Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 16 2024 19:55:23 FOUS30 KWBC 161955 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 355 PM EDT Fri Aug 16 2024 Day 1 Valid 16Z Fri Aug 16 2024 - 12Z Sat Aug 17 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEAST U.S... 16Z Update... Changes to the broad ERO areas were relatively minor with the morning update to account for modest adjustments related to 12Z CAMs. The most substantial adjustment was to extend the MRGL risk back across the Ozarks and into far eastern KS/OK where the high- res has gotten into better agreement that an MCS will develop beneath a shortwave as it interacts with a sharp instability gradient along a stationary front. 20-30 kts of 850mb inflow will support expanding convection within this MCS, and propagation vectors aligned to the mean wind and front suggest training of 1-2"/hr rain rates. This could produce locally more than 3" of rain and instances of flash flooding. Considered a small SLGT risk as well for the urban SE coast of southern FL where 06Z HREF probabilities for 5" were notable. However, the synoptic pattern seems to support enough push SW that the SW coast would be more prone to isolated flash flood instances due to convergence along the sea breeze there. The 12Z HREF probs and CAMs did pivot subtly in that direction, so felt the MRGL risk was well placed and no upgrade was needed. Otherwise, any changes to the inherited were cosmetic and most of the overnight discussion was still valid. ....Southeast U.S... Convection across portions of the lower Ohio Valley early this morning is forecast to reach portions of southern/eastern Kentucky and Tennessee later today and eventually into northern Alabama/Georgia. There is lower confidence on how the activity will hold together or what additional line segments will develop but with plentiful moisture and sufficient instability in place this afternoon, several areas of heavier rainfall will be possible. Based on the latest guidance and 12Z HREF probabilities, pockets of 1-2" hourly totals will be possible, most likely across portions of middle/eastern TN, far southeast KY, and into northern AL/northwest GA where the Slight Risk in the Excessive Rainfall Outlook exists. Some localized totals of 2-4" will be possible within that area and could lead to scattered instances of flash flooding. ....Great Lakes through the Ohio Valley and into the Ozarks... For today and tonight, a cold front is expected to move eastward from the Midwest through the Ohio Valley as the main upper level shortwave energy digs further into the Great Lakes region. This combination should provide plenty of support for scattered to numerous showers and thunderstorms to develop along the advancing cold front across the Ohio Valley into the Central Appalachians. The activity should be fairly progressive and quick moving, limiting the overall duration of any pockets of heavy rainfall at any one location, but a few spots of merging storm cells or training could develop that pose a flash flood risk. Farther north into the Great Lakes (Wisconsin/Michigan), storm motions under the core of the upper trough are likely to be slower and when combined with the higher PWs and somewhat elevated soil moisture percentiles (Wisconsin), a few instances of flash flooding will be possible. Farther south through the Ozarks, the front should align generally W-E while a shortwave moves atop it tonight. As the LLJ surges moisture into this boundary, an MCS is likely to develop with training of 1-2"/hr rain rates possibly causing instances of flash flooding. ....Southwest U.S... Increasing monsoonal moisture will begin to lift northward into portions of the region today, the highest PW anomalies will likely be tied to far southern Arizona but the 12Z guidance is keying on enough moisture and sufficient instability developing today combined with the subtle shortwave lifting through to spark at least isolated thunderstorms into portions of southern NV, especially late this evening and tonight and southern/central New Mexico, although instability may be somewhat marginal this far east. Some of the convection will be capable of producing intense rain rates and could lead to flash flooding over the typical vulnerable locations. ....South Florida... A trailing frontal boundary and trough over the region combined with a very high moisture axis will bring another round of scattered to numerous thunderstorms to South Florida later today through the evening hours. The combination of high PWs and sufficient instability will lead to very intense and efficient rain producing thunderstorms, capable of producing 2-3" hourly totals at times. The 00Z guidance was quite bullish for southeast FLorida coastal areas with potential for some locations to see several inches today, but the 12Z has backed off somewhat as the synoptic flow becoming more E/NE appears sufficient to push convection towards the Everglades and SW coast. There could be some slowing/training of intense rain rates on both urban coasts, but the signal shifting more towards the Everglades prevents any upgrades from the inherited MRGL risk at this time. Weiss/Taylor 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,=20 accompanied by increasing upper diffluence within the RRQ of an=20 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.=20 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=20 guidance, as impressive bulk shear in the favorable thermodynamics=20 could result in these intense rain rates anywhere across AZ/UT and=20 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=20 to north. The HREF neighborhood probabilities for 3" show a local=20 maxima above 40% from near Washington, D.C. northward into the=20 Poconos, so a locally higher risk appears to exist here, especially in urban areas. However, guidance is quite different in the=20 individual evolution from the models, so despite recent CSU first=20 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=20 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=20 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=20 strengthening upper jet streak, and mesoscale upslope ascent=20 occurs into the Cascades. This will result in widespread showers=20 and even scattered thunderstorms in response to MUCAPE reaching=20 500-1000 J/kg. The HREF neighborhood probabilities for 1"/hr reach=20 as high as 30-40% Saturday, so despite generally rapid storm=20 motions, brief torrential rainfall is likely, with total rainfall=20 being enhanced by repeating rounds of cells. August ARs usually do=20 not result in anything more than isolated flood/flash flood=20 instances, but the MRGL risk was extended along the Cascades into=20 northern CA, and rapid runoff/instances of flash flooding are=20 possible, with impactful flash flooding, including debris flows,=20 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=20 be capable of producing hourly totals up to 0.75" at times,=20 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!6w08urrXLO6egPmnijp_bYt7CPiu9tpTdgeRIRg-fObA= PfoOOUnAYeBQb_1QkLwv32RxStwYWu2EW11c8aBg6yR92fE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6w08urrXLO6egPmnijp_bYt7CPiu9tpTdgeRIRg-fObA= PfoOOUnAYeBQb_1QkLwv32RxStwYWu2EW11c8aBg4p_dwYs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6w08urrXLO6egPmnijp_bYt7CPiu9tpTdgeRIRg-fObA= PfoOOUnAYeBQb_1QkLwv32RxStwYWu2EW11c8aBgJy0kO5I$=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) .