Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 17 2024 01:01:02 FOUS30 KWBC 170100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 900 PM EDT Fri Aug 16 2024 Day 1 Valid 01Z Sat Aug 17 2024 - 12Z Sat Aug 17 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST AND THE MID-SOUTH INTO THE TENNESSEE VALLEY AND SOUTHEAST U.S.... ....0100 UTC Update... Given the loss of diurnal heating (ML CAPE trends), supported by observational trends (radar and satellite loops), have been able=20 to drop quite a bit of the Marginal Risk area, i.e. the Ohio=20 Valley and Upper Midwest along with South Florida. This also=20 includes the Slight Risk that had been in place across the TN Valley. The Marginal over the Southwest meanwhile was truncated a bit on the western side (AZ border with NV and CA), again based on the observational trends and latest high-res guidance (HRRRs and HREF probabilities). Still sufficient deep layer instability and low-level moisture pooling/convergence ahead of the quasi west-east front to support maintaining the Marginal Risk and thus localized flash flood threat from the MOKSAROK 4-state area east-southeast to the Mid-South, TN Valley, and the western Carolinas. Hurley Day 2 Valid 12Z Sat Aug 17 2024 - 12Z Sun Aug 18 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE SOUTHWEST INCLUDING THE SLOT CANYON REGION OF UTAH... ....Southwest U.S... The monsoon ridge will amplify while retreating slightly to the east on Saturday to center over New Mexico and Colorado. To the west, an impressive stream of moisture will surge northward from Mexico characterized by PWs reaching to nearly 1.25" as far north as Idaho, and 1.75" across Arizona, as much as +2 to +3 sigma within the plume.. Within this plume, local enhancements to ascent will occur as shortwaves pivot northward around the ridge, accompanied by increasing upper diffluence within the RRQ of an upper jet streak and instability that may peak around 2000 J/kg during the aftn/eve. All of this points to a highly active day of monsoon thunderstorms, and both the HREF and UA WRF suggest hourly rainfall accumulations may peak around 0.75" to 1". Mean storm motions should be progressive on 0-6km winds of 15-25 kts, but repeating rounds are possible which could result in local accumulations as much as 3" as reflected by HREF neighborhood probabilities. The greatest risk for multiple rounds of heavy rain rates appears to be over the sensitive slot canyon region of Utah, and after coordination with WFOs SLC/FGZ/VEF a targeted SLGT risk was added. The remaining inherited MRGL risk was adjusted slightly for newer guidance, as impressive bulk shear in the favorable thermodynamics could result in these intense rain rates anywhere across AZ/UT and vicinity on Saturday resulting in instances of flash flooding. ....Eastern Great Lakes, Mid-Atlantic, and into the Mid-Mississippi Valley... A broad but anomalous upper trough continuing to rotate eastward Saturday will reach the Appalachians by Sunday morning. Downstream ascent through height falls, distant divergence, and diffluence within the RRQ of a poleward arcing and intensifying jet streak will provide plentiful lift for widespread but scattered showers and thunderstorms Saturday. The focus for convection will likely be in response to two primary mechanisms, the upper jet in the Mid-Atlantic, and the surface cold front from the Ohio Valley into the Mid-Mississippi Valley. Although convection is likely to be generally loosely organized and progressive, rainfall rates within favorable thermodynamics characterized by PWs of 1.25 to 1.75 inches and MUCAPE of 1000-2000 J/kg have a high probability of exceeding 1"/hr in many areas. In general, this should result in spotty 1-2" totals and only isolated flash flood responses. However, in the Mid-Atlantic, some enhanced organization beneath the RRQ of the parent jet streak could result in more organized storms on 0-6km bulk shear of 30 kts and some training from south to north. The HREF neighborhood probabilities for 3" show a local maxima above 40% from near Washington, D.C. northward into the Poconos, so a locally higher risk appears to exist here, especially in urban areas. However, guidance is quite different in the individual evolution from the models, so despite recent CSU first guess fields indicating a SLGT risk, opted to hold off and maintain the MRGL for now. However, a local upgrade may be needed with the overnight issuance. Farther south along the stalled boundary, some training of echoes is possible as the mean 0-6km winds and propagation vectors become parallel to the front from the TN VLY into the Mid-South. Despite this, HREF neighborhood probabilities for FFG exceedance are modest and the soils are dry as reflected by 40cm percentiles from NASA SPoRT, so the MRGL was extended into this area, but at this time no further upgrade appears needed. ....Pacific Northwest... An impressive upper level trough will amplify into a closed low off the northern CA coast, dropping 500mb heights to less than -3 sigma Saturday afternoon. Downstream of this low, pinched flow will transition into broad divergence over the Pacific and Interior Northwest, surging moisture and ascent into the region. PWs are progged to reach above 1 inch, and possibly above 1.25" in parts of the Cascades, reaching nearly +3 sigma from climatology. As the upper low pivots slowly northeast, this moisture fetch will be persistent into the region, and IVT values peak around 350 kg/m/s, reflective of a rare but weak August AR into WA/OR/northern CA. During this time, ascent increases within the RRQ of a strengthening upper jet streak, and mesoscale upslope ascent occurs into the Cascades. This will result in widespread showers and even scattered thunderstorms in response to MUCAPE reaching 500-1000 J/kg. The HREF neighborhood probabilities for 1"/hr reach as high as 30-40% Saturday, so despite generally rapid storm motions, brief torrential rainfall is likely, with total rainfall being enhanced by repeating rounds of cells. August ARs usually do not result in anything more than isolated flood/flash flood instances, but the MRGL risk was extended along the Cascades into northern CA, and rapid runoff/instances of flash flooding are possible, with impactful flash flooding, including debris flows, possible across recent burn scars. ....South Florida... Lingering boundary across S FL will again be a focus for deep convection with heavy rain rates exceeding 2/hr. Storm motions will likely be slow, and some training is possible as reflected by propagation vectors anti-parallel to the weak mean 0-6km winds, and HREF neighborhood probabilities reach above 70% for 3 inches. However, the focus on Saturday appears to be primarily across the Everglades, with lesser chances into the urban SE or SW coasts, so the MRGL risk was maintained. Weiss Day 3 Valid 12Z Sun Aug 18 2024 - 12Z Mon Aug 19 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID ATLANTIC... ....Mid Atlantic and Northeast... By Sunday, the cold front being driven eastward by an anomalous upper trough traversing the Great Lakes and Northeast will push into the Mid-Atlantic and New England. This will serve as a focus for another day of scattered to widespread showers and thunderstorms, with enhanced training likely on S-N oriented mean flow parallel to the from from near Washington, D.C. through Upstate NY. Here, PWs will be nearly +2 sigma according to NAEFS, which will be acted upon by the robust synoptic lift to deepen convection. Rainfall rates of 1-2"/hr are likely, which through training could produce locally more than 3" of rainfall as shown by SREF and ECENS probabilities. Where this training occurs across urban areas or sensitive soils, scattered instances of flash flooding are possible. The recent CSU FG fields depict an enhanced risk for flash flooding along I-95 and into the Tri-State area, leading to higher confidence in the inherited SLGT risk, which was extended NW slightly to account for new model guidance. Additionally, the MRGL risk was drawn much farther south into the Piedmont and Coastal Plain of the Carolinas where some short term training across extremely vulnerable soils saturated from recent TS Debby could cause isolated instances of flash flooding. ....Southwest and Intermountain West... Impressive and deep monsoonal moisture will continue to stream around the western periphery of the monsoon ridge and combine with daytime instability to produce scattered to widespread showers and thunderstorms from AZ as far north as WY and ID. Convection will be capable of producing hourly totals up to 0.75" at times, particularly across portions of Utah where the better instability and highest PW anomalies are forecast. Coordinated with WFO SLC and GJT about a SLGT risk for D3, but there is some concern about cloud cover lingering across areas that get hit with heavy rain on Saturday limiting the intensity and overlap. While this limits confidence below a SLGT risk, it is still likely that at least isolated instances of flash flooding will occur Sunday across sensitive soils and terrain features. It is possible that a SLGT risk may be needed for portions of the area with later updates as well. ....South Florida... Yet another day of scattered slow moving thunderstorms with 2-3"/hr rain rates is likely on Sunday as a lingering trough persists. The focus on Sunday appears to be primarily in the vicinity of the Everglades, but some low-end potential exists into both the SE and SW urban coasts, which could also experience heavy rain on D1 and D2. After coordinating with WFO MFL, a small MRGL risk was added for D3 to cover the continued isolated flash flood risk. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7IdubrXEAQL6r181Qpe2LdDTrHjDC7Y5ZRjdeqEj65lB= O5866BJf2mGbDBmzRHQD4yxTREA7rYyK9YG9ONPtt-IySmU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7IdubrXEAQL6r181Qpe2LdDTrHjDC7Y5ZRjdeqEj65lB= O5866BJf2mGbDBmzRHQD4yxTREA7rYyK9YG9ONPt7rWvQxI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7IdubrXEAQL6r181Qpe2LdDTrHjDC7Y5ZRjdeqEj65lB= O5866BJf2mGbDBmzRHQD4yxTREA7rYyK9YG9ONPtqq2c16w$=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) .