Subj : HVYRAIN: Excess Rain D2-3 To : All From : Mike Powell Date : Thu Aug 14 2025 08:52:52 FOUS30 KWBC 140754 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 354 AM EDT Thu Aug 14 2025 Day 2 Valid 12Z Fri Aug 15 2025 - 12Z Sat Aug 16 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF MINNESOTA AND WISCONSIN... ....Upper Mississippi Valley... A cold front will progress slowly eastward Friday across Minnesota, elongated from Wisconsin to South Dakota by the end of D2. This front will elongate and pivot more to a W/E direction as the driving mid-level flow gets squeezed between a trough over northern Canada and a bulging ridge across the Lower/Mid Mississippi Valley. As the flow becomes more zonal to align elongate the front, waves of vorticity will surge eastward within the pinching flow to track west to east along this front from the Dakotas into Wisconsin. This will produce enhanced lift into an environment that will steadily become more favorable for convection, and may feature extreme thermodynamics of PWs above 1.75 inches overlapping MUCAPE of 3000-4000 J/kg later D2. This suggests that thunderstorms will become scattered to widespread, peaking later in the day as the LLJ ramps up to 25-35 kts from the SW. This will occur concurrently with a surge of bulk shear to above 35 kts, and as warm cloud depths climb during this period as well, convection will grow upscale, potentially into one or more MCS, with rainfall rates of 1-3"/hr. Mean 0-6km winds of 15-25 kts suggest cells will move quickly eastward, but redevelopment along the front and any upwind growth on MCS boundaries could prolong this rainfall through backbuilding/training. The inherited MRGL risk was modified cosmetically and a targeted SLGT risk was added to match the highest ensemble probabilities of 3 inches which also overlap some lower FFG. ....Mid-Atlantic through portions of the Southeast... The cold front from Thursday will dissipate into a trough Friday while pushing south towards AL/GA/SC. Despite the weakening of this boundary and increasing heights aloft in the vicinity of an expanding mid-level ridge, showers and thunderstorms are again expected, although in a more scattered fashion than on Thursday. PWs will remain elevated as tropical moisture continues across the region, exceeding 2 inches in a ribbon from eastern NC through the Gulf Coast of MS/LA. This will overlap with impressive MUCAPE above 2000 J/kg during peak heating to drive aftn convection, generally of a pulse nature, across much of the region. Weak impulses are modeled within the flow, but with minimal accompanying height falls or PVA, so any organization will be tied to outflow boundaries or storm mergers within the otherwise pulse environment. Despite rainfall rates that will likely peak at 2-3"/hr thanks to the robust thermodynamics, short lifespans and a lack of organization suggests only a MRGL risk of excessive rainfall is needed. ....Southwest... The flow across the Southwest on Friday becomes increasingly pinched between an amplifying trough digging over California and a bulging ridge centered near the Central Plains. Shortwave energy and accompanying spokes of vorticity will periodically shed from this upstream trough and track SW to NE from southern AZ through the Four Corners, likely resulting in widespread convective development, especially as a MUCAPE plume surges above 1000 J/kg. It is possible that development will be slow to initiate on Friday due to remnant overnight convection Thursday night leaving cloud cover early, but most available models including the GFS-forced UA WRF indicate widespread thunderstorms from southern AZ through UT/CO and even into WY during the evening. Rainfall rates in convection will likely peak above 1"/hr across AZ where instability/PWs are higher, but should reach 0.5"-1"/hr elsewhere. While 0-6km mean winds will be progressive enough to limit individual cells from producing too much rain, sufficient bulk shear of 20-30 kts, especially beneath any shortwave impulses, will help organize convection into clusters to either lengthen rainfall duration or result in repeating rounds of storms. Where this occurs, rainfall probabilities above 1" reach as high as 30-40%, and the MRGL risk was expanded northward from inherited. ....Western Gulf Coast... Invest 98L and its accompanying potent mid- level circulation will lift northwest into northern Mexico or southern Texas Friday afternoon. To the east of this feature, accelerating low-to-mid level winds will advect a plume of tropical moisture northward onto the Texas coast, with PWs potentially exceeding 2.25 inches along much of the Texas coast. While this is only around the 90th percentile from the CFSR climatology, it will be accompanied by MUCAPE