Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 09 2023 01:00:01 FOUS30 KWBC 090059 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 859 PM EDT Tue Aug 08 2023 Day 1 Valid 01Z Wed Aug 09 2023 - 12Z Wed Aug 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS AND THE NORTHEAST... ....Central Plains... The Slight Risk area from eastern CO through southern NE and much of KS remains largely the same with this update. Guidance (with the support of recent observational trends) remain in good agreement on an organizing MCS that picks up speed through the night as it moves across KS/NE. While there is some potential for minor backbuilding/training along the southwestern flank of the MCS, the main threat for flash flooding will likely be driven by short-term (3-hr) totals of 3-5 inches (with rainfall rates of 1-3"/hr), per the 18z HREF guidance. The spatial agreement between HREF members on the QPF axis is greater than normal (as depicted by 18z HREF EAS neighborhood probabilities for 1" exceedance of 40-60% across the Slight risk area). The best odds for 3" exceedance appear to be across the far eastern portions of the Slight risk area (east-central Kansas) through 12z (where 18z HREF neighborhood probabilities for 3" exceedance are as high as 30-40%). ....Northeast... Shower and thunderstorm activity in association with a mature, occluded low pressure system is gradually coming to an end this evening across the Northeast, but widely scattered rainfall rates of 1-2"/hr are still ongoing at this hour (per MRMS estimates) across portions of VT/NH/ME. Decided to keep a Slight risk depicted in these areas for the evening update (with a removal of the inherited Moderate risk area) to account for the wet antecedent conditions and lingering convective activity. The threat for additional flash flooding should largely subside by 06z (with the exception of far northeast ME, where rates up to 1"/hr may persist until 12z). ....Arizona... Monsoonal moisture (with PWATs of 1.0-1.5") continues to drive scattered convection across portions of southeastern AZ early this evening, with MRMS indicating localized 15-min totals of up to 1" in association with the strongest convective cores. This will continue to drive an isolated flash flood risk for at least a couple more hours, as RAP analysis indicates very high levels of instability (per SB CAPE of 2500-4500 J/kg) at 00z. Shower and thunderstorm activity should largely come to an end by 06z with waning instability, but an isolated shower or two may linger well into the night. Churchill Day 2 Valid 12Z Wed Aug 09 2023 - 12Z Thu Aug 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE MID-MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS.... ....2030Z Update... ....Slight Risk Area... No big changes were made to the giant Slight Risk area across much of the nation's mid-section. There remains fair confidence that Moderate risk level impacts may be seen in a portion of the area, particularly along the Mississippi south of St. Louis through about Memphis. Unfortunately there is poor confidence where in that large area those impacts will be felt. The broad weather pattern will feature an MCS moving across MO the Slight Risk area early in the period, likely fizzling out through the midday hours. With afternoon heating, additional storms are likely to break out across central IL and IN, with more storms developing over southern MO and AR as well. In the evening, the storms will likely organize into bowing segments, that while fast moving across the aforementioned highest risk portion of the Slight, are likely to become repeating, with multiple rounds of storms moving over the same areas in quick succession. Some of the guidance places where this will happen as far south as Memphis, while several others are closer to the Cape Girardeau and Paducah areas. Since the Paducah area has been very hard hit in recent weeks, streams are still running high, and several inches of rain falling over this area from the repeating storms are likely to cause flash flooding impacts. The area will continue to be monitored for a probable Moderate Risk upgrade. Elsewhere, the Slight and Marginal were expanded to include more of OH as the aforementioned first round of storms tracks over those areas, along with potential for the western edge where the storms form and are slowest-moving potentially occurring as far west as Kansas City. ....Mogollon Rim of Arizona... Recent guidance suggests more concentrated convection is likely to develop Wednesday afternoon along the Mogollon Rim of Arizona, with a fair bit less convection across southeastern Arizona. The Marginal Risk was shifted accordingly. Wegman ....Previous Discussion... ....Illinois into the Tennessee Valley and portions of the Southeast... Numerical guidance still shows that organized convection should develop during the afternoon or evening as shortwave energy initially over Kansas/Nebraska heads eastward...leading to height falls aloft overspreading the Mid-Mississippi Valley by 10/00Z.=20 Gulf moisture gets pulled northward during the day with both the GFS and NAM showing precipitable water values exceeding 2 inches from central/south central Missouri into Mississippi and Alabama by 09/18Z while the flow aloft ahead of the strengthening shortwave becomes increasingly difluent. This is similar to what was shown by the models on Monday but displaced a bit west. The flow aloft is still expected to become difluent on the east-side of an open wave at 300 mb. With moisture flux some 4+ standard anomalies above climatology...2 to 4 inch amounts still seem like a reasonable range of amounts. As such...a Slight risk area still seems appropriate.=20 Southwest U.S... Similar to Tuesday...west to to southwest flow aloft will draw additional moisture from the Gulf of California into parts of Arizona...leading to increased coverage of late afternoon and evening showers and thunderstorms over/near some of the higher terrain in mainly in Arizona. There may be a few downpours from storms if the instability is sufficiently high to generate some storms. Bann Day 3 Valid 12Z Thu Aug 10 2023 - 12Z Fri Aug 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE SOUTHERN APPALACHIANS... ....2030Z Update... ....Southern Appalachians... An early morning MCS, continuing from the Day 2/Wed period is expected to track over the southern Appalachians Thursday morning. The MCS will likely have embedded convection capable of rainfall rates to 2 inches per hour. Expect the terrain will also add lift on the western faces of the Great Smokies on the Tennessee side of the mountains, which may locally increase rainfall amounts. AHPS data show the area has had about normal precipitation for the past 2 weeks, which should allow the soils to absorb much of the rainfall. Nonetheless the terrain will also help focus that rainfall into the local rivers and streams, likely resulting in rapid river rises. Trends in the guidance have been decreasing/trending drier run-to-run, so the flooding risk has been decreasing. The inherited Slight Risk was trimmed to just the southern Appalachians, since the MCS will be the primary driver of rainfall, and that rainfall will be focused over the small area consisting of the southern Appalachians. ....Mid-Atlantic into the Northeast... Despite moisture advection ahead of the cold front, there remain few signals in the guidance that there will be much training or concentrated storms across the Marginal Risk area. Much of the area has seen quite a bit of rainfall lately, resulting in higher river levels. Thus, think most areas will see less than an inch of rain, but for those few areas where over an inch falls due to convection, isolated flash flooding is possible. The Marginal Risk is considered a lower-end one for this region, and any Slight risk upgrades at the moment appear unlikely. ....Upper Midwest... The Marginal Risk area was shrunk to much of southern MN and eastern areas of SD and northwest IA with this update. There remains good forcing and instability, but moisture this time will be the primary limiting factor. The signal for heavy rain is rather low, and the area has been fairly dry in previous weeks. Thus, think the flash flooding threat is limited to urbanized areas, including Minneapolis-St. Paul, and other poor drainage areas. ....Four Corners Region... The focus for convection Thursday afternoon will shift a bit further to the north to include the Big 5 National Parks in southern UT, as well as the western CO mountains, and the Mogollon Rim of AZ for the second straight day. Once again the signal for heavy rain is limited, but given the proclivity of this area for flash flooding, particularly in slot canyons, burn scars, and dry washes, the Marginal Risk area was expanded with this update. Wegman ....Previous Discussion... ....Eastern U.S.... Surface low pressure...initially associated with the convection over the central U.S. on Wednesday...is expected to move towards the eastern seaboard on Thursday ahead of a cold front dropping out of Canada into the Great Lakes. This will set the stage for increasing coverage of showers and thunderstorms over the eastern third of the country with one area of showers and thunderstorms associated with the surface low pressure center and a second farther south. Precipitation amounts should be fairly light...however rainfall associated with the low could overlap with areas that were soaked recently has the potential for isolated flash flooding or run off problems. Farther south...the area from the Tennessee Valley towards the Carolinas should have a seasonally moist airmass and enough dynamics to support rainfall rates sufficient to produce isolate flooding without such wet antecedent conditions. At this point with timing being so broad...opted for one broad Marginal area given the proximity of the two regions and uncertainty of timing. ....Upper Midwest... Placed a Marginal Risk area across portions of the Upper Midwest in a region where instability is expected to develop ahead of a developing surface low. With moisture pooling over the eastern Dakotas during the period...and precipitable water values increasing to more than 1.6 inches...convection that develops should have sufficiently strong instability to support locally heavy downpours by evening. Given uncertainty in placement and even the coverage of storms precludes more than a Marginal risk area at this point. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-wlBjygDPQxFazmLoSkFqJC_3Mzfe0zFpZH9ViWiVTPD= xr3TDiIONkYQHrL33LNFYOYkf93lck1IaOD_BG572tygWp0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-wlBjygDPQxFazmLoSkFqJC_3Mzfe0zFpZH9ViWiVTPD= xr3TDiIONkYQHrL33LNFYOYkf93lck1IaOD_BG57KW4hJ6M$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-wlBjygDPQxFazmLoSkFqJC_3Mzfe0zFpZH9ViWiVTPD= xr3TDiIONkYQHrL33LNFYOYkf93lck1IaOD_BG570OGfnG8$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .