Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 08 2023 20:18:27 FOUS30 KWBC 082018 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 417 PM EDT Tue Aug 08 2023 Day 1 Valid 16Z Tue Aug 08 2023 - 12Z Wed Aug 09 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN MAINE... ....16Z Update... ....Northeast... The axis of heaviest rainfall currently extends from Boston south and east in MA and across a large portion of southeast Maine. In coordination with GYX/Gray, ME and CAR/Caribou, ME forecast offices, the Moderate Risk was shifted eastward to align with this axis of heaviest rainfall. As we go through the afternoon, expect scattered convection to develop across much of VT/NH, where the Slight risk remains. Abundant cloud cover appears to be the primary factor holding instability down, but through the early afternoon more sunshine may poke through the mostly cloudy skies to increase instability. The shower and thunderstorm activity is well forecast in all of the guidance to remain widely scattered to scattered, with little in the way of organization expected. Thus, despite the favorable antecedent conditions, the flood risk has come down a bit in this region, likely to the lower-end Slight category. With sunset and the end of diurnal heating, the showers and storms should dissipate, which will end the flood threat in the Northeast entirely with the exception of Northern Maine, which will still be ahead of the cold front. However, even in northern ME, the instability will be very limited, so no convection with higher rainfall rates is expected through the overnight. ....Portions of the Mid-South... An MCS is currently racing across MS, northern LA, and entering northern AL this morning. As the MCS was developing overnight last night, there was a period with training thunderstorms that resulted in numerous instances of flash flooding across eastern OK and into AR. However, since then the MCS has become far more quickly-moving, and is weakening, as is typical for the mid-morning. Thus, the inherited Slight Risk was double-downgraded to a no risk for the much of the area as FFGs are very high across much of the Deep South. The Marginal across northwest AR and southwest MO is largely for potential developing convection very early Wednesday morning in that area ahead of the MCS. Across southern/southeast GA, southeast AL, and portions of north Florida, once the remnant MCS reaches this area late this afternoon a stalled front may provide the forcing for localized training storms over this area. High FFGs will keep the flash flooding threat isolated, but can't be ruled out. ....Central Plains... The Slight Risk area from eastern CO through southern NE and much of KS is largely the same with this update. The guidance is in good agreement on a developing MCS that too picks up speed through the night as it crosses through KS. However, there are indications of some backbuilding and secondary lines of heavy storms along the southern side of the MCS, and the up to 2 inch per hour rainfall rates, while being short-lived due to the fast movement of the individual cells, could train over the same areas as multiple rounds move through. There is great agreement on the track of the MCS across the Slight Risk area. The inherited Slight was trimmed on the southwestern side in east-central CO since all the guidance keeps the heaviest rainfall associated with the MCS further north and east. ....Arizona... Monsoonal moisture continues to increase across this area, and widely scattered convection capable of localized flash flooding will be possible across the Marginal Risk area.=20 Wegman ....Previous Discussion... ....Northeast U.S.... On-going heavy to excessive rainfall across portions of New England will continue into the afternoon as deepening low pressure north of New York draws anomalously deep layer moisture and associated instability into New England with the synoptic scale flow pattern still able to support back building storms. Timing of the low and associated precipitation has changed a bit and required adjusting the Moderate Risk area. With storms impacting the terrain, particularly the White Mountains of New Hampshire into western Maine, the combination of rainfall and orographics should result in enhanced rainfall totals and the Moderate risk was largely unchanged. With models beginning to latch of to the speed of the back edge of the precipitation area...trimmed the Moderate out of some areas where QPF had become quite low. One other change was to extend the Slight Risk farther south where CAPE values were closer to 1000 J per kg and the potential for warm rain processes was through a particularly great depth. The western portion of both the Slight and Marginal Risk areas were adjusted. ....Central Plains to the South/Southeast U.S.... The Slight Risk area was maintained over the Plains from Colorado into Kansas and parts of adjoining Kansas. Thunderstorm development in Oklahoma during the overnight hours prompted an introduction of a Slight risk area extending from Oklahoma into Arkansas. It also allowed for the Marginal Risk area in the Plains to be joined with the Southern U.S. Marginal Risk area.=20 Southwest U.S... 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 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 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!8gzKG_4jsAIRAjPNwY0-getuTV8J7c2fZ6wG2PAz6m4C= yt0D74Z5Yd9eDqVj2TyohzQ0Tg6M-aB1DrwMcS-6U-A0wGE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8gzKG_4jsAIRAjPNwY0-getuTV8J7c2fZ6wG2PAz6m4C= yt0D74Z5Yd9eDqVj2TyohzQ0Tg6M-aB1DrwMcS-6hy0GwOY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8gzKG_4jsAIRAjPNwY0-getuTV8J7c2fZ6wG2PAz6m4C= yt0D74Z5Yd9eDqVj2TyohzQ0Tg6M-aB1DrwMcS-6NnI3myI$=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) .