Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 31 2025 16:01:10 FOUS30 KWBC 311600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Thu Jul 31 2025 Day 1 Valid 16Z 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... 16Z Update... Forecast remains on track for numerous instances of flash flooding to impact parts of the Mid-Atlantic, including the I-95 corridor between northern VA and southern CT, some of which could be significant and occur during the afternoon/evening commute. The Moderate Risk was expanded in the 16Z update to include the=20 remainder of southern NJ and more of central DE after evaluating=20 12z CAMs. A >120kt jet streak located across southeast Canada=20 places the Northeast and Mid- Atlantic within the favorable right-=20 entrance region of the upper jet, while a stationary front at the=20 surface was analyzed this morning to stretch from coastal southern=20 New England through southern PA. With forcing in place to trigger=20 thunderstorm development, moisture and instability are also=20 sufficient. SPC's mesoanalysis highlights PWs widely over 1.7" and=20 over 2.0" into northern VA and central MD. This equates to near the 90th climatological percentile. Potentially even more impressive=20 is the available instability this morning. SBCAPE values have risen above 4,000 J/kg throughout northern VA, central MD, and into=20 southeast PA and with southerly 850mb flow feeding this instability even further. A very tight gradient exists along the front with=20 very low instability north of the Lower Hudson Bay and southern New England. However, this tight instability gradient may also be the=20 focus for repeating cells capable of containing torrential=20 rainfall given the mean layer flow parallel to this gradient and=20 out of the west- southwest at 20-30kts. After initial convection that has already developed as of 1530Z this morning across eastern PA into northeast MD, the expectation is backbuilding should occur in a west-southwest to east-northeast orientation and continue through the evening as better upper=20 forcing and associated surface low will sweep through tonight.=20 This could lead to some areas within the MDT falling between maxes=20 and not seeing much rain at all, but areas within these heavier=20 axes could see localized totals up to 5-8". 12z HREF neighborhood=20 probabilities for >3" in 6 hours are very high and widely above 50% between central/southern MD and NJ. Overall, the highest HREF signal for greater than 5" totals exist across central NJ and near parts of MD near the Chesapeake Bay. Additionally, 1 hour=20 probabilities for >2" are also impressive and with scattered values between 40-60%, highlighting the rainfall rates associated with=20 efficient rainfall producing thunderstorms. Should these rainfall=20 rates (2-3"/hr) occur over urbanized locations, the large=20 concentration of pavement and impervious surfaces will likely lead=20 to extremely rapid runoff and potentially significant flash=20 flooding. Even outside of the major urban centers, these rainfall=20 amounts could lead to widespread instances of flash flooding=20 through tonight. Snell Previous Discussion... A potent vorticity max paralleled with an exceptionally strong=20 upper level jet streak over southeast Canada and highly anomalous=20 moisture content will spin up a surface low along a frontal=20 boundary over the northern Mid- Atlantic and spread enhanced=20 rainfall to a vast portion of the East. There are two areas of=20 concern regarding heavy rainfall. The first area is from northeast=20 Pennsylvania on east through the Lower Hudson Valley and into=20 southern New England will have a band of heavy and efficient=20 rainfall. PWATs will rise to the 1.8-2.0" range and low-level WAA=20 ahead of the front will give rise to potential training. There is=20 uncertainty regarding the amount of available instability, which=20 could reach as high as 500 J/kg. Still, the modeled soundings=20 depict warm cloud layers up to 13,000ft deep, so while rates may=20 not be as high as farther to the south, warm rain processes could=20 still provide 1-1.5"/hr rates in these areas Thursday afternoon and into Thursday night. A Slight Risk remains is in place for these=20 regions with locally significant flash flooding possible,=20 particularly for areas closer to the Poconos, higher elevations of=20 southern New England. Farther south, while the Mid-Atlantic region is a little farther=20 removed from the jet-streak dynamics to the north, the region is=20 closer to the deepening surface low with strong surface-925mb=20 southerly theta-e advection directed into the surface front. From=20 northern NJ and southern PA on south to northern VA, a tropical=20 air-mass (depicted by 2-2.25" PWATs and mid 70s dew points) will=20 coincide within a warm sector that becomes