Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 29 2025 00:46:25 FOUS30 KWBC 290045 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 845 PM EDT Mon Jul 28 2025 Day 1 Valid 01Z Tue Jul 29 2025 - 12Z Tue Jul 29 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY.. ....Northern Plains to Upper MS Valley... Convection is in the process of growing upscale into an MCS as it=20 moves across central SD into southern MN and northern IA this evening. A forward propagating MCS (likely to become a derecho) will consolidate and move eastward, where a Slight Risk was maintained. Very impressive instability (SBCAPE of 4000-7500 J/kg)=20 and tropospheric moisture (PWs around 2", near the daily max per OAX sounding climatology) downstream of the MCS will support very=20 efficient, heavy rainfall rates with hourly accumulations up to=20 2-3" locally. As the MCS matures (highly likely, given 30-50 kts of deep layer shear and derecho composite parameter of 6-8) the=20 heaviest rainfall totals will likely end up near the developing=20 comma head/bookend vortex where heavy rainfall duration will be=20 locally longer. The 18z HREF suggests the potential for additional=20 totals (12-hr totals through 12z) in excess of 5" (per 40-km=20 neighborhood probs of ~10%) with a narrow axis of over 3" possible=20 as well (30-50%). The portion of west-central MN into south-central MN where this outcome is most likely is fairly rural and judged to be a higher-end Slight Risk from a probability perspective (as=20 6-hr FFGs suggest some of the more vulnerable terrain across this=20 area with values generally near 2.0").=20 ....Southwest... At least an isolated flash flood threat will continue into the=20 early evening hours with impressive convective coverage ongoing=20 over southern NM and into some surrounding portions of central NM,=20 West TX, and far southwest AZ. Convective coverage should gradually wane into the night with the loss of daytime heating and=20 increasing CIN, but in the meantime highly anomalous moisture (per PWs of 1.25"+ being the 90th percentile or higher locally) will=20 support efficient downpours and a resulting flash flood threat=20 (particularly for burn scars).=20 ....OH Valley into the Carolinas... Isolated to scattered thunderstorms to continue this evening across a broad area stretching from KY/IN/OH to the coastal Carolinas.=20 While overall convective organization is limited with declining coverage and intensity into the late overnight hours, PWs of=20 around 2" (or higher) will continue to support heavy rainfall=20 rates (locally as high as 1-3"/hr) within a broad Marginal risk=20 area. The most concentrated flash flood threat in the near term exists across eastern OH and surrounding (see MPD #820 for more information).=20 Churchill Day 2 Valid 12Z Tue Jul 29 2025 - 12Z Wed Jul 30 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL AND NORTHERN PLAINS AND FOR PORTIONS OF THE SOUTHWEST... ....Central and Northern Plains... 21Z update... The trend of shifting the QPF footprint further east across the central U.S. persisted with the most recent run of guidance. Portions of southern Iowa, northwest Missouri and northern Kansas saw an uptick in coverage and amounts for this period. As such the Slight Risk area was expanded on its eastern boundary to cover this part of the region. Hourly rain rates of 1-2 inches/hour still expected to pass through the Plains toward the Midwest. Campbell A stationary front will likely extend across southern IA/NE and into the High Plains on Tuesday afternoon/evening. Easterly flow to the north of this front will likely help trigger convective development over the High Plains as shortwave energy moves out of the Rockies and upper level divergence increases in the right entrance region of an upper jet. This activity will likely push east into the overnight hours across NE and eventually IA along/north of the stationary front. Upscale development into an MCS appears probable driving a heavy rainfall threat. Any eventual MCS will probably propagate off to the east fairly quickly, however additional development downstream over NE/IA is possible given the convergence near the front and what should be CAPE around 4000 j/kg. PWs should be plenty high enough for heavy rainfall, ranging from 1.5" in the High Plains to locally over 2" from eastern NE into IA. There remains some latitudinal uncertainty with the axis of convection during this period...although the general trend has been for a slightly farther south axis. There is also some question as to how far east the risk exists, with the deterministic and ensemble EC and the 00z RRFS/REFS favoring NE, but the GFS and AIFS extending the threat into IA. Given a favorable track record for the AIFS and the presence of the boundary/instability/PWs over IA, we did want to make sure that the Slight risk at least covered into central IA. ....Southwest... 