253 FOUS30 KWBC 060055 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 855 PM EDT Fri Sep 5 2025 Day 1 Valid 01Z Sat Sep 06 2025 - 12Z Sat Sep 06 2025 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE SOUTHWEST, AND PORTIONS OF SOUTH FLORIDA AS WELL AS PORTIONS OF THE TENNESSEE VALLEY/CENTRAL APPALACHIANS... 0130 UTC EXCESSIVE RAINFALL DISCUSSION... Main change across the western US was to remove Colorado and nearly all of Utah from the Marginal risk area now that stabilization has begun with the weakening of daytime heating. In Nevada...convection should linger a few more hours given being in a region of favored upper level support to the east of an upper low over the eastern North Pacific ocean so no change in forecast reasoning there. Most uncertainty was in the southern portion of the Marginal risk area given little in the way of convection. Opted to maintain the area in deference to the 12Z HREF which still attempts to generate low- end probability values for rainfall to exceed FFG during the overnight hours. Expanded the Marginal area in the Tennessee Valley southwestward given the HRRR depiction of convection becoming increasingly robust as it moves east-southeast from the Plains into Arkansas after 06/06Z. Elsewhere...the latest model runs continue to support a Marginal Risk extending northeastward into parts of the Tennessee Valley and the Central Appalachians, Despite the overall decrease of convective coverage over southern Florida in latest satellite and radar imagery...a Marginal Risk area was maintained over parts of the southwest Florida peninsula where the HREF has been maintaining a signal for additional convection to develop over the Gulf which spreads a low-end threat of 1- and 2-inch per hour rates inland. The signal elsewhere across southern Florida appears to be less than 5 percent for rainfall to exceed flash flood guidance through 06/12Z...and was removed. Bann PREVIOUS EXCESSIVE RAINFALL DISCUSSION... ...Southwest... Another active day of convection expected as monsoonal moisture overlaps with weak impulses lifting within broad ridging across the Southwest to produce widespread ascent within favorable thermodynamics today. Additionally, although the remnants of Lorena have dissipated over Mexico, residual elevated moisture remains as it gets funneled northeast downstream of a trough over CA. This is reflected by PWs that were measured on morning U/A soundings of 1.76" at PHX and 1.41" at VEF. While some cloud cover is noted in the GOES-E visible imagery, in general sky conditions should support rapid destabilization today, with MUCAPE potentially reaching as high as 3000 J/kg, especially in CA/NV/AZ. Weak impulses in the flow will help organize what should be widespread convection this aftn, with rain rates potentially producing 0.75"+ rainfall in less than 1 hour as supported by the HRRR and HREF neighborhood probabilities. In general, convection is expected to be scattered enough and moving fast enough (after initiation and once moving off the higher terrain) to preclude an upgrade to a SLGT risk today so the inherited MRGL risk appears to be still supported. However, there is some potential for some more widespread impacts across NV where the CAMs indicate a greater chance for some storm organization beneath a shortwave trough, and HREF FFG exceedance probabilities peak above 20% despite a subtle moderation since 00Z. After coordination with WFO VEF opted to maintain the MRGL risk at this time. ...Tennessee Valley into the Central Appalachians... A cold front draped from Illinois to Oklahoma this morning will steadily progress eastward through D1 as a shortwave impulse dropping across the Upper Midwest drives increasing height falls downstream of the parent trough. The most impressive synoptic ascent today is progged to stretch from northern TN through eastern KY and into southeast OH/western WV, and this is where weak impulses aloft will interact with the front to produce at least a weak wave of low pressure. The accompanying ascent into favorable thermodynamics (PWs approaching 2" this evening combined with MUCAPE of 2000-3000 J/kg) will likely result in widespread thunderstorms along and ahead of the front. Although storms are likely to move progressively as reflected by 0-6km mean winds of 20-25 kts, this flow will generally be parallel to the front to help incur training of echoes this evening. With the HREF neighborhood probabilities for 1"/hr rain rates reaching above 40%, any training could produce local flash flood impacts, and the MRGL risk was adjusted a bit farther northeast for faster model progressions into more sensitive soils with lower FFG. ...South Florida... Morning soundings at KEY and TBW indicate an impressive PW gradient from 2.34" (KEY) to 1.89" (TBW) reflective of a stalled out stationary front draped across the southern Florida Peninsula. This boundary has been plaguing the region for several days now, leading to 7-day rainfall that has been more than 300% of normal in some areas of the southern peninsula. As this front wavers one again today, and weak mid0-level impulses move along it to drive weak surface low development just to the east, the overlap of this ascent with the anomalous moisture will once again result in scattered to widespread thunderstorms, primarily during peak aftn/eve heating. Storms should generally drift east to west, but with weak bulk shear suggesting pulse-type convection, storm mergers and collisions are likely which will result in chaotic motion at times. As the environment will support 2-3+"/hr rain rates, locally more