exceeding 3000 J/kg drawn inland by 850mb winds of 20-30 kts (equal to or greater than the mean 0-6km wind). Although the deterministic models feature a wide variation in QPF output Friday, and the resulting ensemble probabilities are suppressed in response, it appears the setup is favorable for at least modest inland penetration of training heavy rainfall within thunderstorms, so a MRGL risk was added for portions of the region. ....Pacific Northwest... The core of a rare August atmospheric river (AR) will move onshore WA state on Friday, leading to prolonged heavy rainfall, continuing from the landfall on Thursday and discussed in the D1 ERD. While QPF will likely be greater on Friday (potentially 2-4" and even locally greater in the higher terrain of the Olympics and northern Cascades), the threat for excessive rainfall remains somewhat uncertain. Impressive PWs of 1.25 to 1.5 inches (locally the highest ever recorded in the 5-week CFSR climatology from NAEFS) and formidable 850mb winds will help drive strong upslope ascent supporting this intense rainfall into upwind terrain features. While it is nearly certain rainfall will be heavy, there is less certainty into whether rain rates can be sufficient to cause rapid runoff atop very dry soils since MUCAPE is forecast to be negligible which should keep rain rates to around 0.5"/hr. If this rain were to fall atop recent and sensitive burn scars, instances of flash flooding could result, but at this time the threat appears to be below 5% when considering the climatology of the event as well. After coordination with WFO SEW and OTX, other than some concern about a few sensitive burn scars, agreement was made on keeping a MRGL risk out of the forecast at this time. Weiss Day 3 Valid 12Z Sat Aug 16 2025 - 12Z Sun Aug 17 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE UPPER MIDWEST INTO THE GREAT LAKES... ....Central Plains through the Great Lakes... Pinched mid-level flow will become increasingly squeezed between a trough over Ontario and a ridge expanding from the Gulf Coast. Between these two features, nearly zonal flow will force a cold front to generally align west to east and stall, providing a convergent impetus for convection. Along this front, spokes of vorticity will track west to east leading to locally enhanced ascent to support waves of low pressure, while 850mb inflow returns around the high pressure ridge and reaches 25-35 kts to draw elevated thermodynamics characterized by PWs above 1.75 inches and MUCAPE of 4000 J/kg into the boundary. This will support multiple rounds of thunderstorms through the day, peaking during the evening/Saturday night, with rainfall rates of 1-2"/hr or more at times, leading to scattered instances of flash flooding. A SLGT risk has been added for portions of this area where probabilities from the SREF/GEFS/ECENS align for more than 3" of rain. ....Southwest... Continued S/SW mid-level flow around the periphery of a ridge positioned over the Mississippi River Valley will maintain elevated moisture into the Southwest on Saturday. Guidance continues to suggest that weak shortwaves/vorticity maxima will lift northeast embedded within the flow, although the intensity of these features is generally modest, and weaker than on Friday. Additionally, although 850mb flow will remain southerly over NM, it is progged to shift a bit more to the west over AZ, lowering the overall PW anomalies and indicating the greater risk for any isolated flash flooding will shift east from prior days. MUCAPE of 500-1000 J/kg will support rainfall rates that may approach 1"/hr at times, and if these fall across more sensitive features like burn scars or urban areas, isolated instances of flash flooding could result. ....Southeast... A wave of low pressure along a weakening cold front will drift southeast away from the NC coast on Saturday. At the same time, a building ridge over the Mississippi River Valley will lead to anti-cyclonic mid-level flow out of the NW into the Southeast, setting up a situation with moist confluent flow intensifying over the region. PWs will likely remain elevated above 2" along the Gulf Coast, but be lower elsewhere as drier air tries to advect from the NE around the offshore low, and this will result in less widespread coverage of thunderstorms than prior days. While coverage of convection is expected to generally be scattered, some locally enhanced rainfall along any convergence axes is possible due to rainfall rates that will still reach 1-2"/hr at times. Where these fall across vulnerable soils from widespread convection on prior days, impacts could result, and a MRGL risk continues for portions of the area. Weiss $$ --- SBBSecho 3.28-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .