Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Apr 25 2024 23:35:25 FOUS30 KWBC 252335 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 735 PM EDT Thu Apr 25 2024 Day 1 Valid 01Z Fri Apr 26 2024 - 12Z Fri Apr 26 2024 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF WESTERN HIGH PLAINS INTO THE MISSISSIPPI VALLEY... 01Z Update... Expanded the northern end of the Slight Risk area into arts of southwest Nebraska/northwest Kansas in response to continued=20 convective regeneration along axis of instability during the=20 evening hours. Refer to MPD 0172 issued for the area that is valid=20 until 26/0345Z for additional details. While the HRRR has not been=20 performing particularly well in the placement of the heaviest=20 rainfall this afternoon...it seems to be doing okay with the=20 convection along the warm front early this evening. The latest run=20 of the HRRR and the ARW suggest additional convection will be=20 developing along and ahead of the dryline over portions to Texas=20 that spreads eastward across parts of Oklahoma and Kansas later=20 tonight. Did not make any changes with the southern end of the=20 Slight Risk area because of the amount of soil saturation from=20 areas of heavy to excessive rainfall earlier in the day...and=20 additional rainfall should turn to run off more quickly in spots=20 that receive additional heavy rainfall tonight or where rainfall=20 rates of 1.5 to 2 inches per hour occur. Bann 16Z update... The latest hi-res guidance is suggesting multiple hours of rainfall rates up to 2 inches/hour to pass through southeast Nebraska and northeast Kansas. 3-hr FFGs over this part of the region are as low as 2 inches which given the new hi-res trends, could reach or exceed soil saturation. The Slight Risk area was expanded to the northeast to cover more of southeast Nebraska and northeast Kansas. Meanwhile, latest observations and model trends supported the small trimming of the backside of the Slight Risk across portions of Oklahoma and central Kansas. The best potential for the highest amounts and rainfall rates of 1.5-3 inches/hour remain across eastern Kansas to the borders of Oklahoma/Arkansas/Missouri/Kansas. Campbell Convection will be ongoing at 12z this morning across portions of the central and southern Plains. There remains some spread within the 00z HREF guidance whether to favor a northern training axis across portions of KS and MO, or a southern axis from northeast OK into northwest AR. At this point it remains a bit unclear which axis will become the dominant one, but both will likely see some upscale convective development this morning within an area of increasing moisture convergence in the 850-700mb layer. Storm motions will favor some training and backbuilding, and thus some flash flood risk will probably exist this morning from KS into western MO, and from northeast OK into northwest AR. Stronger mid and upper level forcing will eject east out of the Rockies tonight, which will trigger additional convective development over the Plains. Over KS and OK this convection should be quite progressive off to the east. However high rainfall rates are likely, and so any areas that see heavy rain from the first round of convection this morning/afternoon could potentially see areas of flash flooding tonight. Otherwise the activity should be quick enough to pose only an isolated flash flood risk. There is a better chance of a convective training tonight closer to the warm front near the KS/NE border, which could drive an isolated to scattered flash flood threat. Overall will continue with a broad Slight risk that stretches from near the NE/KS border southeast into northwest AR. This covers both the expected convection this morning/afternoon, and the development expected tonight with the arrival of stronger forcing from the west. Chenard Day 2 Valid 12Z Fri Apr 26 2024 - 12Z Sat Apr 27 2024 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE SOUTHERN PLAINS INTO THE LOWER TO MID MISSISSIPPI VALLEY... 21Z update... Overall the setup for this period has not changed much, just minor shifts of the QPF footprint across the Plains, Midwest and Mississippi Valley. There will be some overlap of the heavy rains from the Day 1 period and where some of the higher amounts are expected for this period, particularly across Missouri, Oklahoma and Arkansas. This will elevate the threat for local flooding concerns. Minor reshaping of the Slight Risk to a more southwest-northeast orientation was made with this update along with an expansion into east-central Missouri. Campbell In what is shaping up to be an active stretch of excessive rainfall potential, Friday may end up being a relative minimum of flash flood impacts. The first system will be rapidly ejecting northeast across the Plains and upper MS Valley. Given the quick forward motion and the fact that the system will be moving east of the stronger instability with time, tend to think any flash flood risk will stay localized in nature from the central/northern Plains into the upper Midwest. The southern extent of the system may pose a slightly greater flash flood risk, but even this remains uncertain. Convection should be ongoing Friday morning across portions of TX/OK/AR/MO, although the intensity and organization of this activity remains unclear. The convection will be outrunning the surface front by this time resulting in a gradual weakening of lower level convergence. In the upper levels the forcing is also on a decline, so quite possible activity is weakening Friday. However with plenty of moisture and instability in place, and little reason for much eastward progression of convection...any activity that is able to persist and/or intensify during the day could end up pretty slow moving and thus capable of producing flash flooding. Thus opted to maintain a Slight risk from northeast TX into southwest MO to account for this threat...while also accounting for the fact that some of these areas may be more sensitive depending on the rainfall that occurs Thursday and Thursday night. Also worth noting our machine learning ERO also has elevated probabilities over approximately this same area...likely indicative of the lingering favorable ingredients in place. Chenard Day 3 Valid 12Z Sat Apr 27 2024 - 12Z Sun Apr 28 2024 ....A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF OKLAHOMA... 21Z update... Areal averages of 2 to 4.25+ inches expected from the Southern Plains to the Mid/Upper Mississippi Valley as convection really ramps up during this period. the 72-hr QPF footprint over this region will be in the 2 to 6 inches range with the highest amounts focused over eastern Oklahoma to southwest Missouri. The latest models and WPC forecast trended QPF higher further south across portions of Oklahoma and eastern Texas, as well as, the girth of the QPF axis. As such the size of the Moderate Risk area was expanded west-east and southwest-northeast. With the southward trend, the Slight Risk was also expanded further south into eastern Texas and a smidgen northeast further into central Iowa. Convection near the Front Range trended a bit further west so in turn, minor adjustments of the Marginal Risk in northeast Colorado was made. Campbell A significant rainfall event appears likely Saturday night across portions of north central TX into central/eastern OK. Large scale forcing really ramps up Saturday night with the approach of the mid level trough and 250mb upper jet streak. With strong ridging to the east, these features lift more northward than eastward during the overnight hours, resulting in a prolonged period of impressive upper level divergence centered over Oklahoma. The persistence of this large scale forcing is typically indicative of a favorable training convective setup. In the lower levels we have a very strong 850mb jet in excess of 50 kts resulting in impressive moisture transport and convergence near a slow moving cold front. This low level flow is nearly equal and parallel to the deep layer mean flow, which typically supports a backbuilding convective risk. The expectation is that with deep layer mean flow nearly parallel to the low level boundary, convective cells will tend to train and also backbuild into the strong low level jet. Convection should become intense and organized enough to develop a stronger cold pool resulting in some eastward cell progression with time. This eastward progression is the main wild card in determining the magnitude of the flash flood risk with this event. We do think there will be some eastward progression, but the persistent forcing and strong low level jet both will be opposing a significant eastward push...and thus tend to think we will see enough persistence of convection to result in an organized flash flood threat. Convection will be capable of 2-3"/hr rainfall, and thus it will not take much persistence to start getting some significant rainfall totals. While widespread 5"+ totals may not occur, swaths of totals exceeding 5" are expected, which should result in an increasing, and locally significant, flash flood threat. Model guidance is in decent agreement for a day 3 forecast, all generally focusing the highest swath of QPF from north central TX into central/eastern OK. Although, as would be expected with a day 3 forecast, there is still some uncertainty on where exactly the highest QPF ends up, as the swath of the most excessive amounts will likely be rather narrow. The current placement of the MDT risk is where the best overlap in ingredients and the model QPF consensus resides. To the north into southeast KS, some expansion may eventually be needed depending on antecedent rainfall over the next couple days leading up to Saturday night. Meanwhile some expansion to the south southwest of the current risk area is a possibility...as the south and southwest flank of convection is often a favored region for more prolonged training/backbuilding in these type of events. Across eastern KS into northwest MO and southern IA locally excessive rainfall is also possible. But in general the environmental ingredients are not as favorable for a prolonged period of heavy rain, and thus a Slight risk should suffice. A Marginal risk extends westward across NE into northeast CO along/near the low track where convection near the warm front may result in a localized flood risk. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9Ip6e_bIZ4i_G9iXxNDP5QChE7G6-UoTfzbmQpo5UQqG= P2V54QQNJxv-nsU4MxylE0BC077sa3gF76TdL8-7JHjvBV4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9Ip6e_bIZ4i_G9iXxNDP5QChE7G6-UoTfzbmQpo5UQqG= P2V54QQNJxv-nsU4MxylE0BC077sa3gF76TdL8-7lLVDS30$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9Ip6e_bIZ4i_G9iXxNDP5QChE7G6-UoTfzbmQpo5UQqG= P2V54QQNJxv-nsU4MxylE0BC077sa3gF76TdL8-7RTXSj3w$=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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