Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 31 2025 00:16:53 FOUS30 KWBC 310015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 815 PM EDT Wed Jul 30 2025 Day 1 Valid 01Z Thu Jul 31 2025 - 12Z Thu Jul 31 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE HIGH PLAINS, CENTRAL PLAINS, & MIDWEST... ....Mid MS Valley/Midwest/High Plains... Convection is scattered nature from portions of southern KS and=20 southern MO and portions of northwest OH and northern IL early this evening. A slowly southward shifting cold front helping focus=20 convergence, the right entrance region of the upper jet adding some divergence aloft, plentiful CAPE near the front and PWs locally=20 over 2" continues to support areas of excessive rainfall. Higher=20 end impacts are still possible in this setup, but far from=20 guaranteed given the questions that remain regarding convective=20 mode & evolution and instability persistence. Hourly amounts to 2.5" and local 5" totals are possible where any mesocyclones form or cells manage to merge/train over a couple hour period. Changes=20 were made to the risk areas to account for the latest HREF & RRFS=20 probabilities, as well as recent radar reflectivity trends. ....Rockies and High Plains...=20 Isolated to scattered convection exists across the region late this afternoon/early this evening. Easterly low level flow into the=20 Front Range of CO will likely result in slow cell motions and the=20 possibility of at least isolated flash flooding. Increased=20 convergence near the front could support a bit more convective=20 coverage and eventual organization from portions of eastern NM into the OK and TX Panhandles. Since the Slight Risk areas here and to the east were getting close, decided to merge them. Hourly rain amounts to 2" with local totals to 4" are possible in this area before activity fades overnight. Changes were made to the risk areas to account for the latest HREF & RRFS probabilities, as well as recent radar reflectivity trends. ....Portions of the East/Southeast/Gulf Coast...=20 A broad Marginal risk extends from the central Gulf Coast all the=20 way into portions of southern NY, mostly for current convection=20 which should fade over the next several hours. An early morning=20 risk looks possible across the coastal Carolinas and the northern=20 Mid-Atlantic states as precipitable water values continue to=20 increase, and in the case of the northern Mid-Atlantic states, some uptick of the low-level inflow is forecast. A lack of organization may keep FFG exceedance coverage down, but high rainfall rates=20 will likely locally approach or exceed FFG. Convective coverage and organization should remain generally lacking outside of the=20 currently organized activity entering southwest LA. With the front=20 slowing cells should be slow moving and could drop a quick 1-2" of=20 rain. Likely enough to drive at least a localized urban flash flood risk. Roth/Chenard Day 2 Valid 12Z Thu Jul 31 2025 - 12Z Fri Aug 01 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN MID-ATLANTIC... ....Mid-Atlantic into New England... An exceptionally strong 250mb jet streak over southeast Canada is placing its diffluent right-entrance region over the Northeast on Thursday will provide act as an excellent source for thunderstorms to not only develop but flourish in an environment that is loaded with highly anomalous moisture content. A potent 500mb vorticity max tracking east will instill its own source of strong lift, prompting a surface low to develop along a frontal boundary that sets up over the northern Mid-Atlantic. There are two areas of concern regarding heavy rainfall-- Northern PA to Southern New England: The first area is from northeast PA on east through the Lower Hudson Valley and into southern New England. Here, there is likely to be a narrow corridor of 850-500mb FGEN that supports a band of heavy and efficient rainfall. PWATs will rise to the 1.8-2.0" range and low-level WAA ahead of the front will give rise to potential training. There is uncertainty regarding the amount of available instability, which could reach as high as 500 J/kg. Still, the modeled soundings depict warm cloud layers up to 13,000ft deep, so while rates may not be as high as farther to the south, warm rain processes could still provide 1-1.5"/hr rates in these areas Thursday afternoon and into Thursday night. A Slight Risk is in place for these regions with locally significant flash flooding possible, particularly for areas closer to the Poconos, higher elevations of southern New England. Northern Mid-Atlantic: Farther south, while the Mid-Atlantic region is a little farther removed from the jet-streak dynamics to the north, the region is closer to the deepening surface low with strong surface-925mb southerly theta-e advection directed into the surface front. From northern NJ and southern PA on south to northern VA, a tropical air-mass (depicted by 2-2.25" PWATs and low-mid 70s dew points) will coincide within a warm sector that becomes increasingly unstable (MUCAPE between 1,000-2,000 J/kg). It is worth noting that SST anomalies along the Mid-Atlantic and Southeast coasts are well above normal too, acting as an abnormally rich moisture source for southerly low-level flow to draw from. By Thursday afternoon, low-level WAA into the front will support storms enveloping the region with additional storms from the Appalachians tracking east. Modest vertical wind shear aloft will also support organized clusters of storms, some of which will have the potential to backbuild and train over the I-95 corridor into Thursday night. Latest 12Z HREF guidance shows 24-hr probabilities for >3" of rainfall are high (>70%) from the DC/Baltimore area on north through northeast MD, southeast PA, and into southern