Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 19 2025 00:48:51 FOUS30 KWBC 190048 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 848 PM EDT Mon Aug 18 2025 Day 1 Valid 01Z Tue Aug 19 2025 - 12Z Tue Aug 19 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST AND SOUTHERN MID-ATLANTIC... ....Upper Midwest/Great Lakes... Thunderstorms across northern IL and southern WI and being supported by a circulation apparent on radar imagery entering southeast WI. They've led to numerous flash flood warnings as hourly rain amounts have reached 3" at time, which is too much for saturated soils and urban areas alike. Adjustments for the new=20 update were to remove portions of the MRGL north and west of this=20 circulation per radar reflectivity trends. Locally significant=20 flash flooding remains possible within the Slight Risk, which=20 includes Chicagoland, overnight.=20 ....Southern Mid-Atlantic & Appalachians... The Slight Risk remains for parts of eastern NC and southeast VA due to active convection in that area which has led to a couple of=20 flash flood warnings. Hourly rain amounts have approached 3" at times. The Marginal Risk was shifted from the Eastern Shore into=20 Tidewater VA per the latest HREF guidance. Concern remains for flash flooding in the NC Appalachians through tonight. Soils are also sensitive with some of the Blue Ridge and=20 Smokeys sporting near 90th percentile saturation per 0-40cm soil=20 moisture on NASA SPoRT- LIS. There is the potential for flash=20 flooding along complex terrain and along streams that historically=20 tend to rise quickly in short- duration downpours. ....Front Range through Central Plains... Series of convective complexes are about to cross the border from CO into KS, while an east-west band of convection lies ahead across KS. Hourly rain amounts as of late have reached 2.5" in spots.=20=20 Only the 18z hi- res NAM seems to be capturing this KS band=20 reasonably well. This activity should cross the Sunflower State=20 overnight within the existing Marginal Risk area. Adjustments were=20 made per the most recent HREF guidance and radar reflectivity=20 trends. ....Southwest into Southern Plains... Monsoonal moisture trapped under the ridge across NM/Southern=20 Plains has led to scattered convective development from southeast=20 AZ up through much of NM, west TX, and western OK -- where a few active flash flood warnings are in effect -- which will be slow to collapse/thin out in coverage overnight. Hourly rain amounts have=20 reached 2" at times. The threat remains for 2-3" locally in any one location remains. Roth Day 2 Valid 12Z Tue Aug 19 2025 - 12Z Wed Aug 20 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTH-CENTRAL PLAINS TO GREAT LAKES, SOUTHERN APPALACHIANS, AND SOUTHWESTERN U.S... 2000Z Update... ....Mid-Atlantic... A Marginal Risk was introduced for the Mid-Atlantic coastal areas that stretch from the I-95 corridor in central VA on east to the Atlantic coast of the DelMarVa. PWATs are likely to still hover around 2" around the Chesapeake Bay and placed close by to the nearby stationary front. MUCAPE will be limited (generally <500 J/kg) but relatively deep warm cloud layers up to 14,000ft could still support localized rainfall rates up to 3"/hr in the most intense cells. Localized flash flooding would have better chances of unfolding should these more potent cells occur over more urbanized environments or in poor drainage areas. Otherwise, minor adjustments to the inherited Marginal Risks were made based on latest QPF trends. The southern Appalachians bear watching as they could be more sensitive to potential flash flooding should the area be hit by heavy rainfall on Monday. Mullinax ---Previous Discussion--- ....Southern Plains to Great Lakes... Mid and upper level progression will undergo a wave break pattern with the eastern flank of the ridge being compressed with an overall shift westward in the ridge axis leading to energy dropping southeast out of the Great Lakes area. Widespread +1 to +2 deviation PWATs will exist in proximity of the Missouri Valley up into the Great Lakes with a cold frontal progression moving southeast out of the Northern Plains into the Central U.S. by Tuesday afternoon. Coupled low and mid-level ascent provided by migrating shortwaves and the aforementioned front will interact with the above normal moisture environment in place allowing for widespread convective development across a large chunk of the Plains up through the Ohio Valley and Great Lakes creating a broad threat for localized flash flooding. Greatest potential will likely exist over the Mid-Mississippi Valley into the western Ohio Valley given the strongest low-level convergence signature and mid-level shortwave focus in tandem for Tuesday afternoon and evening. Areal average QPF output still isn't as robust as one would look for to entice an upgraded risk, however there are some early indications from the end of the CAMs window where perhaps a targeted upgrade could occur if the trends remain favorable for the outlined area. Locally as much as 3" are indicated in that zone with some 2-3" totals littered across the Central U.S in proxy to the advancing cold front. First Guess Fields remain consistent on just a broad MRGL risk area which was general continuity from previous forecast. As a result, maintained that posture, but will monitor for the upgrade potential in the succession of updates. ....Southern Appalachians... Elevated moisture and narrow tongue of instability in the confines