21Z update... The signal for heavy rainfall increased for portions of New Mexico and far southwest Arizona for this period. Much of this part of the region is sensitive to higher rainfall intensities/amounts. In coordination with the local forecast offices concern was raised for not only recent burn scars but also the Sangre de Cristo Mountains and possibly the Gila region of southwest NM. A Slight Risk was raised for this period to cover this increased threat for local flash flooding potential. Campbell Another day of scattered afternoon convection expected Tuesday. Probabilities from both the 00z HREF and REFS show a bit more persistence of high 1"/hr exceedance over the Sacramento Mountains and vicinity. Thus the flash flood risk over these areas, and the susceptible burn scars, may very well be higher Tuesday than Monday. However there are some questions regarding the coverage of heavier rainfall amounts away from these terrain areas, as the overall coverage could be a bit less than Monday. Given the small extent of the higher risk at the moment and the uncertainty on the broader coverage, we will leave the ERO at a Marginal level pending what happens Monday. Although locally significant impacts are possible within/near the Sacramento Mountain burn scars. ....Southeast... Expecting greater convective coverage on Tuesday from the Southeast into the southern Appalachians. Not much in the way of forcing to organize convection, and generally expecting pulse activity that does not last all that long at any one location. This will act as a limiting factor for flash flooding...however there should be enough convective coverage that some cell mergers along outflows will be a possibility. PWs in the region are forecast between 2" and 2.3", with CAPE over 2000 j/kg...so the thermodynamic environment will support heavy rainfall rates. Thus even with a relatively short duration of rain at any one location, rainfall totals locally over 3" appear probable driving a localized flash flood risk. Chenard Day 3 Valid 12Z Wed Jul 30 2025 - 12Z Thu Jul 31 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDDLE MISSISSIPPI VALLEY AND FOR PORTIONS OF THE FRONT RANGE AND ADJACENT HIGH PLAINS... ....Mid MS Valley... 21Z update... The area highlighted for the Slight Risk area remains in good order, only minor reshaping of the boundary made to reflect the latest WPC QPF and model trends. Campbell At 12z Wednesday it seems likely that we will have some degree of organized convection ongoing in the vicinity of IA, although at this lead time getting these specifics right is tricky. Nonetheless, it appears probable that a better defined shortwave/MCV will be associated with this convection and slowly move east across IA/MO/IL and vicinity Wednesday into Wednesday night. Meanwhile we will have a slowly southward shifting cold front helping focus convergence, and the right entrance region of the upper jet adding some divergence aloft. With plentiful CAPE near the front and PWs locally over 2", the ingredients for areas of excessive rainfall are in place. Some uncertainty remains on the details...but 00z models are coming into a better consensus on the favored axis for heavy rainfall..generally stretching across portions of IA, northern MO and into central/northern IL and possibly portions of IN. ....Southwest and High Plains... 21Z update... There is a growing signal for convection to fire up along the Front Range and the adjacent high plains. The nature of coverage still seems to be scattered however guidance has consensus of 1 to 2 inches for areal maximums with isolated locations potentially reaching 3 inches or higher. In coordination with the local forecast offices, opted to raise a Slight Risk for eastern Colorado, southeast Wyoming, western Kansas, small portions of the Oklahoma and Texas Panhandles, and northeast New Mexico. Campbell Isolated to scattered convection will likely develop near areas of terrain from NM into the Rockies. Generally not looking like the greatest forcing, so the degree of convective organization remains unclear. At the moment most models indicate mostly isolated/scattered development, and generally not enough coverage for anything more than a Marginal risk. However there is some potential for a few organized clusters to move off the terrain into the High Plains, and so can not rule a targeted Slight risk or two as we get closer in time and have more high res guidance to interrogate. ....Elsewhere... A broad Marginal risk extends from the central Gulf Coast all the way into portions of New England. High PWs and CAPE and convective coverage will drive the risk from the Gulf Coast into the TN valley. A lack of organization may keep FFG exceedance coverage down, but high rainfall rates will likely locally approach or exceed FFG. A cold front will drive the risk form portions of the OH valley into New England. Generally quick cell motions and the lack of a significant QPF footprint in the 00z models keep this risk at a Marginal level. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7uKUyAFCqjELOeZAxXB6HZgKE9uvfMQLNYq3ywZWyE8D= eT9e1VILQ3GHjlLqCWeAJgBMuNt4CgyIFJURy1BYHKgN_QI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7uKUyAFCqjELOeZAxXB6HZgKE9uvfMQLNYq3ywZWyE8D= eT9e1VILQ3GHjlLqCWeAJgBMuNt4CgyIFJURy1BYL2gubGU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7uKUyAFCqjELOeZAxXB6HZgKE9uvfMQLNYq3ywZWyE8D= eT9e1VILQ3GHjlLqCWeAJgBMuNt4CgyIFJURy1BYHqlyG4Y$=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) .