NJ. Just as concerning is this same region sports 30-60% chances for >5" of rainfall. Impact potential for Moderate Risk Area: These probabilities listed above serve as signals in ensemble guidance that highlight the potential for significant flash flooding throughout the country, let alone over a highly populated urban corridor. Given the atmospheric parameters in place, it is possible that the Mid- Atlantic I-95 corridor witnesses thunderstorms that produce rainfall rates up to 3"/hr, with perhaps the most intense storms potentially generating 2" rainfall totals in as little as 30 minutes. QPF may seem light at first glance given the hourly rainfall rate potential, but this is largely due to lingering uncertainty in the placement of the heaviest rainfall. Many individual convective models show localized totals surpassing 5" with even some localized maxima approaching 8". Residents in the Mid-Atlantic should ensure they have reliable means of receiving warnings issued by their local WFOs throughout the day tomorrow and into Thursday night. ....Other Risk Areas... Aside from some minor adjustments to the Marginal Risk area based on the latest WPC QPF forecast, the forecast remains on track. The Southern Rockies do bear watching, but potential Slight Risk upgrades will be also determined upon how sensitive soils are following today's thunderstorm activity. Mullinax ---Previous Discussion--- ....Gulf Coast into OH Valley... Another day of slow moving pulse thunderstorms is expected over a large swath of the region to the south of the cold front. Similar to Wednesday, high PWs and CAPE and convective coverage will drive a localized flash flood risk. A lack of organization may keep FFG exceedance coverage down, but high rainfall rates will likely locally approach or exceed FFG. ....Rockies into the Plains... Scattered convection is likely from eastern AZ into NM northward all the way into MT. Currently not seeing a strong indication of organization that would lead to Slight risk level flash flood coverage. However localized flash flooding will again be possible over this broad region. There are some height falls beginning to approach from the west, so that could perhaps end up driving a more organized risk somewhere within this broad Marginal risk. But at the moment not seeing enough of a signal in the global models or CAMs to go with any Slight risk areas. Chenard Day 3 Valid 12Z Fri Aug 01 2025 - 12Z Sat Aug 02 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CAROLINAS... Minor tweaks to the Marginal and Slight Risks areas were made based on new WPC QPF and 12Z guidance. Overall, no significant adjustments were made and the forecast remains on track. Will keep an eye on the northern Rockies and central Rockies in future forecasts should any need for Slight Risk upgrades be necessary. Mullinax ---Previous Discussion--- ....Southeast and Mid-Atlantic... A Slight risk was maintained across much of the carolinas. Likely to have a well defined cold front dropping south over NC that will help trigger a more organized corridor of convection Friday afternoon. Meanwhile both terrain driven and coastal convergence driven convection are also likely by afternoon. This combination will result in good convective coverage, which will ultimately likely lead to a decent degree of cell merger activity. PWs are forecast as high as 2.3"-2.5", with plentiful instability. Very heavy rainfall rates are expected in this thermodynamic environment, which combined with the likelihood of cell mergers, should drive an isolated to scattered flash flood risk. There is still some degree of uncertainty with the details by Friday. The 00z ECMWF is an outlier really hanging the front and better upper level support well to the north. Thus it keeps a heavy rainfall risk over NJ into NYC and less of a concentrated risk over the Carolinas. This is not supported by the GFS/UKMET/CMC...and the 00z ECMWF AIFS ensemble also looks more in line with the GFS/GEFS. Thus we will continue to lean the EROs to be more in line with the non ECMWF solution. However we will need to continue to monitor trends going forward. ....Lower MS Valley and Vicinity... A cold front should make it all the way into portions of TX/LA and MS by Friday afternoon. This should act as a focus for convective development, and with a large pool of PWs over 2", heavy rainfall rates are likely. Large scale forcing and moisture transport is weaker here, so not expecting widespread heavy rainfall...but localized heavy rainfall rates should be able to drive an isolated flash flood risk. ....Rockies and Plains... Convection is likely from NM into MT Friday. The best environment for heavy rainfall will probably be across the northern end of this risk area, centered across MT and vicinity. More in the way of mid level troughing will be over this corridor, to go along with the highest PW anomalies in the CONUS...near early August peak values. Combine this with CAPE forecast around 2000 j/kg and the environment appears conducive to areas of flash flooding. However none of the 00z global models are all that aggressive with QPF output, and the one CAM that goes this far (the 00z RRFS) suggests a more progressive convective mode. Thus we will stick with a Marginal risk and continue to monitor trends. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!96pJMKs0mp981AYIBMSpbiWPv1IbQZ4RWp-4N18P2vQx= 2djvGvz-YHQFaqzPDHiqZXmB9YweNriLZp2cC5zk-4UZw28$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!96pJMKs0mp981AYIBMSpbiWPv1IbQZ4RWp-4N18P2vQx= 2djvGvz-YHQFaqzPDHiqZXmB9YweNriLZp2cC5zkdZ0S91Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!96pJMKs0mp981AYIBMSpbiWPv1IbQZ4RWp-4N18P2vQx= 2djvGvz-YHQFaqzPDHiqZXmB9YweNriLZp2cC5zkbctcnFw$=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) .