Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 09 2025 07:55:51 FOUS30 KWBC 090755 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 355 AM EDT Sat Aug 9 2025 Day 1 Valid 12Z Sat Aug 09 2025 - 12Z Sun Aug 10 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM FAR NORTHERN MISSOURI THROUGH MUCH OF EASTERN IOWA AND INTO SOUTHWESTERN WISCONSIN... ....Upper Midwest... The synoptic setup for heavy rainfall is becoming increasingly=20 favorable as a near-textbook Maddox Frontal Pattern develops across the Upper Midwest. This will likely result in numerous instances=20 of flash flooding from northern Missouri through southern Wisconsin. The period begins with a closed 500mb low moving across southern=20 Manitoba/Saskatchewan, which will drive an elongated trough=20 southeast into the Central Plains and Upper Midwest with=20 accompanying height falls. Embedded shortwaves shedding around the=20 primary closed low will help keep the trough axis aligned SW to NE=20 from the Four Corners into the Upper Midwest, and this is a=20 critical piece to the evolving setup. As the trough lingers back to the west, the parent upper jet streak rotating into central Canada will maintain a tail back into the Central Plains, leaving=20 favorable RRQ diffluence atop the region, and aligned with the=20 greatest height falls. Since the trough will move little during=20 this time, this pronounced synoptic support will be quasi-=20 stationary, resulting in prolonged lift across the region. At the surface, a cold front will try to push east, but become=20 generally stationary from MN to KS as the upper trough lingers to=20 the west. This will enable a plume of elevated moisture to=20 continually surge northeast ahead of the front, as PWs of 2 to 2.2=20 inches (above the 97th percentile according to NAEFS) get drawn=20 northward on 850mb inflow of 20-30 kts, pushing an impressive theta- e ridge axis northward into IA. This anomalous moisture will=20 combine with MUCAPE of 2000 J/kg or more to supply extreme=20 thermodynamics into the region, supporting heavy rainfall rates for which both the HREF and REFS indicates has a 30-40% chance of=20 exceeding 2"/hr, and the HRRR (as far out as it goes) suggesting as much as 1"/15min (4"/hr rates) at times.=20 The concern for Saturday, especially within the inherited MDT risk=20 area, then, is that multiple rounds of convection are likely as weak=20 impulses track aloft and 850-300mb winds remain parallel to the=20 front. At the same time, Corfidi vectors become increasingly aligned=20 anti-parallel to this mean wind, indicating a high potential for=20 backbuilding and training, which will support total rainfall above=20 5" (40-60% chance), with locally as much as 8" possible (10-15%=20 chance). However, there is still considerable uncertainty into the=20 exact placement as rounds of convection, especially through any=20 clusters/MCS, drive the effective boundary (merged outflow with=20 front) north and south at times. This could result in a slow=20 southern push of the heaviest rainfall through the day, although=20 the northward advance of the theta-e ridge at times should again=20 push heavy rain northward as well. With a lot of spread in place,=20 the HREF EAS probabilities (which are extremely significant for=20 2"/24hrs at above 35%) were used to help tailor the MDT risk area,=20 which aligns quite well with the latest ECMWF EFI above 0.7. Despite the uncertainty in exact placement, the MDT risk remains=20 needed for the likelihood of training/repeating intense rainfall=20 rates of 1-3+"/hr leading to FFG exceedance probabilities that=20 peak above 40%. The MDT risk was expanded slightly to the SW to=20 account for the latest high res CAMs as well, and numerous=20 instances of flash flooding are likely. ....Southeast...=20 Another active day of showers and thunderstorms is expected across Florida and the coastal Southeast. The driving factor for this=20 widespread convection will be a trough of low pressure centered=20 over the eastern Gulf, with a secondary surface stationary front=20 draped along the Panhandle of FL to off the SC/GA coast. This results in generally weak flow within a col in the vicinity of a=20 mid-level ridge, with modest onshore flow from the Atlantic=20 becoming subtly accelerated by the trough over the Gulf.=20 Additionally, weak impulses rotating around the Gulf trough will=20 push W/NW through the period, providing locally enhanced ascent=20 across the area. Although ascent will be generally modest outside of any impulses or=20 sea breeze/frontal convergence boundaries, the environment will be=20 favorable for periods of heavy rainfall. PWs above 2.25 inches, or=20 above the 90th percentile according to NAEFS, will encompass the=20 region, and overlap with MUCAPE of 1000-2000 J/kg to support robust=20 thermodynamics for heavy rainfall. Any ascent working into this=20 environment will support convection with rainfall rates of 2-3"/hr. Cells that develop will likely move very slowly W/NW on 0-6km mean winds of just 5-10 kts, and as propagation vectors become=20 increasingly aligned back into the higher moisture and anti- parallel to the mean wind, backbuilding and repeating cells will=20 become likely. This could be enhanced by regeneration along sea=20 breeze boundaries or storm mergers, lengthening the duration of=20 these heavy rain rates.=20 Although identifying where the heaviest rainfall will occur today is=20 challenging due to the weakly forced environment and slow-moving=20 storms, the HREF neighborhood probabilities support generally two=20 areas of heavier rainfall. The first is along the Gulf Coast of the=20 FL Peninsula where the W/NW inflow may allow for a more focused=20 region of heavy rainfall as the sea breeze becomes pinned and cells=20 moving westward merge with it. The other region is along the SC/GA=20 coasts where a weak impulse moving onshore within modest WAA will=20 help deepen warm cloud depths and focus heavier rainfall along the=20 frictional land/sea convergence boundary. In both of these areas,=20 HREF probabilities for 5"/24hrs peak above 50%, and this could occur=20 atop primed soils from heavy rainfall on Friday. After coordination with ILM/CHS/JAX, a targeted SLGT risk has been added for the=20 coastal plain of GA/SC where the HREF signal is most impressive and rounds of storms effectively converging along the coast could=20 produce repeating rounds of heavy rainfall. ....Southwest...=20 A mid-level ridge centered across southern NM will gradually=20 weaken today in response to an impressive trough amplifying across=20 the Northern Rockies and a weak mid-level impulse rotating westward from Texas beneath it. This feature is progged to track along the=20 international border before gradually rotating northwest late in=20 the period, and locally enhanced ascent in the vicinity of this=20 impulse will drive renewed and focused convection. As this=20 shortwave tracks W/NW, it will impinge into a narrow corridor of=20 elevated PWs above 1.25 inches aligned with a similar narrow=20 channel of MUCAPE above 1000 J/kg. Together, this will provide an=20 environment that will support thunderstorms with heavy rainfall as=20 reflected by simulated reflectivity from the available CAMs=20 including the HRRR-forced and GFS-forced UA WRF.=20 Storm motions beneath this weakening ridge will be quite weak and=20 variable, as expected, with 0-6km mean wind vectors showing a=20 variety of directions and speeds of just 5-10 kts. With bulk shear=20 remaining minimal, this indicates that pulse-type storms with=20 generally limited lifespans will be the primary storm mode, and in=20 the weakly forced environment this will allow for outflows and storm=20 mergers to result in additional, and at times more intense,=20 redevelopment. Rainfall rates could exceed 1"/hr at times (20-40%=20 chance) which through mergers and slow motions could produce 1-2" of=20 rain with locally higher amounts. Briefly considered a small SLGT=20 risk over the area of greatest instability near Tucson, but wide=20 variation in coverage among the various CAMs combined with recent=20 dryness across the area precluded the confidence for an upgrade at=20 this time. ....Central High Plains...=20 The tail end of a front stretching from the Upper Midwest will=20 linger across the Central High Plains today. As a trough upstream=20 deepens over the Northern Rockies and sheds spokes of vorticity=20 eastward, these will interact with the lingering front to produce=20 scattered showers and thunderstorms late this afternoon through=20 tonight. Impressive bulk shear of 35-50 kts combining with moist=20 E/NE 850mb flow will result in slow moving thunderstorms organizing into clusters, with repeated development likely upstream. Coverage of storms may be isolated to scattered according to high-res CAMs, but the slow motion of any cell that develops combined with=20 anticipated rain rates above 1"/hr may produce isolated flash=20 flooding. Weiss Day 2 Valid 12Z Sun Aug 10 2025 - 12Z Mon Aug 11 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM EASTERN KANSAS NORTHEAST THROUGH SOUTHERN WISCONSIN... ....Central Plains through the Upper Midwest... An extremely active pattern continues across the Plains and Upper Midwest as the synoptic regime from D1 persists through the weekend.