphery of the cyclone as it moves north then northwest. The heavy rain will expand into Southern TN before the end of the forecast period with more rain anticipated in the Tennessee Valley after D2. Because of the growing consensus in the heavy rainfall across the above areas, the MDT risk was expanded further west and northwest into Alabama and over the southern fringes of TN. A large SLGT and MRGL risk will encompass a large area of the Southeast and Southern Mid Atlantic as well as the outer bands of Helene are forecast to impact the region with locally heavy rainfall possible as is customary with these types of tropical evolutions. Kleebauer Day 3 Valid 12Z Fri Sep 27 2024 - 12Z Sat Sep 28 2024 ...INTRODUCED A MODERATE RISK OF EXCESSIVE RAINFALL OVER A PORTION OF THE UPSLOPE REGION INTO THE SOUTHERN APPALACHIANS... 20Z Update: The recent forecast for Helene continues to signal the cyclone weakening rapidly but maintaining an expansive QPF footprint as it migrates northwestward into the Tennessee Valley as it becomes captured within the 500mb trough. The storm will eventually slow begin meandering around Western TN with a continued heavy rain threat circumnavigating around Helene`s general circulation. A SLGT risk still exists further west into the Mid and Lower Mississippi Valley, as well as portions of the Southern Ohio Valley, but the prospects for a higher risk are slowly increasing given the recent ensemble QPF trends. With the threat still some time away and a lack of any short range guidance which can narrow down some of the banding/convective details, the wording will remain a higher-end SLGT risk with a possibility of an upgrade in future updates. To the east, the heaviest rainfall will be coming to an end, but still a prevailing easterly flow banked against the terrain in Western NC will allow for a continued threat of locally heavy rains on top of what will fall the prior period. The previous MDT was maintained given the consistent signal for ongoing rains will occur through early Friday afternoon before coming to an end, outside some scattered shower threats through the evening. To outlined the continued threat of rainfall, did not want to deviate from the previous forecast and kept the previous MDT with a small expansion to the south over Upstate SC where some guidance lingers the rainfall through Friday morning where FFGs will likely be at their lowest points. Kleebauer ..Previous Discussion.. Based on latest NHC track guidance...Helene should be in northern Georgia and tracking on a more northwestward course during the day. This suggests the flow of deep moisture should be on-going to the terrain of western Carolinas into southwest Virginia. Latest model guidance shows several inches of rainfall in addition to what falls in the predecessor 48 hours...felt a focused Moderate risk placed where there was best model consensus was warranted. It was also a multi-day rainfall total farther south along the NHC track in the eastern Tennessee Valley that was the main reason to keep a Slight Risk where model QPF be a little too light to support one. That portion of the Slight Risk area extends westward to the deep- layered low where steepening low level lapse rate and the additional moisture streaming off of Helene could lead to some showers and thunderstorms capable of producing locally heavy downpours. Bann Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt