Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 15 2024 19:51:57 FOUS30 KWBC 151951 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 351 PM EDT Thu Aug 15 2024 Day 1 Valid 16Z Thu Aug 15 2024 - 12Z Fri Aug 16 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID MISSISSIPPI VALLEY AND LOWER OHIO VALLEY... 16Z update... The latest 1hr hires guidance continues to suggest multiple thunderstorms capable of producing 1 to 2.5+ inches/hour=20 rain rates across southeast Missouri to western Kentucky. There are some trends for these storms to track across central Illinois so the northern bounds of the Slight Risk area was expanded northward further into central part of the state. There was also a small nudge westward of the Slight to cover more of southwest/south- central Missouri. Campbell ....Mid MS Valley to Lower Ohio Valley... The active weather associated with the large troughing over the Northern Plains and UPper Midwest this morning will move eastward today. At the start of the period, convection should be ongoing across portions of Missouri into Illinois and portions of Iowa, tied with the overnight convective line segments. This activity should be both relatively progressive and also weakening with the loss of the nocturnal low level jet and less favorable influx of moisture. However, a few pockets of heavier rainfall and rain rates will be possible that could total a couple/few inches through mid afternoon. Further south/southwest, outflow boundaries and the approaching cold front from the west/northwest will help spark an additional round of showers and thunderstorms late this afternoon into the late evening/overnight hours, most likely centered over portions of southern/central Missouri to southern Illinois, far southwest Indiana, and portions of western Kentucky. Here, the setup is much more favorable for 1) robust deep convection, 2) training/backbuilding storms with the flow becoming more parallel to the storm motions, and 3) an impressive evening/night low level jet of 30-35 kts impinging on the boundary in place. THe 00Z HREF neighborhood probabilities indicate potential for some intense rain rates with the storms, where the 2" hourly total probs reach 30 percent for several hours this evening/overnight with a slight signal for 3" hourly totals at times. Overall, isolated totals of 3-5" are possible with some localized higher amounts not out of the question based on the 00Z hi-res guidance and the HREF showing a few areas of 25 percent for 5" over the 24 hour period and a near 15 percent signal for 8" totals. For the Day 1 Excessive Rainfall Outlook, still messaging a Slight Risk for this area for the potential of isolated to scattered instances of flash flooding, especially if the heaviest amounts fall over portions of southern Missouri where soil moisture is already elevated due to recent heavy rainfall events. Taylor Day 2 Valid 12Z Fri Aug 16 2024 - 12Z Sat Aug 17 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS EASTERN=20 PORTIONS OF TENNESSEE VALLEY... 21z update... The latest guidance had a small southward shift of where the higher QPF will track across southern Missouri and and Kentucky so the Marginal Risk area was adjusted south to reflect this change. There is a growing signal for better concentration of showers and thunderstorms across eastern portions of the Tennessee Valley. Consensus shows areal averages of 1 to 3 inches for with=20 hourly rainfall rates of 1/1.5 inches/hour. A Slight Risk area was hoisted for this update to cover northeast Alabama, northwest Georgia, eastern Tennessee, far western North Carolina and Virginia, and southeast Kentucky. Campbell ....Great Lakes to Ohio Valley... By Friday, the cold front and associated upper level troughing will shift eastward toward the Great Lakes and Ohio Valley. Expect another day of scattered to numerous showers and thunderstorms as a result of the higher moisture ahead of the front along with favorable instability developing during the peak of the daytime heating. The set up does favor faster/progressive storm motions but a few repeating rounds or backbuilding storms will be possible so a few instances of flash flooding can't be ruled out so the Marginal Risk remains in place for the area. ....Mid Mississippi Valley to Tennessee Valley...=20 Late in the forecast period, the latest guidance is keying on a=20 subtle shortwave trough embedded in the northwest flow between the=20 upper ridge over Texas and the large troughing over the Great=20 Lakes. With the nocturnal low level jet reaching 25 to 30 kts=20 impinging on the surface boundary expected to be draped across=20 portions of Kansas and Missouri, there is increased likelihood of=20 convection developing early Saturday morning. While confidence in=20 exact placement is a bit higher than normal, the setup and=20 environmental ingredients suggest potential for some localized=20 heavy rainfall and this is showing up in the various deterministic=20 and ensemble probabilities this cycle. Isolated/localized flash=20 flooding will be possible. Taylor Day 3 Valid 12Z Sat Aug 17 2024 - 12Z Sun Aug 18 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST U.S. AND THE EASTERN U.S... 21Z update... Both areas of the country remain in good order with the Marginal Risk areas that are already in effect. Of note, there may be the need for a further upgrade to a Slight Risk for eastern portions of the Tennessee Valley. While some of the guidance shifts the focus to other parts of the region, a few keep additional 1-2 inches over the Tennessee Valley that overlaps the area of focus for the Day 2 period. The level of threat for excessive rainfall will largely be dependent on how much and were the rainfall falls. After collaboration with the local forecast offices opted to=20 maintain the Marginal and continue to monitor for future updates. Campbell ....Southwest U.S... The return of deeper monsoonal moisture is expected across the region, initially late Friday night across portions of southern Arizona, but by Saturday a plume of higher moisture surges north/northeast through the region. This is in response to the anomalous upper ridge axis becoming centered over eastern New Mexico and western Texas while a strong upper trough approaches the coast of California. This will set up a favorable flow of this higher moisture in between across much of Four Corners region during the period. PW anomalies reach +2 to +2.5 standard deviations above normal during the period while the latest model guidance is suggesting a subtle shortwave trough rippling through the flow, which will act to enhance the forcing for ascent across the region. Combined this with the expected instability during peak heating and the set up will become quite conducive for scattered daytime to evening showers and thunderstorms. The latest guidance is favoring northern Arizona and much of Utah for the greatest QPF and potential for higher rain rates, but the threat will exist across the larger region. Isolated to widely scattered instances of flash flooding will be possible, particularly for the more vulnerable locations and a Marginal Risk covers the region. ....Mid Atlantic to Northeast... A pronounced upper trough is forecast to move from the Great Lakes region at the start of the period to the Upper Ohio Valley and Central Appalachians by 12Z on Sunday. At the surface, a cold front is expected to pass through with a ribbon of higher moisture surging ahead of it. The combination of the favorable forcing for ascent and higher moisture, along with the daytime heating instability, should be sufficient to bring scattered to numerous showers and thunderstorms to the region. While the setup should favor relatively progressive storm motions, some areas could see a few repeating rounds or backbuilding storms on the southern/southwest flank of any line segments that may bring locally higher rainfall totals. Based on the latest deterministic and ensemble guidance plus ML first guess fields, a broad Marginal Risk is advertised for the region for localized/isolated flash flooding. ....Southeast... The advancing cold front will become more west to east oriented across the Southeast during the period. The moisture profiles in the region will remain elevated (around 1.5 std deviations above normal) and with robust daytime heating and instability, strong thunderstorms are forecast to develop along/ahead of the frontal boundary then quickly move to the east/southeast. A localized/isolated flash flood threat exists and the Marginal Risk was extended southward to account for pockets of heavy rainfall totaling a few inches in places over a relatively short period of time. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8Yi-3JIqT1kVzhTwMmot6lNPto8BVrj6Qeu3TvWf3H69= 4nwVKXSRZ2kwLaVwe-9bCLJ98d4QxP7WX_CoiNWcKDOYHt4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8Yi-3JIqT1kVzhTwMmot6lNPto8BVrj6Qeu3TvWf3H69= 4nwVKXSRZ2kwLaVwe-9bCLJ98d4QxP7WX_CoiNWcAlmhbsc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8Yi-3JIqT1kVzhTwMmot6lNPto8BVrj6Qeu3TvWf3H69= 4nwVKXSRZ2kwLaVwe-9bCLJ98d4QxP7WX_CoiNWcmVo2JMM$=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. 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