Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 16 2025 15:57:01 FOUS30 KWBC 161556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Sat Aug 16 2025 Day 1 Valid 16Z Sat Aug 16 2025 - 12Z Sun Aug 17 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST AND A SMALL PORTION OF SOUTHERN NEW MEXICO... ....Upper Midwest and Northern Plains... Small changes were made to the outlook to adjust for observational trends; the most consequential was a slight southward shift in the Slight Risk area in the Upper Midwest. The latest radar trends suggest a compact MCS in C WI will continue to progress east- southeast or southeast late this morning and early this afternoon, with potential for some training of higher rain rates on the upshear flank of the cold pool. However, greater concern for backbuilding and training convection would be for convective development expected to occur later -- generally near the instability gradient. However, hi-res model guidance (even runs initialized at 12Z) does not seem to have the best handle on the ongoing situation across N/C WI, and this poses some challenges with placing the most likely area for heavier rainfall. If the ongoing MCS continues to expand southwest with additional development on the upshear flank, and remain forward-propagating, the outflow boundary could push further south than anticipated and set up from N IL back toward NE IA. However, if the MCS remains close to its current dimensions and upstream convection struggles to become established in the next few hours, the outflow and effective front would be more likely to be situated further north, from S WI into SE MN. These differences are crucial.=20 The overall pattern does support training and backbuilding with any later rounds of convection developing along the instability gradient. The nose of the LLJ will be focused back to the west, with a large reservoir of strong instability, and a corridor of deep, anomalously high moisture in place. Although hi-res models are struggling to place things consistently at the moment, they all generally show an overnight round of convection that backbuilds in a nearly-stationary fashion, and this is consistent with the conceptual model for this type of pattern. That scenario can produce more significant impacts if realized, and so a Moderate Risk upgrade was considered. However, given the lack of consistency in guidance and uncertainties around ongoing convection, the Slight Risk was maintained for now. We will be monitoring convectiveand just east)= focused activity as well as directly=20 west, where the Slight Risk remains.trends over the next few=20 hours for a possible unscheduled outlook update this afternoon and=20 a targeted upgrade to Moderate Risk. ....South-Central Texas, Along Gulf Coast and Southeast... Very few changes were made to the outlook along the Gulf Coast region, other than to remove the Slight Risk in C TX, which was also coordinated with WFO EWX. The ongoing convection this morning is very isolated and models have backed off on the heavy rainfall signal in Texas in general. Nevertheless, the Marginal Risk was maintained in these areas. A Slight Risk was also considered from southeast Georgia into northeast Florida where 12Z HREF probabilities of 2-4 inches of rain area most elevated and concentrated. However, simulated reflectivity from the 12Z hi-res members suggests that localized rainfall maxima would be most likely through cell mergers and boundary collisions that would briefly maximize rainfall in a certain area, rather than anything lingering for several hours. Given the transient nature of the heavy rainfall, we opted to maintain the Marginal Risk for now. A Marginal Risk was also briefly considered for portions of the Mid-Atlantic from far N VA into W MD, C/NE PA, and S NY. Isolated convection has already developed and is not moving very quickly. However, 12Z soundings show a significant amount of dry air aloft and precipitable water values, although somewhat elevated, are not highly anomalous. Therefore, any heavy rainfall and resulting flash flooding would be expected to be fairly isolated. In other words, the probability is non-zero, but less than 5% (Marginal Risk). ....Southwest... Monsoonal moisture plume should focus farther east today, shifting the potential for isolated flash flooding across southeastern Arizona, and portions of southern New Mexico. CAMs=20 continue to support more concentrated activity in the Sacramento=20 Mountains, but have backed off on instability and rainfall potential in southwest New Mexico. Therefore, the Slight Risk was trimmed to be concentrated near the Sacramento Mountains specifically. Another area of focus would be the Trans-Pecos and Big Bend region of Texas, where 12Z HREF probabilities of 2 inch rainfall are higher. There is a weakness in the mid-upper level flow over this area, so developing thunderstorms have the potential to be nearly stationary. It will be monitored for a potential Slight Risk upgrade. ....Northwest and Northern Rockies... Marginal risk remains across far north-central California and much of central/eastern Oregon with an expansion across northern ID and western MT. Elevated moisture from the record high moisture atmospheric river (Salem OR had its highest PW on record at 00Z) develops into a ribbon across these areas during peak heating today which should produce isolated flash flood threats. Some of the heavy rainfall could occur atop burn scars, locally enhancing runoff potential. Lamers/Jackson Day 2 Valid 12Z Sun Aug 17 2025 - 12Z Mon Aug 18 2025 Fri Aug 15 2025 - 12Z Sat Aug 16 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF THE UPPER MIDWEST... ....Northern Plains through the Upper Midwest... Continued activity from late tonight into or through Sunday morning is expected along residual/frontal boundaries over portions of the Midwest, likely including potential repeat areas in southern MN and western WI. This activity is developing in an elevated moisture atmosphere along northern extent of upper ridge focused over the central CONUS. Guidance is quite varied across the Upper Midwest with the Canadian Regional keeping the heaviest focus over northern MN/eastern ND, the 00Z FV3LAM a bit farther south from eastern ND to northwestern WI, the 00Z RRFS over southern MN. The 06Z NAMnest and HRRR also have a southern MN focus which is where the Slight Risk remains, though the Marginal Risk was expanded north to accommodate more of the northern solutions. Given all the boundary and mesoscale interactions that will take place today and tonight, there is remaining uncertainty which will hopefully come into better focus with guidance later today. Moisture from the atmospheric river pushing over the Northwest and northern Rockies through tonight extends over the northern Plains on Sunday. 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 the Dakotas. ....New Mexico... Continued monsoonal moisture over New Mexico on Sunday 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 remains for the northern half of the FL Peninsula into southern GA and west through Mobile, AL remains in effect. Jackson Day 3 Valid 12Z Mon Aug 18 2025 - 12Z Tue Aug 19 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR NEW MEXICO AND THE GREAT LAKES... ....Great Lakes... The persistent frontal pattern over the Upper Midwest begins to move east Monday night. Until then, continued organized activity can be expected to redevelop over similar areas from recent days in Iowa/Wisconsin before shifting across Michigan. Elevated moisture rounding upper high pressure centered over the central states will continue to fuel heavy rain potential with impulses/the frontal pattern to aid lift. There is notable uncertainty with most global guidance and the Canadian Regional focusing heavy rain closer to Lake Michigan while the 00Z RRFS and the 03Z SREF is focused more in Iowa/southern Minnesota. The broad Marginal is maintained for now with notes that a more focused area of heavy rain is likely (shrink the outlook area) and if that were to be particularly heavy and/or over areas impacted from heavy rain between now and then, an upgrade would be necessary. ....New Mexico and Far West Texas... Continued monsoonal moisture advection continues to result in terrain based initiation with an isolated flash flood threat. The Marginal Risk was expanded to include most of New Mexico and a little more of Far West Texas and southeast Arizona. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4LAIMLvbEmUrSkwTbODtoSfH7FPmS4BQGSmmYG7hIxKj= pyKlD4fUvsXkY_R-cTO7Bazv6wIcHVvVlAzNrogNweN-AhE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4LAIMLvbEmUrSkwTbODtoSfH7FPmS4BQGSmmYG7hIxKj= pyKlD4fUvsXkY_R-cTO7Bazv6wIcHVvVlAzNrogNURS1iRQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4LAIMLvbEmUrSkwTbODtoSfH7FPmS4BQGSmmYG7hIxKj= pyKlD4fUvsXkY_R-cTO7Bazv6wIcHVvVlAzNrogNTl6ZlTg$=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) .