=20 An amplifying mid-level trough centered from ND through NM will deepen to produce 500mb heights that may locally fall below the 10th percentile according to NAEFS. ALthough the primary vorticity maxima in the core of the trough spins northeast into Ontario, lingering impulses shedding back into the Four Corners maintain the trough axis back to the west. This will persist mid-level SW flow angling across the Central Plains and into the Upper Midwest, with weak impulses rotating through the flow and persistent upper diffluence within the RRQ of a poleward arcing jet streak downstream of the primary trough axis providing deep layer ascent. This pattern matches the conceptual model for a Maddox Frontal rainfall event, and another day of heavy rainfall is likely Sunday in portions of the region. As the synoptic pattern stalls, a surface stationary front will oscillate from WI through OK, providing a focus for convective generation along the low-level convergent boundary. Additionally, any weak impulse transiting the front could result in weak low- pressure development along the boundary. This makes for a challenging QPF and ERO forecast since the environment will be extremely favorable for training heavy rainfall, but the exact placement remains very much uncertain. As far as the environment, thermodynamics are expected to be=20 exceptional for the area supporting intense rainfall rates. PWs above 2", or well above the 90th percentile, are progged to surge northward on return flow ahead of the front, combining with MUCAPE of 2000 J/kg to fuel widespread convective development as the=20 850mb inflow/LLJ propagates northeast. Mean 0-6km winds will again=20 be parallel to the boundary, although with wind speeds are expected to be generally more progressive than on Saturday, reaching 15-20=20 kts. However, bulk shear progged to reach 40 kts will support=20 organized cells, and Corfidi vectors collapsed to just 5 kts and=20 aligned against the mean wind indicate a high threat for=20 backbuilding cells leading to longer duration of heavy rainfall and slow net cell motions. With rainfall rates of 2-3"/hr expected,=20 this could result in axes of 3-5 inches of rain with locally higher amounts. This will likely result in scattered to numerous flash=20 flood instances, with locally significant impacts possible should=20 heavy rain train across urban areas. There remains uncertainty into the placement of the heaviest rainfall, and the inherited MDT risk was adjusted cosmetically for the recent guidance to align both with the highest REFS/SREF exceedance probabilities while also accounting for the expected heavy rainfall footprint from D1. Additionally, some adjustments were made to better reflect the CSU first guess field which pulled the higher risk areas a bit SW from previous runs. ....Southwest... A MRGL risk remains for portions of the Southwest on Sunday as monsoonal moisture begins to return to the area leading to a more active convective pattern. A mid-level trough extending down from the Northern High Plains will continue to amplify Sunday, producing lowered heights across the Four Corners and Desert Southwest, through which weak spokes of vorticity will rotate. These lowered=20 heights and locally enhanced ascent will impinge into an=20 increasingly favorable thermodynamic environment to support=20 convection with heavy rainfall. Low-to-mid level flow will=20 gradually back more to the SW, allowing for a better draw northward of PWs exceeding 1", which may spread as far north as the Mogollon Rim and combine with MUCAPE of 500-1000 J/kg across the southern=20 half of AZ and NM. This will support scattered to widespread=20 showers and thunderstorms which will move slowly eastward through=20 the day. With rainfall rates as much as 1"/hr at times, this could=20 result in at least isolated instances of flash flooding, especially across more vulnerable soils or terrain, and the MRGL risk was=20 maintained and adjusted for new guidance. ....Southeast... Another very active day across portions of the Southeast is expected as tropical moisture (PWs above 2.25 inches) persists in response to a wave of low pressure drifting across the eastern Gulf. The mid-level pattern remains quite diffuse into Sunday, with a col sitting over much of the Southeast in response to broad ridging along the Atlantic Seaboard. This suggests that features will move little during the period, and guidance indicates a weak shortwave drifting along the GA/SC coast once again on Sunday. 850mb inflow from the southeast will maintain impressive moist advection onshore, and this wind, while modest at 15-20 kts, will generally exceed the mean 0-6km winds which will be just 5-10 kts. This will provide a two-pronged effect on the heavy rainfall threat in that the 850mb wind exceeding the mean wind will drive large scale enhanced ascent, while the weak mean winds will additionally support slow movement of convection that develops. With rainfall rates likely peaking at 2-3"/hr (30% chance for 2"/hr from the HREF), maximum rainfall of 3-5" with locally higher amounts is=20 again possible on Sunday as reflected by SREF/GEFS/HREF/REFS probabilities. Some of this may fall atop soils that are sensitive from prior heavy rainfall on Friday and Saturday, leading to a locally higher flash flood risk. The latest UFVS CSU First guess field has a SLGT risk over coastal SC, which overlaps almost identically with the SLGT risk on D1 and the highest REFS/HREF/GEFS probabilities into D2 (when available). For this reason a targeted SLGT risk was again added, primarily for the SC coastal plain. Weiss Day 3 Valid 12Z Mon Aug 11 2025 - 12Z Tue Aug 12 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHERN MISSOURI THROUGH NORTHERN ILLINOIS AND SOUTHERN WISCONSIN... ....Central Plains through the Great Lakes... The amplifying trough across the High Plains and the accompanying cold front producing the heavy rainfall across the Plains/Upper Midwest Saturday and Sunday will gradually pivot eastward on D3 /Monday/. Despite this eastward translation, pinched 850-500mb flow between this trough and a ridge to the east will maintain efficient moist advection out of the Gulf, albeit with PW anomalies lower than forecast for the weekend. This suggests that another day of training convection is likely along the front from Oklahoma=20 through Michigan, but with somewhat reduced intensity. Rainfall=20 rates will likely still peak above 1"/hr at times, which through=20 training could produce 1-3 of rainfall (ECENS/GEFS probabilities=20 for 3+ inches below 10%), and where this rain overlaps with heavy=20 rain from the weekend (most likely on the western edge of the SLGT=20 risk area), scattered instances of flash flooding are possible.=20 Otherwise, any cells training along the front could produce=20 isolated responses and a broad MRGL risk draped along the length of the front into the southern High Plains continues. ....Southeast... Rinse and repeat for the Southeast, especially in the vicinity of South Carolina, Georgia, and the Florida Panhandle, as onshore=20 return flow around an elongated high over the Atlantic spreads=20 tropical moisture onshore. PWs are again progged to be 2-2.25 inches with overlapping MUCAPE of 1000-2000 J/kg. Within these impressive thermodynamics, forcing for ascent will be provided both by land/sea convergence axes but also downstream of two=20 shortwaves: one moving near the SC coast and a second rotating=20 northward from the Gulf into the FL Panhandle. Both of these mid- level features may reflect to the surface as inverted troughs,=20 providing additional convergence for slow moving thunderstorms=20 containing rainfall rates of 1-3"/hr. There is quite a bit of=20 spread among the global ensembles by D3 as to how far inland=20 convection will track, and as to whether there will be any focused=20 areas of heavier rainfall. For these reasons a broad MRGL risk=20 remains across the area for at least isolated flash flood=20 potential, especially since soils will be primed from heavy=20 rainfall anticipated on D1 and D2. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5sXTOmCMFKwfCj-OXSIAOq3z_V3-Ipxk4x-qzGNrqUTK= rhoyE7ohHE_noUyvOKkSq72sTD2oM6-Lm5rvximRgWg4lqE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5sXTOmCMFKwfCj-OXSIAOq3z_V3-Ipxk4x-qzGNrqUTK= rhoyE7ohHE_noUyvOKkSq72sTD2oM6-Lm5rvximR6cuG1YU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5sXTOmCMFKwfCj-OXSIAOq3z_V3-Ipxk4x-qzGNrqUTK= rhoyE7ohHE_noUyvOKkSq72sTD2oM6-Lm5rvximRSM2Pt2Y$=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. 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