Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 02 2024 00:42:24 FOUS30 KWBC 020041 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 841 PM EDT Fri Nov 1 2024 Day 1 Valid 01Z Sat Nov 02 2024 - 12Z Sat Nov 02 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 SOUTHEASTERN NEW MEXICO INTO FAR WEST TEXAS... Training convection this evening has resulted in isolated flash flooding across portions of central LA. HRRR guidance indicates=20 the convergence axis driving this activity should weaken over the=20 next couple hours, with convective intensity and training=20 potential expected to diminish as well. Cloud tops are beginning to show some signs of warming, so maybe this weakening trend is=20 beginning. Nonetheless, a localized flash flood risk is possible=20 for the next several hours. Otherwise attention turns to portions of southeast NM for the overnight hours into Saturday morning. An increasing and robust southeasterly low level jet is rapidly returning moisture into the region, and convergence on the nose of this moisture transport=20 axis should help initiate convection within the next several=20 hours. Larger scale forcing in the form an an approaching longwave trough and upper jet streak to the north should increase=20 overnight, which should help convection grow upscale in nature=20 after 06z. Some southwest to northeast training appears likely, and as is seemingly often the case over this region, would not be=20 surprised if convective coverage ends up a bit larger than depicted by the HRRR. Overall expect 1-3" of rainfall through 12z Saturday, but a narrow swath of 3-6" seems possible along the convergence=20 axis where training is maximized.=20 Recent HRRR runs have trended a bit north with the maximum=20 QPF swath in recent runs, and so have also expanded the Slight risk in this direction as well. Will not cut back on the southern=20 extent of the Slight risk as some model guidance still favors this corridor. Overall the updated Slight risk encompasses the model=20 spread and the bit broader nature accounts for the potential of a=20 more organized convective area as the night progresses. Chenard Day 2 Valid 12Z Sat Nov 02 2024 - 12Z Sun Nov 03 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS... 2030Z Excessive Rainfall Discussion Update... There was a subtle westward/southward shift in the axis of heaviest rainfall shown in the 12Z numerical suite of guidance. The axis of the high-end Slight risk area was elongated in a similar fashion with a more subtle shift in the axis, The expectation about the very dry antecedent conditions helping to mitigate the overall excessive rainfall impacts still hold. Bann Previous Discussion... Additional Gulf moisture will track north on a LLJ across Texas and into the Slight Risk area on Saturday. PWATs may exceed 1.75 inches. The trough over the intermountain west will slow to a crawl, while much faster moving shortwaves round the base of the trough and move north up the Plains. Multiple rounds of storms are expected from north Texas to the Kansas City metro. The heaviest rains are likely across central Oklahoma, where a higher-end Slight is in place. As on Day 1, extremely dry antecedent conditions are in place over much of the Slight Risk area, with soil moisture levels from Nasa Sport at under 10%. While clay soils may start out hydrophobic, for the most part, the empty creeks and streams should work to prevent much flash flooding. Given the available moisture for the storms to work with, training is likely, which should raise the flash flooding potential to widely scattered. The Slight Risk area was expanded a bit towards the northeast and now approaches the Kansas City metro. Meanwhile a portion of north Texas was removed based on the latest guidance. Nonetheless a slower forward progression of the entire pattern should concentrate the worst impacts from any flash flooding in a smaller corridor highlighted by the Slight Risk. No major changes were made to the inherited risk areas. The greatest uncertainty is how well the dry soils will absorb the sudden abundance of moisture as the drought conditions in place now across the southern Plains will help in many cases to mitigate any potential flooding impacts. Wegman Day 3 Valid 12Z Sun Nov 03 2024 - 12Z Mon Nov 04 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN OKLAHOMA AND THE OZARKS... 2030Z Excessive Rainfall Discussion Update... The 12Z suite of guidance offered good support for the placement of the Moderate Risk area. Uncertainty remains as to exactly how well the dry soils will absorb the rainfall and exactly how much of an overlap there will be with the rainfall on Saturday. The model run to run consistency to maintain the Moderate risk outlook area. Bann Previous Discussion... The inherited Moderate Risk area was shifted to the southeast a few rows of counties in Oklahoma with this update. This is in response to the latest guidance suggesting the heaviest rainfall will fall more across eastern Oklahoma through the Ozarks of western Missouri. A big factor as to the flooding potential will be rainfall from the Day 2/Saturday period, which alone is expected to result in a widespread 3-5 inches of rain across the Moderate Risk area. When added to the 2-4 inches of rain expected Sunday for many of the same areas, even the antecedent drought conditions in place now should not be able to handle two consecutive days of heavy rain. The primary point of uncertainty is how much overlap there will be between the two days. The slow moving nature of the entire system should mean much of the Moderate Risk area will be in the overlap. Urban and flood sensitive areas are likely to have the greatest risk of impacts from training storms, with the strongest storms and heaviest rains expected Sunday night. The storms will continue north and east across northern Missouri and into Iowa and Illinois, where the Slight Risk was extended based partially on prior days' rainfall. Regardless, outside of the Moderate Risk area, antecedent dry conditions will have a bigger mitigating impact on the flooding. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5kmUC1SiAotJQqvC9m6TUNm7Jf4ADeIfkR2nhhQM7Ju2= l4G2TBzZuHq_bBaqLU-8qxmrkUzUcjfhtOsvvgzzFUbXPMc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5kmUC1SiAotJQqvC9m6TUNm7Jf4ADeIfkR2nhhQM7Ju2= l4G2TBzZuHq_bBaqLU-8qxmrkUzUcjfhtOsvvgzziP_rrww$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5kmUC1SiAotJQqvC9m6TUNm7Jf4ADeIfkR2nhhQM7Ju2= l4G2TBzZuHq_bBaqLU-8qxmrkUzUcjfhtOsvvgzz8U9ole4$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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