of the Southern Appalachians will allow for a threat of focused heavy rainfall across the western Carolinas down into GA. A quick moving mid-level perturbation dropping south out of the Ohio Valley will enhance the potential providing just enough forcing and weak shear to warrant some heavier convective outputs capable of dropping 2-3" in a short time that would generate some flash flood prospects in the vicinity of the complex terrain. Previous forecast was still on target with the latest model trends leading to relative continuity in the MRGL in place. ....Southwest... Little to no change in the expected pattern evolution across portions of the Southwest U.S. with regards to convective opportunity. Remnant moisture with PWAT anomalies around 1 standard deviation above normal will be situated over eastern AZ/much of NM on Tuesday. Considering the moist environment in place, hourly amounts between 1-2" are plausible where cells stall, backbuild, merge, or train. An isolated terrain driven flash flood risk is possible in the diurnal convection, especially over the Sangre de Cristos, Sacramento's, and eastern Mogollon Rim. Considering run to run variance being limited over the past 24 hrs., there was no reason to deviate much from the inherited forecast. The MRGL risk was generally maintained with some minor adjustments on the northern periphery to account for QPF enhancement in-of the Sangre de Cristos. Kleebauer Day 3 Valid 12Z Wed Aug 20 2025 - 12Z Thu Aug 21 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN EFFECT FOR THE SOUTHERN PLAINS, MID-SOUTH, CENTRAL AND SOUTHERN APPALACHIANS, THE NORTHEAST, AND SOUTHWEST U.S... 2000Z Update... ....Northeast... Latest trends over the past 12 hours in the ECMWF, EC-AIFS, UKMET, and NAM have come in wetter over portions of the more populated corridor of the Northeast. There is discussion as to whether or not this ultimately becomes a predecessor rain event (PRE). This is largely due to Erin's proximity to the region, the placement of a 250mb jet streak over the Gulf of Maine placing its diffluent right-entrance region over the Northeast, and a nearby coastal front acting as a trigger for storms that tap into PWATs that are hovering around 2". However, the bulk of the PWAT, 700-300mb moisture field, and theta-e ridge associated with Erin reside farther south and over the Atlantic. Whether it decides to PRE or not to PRE, the facts are that anomalous PWATs and modest lift aloft will foster efficient warm- rain processes over a highly populated section of the country that contains plenty of hydrophobic surfaces. There is also an increasing signal in AI guidance and ensembles as well. At the moment, the lack of instability and PWATs mainly associated with the D1 Midwest system are making this flash flood setup more localized in nature. However, these trends have only just started in some members of guidance. This setup will continue to be closely monitored over the next 24-48 hours. Mullinax ---Previous Discussion--- ....Southern Plains to Appalachians... Slow migrating cold front will become the focal point for convective development over the course of Wednesday into early Thursday leading to widely scattered thunderstorms capable of localized flash flooding. Signal is really diffuse overall, but environmental conditions are still relevant for pockets of heavy rain that could cause some flash flood concerns in more urbanized corridors of the Southern Plains to the southern half of the Appalachian front. The best potential likely exists in the mountains where complex terrain and convergent flow will aid in upslope forcing capable of slow-moving cell motions and heavy rainfall over smaller foci (mountain ridge tops). A low-end MRGL exists over a broad area with potential for a more concrete area of interest pending how the broad synoptic pattern evolves and any focal points within the mesoscale (remnant outflows). ....Southwest... Monsoonal moisture presence and general diurnal instability will aid in traditional afternoon and evening convective output across east and southeast AZ into west-central NM on Wednesday. Signals for heavy precip are very isolated leading to the threat lying just inside the MRGL risk threshold. There is a chance the areal coverage shrinks or the threat goes away altogether, but model output maintains the prospects at this juncture, so maintained continuity from previous forecast. ....Northern Mid Atlantic... As of this update, there is no risk area in place across Central PA into New York State, however will be monitoring the pattern progression of a shortwave ejecting out of Ontario by the end of D2 into D3 that could entice a targeted risk in these two specific zones. Models are truly split on the threat, and with antecedent conditions running very dry over the areas in question, it will take guidance amping up the precip output to consider any upgrades. This has some definition within the ML guidance and EC ENS, so wanted to provide some insight on the threat despite a lack of a risk area. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-Da3svooXOlAXd3ttpotjKcB3fldCmn-9asr-vYkPE52= AQdZ0dmvgbkaJWO_5tdCdE8pyCFVpm3drlpTn3R9GlJs-M0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-Da3svooXOlAXd3ttpotjKcB3fldCmn-9asr-vYkPE52= AQdZ0dmvgbkaJWO_5tdCdE8pyCFVpm3drlpTn3R9IdomrMc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-Da3svooXOlAXd3ttpotjKcB3fldCmn-9asr-vYkPE52= AQdZ0dmvgbkaJWO_5tdCdE8pyCFVpm3drlpTn3R9CFbAp-s$=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) .