Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
693 FOUS30 KWBC 251544 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1144 AM EDT Wed Sep 25 2024 Day 1 Valid 16Z Wed Sep 25 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 SOUTHWEST NORTH CAROLINA... ..LIFE-THREATENING FLASH FLOODING INCREASINGLY LIKELY OVER THE SOUTHERN APPALACHIANS.. Deep layer south to south-southwest flow will allow for the advection of increasingly moist, unstable air to funnel poleward ahead of Helene thanks to a closed upper low/trough centered over the Mid-Mississippi Valley. Afternoon destabilization within the budding warm sector along with sufficient left-exit region (LER) jet dynamics from the trough will translate to more widespread convective coverage across the Southeastern U.S into the Central/Southern Appalachians as we move into the late-afternoon and evening time frame. Areas across the Escarpment located over far Northeast GA into the Western SC/NC border have seen 2" to as much as 5" of rainfall in the last 24-36 hrs leading to a priming of the top soil layer as indicated by the latest NASA SPoRT soil moisture percentiles approaching 60-70% over the areas affected. Recent CAMs and ensemble output have increased the QPF signal across the Southeast with the axis of heaviest rainfall bisecting Eastern AL up through Western GA into North GA and the Southern Apps/Smokies bordering TN/SC/NC. HREF blended mean QPF indicates widespread 3-6" of rainfall from this predecessor rainfall event (PRE) that correlates with the strong upper forcing and deep tropical connection to north of Helene as it wanders into the Eastern Gulf. Probability fields are also very aggressive with the signature for enhanced rainfall rates and totals with the EAS signal for 3" settling at 50-80% within the Escarpment area into Northeast GA. Neighborhood probabilities of >5" between 70-90+% and >8" between 20-35% are more than sufficient for a very impactful event expecting to unfold across the complex terrain in-of the Southern Appalachians. Given the signals expressed via the hi-res ensemble, along with ample support from the various ML output for the heaviest rainfall, as well as coordination with the local Greenville-Spartanburg WFO, a targeted High Risk was added for the D1 period. The MDT risk was expanded a bit further southwest and west based on the latest trends with the axis of heavier QPF a touch further west within the 12z deterministic and adjusted HREF mean QPF output. This is likely due to the deeper trough axis to the west and slower progression of the overall upper pattern placing the best LER jet dynamics a bit more in the interior than what was progged the last 24 hrs. The MDT still encompasses much of Central and Northern GA, but does now expand down into a small area of Eastern AL and Eastern TN along the Great Smoky Mountains. SLGT risks still align within the Apalachicola region in the Panhandle, as well as the Western coast of FL as the outer bands of now Hurricane Helene begin approaching the area(s) overnight with locally heavy rainfall (>3"/hr) in any banding structures. Kleebauer 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... Given the increasing confidence about the overlap of the axis of a predecessor rainfall event on Day 1 period and the amount of rainfall expected as the center of Helene approaches the southern Appalachians during the latter part of the Day 2 period...there has been a growing number of models generating close to 10 or 11 inches of rainfall for the 2-days combined in the complex terrain. That raises concerns about impactful flooding becoming widespread with considerable and potentially life-threatening flash and urban flooding as well as areas of significant river flooding over portions of Florida, the Southeast U.S. and the southern Appalachians. Widespread minor to moderate river flooding is likely and major river flooding is possible. Landslides are possible in areas of steep terrain in the southern Appalachians given these rainfall amounts. This prompted the upgrade to a high risk over portions of northwestern Georgia into far western South Carolina that was mainly confined to the terrain. What effectively amounts to a high-end moderate risk surrounds the high that extends along much of the track as Helene makes landfall along the Florida panhandle coastline around 27/00Z (give or take a couple of hours) based on the latest NHC guidance. In terms of model guidance...the tracks tended to be clustered well resulting in NHC guidance being close to most model runs. For the Day 2 period...that resulted in the highest rainfall totals being close to each other where the orographic forcing was consistent. A second axis of excessive rainfall branched off to the northwest from Helene`s track towards a deep-layered low developing over the southern portion of the Mississippi Valley as moisture gets entrained into that system leading to scattered convection that could produce locally heavy amounts. Mainly confined the Slight risk to the Tennessee Valley and only a Marginal risk extending back towards the core of the upper system. Bann 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... 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