Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 16 2025 20:05:34 FOUS30 KWBC 162005 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 405 PM 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. 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 convective trends over the next few hours for a possible unscheduled outlook update=20 this afternoon and 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 continue to support more concentrated activity in the Sacramento 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 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF THE UPPER MIDWEST... ....Upper Midwest...=20 A backbuilding band of thunderstorms may be ongoing at the beginning of the period (12Z Sunday), however there is considerable uncertainty. The latest hi-res model guidance suggests this would be most likely from S MN into SW WI, but there is placement uncertainty related to the mesoscale evolution and outflow boundaries over the next 12-18 hours, and uncertainty around how well convection will be able to sustain itself. Models fairly=20 consistently show warming mid-level temperatures Saturday Night in=20 the same area. This generally reduces the size of the CAPE profile=20 during a critical time of convective initiation, and could increase the strength of a cap. However, if thunderstorm activity becomes more organized, the overall pattern is favorable for training and backbuilding, with the nose of a LLJ pointed into S MN, and an instability maximum centered just to the southwest (upstream) of the most likely area for convective initiation. Organized clusters of storms training in a region of strong instability and anomalously high PWs could lead to rapid accumulation of rain in a narrow corridor. Rain rates may reach 2 inches per hour in that scenario, and thus there is a conditional threat for some significant flash flooding. Given the uncertainties expressed above, the overall categorical risk is being held at Slight for now. Additional thunderstorm development is expected late in the afternoon or early in the evening on Sunday in eastern South Dakota (or adjacent areas), and a continuation of strong instability and anomalously high PWs would support high rain rates and a continued flash flood threat overnight in the Slight Risk area. ....Elsewhere in the Country... Multiple other Marginal Risk areas have been maintained -- from Montana into the Dakotas, in portions of New Mexico and West Texas, and in portions of Florida and southeast Georgia. A Marginal Risk area was also added in the Mid Atlantic for Pennsylvania and adjacent portions of neighboring states. The common factor for all these areas is that, although there is some risk of flash flooding, any higher rainfall rates and thunderstorm activity should be relatively brief, and therefore=20 impacts should be relatively isolated.=20 Lamers Day 3 Valid 12Z Mon Aug 18 2025 - 12Z Tue Aug 19 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR SOUTHERN NEW MEXICO AND FAR WEST TEXAS, AND THE GREAT LAKES... Confidence is lower in concentrated areas of excessive rainfall=20 and flash flooding for the Day 3 period, and thus the previous Marginal Risk areas have largely been maintained. The Western Great Lakes and the New Mexico and West Texas regions both are projected to have precipitable water values near or above the 90th percentile, and abundant instability. Therefore, even though the details are unclear, the environment in each area is broadly supportive of organized convection with high rain rates above 1 inch per hour at times. A future Slight Risk upgrade seems most likely in the Great Lakes region due to much stronger low-mid level inflow, supported by anomalously high integrated vapor transport (above the 90th percentile based on the ECMWF ensemble mean). Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7EpgOjSwZr7vKWZE2Yam_asPJo25JVuD7q0dJDKM3z_r= sCaFChdzoASzYkqGuvjo4uSJ2D7rNKXyeQ47YC1w_Q2BC6E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7EpgOjSwZr7vKWZE2Yam_asPJo25JVuD7q0dJDKM3z_r= sCaFChdzoASzYkqGuvjo4uSJ2D7rNKXyeQ47YC1wMWl_2qs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7EpgOjSwZr7vKWZE2Yam_asPJo25JVuD7q0dJDKM3z_r= sCaFChdzoASzYkqGuvjo4uSJ2D7rNKXyeQ47YC1wMaHMXD0$=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) .