than 3" of rain is possible which could produce isolated instances of flash flooding, primarily should these amounts fall atop urban areas. Weiss Day 2 Valid 12Z Sat Sep 06 2025 - 12Z Sun Sep 07 2025 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE INTERMOUNTAIN WEST, FLORIDA, AND THE MID-ATLANTIC/NORTHEAST... ...Intermountain West... Monsoonal moisture will spread even farther north on Saturday, led by a modest stationary front that will waver but gradually shift into the Central Rockies by later D2. The greatest PW anomalies appear to focus along this front which will drive a better chance for thunderstorms with at least briefly heavy rainfall into the interior Northwest and Central Rockies, with continued scattered thunderstorms again likely across the Southwest. Some drier air noted in PW anomalies and 700-300mb RH fields advecting from the SW may lower coverage and intensity in the SW than on previous days, but slow moving storms with subtle organization could still produce at least isolated impacts in the Southwest and Great Basin. Farther north and east, instability is a lower (less than 500 J/kg) and storms will move faster (mean 0-6km winds 10-15 kts) but isolated impacts are still possible due to the elevated PWs, so the inherited MRGL risk was only cosmetically adjusted. ...Mid-Atlantic and Northeast... An impressive mid-level trough (500mb heights falling below the 1st percentile across the Great Lakes according to NAEFS) will swing eastward on Saturday, driving a cold front eastward. The impressive height falls accompanying this trough will push the cold front relatively quickly eastward, but provide impressive deep layer ascent, especially where the RRQ of the associated jet streak overlaps the height falls and weak impulses within the flow. This is most likely across the Mid-Atlantic states and interior Northeast states. This robust ascent will tap into favorable thermodynamics as SW flow downstream of the trough axis will draw a ribbon of elevated PWs around 1.75 inches in conjunction with MUCAPE above 1000 J/kg northward. This will result in scattered to numerous thunderstorms, with modest organization into clusters likely through bulk shear of 25-35 kts. While mean 0-6km winds of 30-40 kts will support fast moving thunderstorms, the presence of this favorable bulk shear and Corfidi vectors aligned to the mean wind indicate an elevated training potential. As rain rates surge above 1"/hr at times (30-40% chance of exceedance), this will promote repeating rounds which could produce as much as 3+" of rain in a few areas. This region has been very dry recently as reflected by 0% rainfall the past 7 days from VA to MA, and even where rainfall has occurred in Maine 0-40cm soil moisture percentiles are less than 10%. After coordination with BTV/GYX/CAR and analysis of 12Z HREF FFG exceedance, the MRGL risk was trimmed to encompass primarily just the urban areas of I-95 from VA to MA with this update. ...Florida... The stalled front will continue to drift across the Florida Peninsula Saturday, but some subtle return NW of this feature is expected as it gradually dissipates. The weakening of this feature will do little to change the overall environment and rainfall potential, however, as a weak tropical wave will persist in the vicinity, with ascent acting upon impressive PWs remaining above 2.25 inches. This suggests another day of scattered to numerous showers and thunderstorms with rainfall rates as much as 3"/hr at times, with a subtle expansion north/northwest expected on Saturday when compared to Friday. At this time the MRGL risk remains, but after several days of heavy rain, and slow moving convection atop urban areas or saturated soils from prior rains could become increasingly problematic. Weiss Day 3 Valid 12Z Sun Sep 07 2025 - 12Z Mon Sep 08 2025 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF TEXAS AND THE FLORIDA PENINSULA... ...Texas... The inherited MRGL risk was trimmed from the north to account for latest model trends and a slightly stronger trough which pushes the cold front farther south/east across Texas. This front will impinge into favorable thermodynamics characterized by PWs of 2-2.25" and MUCAPE of 1000-2000 J/kg. The biggest change to this risk area was to cut out the northern half of Texas and into the Southern Plains where instability is forecast to be quite limited, so even if convection does blossom north of the front in response to weak impulses within the NW flow, they should be less intense and not produce an excessive rain risk. Farther south, however, the elevated PWs will support rainfall rates of 2"/hr at times, which if clusters organize and move across sensitive regions, could cause impacts related to flash flooding. ...Florida Peninsula... The impressive mid-level trough centered over the Great Lakes/Ohio Valley will continue to amplify Sunday, pushing height falls deeper into the Gulf Coast and the Florida Peninsula. This will have the additional effect of driving a cold front into the central Florida Peninsula, while this trough evolution also pushes the associated jet streak farther northeast leaving just distant, but lingering, jet level diffluence across the peninsula. The environment will remain supportive to convection with heavy rain rates of 2-3"/hr, but coverage may be a but more confined to the eastern half of the peninsula than the prior 2 days. Still, since the area has been wet recently, the MRGL risk was tailored slightly but remains across most of the southern 2/3 of Florida. Weiss Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt