Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 11 2023 20:10:52 FOUS30 KWBC 112010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 409 PM EDT Fri Aug 11 2023 Day 1 Valid 16Z 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... ....Northeast, to Ohio Valley, to Central Plains, to Four Corners... The large Marginal Risk area was only changed a small amount from the previous outlook. An expansive front will be draped across much of the country on Saturday, with primarily diurnally driven clusters and lines of convection peaking in the afternoon and evening hours. The 12Z HREF shows a scattering of low 40km neighborhood probabilities of flash flood guidance (FFG) exceedance across much of the Marginal Risk area between 18 UTC and 08 UTC, generally below 15 percent. This indicates that while hi-res models are showing some localized intense rainfall, particularly on very short time scales (30-60 minutes), it is not very organized or sustained, and the models do not agree much on placement. The run-to-run consistency has also been lacking. Nevertheless, the hi-res models are also showing maximum rainfall rates of 1-2 inches per hour over much of the Marginal Risk area as well, fairly consistent with an isolated threat of excessive rainfall and flash flooding. The Marginal Risk was fine-tuned based on the HREF probabilities, as well as updated CSU machine learning guidance, and ensemble CAPE and PW probabilities to identify the most supportive environment. A Slight Risk upgrade was considered in a couple areas, generally where HREF EAS (ensemble agreement) probabilities suggested more agreement on a sub-regional concentration of intense rainfall. One such area was in NE PA, N NJ, S NY (including NYC), CT, and W MA. HREF neighborhood probabilities of 1 inch per hour rain rates were higher in this area, including as high as 35 percent in NYC. However, there has been a lack of run-to-run consistency, so confidence in placement that precise is generally lower. Another area considered was the central High Plains, eastward into the Ozarks and Mid Mississippi Valley. Hi-res models do have a signal for streaks of higher QPF in that area, but in the Plains FFG is relatively high, and further east there is lower confidence on consistent placement. However, both these areas will be monitored for potential future upgrades. ...South Dakota and Vicinity... 20 UTC Update: The only change was to shift the Marginal Risk area slightly to the west to better match new guidance. The 12Z HREF, now extending into the Day 2 period more fully, did not show very high probabilities of FFG exceedance, so it is possible this Marginal Risk could be removed in the future. Previous Discussion: 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. ....Central Sierra Nevada... A Marginal Risk area was added in the Central Sierra Nevada for the second consecutive day. ECENS and GEFS precipitable water values are both forecast to be near or above the 95th percentile in this area, and hi-res model guidance indicated CAPE values reaching close to 1000 j/kg again in the afternoon hours. This should be sufficient for some localized rainfall rates around 1 inch per hour, which could lead to isolated flash flooding. Lamers/Wegman Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9dJ8tDbt1KCgwNTA5dTBU5hgbCggt37pIy3zrlHR0zay= BanaCWbC8MDeL9DDmOVNVfChjjHC69Dukq4eZno1dloyTiw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9dJ8tDbt1KCgwNTA5dTBU5hgbCggt37pIy3zrlHR0zay= BanaCWbC8MDeL9DDmOVNVfChjjHC69Dukq4eZno1NUb5FKY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9dJ8tDbt1KCgwNTA5dTBU5hgbCggt37pIy3zrlHR0zay= BanaCWbC8MDeL9DDmOVNVfChjjHC69Dukq4eZno1UFPn5wU$=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) .