Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 04 2025 20:00:56 FOUS30 KWBC 042000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 PM EDT Thu Sep 4 2025 Day 1 Valid 16Z Thu Sep 04 2025 - 12Z Fri Sep 05 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST CALIFORNIA AND CENTRAL AND SOUTHERN ARIZONA... ....Southwest... The Slight Risk area remains in place as satellite imagery shows much of southwest AZ and the Lower CO River Valley are witnessing strong surface based heating and increasing PWATs over the region as Lorena's moisture moves northward. The 12Z Yuma, AZ sounding was loaded with moisture (1.95" PWAT) and highly unstable (MLCAPE >3,000 J/kg). PWATs throughout the Slight Risk area are above the 90th climatological percentile with isolated pockets of >90th climatological percentile PWATs located as far north as the northern Great Basin. With much of southwest AZ and the Mojave in Southern CA seeing strong surface based heating, the Slight Risk was tweaked to include more of the areas witnessing the strongest daytime heating. The Slight was scaled back subtly in southeast AZ where extensive cloud cover should limit the amount of instability aloft. Mean wind flow within the 850-300mb layer is very light (<5 kts) so storms will be capable of producing torrential downpours within a short time frame over affected areas before dissipating. Storms should stick around into Thursday evening as residual outflows trigger additional storms that should still have lingering MUCAPE to work with after sunset. The latest 12Z HREF guidance supports the idea that southwest AZ and far southern CA are more susceptible to Excessive Rainfall rates that can top 2"/hr in areas where dry washes and arroyos can be quickly inundated with water. ....South Florida... A stationary front in the low levels and an upper jet to the north providing some right entrance region ascent continues to foster a very wet pattern across South Florida today. Strong thunderstorms will have PWATs up to 2.3" to work with and as much as 2,000 J/kg of MLCAPE aloft. HRRR soundings show warm cloud layers that are at least 15,000ft deep and low-mid level RH values that surpass 85%. 12Z HREF guidance shows moderate-to-high chances (50-70%) for rainfall totals >3" today with some low-to-moderate chances (20-40%) for localized areas surpassing 5". A Marginal Risk area remains in effect for southern Florida and the Keys given the bulk of the heaviest rainfall is likely to occur in less urbanized environments, but there is the potential for some instances of flash flooding within urbanized areas that do include the Miami metro area. Mullinax Day 2 Valid 12Z Fri Sep 05 2025 - 12Z Sat Sep 06 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST ARIZONA AND SOUTHWEST NEW MEXICO.... ....Southwest... Latest guidance has come into better agreement that Lorena will steadily weaken while staying west of the Baja Peninsula. This is good news as it keeps its more tropical air-mass and potent mid- upper level vortex away from southern AZ and southern NM. That=20 said, its remnant moisture will be caught up in the mean 250-500mb=20 trough axis located over northern Baja and Southern CA and will be=20 directed across northern Mexico that is likely to bleed over into=20 far southern AZ, southern NM, and into western TX. PWATs will=20 remain at their highest in the Lower CO River Basin on east into=20 south-central AZ where PWATs will reach as high as 1.8". From=20 southeast AZ on east into southwest NM, PWATs will generally be in=20 the 1.25-1.5" range which is hovering near the 90th climatological=20 percentile. The expansive cloud cover east of the Lower CO River=20 Basin should limit instability enough to limit the intensity and=20 areal coverage of potential flash flooding. Still, some of these=20 areas are still sporting locally sensitive soils from the past=20 couple weeks of rainfall, not including what rainfall occurs today=20 (9/4). With the residual anomalous moisture from Lorena nearby,=20 opted to maintain the Slight Risk this forecast period as it will=20 not take much (1-hr FFGs generally less than 1.5") to produce flash flooding. The large Marginal risk area spans from Oregon and as far east as Colorado and New Mexico. The overall synoptic