increasingly unstable=20 (MUCAPE between 1,000-2,000 J/kg). It is worth noting that SST=20 anomalies along the Mid-Atlantic and Southeast coasts are well=20 above normal too, acting as an abnormally rich moisture source for=20 southerly low-level flow to draw from. By this afternoon, low-=20 level WAA into the front will support storms enveloping the region=20 with additional storms from the Appalachians tracking east. Modest=20 vertical wind shear aloft will also support organized clusters of=20 storms, some of which will have the potential to backbuild and=20 train over the I-95 corridor into Thursday night. Latest 12Z HREF=20 guidance shows 24-hr probabilities for >3" of rainfall are high=20 (>70%) from the DC/Baltimore area on north through northeast MD,=20 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 and into Thursday night. ....Gulf Coast into Ohio Valley... Slow moving thunderstorms will persist today and tonight over a vast area south of the cold front. 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. This region is covered by a Marginal Risk for excessive rainfall and isolated flash flooding. ....Rockies into the Plains... Scattered thunderstorms expected to fire up from eastern Arizona/New Mexico northward to Montana. In general, the storms will likely lack organization for a more elevated threat for flash flooding. The Marginal Risk was expanded to include more of WA/OR up towards the eastern Cascades as an upper low lifts across the region and intersects with PW values above the 75th climatological percentile. Snell/Mullinax/Campbell Day 2 Valid 12Z Fri Aug 01 2025 - 12Z Sat Aug 02 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CAROLINAS... ....Southeast and Mid-Atlantic... During this period a cold front will be dropping south through the region which will 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. Much of the area will have PW values nearing 2.5 inches along with abundant instability. 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. ....Lower MS Valley and Vicinity... Convection will fire along the cold front as is continues to drop south during this period. Pooled PW values exceeding 2 inches will aid in the development and enhancement of heavy rainfall across the region. 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... Scattered convection is expected to span from Montana to New Mexico into Friday. The most favorable environment for heavy rainfall will likely setup from Montana into the Dakotas, which is where mid level troughing will be paired 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 localized flash flooding. Hourly rainfall rates up to 1.25-1.5 inches/hour possible over the Northern Rockies/Montana and up to 2 inches/hour across the Dakotas. Campbell/Chenard Day 3 Valid 12Z Sat Aug 02 2025 - 12Z Sun Aug 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST... ....South and Southeast... The strong front mentioned will continue pressing southward, draped from the Southeast westward across the Deep South, Lower Mississippi Valley and over Texas. High moisture content and instability along and ahead of the front interacting with the right entrance region of the upper jet will promote heavy to excessive rainfall in thunderstorms that could cause localized flash flooding. Very heavy rainfall rates are expected and should drive an isolated to scattered flash flood risk. A Slight Risk remains in effect for portions of South Carolina, southern Georgia and extreme northeast Florida. The front stretching west across the Gulf Coast states in a moist and unstable environment could promote scattered diurnal thunderstorms into Saturday thus maintaining the Marginal Risk. ....Rockies and Plains... Shortwaves rounding the eastern edge of the upper ridge in the western U.S. is forecast to interact with the same front that bends back across the Great Plains, where rounds of rain and thunderstorms are likely with possibly multiple MCSs. High rain rates should be supported at times with the above normal moisture and instability with some areas possibly seeing repeated rounds of storms as the front begins to lift as a warm front. The region is covered by a Marginal Risk. Some lingering monsoonal moisture will keep convection possible around southeastern New Mexico resulting in isolated flooding concerns over sensitive burn scars. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_P10bbXZXpcYW2vjnwIC5XcxUgTDniVnvRCd0Dv2UJd9= vVSq8dN8_IrWqZrq3JhQpxqXXsDAB4fTUK0g-uhuCxnLxr4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_P10bbXZXpcYW2vjnwIC5XcxUgTDniVnvRCd0Dv2UJd9= vVSq8dN8_IrWqZrq3JhQpxqXXsDAB4fTUK0g-uhuBl_nKjU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_P10bbXZXpcYW2vjnwIC5XcxUgTDniVnvRCd0Dv2UJd9= vVSq8dN8_IrWqZrq3JhQpxqXXsDAB4fTUK0g-uhucvw7VAc$=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) .