Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 11 2023 16:01:48 FOUS30 KWBC 111601 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Fri Aug 11 2023 Day 1 Valid 12Z Fri Aug 11 2023 - 12Z Sat Aug 12 2023 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL IN THE SOUTHEAST, OZARKS, CENTRAL GREAT LAKES, DESERT SOUTHWEST, AND CENTRAL SIERRA NEVADA... ....16Z Update... Minor adjustments were made to the overall forecast as quasi-zonal flow persists across much of the Lower-48, as a broad upper-trough arrives into the Northern Tier. After an initial band of warm-advection showers developed this morning across the Upper-Midwest, potentially several rounds of thunderstorms are possible this afternoon across portions of central and southern Michigan along an approaching cold front. Earlier thinking remains on track for southwesterly inflow associated with a lifting warm front to supply MUCAPE around 1500 J/kg and 1.5" PWATS to support some heavy rainfall with later storms along the front. Based off trends in the 12Z high-res guidance, the Marginal Risk was trimmed back a bit to encompass central Michigan, northwest Ohio, and northeast Indiana where coverage could be greater. To the southwest across Kansas and Missouri, the Marginal was expanded slightly to the northwest based on 12Z guidance for some potential training activity later tonight as a strengthening LLJ overruns a lifting warm front. There remains some uncertainty regarding specifics with convective coverage and placement, although the northwest and southwest corners of Missouri remain relatively more vulnerable to rainfall in light of recent storms. Across Arizona and New Mexico, shower activity continues along a plume of Monsoonal moisture returning northward. An uptick in QPF was noted into northwest Arizona in the 12z guidance, where plenty of insolation and moistening is underway on the edge of the cloud shield from ongoing rain. Thus, expanded the Marginal Risk northwest to account for potentially increased coverage which could cause issues later today. Asherman=20 ....Michigan, Northern Indiana, Northwest Ohio... A 15-25 kt LLJ will transport a bit of moisture from the Midwest that will be lacking a connection with the Gulf into portions of the Midwest. Convection is likely to break out along this front, especially during peak heating, and continuing into the evening. CAMs guidance is in decent agreement that multiple rounds of showers with some embedded storms will form along the southern boundary between steadier rain and forcing to the north, and isolated convection with little forcing to the south. Thus, this region is likely to be in the middle, and thus subject to multiple rounds of storms. The southwesterly flow will also transport air with MUCAPE around 1,500 J/kg, and PWATs to around 1.5 inches. This is far from impressive for this time of year, but will be enough moisture availability for the storms. ....Ozarks and Mid-Mississippi Valley into the Southeast... Unlike further north, this area will have plenty of moisture and instability, with PWATs exceeding 2 inches in some areas, and instability exceeding 4,000 J/kg of MUCAPE, which is extremely unstable. Thus, what will be missing in this area is a strong enough source of forcing to allow the storms that will form over this area to organize. Scattered storms are already over portions of northern AL this morning. They will track southeastward, following the greatest moisture and instability. However, other than localized outflow boundaries, the forcing will be limited. Thus, expect the storms to form and reform as the broader area of scattered storms moves southeastward, but the storms will be largely independent of each other. While the plentiful moisture and instability will support individual storms becoming strong enough to produce heavy rainfall, the storms will be moving enough and will be widely scattered enough that the flash flooding threat will be largely confined to urban and poor drainage areas. High FFGs common across the Deep South will also limit the flooding threat. ....Eastern Arizona and Western New Mexico... A very diffuse monsoonal signature is the best the guidance is showing as far as convective coverage goes this morning for the return of storms again this afternoon. With limited forcing and capping aloft, the convection may not be quite as strong today as they were yesterday evening. Nonetheless given the sensitivity of the area to flash flooding, particularly over burn scars, slot canyons, dry washes, and any urban areas, the Marginal Risk was maintained with this update. ....Central Sierras into Nevada... The guidance continues to indicate the potential for isolated flash flooding from the Central Sierra Nevada eastward into central NV. No significant changes were noted with this update, as any storms capable of flash flooding will only be able to form if there is enough break in the cloud cover to allow instability to increase. Wegman Day 2 Valid 12Z Sat Aug 12 2023 - 12Z Sun Aug 13 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A