pattern continues to favor isolated to scattered thunderstorms from the Great Basin on east to the central Rockies, and elevated PWs and meager instability aloft support the potential for localized Excessive Rainfall rates. ....South Florida... The overall pattern described during the Day 1 period is likely to be a near carbon-copy look for Day 2 with locally heavy rainfall=20 between 2-4" are forecast in the period. With upper level flow=20 becoming more southerly over south Florida and the upper jet=20 orientation favoring a bit more upper level divergence, there could be a larger swath of storms across South FL and the FL Keys. While PWATs will remain within the 2.0-2.2" range, extensive cloud cover may play a role in limiting instability. Given warm cloud layers remain rather deep and Upshear Corfidi Vectors are generally <10 knots, torrential downpours within training convection over South FL is possible. The bulk of the heavier rainfall should occur over the Everglades and more rural areas, but there is a chance storms containing up to 3"/hr rainfall rates could occur within the Miami metro area. A Marginal Risk remains in effect for southern Florida and the Keys. ....Ohio Valley... There was some thought on introducing a Marginal Risk for portions of the OH Valley that stretched from eastern KY on north and east=20 through western WV, eastern OH, and southwest PA. This region has=20 been rather dry in recent weeks with NASA SPoRT-LIS showing 0-40cm=20 soil moisture percentile below 10% in some cases, and storm motions should be progressive. It is worth noting there is likely to be=20 modest instability present (MUCAPE 1,000-2,000 J/kg) and PWATs=20 closer around 1.5" Friday afternoon. Should new CAMs guidance=20 support a stronger signal for excessive rainfall rates in future=20 forecast cycles, a categorical risk upgrade to Marginal may be=20 needed. At this moment, the rainfall looks more beneficial than=20 harmful, and thus a Marginal Risk was not introduced at this time. Mullinax Day 3 Valid 12Z Sat Sep 06 2025 - 12Z Sun Sep 07 2025 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM TEXAS TO THE PACIFIC NORTHWEST, SOUTHERN FLORIDA AND NEW ENGLAND... 2030Z Update: The Marginal Risks from overnight shift remain on track and were tweaked in some areas to account for the latest changes in WPC QPF. The Northeast will have an anomalous plume of PWATs that surpass the 90th climatological percentile that produce some localized Excessive Rainfall rates, but the region has been=20 quite dry in recent weeks and storm motions should be progressive.=20 The flash flood threat is likely to be localized with the majority=20 of the rainfall in the Northeast being more beneficial than=20 harmful. Similar anomalous PWATs will be present over the Northwest and northern Great Basin, but model spread in where the heaviest=20 QPF and best instability shapes up remains unclear. Mullinax ---Previous Discussion--- ....West, Southwest and Southern Plains... During this period a frontal boundary will stretch from the Gulf westward into Texas. Some enhanced moisture left over from Lorena and instability will be present over the Southern Plains, which could allow for locally heavy rainfall to occur. A broad Marginal is in effect for the western half of Texas into the southern High Plains, Colorado, Southwest, Great Basin, Idaho and Oregon. ....Florida and New England... The frontal system over Florida and northeast in the western Atlantic will provide a focus for moisture to pool, leading to at least scattered diurnal convection each day. Excessive rainfall across Florida will likely focus across the southeast peninsula in Miami-Dade County and vicinity. The Marginal Risk up north covers most of Vermont, New Hampshire and Maine. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8n_SFui6eYeZEle8lc6TwuLUAZcl_4XzgVNafDx43yTA= _WZRJ6hLZAVTfyHzFciD50uas60elw5DRvLyv9_H2aGbk9k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8n_SFui6eYeZEle8lc6TwuLUAZcl_4XzgVNafDx43yTA= _WZRJ6hLZAVTfyHzFciD50uas60elw5DRvLyv9_H171PyII$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8n_SFui6eYeZEle8lc6TwuLUAZcl_4XzgVNafDx43yTA= _WZRJ6hLZAVTfyHzFciD50uas60elw5DRvLyv9_HUtg0PFE$=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) .