Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
905 FOUS30 KWBC 230032 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 832 PM EDT Sun Sep 22 2024 Day 1 Valid 01Z Mon Sep 23 2024 - 12Z Mon Sep 23 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID MISSISSIPPI VALLEY AND THE SOUTHERN HIGH PLAINS... ...0100 UTC Update... Few refinements made to the Day 1 ERO update, based on the latest observational and mesoanalysis trends, recent HRRRs/trends, and 18Z HREF probabilistic suite. This included expanding the Slight Risk slightly east of the TX Big Bend (toward the Mexican border), where sufficient upper level difluence and increased low-level FGEN ahead of the flat mid-upper trough and approaching surface cold front is resulting in an uptick (more focused area) of low-level moisture transport/flux convergence. Latest longwave IR trends depict this with the slow expansion of cooling cloud tops, while more recent CAMs (including 18Z NAM Nest, WRF-ARW2, and HRRRs) show isolated totals of 3-5+ inch totals overnight. Elsewhere, have removed the Marginal Risk area across northeast IL-northern IN-southwest Lower MI, given the low-level stabilization behind the cold front. Hurley ...Previous Discussion... The focus for areas of heavy rainfall will continue to be within a plume of deeper moisture ahead of a large-scale upper trough making its way out over the plains towards the Mid-Mississippi Valley. The airmass already in place had precipitable water values generally at or above 1.75 inches...from the western portion of Texas northeastward to where it encounters a cold front moving in from the central Plains/Corn Belt. Maintained the Slight Risk area over western Texas, driven by diurnal instability strong enough to support spotty 2 or 3 inch rainfall amounts and rainfall rates topping 1-2 inches per hour per the latest 12Z HREF neighborhood probs. FFG values are around 2.5"/hr but some CAMs do show ~3"/hr rates at times within/near the Slight Risk outline. Also extended the risk area northeastward into the OKC Metro area per some heavier CAM signals and coordination with WFO Norman. Over southern MO eastward, surface front will help focus moisture convergence in the warm sector but the upper support is not expected to be as strong as it was on Saturday. Still, 12Z sounding from SGF does show plenty of moisture (fully saturated profile, PW ~1.8" or around the 95th percentile). Morning activity has pushed through central MO into IL and have trimmed/shifted the Slight Risk outline to align better in the more unstable air/better instability farther south where there is more clearing. Models have trended a little less with QPF in this area, but still feel there is a low- end Slight Risk threat for at least southern MO and just into northern AR. 12Z CAMs show some 1-2"/hr (even 3"/hr) rates this afternoon and 1-hr FFG values are as low as 1.50-2" in this region (southern MO into northern AR). Fracasso/Bann Day 2 Valid 12Z Mon Sep 23 2024 - 12Z Tue Sep 24 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-MISSISSIPPI VALLEY EASTWARD TO THE CENTRAL APPALACHIANS... 2030Z Update...Expanded the Marginal risk area eastward to the central Appalachians into areas of lower FFG values (~1.5"/hr) and 12Z HREF neighborhood probs of exceeding that around 10-25%. First guess fields also suggest this expansion. Otherwise, no change to the overall thinking but will have to watch southern MO for any potential upgrade depending on D1 rainfall and the next set of CAM guidance. Fracasso Previous discussion from 0830Z... Surface low pressure begins to develop on Monday as shortwave energy moving through the southern stream trough...leading to renewed showers and thunderstorms in the Mid-Mississippi Valley early in the day that shifts into the Ohio Valley by late Monday night/early Tuesday morning. The best overlap of ingredients and forcing looks appears to be from the afternoon into the early evening across portions of southeast Missouri into parts of southern Illinois. After that...low level flow should take on an increasing component parallel to the front. Between the weakening magnitude of moisture flux convergence at the front and the loss of daytime heating/destabilization...rainfall rates should be waning. There are a few ensemble members which generate over an inch of rain over the Ohio Valley as upper flow backs in response to energy in the northern stream...but the expectation is that the flash flooding risk is minimal with those rainfall rates and dry antecedent conditions. Bann Day 3 Valid 12Z Tue Sep 24 2024 - 12Z Wed Sep 25 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE EASTERN OHIO VALLEY AND ACROSS CENTRAL TEXAS... 2030Z Update... Expanded the Marginal Risk for the Ohio Valley into the central Appalachians based on uncertainty in the latest guidance, more or less matching near the new first guess fields as well. For Texas, shrank the area a bit to concentrate on the lower FFG areas but the QPF signal continues to decrease. This area could be removed in future cycles. Fracasso Previous Discussion from 0830Z... Ohio Valley... As energy in the southern stream interacts with digging energy in the northern stream late on Day2...the flow begins to back and become diffluent leading to increasing coverage of rainfall by the time Day 3 begins. The flow should continue to become increasingly diffluent aloft with precipitable water values now about 1.75 inches in place. Spaghetti plots of QPF from the ensembles show minimal support for amounts greater than 2 inches...and most of that support is from just a few GEFS members. With antecedent conditions being so dry...it is presumed that only isolated instances will occur that run off problems occur. Texas Light southeasterly on-shore flow across far southern Texas will start to draw moisture northward again on Tuesday afternoon. As it does so..a weakening cold front will help provide some focus for showers and a few thunderstorms in central or western Texas. The NAM and ECMWF were the only two operational runs from the 22/00Z model suite to generate 2+ inch amounts of QPF while the GFS had support from the GEFS favoring a weaker/drier solution. Given the uncertainty...opted to maintain a Marginal risk for this cycle. 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