Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 14 2025 15:55:27 FOUS30 KWBC 141555 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1155 AM EDT Thu Aug 14 2025 Day 1 Valid 16Z Thu Aug 14 2025 - 12Z Fri Aug 15 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC STATES... ....16Z Update... ....Mid-Atlantic... Based on some of the latest CAMs guidance, featuring notable agreement with the development of training thunderstorms across portions of southern and eastern North Carolina, a higher end Slight is in effect from the Hampton Roads area southwest through far northeastern South Carolina. Confidence has therefore=20 increased on the potential for flooding, especially considering=20 recent heavy rainfall has saturated the soils in the area. Abundant moisture with PWATs near 2.25 inches will support highly efficient warm rain processes which will allow any storms, and especially training storms to produce multiple inch per hour rainfall rates with the heaviest cores. One caveat will be somewhat reduced visibility from cloud cover from the ongoing MCS, which may help portions of SC avoid the greatest impacts from flooding in favor of areas further northeast into NC.=20 Confidence decreases just a bit across far northeastern North Carolina and southeast Virginia, where much of the convective activity expected will develop along the James River and the storms may then progress a bit south of due east. There remains some potential that those storms' cold pools help fuel additional convective development over the more sensitive urban Hampton Roads area. ....Northeast... The Marginal Risk area was split around the NYC metro and Long Island, noting a stationary boundary persisting across eastern MA and RI, and more moisture from central NJ southward, resulting in a relative minimum of rainfall over the NYC metro. ....MS/AL... A Slight Risk upgrade was considered for this area for this afternoon's expected convection across central MS and extending into central AL. Plentiful atmospheric moisture and instability=20 will be available, however the lack of a strong enough forcing mechanism should limit any flash flooding impacts to isolated areas. Any slow moving convection along the immediate Gulf Coast could result in isolated flash flooding in any urban areas, so the Marginal Risk was expanded to include the Gulf Coast. ....Upper Midwest... The Marginal Risk remains in place across much of Minnesota and adjacent counties with this update. Expect some limited convection to develop across the state within the broader low level jet this afternoon, but any storms that develop should be fast moving. Additional storms develop across northern Minnesota into tonight, but these too should also be fast moving. Regardless, expected coverage over the coming days will be increasing, along with the flash flooding threat. ....Southwest... No major changes were made, but there is some potential for convective development over the Phoenix metro, or from outflows from likely more numerous showers and storms along the Mogollon Rim as those outflows move southwestward into the Valley. The Marginal Risk was expanded to include these lower elevation areas with this update. Wegman ....Previous Discussion... ....Southern New England through the Lower Mississippi Valley... Surface cold front draped from New England through the Ozarks will drift gradually southeast but generally stall across the region Thursday as the parent trough begins to fill, leaving gradually more zonal flow over the eastern CONUS. Although heights will gradually rise, weak shortwave impulses embedded within the flow will maintain periodic ascent atop the front to create waves of low pressure rippling northeast to produce locally enhanced lift, and scattered to numerous thunderstorms are again expected across the east on Thursday, primarily from southern New England through the Gulf Coast. Thunderstorms that develop will all have the potential to produce pockets of heavy rainfall as thermodynamics remain impressive. PWs of more than 2 inches will remain in a plume arcing within return flow around offshore high pressure from the Florida Panhandle through the northern Mid-Atlantic states. This PW will overlap with elevated MUCAPE of 1000-2000 J/kg to create the favorable thermodynamics for heavy rainfall, and the HREF indicates scattered neighborhood probabilities for 2"/hr rain rates across much of the area supporting the broad inherited MRGL risk. However, there appears to be at least a signal for some more organized convection with a greater threat for excessive rainfall across the eastern Carolinas and into Southeast Virginia. This region will benefit from additionally focused ascent through the presence of the cold front and a surface trough, upon which a more pronounced shortwave is progged by the consensus of the models to lift northeast during peak heating. This will promote at least modest bulk shear to support multi-cell clusters across this area, while most of the rest of the east should generally feature pulse type thunderstorms. Additionally, mean 0-6km winds and propagation vectors are aligned and parallel to the boundaries, suggesting an enhanced training risk as well, which additionally supports the elevated HREF and REFS probabilities for 3"/24hrs (and lower but still notable 5"/24 hrs). For this reason the inherited SLGT risk was adjusted to spread farther south into the eastern Carolinas. ....Southwest... Mid-level ridge centered over NM to start Thursday morning will weaken in response to height falls approaching from CA as a trough digs down the Pacific coast. This will have the two- pronged affect of lowering heights across the Desert Southwest/Four Corners region, while also leading to increased cyclonic flow emerging from near Baja to push a potent shortwave northward across AZ and into UT. Additionally, this increasingly south flow, especially between 850-700mb, will transport elevated PWs northward, and a plume above 1" is progged to surge almost to UT by the end of the forecast period. Where this combines with MUCAPE above 500 J/kg, slow moving storms (0-6km mean winds of just around 5 kts to the north) could become more organized leading to a longer duration of heavy rainfall rates of 0.5-1"/hr. The total coverage is still somewhat uncertain so the inherited MRGL risk still appears appropriate, but this was expanded north to match recent guidance, and convection is likely to be more widespread Thursday than it was on Wednesday. ...Upper Mississippi Valley... A warm front will lift northeast towards Minnesota today, accompanied by a surge of 850mb winds to 20-30 kts advecting anomalous PWs of 1.5 to 1.75 inches northeastward (+1 to +1.5 standard deviations above the climo mean). This front will be driven eastward by a potent but flat and fast- moving shortwave embedded within the nearly zonal flow aloft, leading to a period of enhanced ascent aided by the warm front and accompanying isentropic lift of the 850mb wind. This will result in scattered thunderstorms across the area, with bulk shear of 25-35 kts potentially supporting some organization into clusters. The guidance is highly variable in both the placement and intensity of convection today, and the QPF footprint and ensemble probabilities suffer as a result. Still, with the impressive thermodynamics in place, thunderstorms that develop could support rainfall rates of 1-2"/hr with at least short-term training possible, and the inherited MRGL risk was curtailed for latest guidance but continued. ....Pacific Northwest... A rare August atmospheric river (AR) is progged to make landfall this evening north of 45N into WA state with IVT above 500 kg/m/s as reflected by both GEFS and ECENS probabilities above 90%. A weak shortwave embedded within otherwise zonal flow aloft will help produce ascent into this plume of IVT, but instability will be negligible and jet level ascent appears to be displaced north of the core of the AR. While deterministic model QPF output and accompanying ensembles support a threat for heavy rainfall, especially in the Olympics, this will be occurring atop extremely dry soils and should generally be beneficial rainfall. Additionally, in the AR/ERO verification data set since 2019, there has never been an impactful (MRGL risk or higher) AR in the month of August. For this reason, despite some heavy rain spreading onshore this evening, no excessive rainfall impacts are expected. Weiss 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 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7ccEY7Y9vN5NNLx_y2iIoep5_MWNnK1n7qkXhLEWGxkR= 9XglO8yc4urUWKd3nKmNESGpFLGmLzG3YYcGvSq9GTzJQFU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7ccEY7Y9vN5NNLx_y2iIoep5_MWNnK1n7qkXhLEWGxkR= 9XglO8yc4urUWKd3nKmNESGpFLGmLzG3YYcGvSq9cYEsgRA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7ccEY7Y9vN5NNLx_y2iIoep5_MWNnK1n7qkXhLEWGxkR= 9XglO8yc4urUWKd3nKmNESGpFLGmLzG3YYcGvSq9trHy9ZI$=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) .