Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 08 2025 20:03:10 FOUS30 KWBC 082002 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 402 PM EDT Fri Aug 8 2025 Day 1 Valid 16Z Fri Aug 08 2025 - 12Z Sat Aug 09 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN MINNESOTA INTO FAR NORTHWESTERN WISCONSIN... ....16Z Update... ....Upper Midwest... In coordination with FGF/Fargo, ND, MPX/Minneapolis, MN and DLH/Duluth, MN forecast offices, the Slight Risk was trimmed with this update to portions of northern Minnesota and far northwestern Wisconsin. A progressive and weakening MCS currently moving into the Arrowhead and the western Lake Superior areas will continue weakening and dissipate over the next few hours. Another MCS is expected to develop this evening near or just east of the ND/MN border, and rapidly progress east, in very similar fashion to the behavior of the current MCS earlier this morning. The MCS is likely to take the form of a squall line, and therefore be a minimal flooding threat, despite the storms having ample (up to 3 sigma above normal) moisture to work with. PWATs will approach 2 inches in a few locations as the storms move through. The primary flooding concern is where any storms get hung up on the southern end of the line, resulting in localized training. If this does occur, then the southern portion of the Slight along the MN/WI border would have the highest flash flooding risk. Elsewhere, the surrounding Marginal largely covers the potential for isolated training or if a particularly strong storm moves over a particularly vulnerable area. ....Southeast... The Marginal Risk in the Southeast remains unchanged with this update. Rounds of storms associated with a tropical wave moving over Florida will take advantage of a very moisture-rich air mass, resulting in scattered showers and thunderstorms. The area of greatest concern will be around the Tampa-St. Pete metro, where slow-moving storms associated with a convergence boundary appear likely to impact the metro later this afternoon. This is highlighted in much of the 12Z CAMs guidance. Very high FFGs (over 3 inches in 1 hour) should mitigate the coverage and severity of any flash flooding with any storms. While the storms should be slow-moving, the cores and their mergers and interactions should hold the flooding threat even along the west coast to a Marginal, albeit a higher-end Marginal. Wegman ....Previous Discussion... ....Northern Plains through Upper Mississippi Valley... An anomalous 500 mb low will track eastward from the Montana/Canada border today with strong height falls moving into the Dakotas later today and tonight. At the surface, low pressure over South Dakota will track eastward toward Minnesota today before advancing into southern Canada overnight as a cold front becomes more defined to its south. Strong to extreme instability is forecast again with this system today, ahead of the low/front over central/southern Minnesota along with highly anomalous moisture over central and northern Minnesota (+2 to +4 standardized anomalies via 00Z GFS). The front will steadily track eastward between 00-12Z Saturday but potential will exist for high rainfall rates given the environment. An MCS from early Friday morning is expected to continue eastward into the northern half of Minnesota early in the period but with a weakening trend as the low level jet weakens and veers. While this first round of rainfall is not expected to pose much in the way of a flash flood threat prior to 18Z, high short term rates and perhaps an inch or so of rain may prime soils for a second round Friday night. A second round of thunderstorms is expected to form over eastern North Dakota by late evening with convection orienting from NNE to SSW with the advancing cold front. Numerous thunderstorms are expected ahead of this boundary for Friday night as low level forcing combines with increasing diffluence/divergence aloft ahead of a jet max east of the upper low. Should line orientation briefly match the mean steering flow, allowing for training, high rain rates of perhaps 2+ in/hr can be realized as the convective axis shifts eastward into northwestern Wisconsin by Saturday morning. A Slight Risk was introduced for the potential for flash flooding and potential for 2-4 inches in central to northern Minnesota. The Marginal Risk surrounds this region for lower confidence in excessive rainfall potential. ....Southeastern Coast into Northern and Central Florida... High moisture with precipitable water values near 2 inches (a bit higher over central/northern Florida) will be in place again today in the vicinity of a lingering stationary front that extended from offshore of the Carolinas into northern Florida. Daytime heating should allow MLCAPE values to rise into roughly the 1000-2000 J/kg range. Weak mid-level ridging over Florida will be accompanied by weak deeper layer mean flow, supportive of slow storm motions. 00Z models support the potential for some slightly stronger low level flow over the western Peninsula, northeast of an inverted trough axis in the eastern Gulf, which could increase potential for slow cell movement/backbuilding. The environment could support rain rates of 2 to 4 in/hr, but the threat appears rather isolated to widely scattered, only supporting a Marginal Risk. Otto Day 2 Valid 12Z Sat Aug 09 2025 - 12Z Sun Aug 10 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR EASTERN IOWA, SOUTHERN WISCONSIN, AND FAR NORTHWESTERN ILLINOIS... ....20Z Update... ....Upper Midwest... In coordination with DVN/Quad Cities, IA; MKX/Milwaukee, WI; ARX/LaCrosse, WI; and DMX/Des Moines, IA forecast offices, a Moderate Risk upgrade was introduced with this update for much of eastern Iowa, southern Wisconsin, and far northwestern Illinois. A cold front will move eastward into the upper Midwest from the northern Plains tonight. This front will stall out as both its parent surface low tracks northeast, and the upper level low supporting the cold air pushing the front east also retreats towards the north. This will leave the front stalled out over central Iowa northeast into far western Wisconsin. Meanwhile, abundant Gulf moisture is already in place across the upper Midwest, and will further increase overnight as the cold front both pushes it eastward, and allows the moisture to pool along the front in the form of showers and thunderstorms. The initial time the front stalls will be when it is strongest, and able to support the greatest coverage of storms. Instability will be incredibly high south and east of the front in the warm and very humid air mass, with the high resolution guidance on average showing MUCAPE values north of 4,000 J/kg. This will support explosive development of the storms as they form, with the cold pools of those storms supporting additional storm development. Guidance is in decent agreement that the storms will initiate to the north in Wisconsin, then like a chain reaction, pop down the front towards the southwest. The predominant flow will be southwesterly, parallel to the front, with Corfidi vectors perpendicular to the flow, out of the northwest. Thus, expect some training of storms towards the northeast as any lines of storms track towards the southeast. Since the forcing front will be stalling out, the steady advection of additional Gulf moisture and instability on a 25 kt southwesterly LLJ at 850 mb will support additional convective development towards the south and west, with those storms then tracking southeast. This will result in areas where there are multiple hours of heavy rain over individual areas. While the exact location of those areas is very difficult to pinpoint much before the event begins, the Moderate Risk area highlights where the greatest density of locations subject to multiple hours of heavy rain will be. Previous heavy rainfall over Iowa have led to rivers in the area already higher than normal, so this significant addition of rain will greatly increase stream, creek, and river levels all across the area. Considerable flash and urban flooding is likely, resulting from 1-3 inch per hour rainfall rates with the stronger storms, widespread rainfall totals of 4-6 inches, and isolated higher rainfall amounts possible. As is typical of these events, the greatest coverage and intensity storms are likely to occur overnight Saturday night into Sunday, as the heaviest rains have pushed into eastern Iowa. For the storms in Wisconsin, the storms will be through the day, but with a similar setup for movement, coverage, and intensity of storms.=20 Expect 2-3 rounds of storms over the heaviest impacted spots, which appears likeliest to occur over eastern and southeastern Iowa. ARIs are near 25 year recurrence intervals along the Mississippi River, and up to a 70% chance of exceeding flash flood guidance over much of eastern Iowa. The flash flood guidance numbers are likely to decrease as a result of the heavy rain on D2/Saturday. For the surrounding Slight Risk area, much of it factors in uncertainty with how far away from the main front the heaviest rain can get, and stays largely equidistant from the Moderate Risk area. That said, lesser amounts of training and storms are still expected, and could cause widely scattered instances of flash flooding. ....Southwest... The Marginal Risk across the Southwest was greatly trimmed in coordination with ABQ/Albuquerque, NM and EPZ/El Paso, TX forecast offices. The monsoon will be waning through the period, which should keep any wet thunderstorms confined to mostly southeastern Arizona on Saturday, elsewhere, abundant dry air should keep most convection as dry thunderstorms. Guidance has greatly increased the precipitation footprint across that area, which too contributed to the downgrade. ....Southeast... No changes were made to the Marginal Risk from southern South Carolina through much of the Florida Peninsula. Scattered to numerous storms are likely to combat very high FFGs in the area, resulting in isolated instances of flash flooding. The storms could organize into local clusters, but heavy rain is unlikely to remain over any one area for an extended period of time, despite the abundance of moisture (PWATs well over 2 inches), so for now the Marginal Risk remains in place with little agreement on where any embedded higher risk areas will be. ....High Plains of Colorado and Western Kansas... Widely scattered convection across eastern Colorado could pose an isolated flash flooding risk over the area on Saturday. While a few storms may initiate over the area during the afternoon, the greater coverage of storms will occur overnight Saturday night. Storm=20 motions should be fast enough to the southeast to only pose an=20 isolated flash flooding risk. Wegman Day 3 Valid 12Z Sun Aug 10 2025 - 12Z Mon Aug 11 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM FAR NORTHERN MISSOURI THROUGH MUCH OF EASTERN IOWA AND INTO SOUTHWESTERN WISCONSIN... ....20Z Update... In coordination with DVN/Quad Cities, IA; MKX/Milwaukee, WI; ARX/LaCrosse, WI; and DMX/Des Moines, IA; and EAX/Pleasant Hill, MO=20 forecast offices, a Moderate Risk upgrade was introduced with this=20 update for far northern Missouri, much of eastern Iowa, southern=20 Wisconsin, and far northwestern Illinois. This area is very similar to the Moderate Risk area from D2/Saturday, as the front forcing all of the storms that will cause the flash flooding will be ongoing across much of eastern Iowa Sunday morning, weaken during the day, with more storms moving in from the west on Sunday night. The storms are likely to develop along the same stalled out front again Sunday night, but will extend well southwest of the D2/Saturday period across much of northern Missouri and likely eastern Kansas as well. The Moderate Risk was kept to mostly Iowa for now since it will be a continuation of very heavy rainfall from the D2/Saturday period. However, expect multiple inches of rain to impact much of northern Missouri, including the Kansas City and St. Joseph metros. Since relatively little rain is expected in those metros on Saturday, the areas may take a round of storms before soils are moistened up, but even there, training and multiple rounds of storms are likely. Thus, while Kansas City and St. Joseph are currently in a higher-end Slight, it's very possible the area may need to be upgraded to a Moderate as well with future updates. Regardless, the continuation of very heavy rain in already hard hit areas will likely result in more impactful and widespread flooding impacts across Iowa, far western Illinois, and far southwestern Wisconsin on Sunday as compared with Saturday. More widespread convection is also likely across much of Kansas and into far eastern Colorado on Sunday. The storms are unlikely to result in more flooding on Sunday since the storms on Saturday should be relatively few and far between, but the area will be monitored for potential need to upgrade to a Slight Risk, extending from the current Slight across eastern Kansas. The Marginals in the Southeast and Southwest were largely unchanged. See the previous discussion below for more details on those areas. Wegman ....Previous Discussion... ....Southern Mid-Atlantic Coast to Northeastern Gulf Coast... A lingering frontal boundary over northern Florida and high pressure anchored over the northern Mid-Atlantic will maintain low level southeasterly winds into the southeastern U.S. on Sunday. Precipitable water values of 2.0 to 2.3 inches over the Southeast and Florida will again support intense tropical downpours as thunderstorms increase with daytime heating. Low level southeasterly winds and confluent axes will combine with sea breeze convergence and other mesoscale boundaries to support scattered thunderstorms with isolated flash flood potential from 2-4 in/hr rainfall rates. ....Southwest... Continued advancing of the base of an upper trough from the central Rockies into the central Plains and an elongated ridge over California will favor northwesterly mid-level flow into the Southwest. While this may reduce moisture availability across the Four Corners region, low level southerly flow is expected to be in place over the northern Gulf of California into Arizona, with westerly flow advecting low to mid-level moisture downstream toward New Mexico. Meanwhile, the front over the High Plains from Saturday night is forecast to again sink down to the south with an upslope flow regime setting up over northeastern New Mexico, helping to increase low level moisture into the region. Moisture values for much of central/southern Arizona and New Mexico are expected to be near average and monsoon thunderstorms are expected during the late afternoon and evening hours with perhaps greater coverage than previous days. Typical potential for high short term rain rates (1-2 inches in an hour or less) will set up with isolated to widely scattered flash flood potential. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_03anSBAtKxDGA7e6hGcoFk6rs3YesCrFhJt7NNX5vbo= pjH2Nmw03iBzSzVm-Kpg-NGyICVigYrTU-CaNZ_pq7xhsbA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_03anSBAtKxDGA7e6hGcoFk6rs3YesCrFhJt7NNX5vbo= pjH2Nmw03iBzSzVm-Kpg-NGyICVigYrTU-CaNZ_pMpGxv44$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_03anSBAtKxDGA7e6hGcoFk6rs3YesCrFhJt7NNX5vbo= pjH2Nmw03iBzSzVm-Kpg-NGyICVigYrTU-CaNZ_p3Dh5wN4$=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) .