Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 25 2025 01:00:00 FOUS30 KWBC 250059 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 859 PM EDT Sun Aug 24 2025 Day 1 Valid 01Z Mon Aug 25 2025 - 12Z Mon Aug 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE SIERRA, SOUTHERN UTAH, AND THE WESTERN HIGH PLAINS... ....Intermountain West to the Central Plains... 0100Z Update...Based on the latest observational and mesoanalysis trends, along with the most recent (18Z) HREF rainfall exceedance probabilities, was able to shrink the Slight Risk across parts of=20 the Central Plains (southwest NE and western KS in particular).=20 Areas farther north/northeast of the surface frontal boundary are a bit more stable in the near-surface layer, though mid-level lapse=20 rates of 7-7.5+ C/Km is maintaining MUCAPEs of 1000-2000 J/Kg.=20 Enhanced deep-layer lift (right-entrance region upper jet forcing)=20 overnight along and north of the surface front will favor=20 additional widespread convection and maintain a more elevated risk=20 of excessive rainfall within the Slight Risk area. By the same=20 token, while the latest CAMs does indicate additional isolated max=20 rainfall amounts of 3-5" overnight, there is considerable spread=20 amongst the guidance, and in many areas, the soil moisture=20 percentiles are running 30% or lower. Therefore, expect more of a=20 "low-end" Slight outlook. ....Northern Mid-Atlantic and Pennsylvania/New York... 0100Z Update...Pared the Marginal Risk area a bit based on the latest observational (convective) trends. Mid-level lapse rates are running around 6 C/Km, which is not very steep, so with the loss of daytime heading continue to expect an overall diminishing trends=20 in both coverage and intensity. Based on the 18Z HREF=20 probabilities of 0.50-1.00+ in/hr rainfall rates (which are=20 decreasing with time this evening), expect the excessive rainfall=20 threat to cease between 04-06Z.=20 ....Western Florida... Onshore flow of moist Gulf air along the axis of maximum TWP (2-2.25") interacting with the southern end of the mid-upper level trough and nearby surface front to the north will focus/support=20 convection capable of producing additional rounds of heavy rainfall overnight across the FL Peninsula's west coast. Low-level westerly flow will be able to tap the deep-layer instability over the eastern Gulf. Meanwhile, 925-850 mb westerly flow increasing to 15-20 kts will essentially equal or exceed the mean 850-300 mb wind, which will allow Corfidi Vectors to become easterly at 5 kts or less. This scenario/setup (upwind propagation) will be favorable for cell training, as is noted by the increasing 1 and 3 hour=20 rainfall exceedance probabilities from the HREF and RRFS after 04Z. Hurley Day 2 Valid 12Z Mon Aug 25 2025 - 12Z Tue Aug 26 2025 ....THERE ARE SLIGHT RISK OF EXCESSIVE RAINFALL AREAS FOR SOUTHERN ARIZONA AND OVER PORTIONS OF THE CENTRAL ROCKIES FRONT RANGE AND THE SOUTHERN PLAINS ON MONDAY... 21Z update... Anomalous monsoonal moisture advecting northward into southern Arizona will help trigger enhanced rainfall that may increase the threat for flash flooding. In coordination with the local forecast offices, a Slight Risk area was raised for this increased threat. The latest guidance continue to focus scattered convection over the higher terrain and surrounding areas of central Colorado and northern New Mexico. A increase across north- central new Mexico was noted therefore the Slight Risk was expanded westward to cover this area. Meanwhile, guidance depicted a small eastward shift of the QPF footprint across the Plains. In turn, the Slight Risk area was adjusted to cover more of central Arkansas. Campbell Broad cyclonic flow aloft will be in place across the Plains on Monday while south to southeasterly low level flow continues to be drawn northward at low levels across the Southern Plains, with a boundary extending from northern Texas into northeast New Mexico and southern Colorado helping to focus another round of late day storms once an MCS weakens and dissipates in the morning. The airmass south of the front is expected to have PWs approaching 2 inches across the region and values potentially in the 2 to 2.25 inch range near the Red River. The overnight numerical guidance still supports the idea of two distinct regions of rainfall and saw little reason to make more than minor adjustments to the two Slight Risk areas from the previous outlook. Instability does not appear impressive at this point, but the atmosphere should support locally heavy rainfall should convection develop anywhere between Oklahoma/northern Texas westward into the plains of eastern Colorado and northeast New Mexico. The Slight Risk areas were embedded within a broader Marginal Risk area extending across much of the central and southern portion of the Great Basin and eastward to the central and southern Rockies Front Range. Once again...weak shortwave energy embedded within a broad upper leads to yet another round of late day showers and thunderstorms. Slot canyons...normally dry washes and burn scars remain most at risk for flooding. Bann Day 3 Valid 12Z Tue Aug 26 2025 - 12Z Wed Aug 27 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL AND SOUTHERN ROCKIES AS WELL AS ADJACENT PLAINS... 21Z update... The increased influx of monsoonal moisture will persist, yet shifting to the east and adjusting where the enhanced rainfall potential will setup. A Slight was raised for portions of southeast Arizona for this period. The latest guidance and WPC QPF depicted a small northwest shift of the higher QPF footprint over Colorado and New Mexico and so the Slight Risk area was altered accordingly. The Marginal Risk was reduced some from north-central Texas and increase across southern Kansas and central Oklahoma. Campbell With a sprawling upper ridge in place...the potential exists for excessive rainfall will exist as anomalously deep moisture provides an environment supportive of brief but locally intense rainfall rates from any daytime or evening storms that form. The numerical guidance showed a more coherent signal for organized heavy rainfall from southern Colorado into northern New Mexico where low level flow is transporting moisture upslope and instability peaks at around 1000 J per kg in the late day with shortwave energy moving north-/northeastward across western New Mexico. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5uoY24L0FMWxvJ_1hAIvJU1mYE4_vcs05DZt8IHTX0QV= wSZvOxSOW20Pw0uuHzsOwQrsww8ZuiMej30QbPlasAbCvAk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5uoY24L0FMWxvJ_1hAIvJU1mYE4_vcs05DZt8IHTX0QV= wSZvOxSOW20Pw0uuHzsOwQrsww8ZuiMej30QbPlagpGYsUM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5uoY24L0FMWxvJ_1hAIvJU1mYE4_vcs05DZt8IHTX0QV= wSZvOxSOW20Pw0uuHzsOwQrsww8ZuiMej30QbPlaQba_sEg$=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. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .