Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 24 2025 00:58:32 FOUS30 KWBC 240057 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Wed Jul 23 2025 Day 1 Valid 01Z Thu Jul 24 2025 - 12Z Thu Jul 24 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS AND UPPER MIDWEST... Changes to the Slight Risk extending from south-central NE to=20 central WI were primarily based off current radar trends and latest HRRR/RRFS runs, which are mostly aligned and match environmental conditions. A well-defined instability gradient exists across this region, but weakens a bit over WI and is the driver for heavy rainfall tonight along with an upper shortwave ejecting out of the central Plains. Additional 2-4" totals are possible where convection can remain mostly aligned with the mean layer west- southwesterly flow as hourly rainfall rates also reach near 3" and PWs approach 2". This is most likely to occur across eastern NE into northern IA and southern MN, where ongoing convection is estimated by MRMS to contain pockets of 2-3"/hr rainfall rates and slow forward propagation. 3-hr FFG in this region is generally below 3", so scattered instances of flash flooding are likely. Mostly maintained the larger Marginal Risk across the High Plains and adjacent Rockies as lingering convection maintains itself for a few additional hours. Additionally, locally heavy rain is possible early tonight across the Florida Panhandle within a very moist tropical environment as PWs exceed 2.5". Therefore, the Marginal Risk was tightened, but maintained in the Southeast as well given the potential for slow-moving redevelopment along coastal regions overnight. Snell Day 2 Valid 12Z Thu Jul 24 2025 - 12Z Fri Jul 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL PLAINS INTO THE MIDWEST... ....Central Plains to Upper Midwest... 20Z Update: No significant changes to the outlook areas for the Day 2 ERO. As the front slowly sags southward, moisture will pool along and ahead of the front. Convection is likely at the beginning of the period, with additional development expected in the afternoon. The flow supports training convection and a few repeating rounds over KS/MO may lead to some higher end totals in the 3-5" range (12Z HREF probs for those totals between 30-50%). The Slight Risk looks good and within that, some locally significant flooding is possible from northeastern KS into northern MO. ---previous discussion--- The previous forecast maintains a consistent signature for heavy rainfall in-of the Central Plains up through portions of the Midwest. The pattern is being driven by stout ridging over the Southeastern CONUS extending up into the Mississippi and Ohio Valleys with a cold front gradually gaining latitude as it pushes through the Plains and Midwest. Frontal positioning and very anomalous moisture (PWATs between 90-99th percentile according to the latest NAEFS) will aid in the persistence of heavy convection developing and training within the confines of the front leading to rainfall totals locally exceeding 3" in areas from KS up through the southwest side of Lake Michigan. There's a growing consensus within the ensemble means of the corridor of heavy precip generally aligned southwest to northeast where the front will be located, a classic signature for training given the expected mean flow to be parallel to the front for a majority of the period. 00z EC AIFS ensemble has continued to trend upward in its mean QPF output across KS into MO where the deepest moisture presence is noted as >2" PWATs are running between 80-100% in the prob fields from all three HREF/ECENS/EC AIFS Ens. from central KS up through northwestern MO. This is the area of greatest concern for highest totals due to the environmental factors at play with rates likely to top 2"/hr in the strongest convective cores during the period. Enough of a signal was present to extend the SLGT risk further southwest to account for the better probabilities for >1" within the ensemble with a continuation of the SLGT to the northeast as heavy rain is likely to occur over northern IL towards the Chicago/Milwaukee corridor. ....Central High Plains and Central and Southern Rockies... Isolated to widely scattered convection will initiate, once again across the Front Range back into the Central and Southern Rockies with localized maxima likely contributing to a few flash flood prospects in their wake. The primary ascent is still within that diffluent area ahead of the closed reflection over CA through the forecast period. Enough of a deep moisture presence and modest instability up through the Continental Divide and points east will be suitable for another day of convection with the best opportunity for flash flooding likely located in proximity of the complex topography centered over northern NM through CO and southeast WY. A MRGL risk was relatively unchanged from previous forecast highlighting the threat. ....Southeast... The presence of a slow-moving surface reflection and eastward progressing upper trough across the northern Gulf will lead to another period of heavy rainfall within the confines of the Southeast CONUS. Greatest opportunity will lie along the central Gulf coast where the proximity of the surface low and attendant surface trough could allow for short periods of convergence in-of areas along and south of I-10 from LA back into the western FL panhandle. Guidance remains inconsistent on any organized convective placement with a smattering of heavy QPF bullseyes anywhere along the Gulf coast with even some extending back up through the GA and portions of the Carolina's. With PWATs remaining up inside the 90-99th percentile along the immediate coast and suitable instability presence, the previous MRGL was maintained with general continuity. ....Northern California and Adjacent Areas... A narrow tongue of elevated moisture on the northern periphery of a closed upper reflection along the CA coast will lead to a period of convective development within the northern Sierra's through northern CA and southern OR. PWATs running ~1 standard deviation above normal and MUCAPE between 500-1000 J/kg should be sufficient for scattered convective initiation along the ridges which could easily produce local 1+" of precip in a short time. Given the favorable pattern into the terrain still dealing with remnant burn scars and complex drainage, the risk for at least isolated flash flooding still has merit. The previous MRGL risk was generally maintained with only minor adjustments. Kleebauer/Taylor Day 3 Valid 12Z Fri Jul 25 2025 - 12Z Sat Jul 26 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST AND OHIO VALLEY... ....Midwest and Ohio Valley... 20Z Update: Overall, minor adjustments to the risk areas for the Day 3 period across the region, though there remains some uncertainty in the placement of the heaviest rainfall spatially, likely dependent on how convection plays out in the next 24 to 36 hours. The consensus among the 12Z guidance is for convection to track and potentially train over portions of southern to eastern IA into north-central IL, however some scenarios are further north/northwest while others are south. ---previous discussion--- Persistence within the upper pattern and a slowly trudging cold front over the Midwest and Ohio Valley will lead to yet another period of heavy convective development across the Mid-Mississippi Valley to points east, mainly in the confines of the front. General consensus within the means on there being a more defined convective element over IL through IN and western OH on Friday into possibly early Saturday morning. A sheared mid-level vorticity maxima is forecast to meander over the region with the frontal positioning likely the focal point for convective development/maintenance. Surprisingly, there is a pretty good signal in the global deterministic for between 2-4" of rainfall in spots within the above area, but the placement of course is subject to the exact frontal positioning. The greatest moisture anomalies between 2-3 deviations above normal will likely be centered in-of the Ohio Valley with the maxima somewhere along I-80 and points north when looking at ensemble depictions and various ML. The environment will be more than capable of continuing the theme of higher rates between 2-3"/hr at times, so any training could spur flash flood concerns during any time frames of impact. First Guess Fields have a SLGT risk situated over IL into IN for the D3, aligning well with the current proxy of highest QPF being forecast in the means. This was enough to warrant a SLGT risk upgrade from the Mississippi Valley over into portions of the Ohio Valley. ....Gulf Coast... The disturbance across the northern Gulf will continue to slowly migrate to the west with an attendant surface low remaining situated just south of the west-central Gulf coast. Low center proxy will be the ultimate factor in the prospects of very heavy rainfall along the immediate Upper TX coast back into LA. So long as the low center remains far enough offshore, the greatest low- level convergence pattern and persistent area of convection could remain just off the coast to limit impacts. However, the airmass surrounding the disturbance will still be pretty robust with PWATs remaining very high (>2.3") for much of the immediate coastal plain over TX back through the central Gulf coast. The National Hurricane Center is maintaining a 10% within the latest Tropical Weather Outlook over the northern Gulf for the next 3-4 days, so any tropical characteristics from this disturbance could enhance flash flood prospects further, especially when it moves closer to the Upper TX coast. ....Northern Plains... A quick moving shortwave ejecting out of the Rockies is forecast to impact the Northern Plains on Friday night into early Saturday with scattered thunderstorms likely to roll through the Dakotas after sunset. Increased regional PWATs upwards of +2 deviations along with ample mid-level forcing will be enough for a period of enhanced heavy rainfall prospects during the time frame of impact. Deterministic output is all over the place in exact convective placement and magnitude, however signals for 2-4" of rainfall in a short period of time would be sufficient for at least low-end MRGL prospects up across parts of the Dakotas. Timing of the shortwave will be key for the threat, but there was enough consensus on their being a chance for a maintenance of the MRGL risk inherited. Kleebauer/Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_K73_dWBG14YGq7_M3dq6jH3y9D4Y0UztwL0h8wsVJAp= _jZ0hVTgDqE4c2BDzP5iYHDTZIQQhypOZqiN_IKR2UlQi3M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_K73_dWBG14YGq7_M3dq6jH3y9D4Y0UztwL0h8wsVJAp= _jZ0hVTgDqE4c2BDzP5iYHDTZIQQhypOZqiN_IKRNH1x3gI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_K73_dWBG14YGq7_M3dq6jH3y9D4Y0UztwL0h8wsVJAp= _jZ0hVTgDqE4c2BDzP5iYHDTZIQQhypOZqiN_IKRGU3FVtM$=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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