Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 27 2024 23:25:27 FOUS30 KWBC 272325 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 725 PM EDT Sat Jul 27 2024 Day 1 Valid 01Z Sun Jul 28 2024 - 12Z Sun Jul 28 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE TEXAS GULF COAST... ....Texas Coast to Southeast US... Maintained the Slight Risk area with onshore southerly flow of=20 deep tropical moisture off the Gulf set to bring a round of=20 convection capable of producing heavy rain during the early morning hours of Sunday in an area where flash flood guidance has been=20 lowered by several days of moderate to heavy rainfall...with the=20 expectation that the convective organization should be less today=20 compared with previous days. Elsewhere, Marginal Risk areas were=20 reduced based on radar reflectivity trends and 18z HREF output. ....Portions of the West & Central Plains...=20 Excessive rainfall remains possible for several more hours before=20 CIN sets in and leads to a convective downturn after sunset.=20 The previous large Marginal Risk area was split in two based on radar reflectivity trends and 18z HREF output. ....Upper Midwest... Maintained the Marginal Risk area mainly over northern Minnesota as given the presence of increasing southwesterly low level flow feeding an airmass with precipitable water values locally in excess of 1.75 inches ahead of a progressive cold front. Areal coverage was trimmed based on radar reflectivity trends and 18z=20 HREF output. Roth Day 2 Valid 12Z Sun Jul 28 2024 - 12Z Mon Jul 29 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST AS WELL AS THE TENNESSEE VALLEY AND SOUTHERN APPALACHIANS... ....2030Z Update... ....Tennessee Valley and Southern Appalachians... The inherited Slight Risk was expanded east to include much of the flood sensitive areas of the southern Appalachians with this update. A slow-moving warm front will provide the forcing for increasing moisture advection out of the Gulf to support widespread storms for much of the Tennessee Valley. With PWATs expected to increase to around 2 inches, scattered showers and thunderstorms that develop in this area with the warm front will have ample moisture to support very heavy rain. FFGs are very low for eastern KY, TN and western VA & NC. Thus, expect even one slow moving storms to cause flash flooding, and with the potential for training, widely scattered instances of flash flooding are likely. ....Minnesota... A low ejecting out of the Plains and a stalled front over Minnesota will also take advantage of plentiful Gulf moisture over much of the nation's mid-section to result in multiple rounds of training storms Saturday night into Sunday morning, with potential for renewed convection again Sunday night. CAMs guidance has resulted in a typical south and eastward shift in the axis of heaviest rainfall away from the Dakotas but drawing southward into southern Minnesota. Thus, the Slight was adjusted accordingly. Elsewhere in the upper Midwest, while there will be less forcing than areas further north, expect widely scattered shower and thunderstorm coverage over eastern Iowa and western Illinois. Thus, the Marginal risks were combined with this update, especially considering there is still some recovery in the soil moisture from previous heavy rains in this area still to go. ....South Texas... Continued onshore flow of deep tropical moisture will continue across south Texas again on Sunday. However, expect limited forcing as the lows of previous days that had formed are not expected to be a significant factor on Sunday. The storms that form, if disorganized and widely scattered will still have plenty of Gulf moisture to work with, so isolated flash flooding is quite possible, especially if the storms hit areas still recovering from previous days' heavy rains. ....Arizona/New Mexico... The Marginal Risk in this area was extended east to once again include the Sacramento Mountains of south-central New Mexico, given the sensitivities of the soils in this area. Expect otherwise typical afternoon convection in the monsoonal moisture to develop once again, with several very sensitive areas into southern Arizona and New Mexico. The storm coverage and organization should be lesser than in previous days...thus reducing the threat of flash flooding. Wegman ....Previous Discussion... ....Northern Minnesota... Rain associated with a cold front moving across the northern tier of the country should be ongoing across northern Minnesota. While there is likely to diminish in intensity for a period early in the period...there should be a renewed risk of excessive rainfall later in the day and evening. With precipitable water values increasing to 1.75 inches or greater continuing to be fed into the region by south to southwesterly 850 mb flow with divergence aloft being provided by the right entrance region of an upper level jet...the 27/00Z HREF neighborhood probability values of 10 to 15 percent for 2+ inch amounts in an hour seem pretty reasonable. Assuming that the GFS was too fast with the forward speed of the front during the period...he risk of training cells and repeat convection are additional concerns for excessive rainfall in addition to the intense rainfall rates. ....Tennessee Valley and Surrounding Areas... A stationary front turned warm front will track northeastward up the Tennessee Valley through late Sunday night/early Sunday morning. Increasing atmospheric moisture associated with the advance of an airmass originating off the Gulf will allow for potential training and backbuilding of strong storms capable of increasingly heavy rainfall. The surface warm front will likely slow as it approaches the southern Appalachians...supporting slower moving storms that with interactions with the terrain will be capable of flash flooding. With very efficient warm rain processes at work...multiple inch per hour rainfall rates . Urban areas are at a higher risk for localized flash flooding with any repeating storms. ....Southwest US... The risk of excessive rainfall continues to get less conducive with time and gets shunted eastward as mid-level westerly flow strengthens over much of the West outside of Arizona and New Mexico. Maintained a Marginal Risk here given the lingering moisture. Bann Day 3 Valid 12Z Mon Jul 29 2024 - 12Z Tue Jul 30 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO VALLEY, SOUTHERN APPALACHIANS AND NORTHERN GEORGIA... ....2030Z Update... The Slight Risk across this area was shifted eastward largely due to guidance shifts in the axis of heaviest rainfall, which now appears to be focused along much of the central and southern Appalachians, but extending back across much middle and upper Ohio Valley. Low FFGs in much of this region due to recent heavy rainfall as well as terrain may require additional upgrades in the future, but it's possible the heaviest rains may shift eastward further with future forecasts. As such, confidence is not quite there for consideration for additional upgrades, but abundant moisture, forcing, and instability all appear to be present in great enough supply to eventually need to consider it. Elsewhere, no significant changes were made to the upper Midwest, where the rainfall should be winding down Monday morning, and again for southern Arizona and New Mexico for monsoonal thunderstorms Monday afternoon. Wegman ....Previous Discussion... ....Southeast US... Focus for excessive rainfall shifts eastward and is mainly focused in the central and eastern portion of the Tennessee Valley as mid level height falls approach from the west and precipitable water values at or above 1.75 inches will already be in place. While the height falls are associated with a trough that was beginning to fill...the diffluent flow located above an instability axis should be enough to support downpours from slow moving storms in the afternoon and evening. Elsewhere in the Southeast US...the forcing remains weak and nearly impossible to time with any accuracy at this range but localized downpours from any convection that can develop and was covered by an expansive Marginal Risk area. ....Upper Midwest to far Western Great Lakes... The same cold front helping to focus heavy to potentially excessive rainfall on Sunday/Sunday night will continue to make its way eastward on Monday. The pattern becomes somewhat less favorable for heavy to excessive rainfall but the potential for some isolated enhanced amounts linger...especially from far northern Minnesota into the western Great Lakes where a Marginal Risk was placed. ....Southwest US... Steering flow in the mid and upper levels begin to weaken as heights begin to rise while precipitable water values linger at or above an inch (even 1.25 inches in places over the desert)...meaning a slight westward expansion in the Marginal risk area over portions of Arizona and New Mexico compared with the previous couple of days. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8e_iLyBDA3_eNAlysAjiHDCZs6YTUb3ExXp2Kk1M3quz= aG5DomZZZu3Q_VNLXXfcWezckltp1LONNynADyfGPQkHbdI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8e_iLyBDA3_eNAlysAjiHDCZs6YTUb3ExXp2Kk1M3quz= aG5DomZZZu3Q_VNLXXfcWezckltp1LONNynADyfGL2VRCmQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8e_iLyBDA3_eNAlysAjiHDCZs6YTUb3ExXp2Kk1M3quz= aG5DomZZZu3Q_VNLXXfcWezckltp1LONNynADyfG_P1kxnM$=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) .