Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 07 2025 16:04:24 FOUS30 KWBC 071604 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1204 PM EDT Mon Jul 7 2025 Day 1 Valid 16Z Mon Jul 07 2025 - 12Z Tue Jul 08 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE HILL COUNTRY OF TEXAS... ....South Central Texas... After coordination with WFOs San Antonio and San Angelo (Texas),=20 this special update was created to add a targeted MDT risk for=20 portions of the Texas Hill Country. Although morning CAMs (pre-12Z=20 runs) are struggling to resolve the current activity, there is=20 enough signal, despite timing and placement differences, in the=20 guidance that concern is elevated for a MDT risk. A 700mb trough aligned SW to NE will string out across central Texas through today and remain nearly in place through the evening. Into this trough, impressive convergence will focus as return flow emerges out of the Gulf while NW flow concurrently progresses from the Southern Rockies. Not only will this force ascent, but examination of CIRA ALPW percentiles reveals that elevated PW in the low levels (sfc-850mb above the 95th percentile) shifting=20 NW while PWs in the 700-500mb layer also above the 95th percentile=20 drop SE. This will force intense moisture confluence into this=20 trough axis, and the result is very weak 0-6km mean winds of just 5 kts with anti-parallel Corfidi vectors of 10-15 kts. This suggests that as convection blossoms during the next few hours storms=20 should move slowly and generally drift W/SW across the Hill=20 Country. This results in both HREF and REFS probabilities that=20 reach 60% and 30%, respectively (06Z runs) for 5" of rainfall=20 through the evening, despite uncertainty in the individual CAMs. Rainfall already this morning has been expanding with radar- estimated rainfall rates above 3"/hr, and as instability climbs this aftn, expect these rain rates to continue as PWs remain above 2 inches. These slow moving storms with these intense rain rates will additionally move atop extremely vulnerable soils due to 72-hr rainfall that has been 5-15", causing catastrophic flash flooding over the weekend and resulting in saturated 0-10cm RSM from NASA SPoRT and FFG below 2"/3hrs. Any storms that move across this extremely vulnerable region will rapidly cause flash flooding, some of which could be significant if cells repeat or stall over the already hard-hit regions. ....Mid-Atlantic... Post Tropical Cyclone Chantal, currently centered over southeast=20 Virginia will gradually accelerate northeastward this afternoon as=20 it gets picked up by an approaching longwave trough from the west.=20 Despite the slow acceleration, significant rainfall is still likely along and north of the track which could result in flash flooding=20 through tonight. As Chantal lifts northeast, the accompanying tropical environment will advect with it. PWs measured via 12Z U/A soundings were near daily records at both IAD and OKX, with deep saturation noted through and above 500mb. This will support extremely efficient rain rates in the vicinity of Chantal today, with 2+"/hr rates probable (>50%) along and east of the track of this weakening depression. As instability climbs this afternoon, convective coverage near Chantal will expand as reflected by the available CAMs, and there may be two areas of maximum rainfall today. The first will be along the track of Chantal where the efficient rain rates driven by warm rain collision processes will train through weak Corfidi vectors and 850mb inflow that may exceed the otherwise weak mean 0-6km winds. Additionally, there is likely to be an axis of heavy rain extending northeast from Chantal where onshore S/SE flow impinges into a surface trough driving strong mass convergence and moisture confluence. Storms along this boundary will likely regenerate and train to the northeast, providing additional focus for an axis of heavy rainfall. With rain rates expected to peak above 2"/hr, there is likely to be a corridor of rainfall today exceeding 3" (60%+ chance from both HREF and REFS) with locally above 5" possible as reflected by 20-30% probabilities and 20-hr PMMs. The greatest risk for the heaviest rainfall appears to be focused from the DelMarVa northeast through NJ and southern Upstate NY. The slight risk was adjusted cosmetically across this area, and locally some higher-end slight risk type impacts are possible, especially for eastern MD/DE this aftn, where training along the surface trough will be followed by the center of Chantal, enhancing the rainfall and flash flood risk. ....Northeast back into the Ozarks... Broad 500mb trough extending from New England back into the Mid- South will push a cold front steadily southeast today before stalling tonight. This front is progged to reach as far as eastern Maine before D2, angling SW through the eastern Ohio Valley and Mid-Mississippi Valley. Along this front, a weak shortwave will lift northeast along the low-level baroclinic gradient, with at least modest upper diffluence from the distant tail of an upper jet streak also providing ascent. The result of this setup will be broad but locally impressive deep layer lift from Arkansas through Maine, leading to scattered to numerous thunderstorms developing during the afternoon/evening. These thunderstorms will blossom within a corridor of robust thermodynamics characterized by PWs of 1.75 to 2 inches, or as much as +2.5 sigma according to GEFS standardized anomalies, overlapping MUCAPE of 1000-2000 J/kg. This will support rain rates that have a high chance (40-70%) of exceeding 1"/hr, with locally higher than 2"/hr also possible.=20 There remains some concern about the coverage of convection today=20 as the CAMs are generally modest and inconsistent. However, the=20 overlap of impressive deep layer lift into the strong PW/CAPE=20 should support at least scattered, if not numerous thunderstorms=20 today. With rain rates likely reaching 1-2"/hr and training=20 expected along the front, this could result in instances of flash=20 flooding anywhere along the boundary. There appears to be a locally higher risk from eastern Ohio into Upstate New York where training may be more impressive and bulk shear reaches 25 kts to support more organization than along other portions of the front. This is also where the HREF and REFS 3"/24hr and 5"/24 hr probabilities peak, so the inherited SLGT risk was just adjusted cosmetically across this area. There is also the potential for some higher rainfall over the Ozarks where the CAMs suggest the potential for some more organized convection tonight on the tail end of the front as it stalls over northern AR. This could be collocated with a weak shortwave moving through the flow and the increasing 850mb LLJ which, while remaining of modest intensity 15-20 kts, could lead to some impressive convergence to enhance and temporally extend rainfall. At this time, confidence is not high enough for an upgrade, but this area will need to be monitored for a possible SLGT risk later today/tonight. ....Northern Plains and Upper Midwest to the Central Plains... A mid-level trough axis will push across the Northern Plains this evening, reaching the Upper Midwest by the end of D1. This will drive a surface cold front south and a wave of low pressure/surface trough east at the same time. Additional ascent will be provided by a modest jet streak pivoting across the northern Great Lakes, but with well aligned RRQ diffluence atop the eastward translating trough axis. This ascent will impinge into a moistening column where 850mb inflow from the S/SW reaches 15-20 kts, driving PWs to above 1.5", or around +1 sigma above the climatological mean.=20 Convection that arises from this evolution will have the potential to produce rainfall rates above 1"/hr, with organization into=20 clusters likely as 0-6km bulk shear rises to 30-40 kts. 0-6km mean winds of 15-25 kts aligned with rapid Corfidi vectors which are generally perpendicular to the boundary suggest training will be minimal and the fast motion should limit the overall flash flood risk. However, locally enhanced training along any outflows or cluster boundaries could cause isolated heavier rainfall exceeding 3 inches in a few areas, with a locally higher potential for this across parts of Nebraska. However, at this time confidence is not high enough for any upgrades as these higher probabilities in general lay atop drier soils with higher infiltration capacity. Still, these intense rain rates could cause at least isolated impacts anywhere within the MRGL risk area today and tonight. ....Southwest and Southern Rockies/High Plains... Isolated to scattered showers and thunderstorms will develop around