Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 27 2024 00:32:55 FOUS30 KWBC 270032 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 832 PM EDT Mon Aug 26 2024 Day 1 Valid 01Z Tue Aug 27 2024 - 12Z Tue Aug 27 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN PLAINS INTO THE UPPER MISSISSIPPI VALLEY... ....Northern Plains into the Upper Mississippi Valley... Stout upper trough over the western Dakotas will continue eastward tonight, with SW flow ahead of it in the mid-levels atop a surface boundary draped along the SD/NE line into MN. Precipitable water=20 values ahead of the upper trough and near the boundary are=20 1.75-2.00+ inches over the outlook areas. Initial round of=20 convection was pushing through the MSP region as of 0030Z into a=20 very moist/unstable atmosphere (surface Tds near and above 80F with MUCAPE > 4000 J/kg). 1-2"/hr rates have been observed with this=20 round but it has been rather brief as the most intense updrafts=20 continue eastward. However, given the upper trough to the west=20 associated with another area of severe weather/heavier rain over=20 central SD, expect this to move across eastern SD into southern MN=20 later overnight where instability remains per SPC Mesoanalysis=20 (~1000 J/kg). 18Z HREF probs for >1"/hr rates are 20-50% within the Slight Risk outline. The aggregate of both events will likely=20 enhance the flash flood risk within this area.=20 Extended the Marginal Risk a bit to the east into the U.P. of=20 Michigan per expected activity with the front near Lake Superior=20 and per recent MPD #932. ....Southwest US... SW to NE axis of ongoing late afternoon convection persists from=20 south central NM northeastward to southeastern CO and northwestern=20 KS. Overall trend in convection is down in NM, but still lies over=20 sensitive areas. To the northeast, convection continues rather=20 robustly into western KS where sufficiently high PW values support=20 a continuation of localized heavier rainfall. Will continue the=20 Marginal Risk outline in this region with a small expansion into KS where some flooding has already occurred just across the border in CO.=20 Fracasso/Hurley Day 2 Valid 12Z Tue Aug 27 2024 - 12Z Wed Aug 28 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE SOUTHWEST AND UPPER MIDWEST... 21Z update... The latest guidance did show an uptick in QPF for the Minnesota arrowhead and into north-central Minnesota. The WPC forecast did increase amounts and coverage over that part of the state therefore the Marginal Risk area was expanded a little to the west to account for this adjustment. For the Southwest Marginal Risk, no changes were needed at this times. Campbell ....Summary... Have expanded the Marginal Risk southward across the Upper Midwest, to account for the potential of locally heavy rainfall before the frontal passage. ....Upper Midwest-Upper Great Lakes... Upper shortwave will continue to dampen somewhat by Day 2, which will weaken the deep-layer Qs forcing. Prior to the surface frontal passage, PWs will remain between 1.75-2.00 inches (around 2 standard deviations above normal). Instability will be much greater over the southern portion of the outlook area (down towards the I-80 corridor, which was a big reason to nudge the ERO Marginal Risk farther south (matching up with SPC's Day 2 Severe Slight Risk). Expect the activity to move rather swiftly before dropping south of the outlook area Tue night. Still appears to be a localized flash flood risk though, given the weakening deep-layer forcing and degree of downwind propagation. Therefore, have continued with the Marginal Risk. ....Southwest US... The risk of excessive rainfall will linger on Tuesday afternoon and evening across portions of the Southwest US as a moisture plume lingers across the area and winds aloft/steering flow weakens in response to the a mid level wave moves away over the Northern Plains as it begins to fill. Model based ingredients suggest the potential for locally heavy downpours may extend a bit west of the area shown by the deterministic runs...so made a minor expansion that way. Hurley Day 3 Valid 12Z Wed Aug 28 2024 - 12Z Thu Aug 29 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST, NORTHERN PLAINS TO UPPER MISSISSIPPI VALLEY, AND EASTERN OHIO VALLEY INTO THE NORTHERN MID-ATLANTIC... 21Z update... The three Marginal Risk areas were not changed for this cycle. The level threat given the expected environmental setup, model trends and latest WPC forecast are appropriately assigned. Campbell ....Summary... Compared to yesterday's Day 4 ERO, the Southwest and northern-tier Marginal Risk areas were largely unchanged. However, the Marginal Risk area across the OH Valley into the northern Mid Atlantic was pared a bit given the latest guidance/trends. ....Southwest... Guidance mid-level vorticity progs support a more north-south outlook area compared to Days 1 and 2, though nonetheless still quite similar. Instability and moisture parameters (afternoon and evening CAPE and PW profiles) remain similar, and thus the anticipation of more isolated/localized instances of flash flooding than otherwise, particularly after 20Z coinciding with peak diurnal heating. ....Upper Midwest... Strong deep-layer warm and moisture advection ahead of the next vigorous mid-upper level trough and associated surface fronts will favor more organized/widespread elevated convection by Wed night ahead of the advancing low-level warm front. Right now, given the model spread with QPFs, have kept the Marginal Risk area, however later shifts will evaluate the need to upgrade at least a portion of the outlook area to a Slight. Given the dynamical forcing and thermodynamical profiles, the potential is there. ....Eastern Ohio Valley to Northern Mid Atlantic... W-E frontal boundary will be sinking slowly southward during day 3. Pre-frontal, deep-layer moisture and instability will be sufficient for areas of heavy rainfall, while the easterly storm motions (parallel to the low-mid level fronts) would support the risk of cell training. The guidance at this point isn't overly robust with the QPF, at least not yet, despite the uptick in right- entrance region forcing from the upper level jet streak over northern New England. Thus for now, have kept the ERO risk at Marginal. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9zRoCAt1_-L4dDCZstzNT1m98rPXrWUv8TWTdUq7LAX5= H2O7IRM76yFvbYxvZzAratZm1a3ZccvS9UCurAhixwIEm3A$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9zRoCAt1_-L4dDCZstzNT1m98rPXrWUv8TWTdUq7LAX5= H2O7IRM76yFvbYxvZzAratZm1a3ZccvS9UCurAhiE5rehAM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9zRoCAt1_-L4dDCZstzNT1m98rPXrWUv8TWTdUq7LAX5= H2O7IRM76yFvbYxvZzAratZm1a3ZccvS9UCurAhiwEdGrek$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .