Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 26 2024 20:11:45 FOUS30 KWBC 262011 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 411 PM EDT Mon Aug 26 2024 Day 1 Valid 16Z Mon Aug 26 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... 16Z update... The latest guidance and radar trends continue to support the Risk areas highlighted across Florida, New England and the north-central part of the country well and did not require any adjustments at this time. Convective activity has been picking up along the Gulf Coast late this morning and WPC has issued a Mesoscale Precipitation Discussion (MPD #928) for much of the Texas and Louisiana coast that was in the Marginal Risk area from overnight. Minor expansions were made to include areas discussed in the MPD. A small nudge of the Marginal was made across southeast Colorado to reflect an uptick in some of the guidance near the High Plains. Campbell ....Summary... Per collaboration with WFOs ABR, FSD, MPX, and ARX, have hoisted a Slight Risk across eastern SD into southern MN, western WI, and a small portion of northeast IA. Have also added Marginal Risk areas across the Mid-Upper TX Coast to eastern portions of the Hill Country, much of the west coast of FL, and across parts of New England. ....Northern Plains into the Upper Mississippi Valley... An upper trough over the Intermountain region will begin lifting northeastward across the High Plains on Monday, becoming less amplified with time as it encounters the central U.S. ridge. Precipitable water ahead of the upper trough and associated surface cold front area expected to peak between 1.75-2.00 inches over the outlook areas. Pre-frontal surface low tracking slowly from northeast NE-southwest SD early Mon to the eastern MN-western WI border by 12Z Tue will coincide with a slower eastward frontal progression later Monday and especially Monday night. As a result expect multiple rounds of organized convection across the outlook areas, with multicellular upscale growth potential along with some supercells. MLCAPEs of 3000-4000+ J/Kg this afternoon-evening within the very moist environment will support intense instantaneous rainfall rates; however, given the relatively weak llvl flow compared to the mid-upper levels, expect the convection to be forward propagating at a fairly decent clip through this evening. This will cut down on the rainfall potential during the daylight hours. However, given the slow moving surface low and front, again expect another round of convection Mon night as PWs remain between 1.75-2.00 inches with MUCAPEs still between 1000-2000 J/Kg. The aggregate of both events will likely enhance the flash flood risk within the Slight Risk area, given the growing support from the high-res CAMs. Elsewhere, the Marginal Risk area remains, though did pull the southern edge south a bit into much of southern WI (including the MKE area) towards the max instability gradient. ....Mid-Upper TX Coast to Eastern Portions of the Hill Country... Well defined low-mid level circulation near the TX Gulf Coast early this morning will traverse slowly into South-Central TX this evening and overnight. Instability will remain greatest offshore, though SBCAPEs near 1000 J/Kg distributed in a tall/skinny profile, with WBZ levels over 13 kft and PWs of 2.00-2.25 inches, will produces areas of heavy rain. Given the weak deep-layer flow (0-6km bulk shear 20kts or less), expect the more intense rainfall rates to be short lived before the more intense updrafts collapse as the cells become outflow dominated. Therefore have opted for a Marginal Risk over this region, with the anticipation that any flash flooding/short term runoff concerns will be isolated. ....Western FL Peninsula... The guidance shows a more enhanced ECSB (East Coast Sea Breeze) given the added synoptic low-level easterly component. This will focus peak diurnal convective coverage along the west coast of the FL Peninsula, which is supported by the model QPFs (especially CAMs). PWs between 2.1-2.3 inches, along with SB CAPEs peaking between 1500-2500 during the afternoon and early evening will support intense short term rainfall rates. In fact, the 00Z HREF 40km neighborhood probability of 2"/hr rainfall rates climbs to 40-60% over/near the Tampa Bay area between 20-00Z (while 3"/hr rain rates climb to around 25%). Per the CAMs, localized totals of 3-5" can be expected, much of which would be within a 3 hour period. Cannot rule out localized flash flooding as a result, particularly in urban areas in and around the Tampa Bay area. ....Parts of New England... Vigorous shortwave energy and compact LFQ forcing pivoting south- southeast on the western periphery of the upper low will set the stage for fairly robust deep-layer forcing over a relatively narrow corridor across parts of New England Today. The mid/upper ridge impinging from the west is likely causing the uptick in upper level flow (localized 90-100kt 250mb jet streak) across the Hudson Valley/eastern NY. Steepening mid level lapse rates along with MLCAPEs ~1000 J/Kg will help fuel numerous showers/tstms during the peak heating hours this afternoon, with storm motions south- southeast. The 00Z HREF depicts high probabilities of >1"/hr rates between 18-00Z, peaking between 60-70% across eastern CT/RI/southeast MA. Localized storm totals of 3+ inches may lead to isolated instances of flash flooding. ....Southwest US... The threat of excessive rainfall will linger into Monday across portions of the Southwest US as precipitable water values of 0.75 to 1.00 inches remain in place from extreme southeast Arizona into New Mexico. Scattered daytime storms remain possible within the plume if deeper moisture...but anomalously strong steering flow should help limit residence time to mitigate some of the flash flooding concern. As mentioned earlier, although even brief heavy rainfall rates in a short period of time may still lead to flash flooding, and a Marginal Risk was maintained in the region. 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=20 this cycle. The level threat given the expected environmental setup, model trends and latest WPC forecast are appropriately assigned.=20 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=20 3. Pre-frontal, deep-layer moisture and instability will be=20 sufficient for areas of heavy rainfall, while the easterly storm=20 motions (parallel to the low-mid level fronts) would support the=20 risk of cell training. The guidance at this point isn't overly=20 robust with the QPF, at least not yet, despite the uptick in right- entrance region forcing from the upper level jet streak over=20 northern New England. Thus for now, have kept the ERO risk at=20 Marginal. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7GL9mrFa0mqvvTCXJnmFgX2BmIX-Di8EQPKiwxnNkYoK= 2iu6_6_0ZD6IguwuklYea3Uf6hsdU0fpLPSmLTDivVnI0L8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7GL9mrFa0mqvvTCXJnmFgX2BmIX-Di8EQPKiwxnNkYoK= 2iu6_6_0ZD6IguwuklYea3Uf6hsdU0fpLPSmLTDi4LN73jw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7GL9mrFa0mqvvTCXJnmFgX2BmIX-Di8EQPKiwxnNkYoK= 2iu6_6_0ZD6IguwuklYea3Uf6hsdU0fpLPSmLTDi_Jq3NJg$=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) .