the periphery of a mid-level ridge today, blossoming across CO and NM before shifting southeast through the day. Although coverage is not expected to be widespread, any showers and thunderstorms that develop could produce briefly heavy rain rates above 0.5-1"/hr, with motions initially slow during development, especially across higher terrain features. Eventually, storms should move more progressively S/SE off the terrain, limiting the overall flash flood risk. However, should any slow moving storm (during initiation) or faster cell with intense rain rates move atop sensitive terrain or vulnerable burn scars, isolated instances of flash flooding could result. Weiss Day 2 Valid 12Z Tue Jul 08 2025 - 12Z Wed Jul 09 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE NORTHEAST AND MID ATLANTIC THROUGH THE TENNESSEE VALLEY, AND FROM THE UPPER GREAT LAKES TO THE CENTRAL AND SOUTHERN PLAINS, AND ACROSS PORTIONS OF THE SOUTHWEST, SOUTHERN ROCKIES, AND HIGH PLAINS... ....Northeast through the Tennessee Valley... Deep moisture ahead of a slow-moving front will continue to provide fuel for training storms and potentially heavy amounts as it slips further south across the Northeast and the Mid Atlantic. Further to the west the front is expected to return north ahead of a shortwave moving into the lower Ohio Valley. A broad Marginal Risk was maintained for now, however embedded upgrades to a Slight Risk may be forthcoming in future issuances with the arrival of new guidance. This may include portions of the Mid Atlantic to coastal New England. The airmass will remain quite moist (PWs 1.75-2 inches), with some of the guidance indicating an uptick in southerly low level inflow and moisture across the region. Away from the front, the Marginal Risk was extended further south to include portions of the eastern Carolinas. A lot of the guidance shows a low level trough becoming the focus for deeper moisture and afternoon-developing, slow-moving storms. This may include some of the areas impacted by heavy rains associated with Chantal. ....Upper Great Lakes to the Southern and Central Plains... A mid-to-upper level shortwave will move out of the Mississippi Valley and into the Great Lakes and lower Ohio Valley this period. This will drive its associated frontal boundary and preceding plume of deeper moisture further south and east across the region. Similar to areas further east, a broad Marginal Risk was maintained, recognizing that embedded Slight Risk area(s) may be forthcoming if confidence increases as newer guidance arrives. One potential area centers over the Ozark Region into the mid Mississippi Valley, where deeper moisture and the ascent provided by a mid level shortwave moving through the base of the broader scale trough may generate more widespread heavier amounts. .......Southwest and Southern Rockies/High Plains... Similar to the previous day, another daily round of showers and thunderstorms are expected to develop and move east from the central New Mexico ranges into the High Plains. Some isolated to scattered storms are expected to develop further southwest across southeastern Arizona as well. Isolated areas of flash flooding will remain a concern, especially across burn scar, complex terrain, and poor drainage areas. Pereira Day 3 Valid 12Z Wed Jul 09 2025 - 12Z Thu Jul 10 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE MID ATLANTIC BACK THROUGH THE LOWER MISSISSIPPI VALLEY INTO EASTERN TEXAS... An elongated axis of deep moisture will provide the corridor for showers and storms, capable of producing locally heavy rainfall. A well-defined shortwave moving out of Ohio Valley may provide the focus for organized heavier amounts as it moves through the Mid Atlantic on Wednesday. An upgrade beyond a Marginal may be forthcoming if the models begin to show greater agreement and/or back-to-back days of heavy amounts. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-vuAU1Tz6l7dmQod_khvzpmv6rPSYn8tdeois29VpbUB= yZuZEsf0uZfUBGNgLo4kJe-_Yk-Khx91446bBWz5mDmYVQ0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-vuAU1Tz6l7dmQod_khvzpmv6rPSYn8tdeois29VpbUB= yZuZEsf0uZfUBGNgLo4kJe-_Yk-Khx91446bBWz58EmG-F8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-vuAU1Tz6l7dmQod_khvzpmv6rPSYn8tdeois29VpbUB= yZuZEsf0uZfUBGNgLo4kJe-_Yk-Khx91446bBWz5YlbbcOo$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .