Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 15 2025 19:29:15 FOUS30 KWBC 151928 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 328 PM EDT Fri Aug 15 2025 Day 1 Valid 16Z Fri Aug 15 2025 - 12Z Sat Aug 16 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST, THE LOWER RIO GRANDE VALLEY, AND FAR SOUTHERN ARIZONA... ....16Z Outlook Update... The ongoing forecast philosophy is on track, with a few changes made to risk areas across the southern U.S. The Slight Risk area across Deep South Texas was confined to coastal areas near Brownsville and along the Rio Grande. The convective pattern near a tropical disturbance just southeast of Brownsville was exhibiting signs of relatively dry air over land inhibiting heavy rainfall potential over land areas so far this morning. Models are still insistent on an inland push of higher PW air (exceeding 2.3 inch PW) in tandem with the northwestward movement of the disturbance. This moistening, pockets of surface heating, and remnant MCV(s) in the region suggests potential for a focused band (or two) of heavy rainfall materializing across inland areas near the coast that could result in flash flooding. The Slight Risk area represents locations with the greatest potential for this scenario to play out, though additional, propagating bands of convection area also expected to propagate northward across a larger part of southeast Texas through the afternoon and evening hours. The second area of larger-scale ERO changes extend from central Mississippi through Alabama and southwestern Georgia. 12Z HREF guidance suggests the potential for scattered thunderstorms to drift/move erratically through the afternoon and early evening amid a very moist and destabilizing airmass (2+ inch PW values). Locally heavy rainfall and spots of 2 inch/hr rain rates are expected, which may impact low-lying and/or urbanized/flashy areas across the Southeast. Heavy rain/flash flood threats remain across the Mid-Atlantic, Desert Southwest, and upper Midwest through 12Z Saturday. Additional information is located in the previous discussion text below. Cook ....Previous Forecast Discussion... ....Northern Plains through Upper Midwest... Organized convective activity of a complex nature is expected today and tonight with the frontal zones over South Dakota and southern Minnesota. Ongoing activity over SD should help determine what happens today with additional activity overnight. Considerable spread in capturing this MCS activity is seen among 00Z HREF guidance with a bit of a south shift from previous runs. This warrants focusing the Slight Risk from southern MN over northern IA with western WI left in given the potential for overnight movement there. Much of this area (MN/WI) will continue to be a focus for heavy rainfall into Monday. ....Lower Rio Grande Valley through the Central Gulf Coast... AL98 redeveloped in the western Gulf overnight which is still poorly handled by CAM guidance (though the 00Z ARW and RRFS have some semblance of it). The northwestward trajectory still puts it on a path to near Brownsville today (03Z RAP suggests by 18Z) which poses a flash flood threat for the Lower Rio Grande Valley. The Slight Risk is maintained for the LRGV where there may be a low center to analyze by landfall. The Marginal risk extends up the Texas Coast and now through much of Louisiana for the tropical moisture plume shifting north with the surface trough. Please see the National Hurricane Center website at hurricanes.gov for any more details about Invest 98L. ....Southwest... Monsoonal moisture is in place across Arizona with ongoing nocturnal convection from central to southeastern portions of the state. It appears some stratus will persist into the morning and could setup differential heating boundaries, so adjustments to the flash flood threat areas are likely. An upper level vort max from northern Mexico will aid diurnal redevelopment. Given the push of above normal PW (rising to 1.5 sigma above normal) the Slight Risk area was maintained and expanded a bit based on the 00Z HREF. The eastern extent of the moisture does look to expand east of the southern Rockies, so the Marginal Risk which covers much of the rest of AZ through the Slot Canyon region of southern UT was expanded a bit to include more of central NM and far western TX. ....Mid-Atlantic through Southeast Coast and southern Appalachians... Continued broad area of elevated moisture from southeast PA to northern FL (where a Marginal Risk is in place) which is north of an upper ridge extending zonally from the FL Peninsula out into the Atlantic. Airmass storms with light steering flow make for slow- moving heavy development. 00Z HREF had a noted decrease in high rainfall probs over the southern Appalachians, so in coordination with WFO GSP, the Slight Risk was downgraded to a Marginal. ....Pacific Northwest... An extremely anomalous moisture plume associated with an atmospheric river continues to through western Washington into northwest OR through today. PW of 1.75" is 5 sigma above climatological normals, which if verified, would set both monthly records for August and threaten all time records. This incredible abundance of moisture will continue to create highly efficient rainfall through tonight, streaming primarily into the west facing slopes of the Olympics and Cascades Mountains. General shadowing is expected for the Seattle Metro, but still significant amounts of rainfall are possible in valley locations. Nearly all of western Washington is in some amount of drought, ranging from Minor to Extreme here at the end of the dry season which should limit flooding impacts. The Marginal Risk remains in effect. Jackson Day 2 Valid 12Z Sat Aug 16 2025 - 12Z Sun Aug 17 2025 Fri Aug 15 2025 - 12Z Sat Aug 16 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE UPPER MIDWEST AND SOUTHERN THROUGH SOUTHWESTERN NEW MEXICO... ....21Z Outlook Update... The ongoing forecast is generally on track,=20 and a few changes to the outlook have been made: 1) Slight Risk has been expanded to cover more area across=20 southern/southwestern New Mexico. Areas of 0.5-1.5 inch rainfall=20 totals are expected across this region, and some of this rainfall is=20 expected to occur across burn scars that have promoted extreme=20 runoff at times especially near the Sacramento Mountains and across=20 the Black Range. Locally significant impacts are possible in this=20 regime. 2) Marginal risk has been expanded northward and westward across=20 portions of western and central Texas. Most models/QPFs depict=20 relatively modest rainfall amounts across the region during the=20 forecast period. However, heavier rainfall has materialized near the=20 center of a tropical disturbance near Brownsville, and there is some=20 concern that the mid-level organization of this disturbance may hold=20 together and spawn thunderstorms over sensitive areas of the Hill=20 Country and vicinity, which remains sensitive from prior rainfall=20 over the past couple months. The flash flood threat appears to be=20 relatively conditional at this time, but will continue to be=20 closely monitored. 3) Marginal risk has been added across northern California and far=20 southern Oregon. Point forecast soundings depict heavy rain=20 potential amid 1.25+ inch PW values and mid-level instability. A few=20 thunderstorms are expected as a mid/upper wave approaches the=20 region. Some of the heavy rainfall could occur atop burn scars,=20 locally enhancing runoff potential. 4) Marginal risk has been expanded across southeastern Montana and western South Dakota. Models depict scattered convection to develop during the afternoon and evening across these areas, although relatively high cloud bases and the progressive nature of=20 convection may limit the overall flash flood threat to isolated=20 spots where training and/or cell mergers can occur. Outside of these areas, Marginal and Slight Risks remain in place across the Upper Midwest. Another round of heavy rainfall is expected especially in the Slight Risk area from a convective complex. Ground conditions should be wet from prior rainfall, and increased sensitivity to heavy rainfall should result in an appreciable flash flood threat especially after 00Z. Some uncertainty regarding specific axes of heavy rainfall preclude any Moderate risk upgrade at this time, although model and observational trends will continue to be monitored. See the previous discussion below for more information. Cook ....Upper Midwest... Continued organized heavy rainfall from MCS development on Saturday which may overlap with heavy rain areas from today over MN and possibly WI. 00Z guidance is a bit south of prior runs which warrants focusing the Slight Risk from central MN through much of WI and west of Lake Michigan. Considering this being the second day of heavy rain, this is considered a higher end Slight Risk. Depending on how much rain falls today and the updated forecasts with more CAMs available a Moderate Risk are may become warranted. ....Gulf Coast and Southeast... Trimmed the Marginal Risk from south-central TX through the Carolinas a bit south based on 00Z consensus. Uncertainty remains with the track and impacts of the moisture plume from AL98 currently over the western Gulf. The associated moisture will push northward into central Texas on Saturday, but uncertainty remains with storm organization and intensity. The moisture plume does extend as far west as the Texas Hill Country and along the Gulf Coast. ....Southwest... Monsoonal moisture will begin to push east on Saturday, expanding the potential for isolated flash flooding across much of southern New Mexico. Given the persistence of 1.5 sigma above normal PW, the Marginal Risk is expanded into northern NM and the Sacramento Mountains are placed in a targeted Slight Risk with 00Z CAM support for focused heavy rain there where highly sensitive burn scars remain. Jackson Day 3 Valid 12Z Sun Aug 17 2025 - 12Z Mon Aug 18 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE UPPER MIDWEST... ....21Z Outlook Update... Minimal changes have been made to the ongoing outlook for this update. Portions of the Upper Midwest continue to remain favorable for both 1) multiple rounds of heavy rainfall in the Days 1-2 forecast periods and 2) substantial redevelopment of heavy rainfall during the Day 3 forecast period. The prior rainfall should create critically sensitive ground conditions that readily promote excessive runoff, and a Moderate Risk may eventually be meeded across portions of Minnesota and vicinity. Too much uncertainty=20 remains regarding specific convective evolution for any upgrades in this forecast cycle, and convective development may be heavily modulated by convection over the prior 24 hours and any remnant outflows that might shift subsequent convective redevelopment.=20=20 Scattered thunderstorms remain likely across New Mexico and vicinity, although coverage may be a bit lower than in prior days - suggesting a Marginal flash flood risk. Additional heavy downpours may cause isolated flooding concerns in north/central Florida and southern Georgia as well. See the prior discussion for additional information. Cook ....Previous Discussion... ....Northern Plains through the Upper Midwest... Continued frontal boundaries along northern extent of upper ridge focused over the central CONUS allows third consecutive day for organized convective activity over southern MN with additional heavy rain threats for western WI. The Slight Risk was expanded a bit south given input from the Canadian Regional which remains north and RRFS which focuses heavy rain more toward the MN/IA border. Moisture from the atmospheric river pushing into western WA today extends over the northern Plains. Given broad area of instability, frontal forcing, and PW of 1 to 2.5 sigma above normal, a Marginal Risk remains for much of MT and along the Dakota border. ....New Mexico... Continued monsoonal moisture between flow from a trough off the West Coast and a ridge over the central US. Will need to monitor areas for upgrades, but for now a Marginal Risk remains in effect for much of NM and far west TX. ....Florida to the Central Gulf Coast... Focus of elevated PW (2 to 2.25" which is 2 sigma above normal) across northern FL through the central Gulf Coast from ridge centered over the central states. The Marginal Risk for the northern half of the FL Peninsula into southern GA was expanded west through Mobile, AL given this high moisture with further expansion possible, particularly if remnants of AL98 will aid organization of activity. Jackson Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6TdZZihUNNXnRyhO3WwSUeIzkjvlMVvqkG8JFX8IQ0y-= G7GE8oREWjI2SNUtBeATdt93NSrpk8PMFqPAjdcmdlV1iSE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6TdZZihUNNXnRyhO3WwSUeIzkjvlMVvqkG8JFX8IQ0y-= G7GE8oREWjI2SNUtBeATdt93NSrpk8PMFqPAjdcmugTGrfU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6TdZZihUNNXnRyhO3WwSUeIzkjvlMVvqkG8JFX8IQ0y-= G7GE8oREWjI2SNUtBeATdt93NSrpk8PMFqPAjdcmwa_byIU$=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) .