BROAD AREA FROM THE NORTHEAST, TO THE OHIO VALLEY, THE CENTRAL PLAINS, AND INTO THE FOUR CORNERS... Few changes were needed to the Day 2/Saturday ERO. An expansive front will be draped across much of the country on Saturday. While the front itself will provide some forcing, the lack of upper level support for most of the country will once again limit the potential for flooding. One exception will be across the Northeast, where an approaching shortwave trough may help to organize some convection. Moisture will be the primary ingredient lacking across the area, as PWATs struggle to approach 1.5 inches. PWAT values around 1.25 inches is enough for showers and storms to form, but both the lack of advection and that not being a huge amount of available atmospheric moisture will cut down on the coverage of storms. Further, there will be quite a bit of cloud cover over the area, which will limit how much instability can develop in the afternoon ahead of the shortwave trough. Thus, despite much of the area being quite a bit wetter than normal with NASA Sport soil moisture imagery showing almost all of northern NY and New England having soils at or wetter than normal, think the flooding threat will be limited due to the fast movement of storms and lack of coverage. Widely scattered convection is likely to track along the front across the middle of the country as well, but with all the forcing confined to the north, expect the storms to struggle to organize. Thus, the signal for heavy rainfall in any one area is weak. Isolated flash flooding will be possible in flood-sensitive areas should one or more rounds of storms move over the area, but most areas will see sub-flooding level amounts of rain Saturday. Scattered showers and storms are also likely to redevelop Saturday afternoon and evening across the Four Corners, with the tail end of the front possibly helping focus and organize the convection. Nonetheless expect limited flash flooding as storms try to focus along the Mogollon Rim. ...South Dakota and Vicinity... Much of the guidance shows showers and storms forming along a 20 kt LLJ tracking up the Missouri Valley and into eastern SD Saturday night. The increased moisture along with upper level support from a strong shortwave trough diving southeastward out of the Canadian Prairies should allow for locally heavy rain to develop over this area. Soils are wetter than normal after recent rains from a few days ago in this area, and while the area is notoriously hard to flood, think the antecedent conditions may allow for isolated instances of flash flooding due to the more organized nature of the storms over a rather confined area of eastern SD. A new Marginal Risk was introduced with this forecast update. Wegman Day 3 Valid 12Z Sun Aug 13 2023 - 12Z Mon Aug 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN KANSAS THROUGH THE LOWER OHIO VALLEY... The "break" from more widespread flooding concerns from today and Saturday will end on Day 3/Sunday. The stalled out front draped across the middle of the country will continue to gather atmospheric moisture along it on a southwesterly LLJ was it strengthens to as strong as 45 kt across MO Sunday night. A strengthening shortwave that starts the day Sunday near the MT/ND border strengthens as it tracks southeastward toward the Slight Risk area. KS and MO may start the day with ongoing showers and storms Sunday morning as moisture increases, but the real threat for flooding will come Sunday night with the arrival of the shortwave and PWATs increase to a tropical 2.25 inches in some areas. The combination of organized storms ahead of the trough and the abundant moisture will increase the flooding threat. Further, antecedent conditions from the past several weeks of heavy rainfall events have left much of the area still at or above normal for soil conditions. The inherited Slight risk area was trimmed from the north to favor areas from Kansas eastward to southwest OH. While areas further north from eastern ND east across MN and into the U.P. of MI will also likely get 1-3 inches of rain as a low wraps up over that area, antecedent soil conditions are much drier in this area, meaning the soils should be more likely to absorb much of the needed rainfall, limiting the flooding impacts. It's likely the axis of heaviest rain may shift north or south in the coming days, so the Slight was determined the most likely flooding risk at this point, but upgrades may be needed with better confidence in the CAMs as the event draws closer. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4f6mNRa8K3jwptjn5ER1detUDlGpkb_UYbFxs8DywBXX= 1IjhDURtlmCOJQcw0nHJlvPDYQtMOeg6B4L4rQVLXsybvaY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4f6mNRa8K3jwptjn5ER1detUDlGpkb_UYbFxs8DywBXX= 1IjhDURtlmCOJQcw0nHJlvPDYQtMOeg6B4L4rQVLcK4vlxE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4f6mNRa8K3jwptjn5ER1detUDlGpkb_UYbFxs8DywBXX= 1IjhDURtlmCOJQcw0nHJlvPDYQtMOeg6B4L4rQVLgRFWI8w$=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) .