Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
717 FOUS30 KWBC 240100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 900 PM EDT Mon Sep 23 2024 Day 1 Valid 01Z Tue Sep 24 2024 - 12Z Tue Sep 24 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-MISSISSIPPI VALLEY EASTWARD TO THE SOUTHERN APPALACHIANS... ...0100 UTC Update... Only minor changes were made to the Day 1 Marginal Risk area, mainly to remove the far western portion (across central MO and northern IL), based on the latest observational, mesoanalysis, and more recent HRRR/RAP guidance trends. TPWs between 1.5-1.75" along with MLCAPE values of 1000-1500 J/Kg will continue to result in an isolated/localized flash flood risk with any slow-moving clusters of convection forming within the warm sector along/ahead of the pivoting mid-upper trough and slow-moving surface cold front. Hurley Day 2 Valid 12Z Tue Sep 24 2024 - 12Z Wed Sep 25 2024 ..THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE OHIO VALLEY AND THE MID-ATLANTIC... 20z Update: The Marginal risk was expanded with this update. The northwestern extent was expanded near the expected low track. We should have convection ongoing at 12z Tuesday over portions of IL into the OH valley, and this may be producing a localized flash flood risk. Then do expect scattered convection to redevelop during the afternoon hours within the warm sector across portions of IN and OH. Th risk was also expanded to the southeast across more of VA and NC. Convection will likely move into this region Tuesday afternoon and evening. These cells should initially be quick moving, however there is some concern cells could grow upscale and backbuild for a period of time near the backdoor front in the vicinity. If this occurs then a smaller corridor of more organized flash flooding could evolve. For now will carry a Marginal risk to cover the expected isolated flash flood threat...but will need to monitor trends in the new HREF. If confidence in a more concentrated area of organized convection increases then a targeted Slight risk may be needed. ...Previous Discussion... The shortwave trough moving across the mid Mississippi Valley on Day 1 will begin to interact with a northern stream trough amplifying over the northern Plains early in the period. Models show the upper pattern quickly amplifying over the Mississippi Valley on Tuesday. At the surface, low pressure is expected to track northeast from the mid Mississippi Valley to the Great Lakes, with a trailing cold front extending back through the Ohio Valley. An axis of deeper moisture (PWs ~1.50-1.75 inches) coinciding with deep southeasterly flow ahead of the upper trough will fuel showers and storms along the low track and ahead of the trailing cold front. Models do not present a signal for widespread heavy amounts but show the potential for locally heavy amounts, including for portions of the central to southern Appalachians, where relative soil moisture is slightly higher and flash flood guidance values lower. Pereira Day 3 Valid 12Z Wed Sep 25 2024 - 12Z Thu Sep 26 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEAST, TENNESSEE VALLEY, AND THE SOUTHERN APPALACHIANS... 20z Update: The Slight risk over portions of AL, northern GA and southeast TN looks in good shape. We very well could be looking at a PRE event ahead of Helene as moisture associated with the tropical system interacts with a front and upper jet. Overall the synoptic setup looks quite favorable for excessive rainfall and global deterministic model QPF is as high as 4-6" over portions of the area. Given the convective nature of the QPF, it seems plausible that rainfall totals could even exceed these values on a localized basis. There is still some uncertainty on the exact location of this higher rainfall axis. Todays model runs have shifted a bit east, focusing more from eastern AL into northern GA and southeast TN. The Slight risk remains a bit broad to account for this uncertainty, but it does seem to cover the areas at greatest risk. The ingredients in place and overall setup does suggest a MDT risk could eventually be needed. However this depends on exactly how the moisture streaming north ahead Helene ends up interacting with the approaching trough/closed low and where exactly this interaction happens. So while confidence in a flash flood risk is increasing, think a higher end Slight risk is the best option for now. If Helene speeds up then a Slight risk may be needed ahead of landfall along portions of the FL Panhandle. Although at this time it looks like any rain Wed into Wed night is more preconditioning ahead of Thursday, which will have a bigger flood risk. Thus kept this area within a Marginal for now. Chenard ...Previous Discussion... Although disagreements on the track continue, the consensus of the models show a powerful tropical cyclone tracking north through the eastern Gulf of Mexico during this period. Apart from the faster Canadian solutions, which bring heavy amounts into the Florida Panhandle, most of the deterministic models and their ensemble means show moderate to heavy rains brushing the coast, but the heaviest amounts remaining out over the open waters through 12Z Thu. Therefore, just a Marginal was maintained from South Florida and the Keys northward along the Florida Gulf Coast through the Panhandle. However, should more guidance begin to trend toward a faster solution, future upgrades may be necessary. Despite their disagreements with the cyclone track, the models are presenting a growing signal for heavy amounts developing well to the north. The cold front pushing east into the Ohio, Tennessee, and lower Mississippi Valley prior to the period is forecast to slow across the region, providing a focus for moisture ushered in by strong southeasterly to easterly flow ahead of the system. With some longitudinal disagreement, the GFS and ECMWF show PWs increasing to 1.75-2 inches along an axis of strong low level convergence extending north from the northern Gulf Coast into the Tennessee Valley. Meanwhile, an upper low that will begin to develop over the mid Mississippi Valley on Wednesday is expected to drop south, enhancing lift across the region. While there is some west-east disagreement on its placement, most of the 00Z deterministic runs showed a stripe of 2-4 inch accumulations extending northeastward from northern Alabama into the southern Appalachians. Pereira 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