Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 29 2024 23:58:21 FOUS30 KWBC 292358 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 758 PM EDT Mon Jul 29 2024 Day 1 Valid 01Z Tue Jul 30 2024 - 12Z Tue Jul 30 2024 .....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE OHIO VALLEY, UPPER TENNESSEE VALLEY INTO THE SOUTHERN APPALACHIANS... ....Ohio/Tennessee Valleys & Southern Appalachians... West-northwest mid to upper level flow will persist from the=20 Northern Plains through much of the Mississippi Valley, Ohio=20 Valley, Southern to Central Appalachians, Mid- Atlantic and=20 Southeast. Embedded shortwaves in this flow will support another=20 day of active convection across much of these areas where PW values will be mostly above seasonal averages. A Slight risk area remains where the latest HREF neighborhood probabilities were the highest=20 for 1, 2 and 3"+ totals and overlapped the lower FFG values and=20 observed precip axis from recent rains. This results in a slight=20 risk from southern Illinois, east-southeast through the Ohio=20 Valley, Upper Tennessee Valley into the Southern Appalachians.=20 Changes from the previous issuance were minimal.=20 ....Southeast Arizona into southwest New Mexico... The Marginal Risk area across the Southwest remains in place.=20 Existing scattered convection should persist past sunset, when CIN will set in and begin to diminish coverage and the heavy rainfall=20 potential. In the meantime, there is the potential for isolated=20 heavy totals and localized runoff issues. Roth/Oravec Day 2 Valid 12Z Tue Jul 30 2024 - 12Z Wed Jul 31 2024 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN MID-ATLANTIC INTO THE CENTRAL APPALACHIANS... ....2030Z Update... ....Northern PA & Southern NY... An approaching 500mb vort max that is also associated with a remnant MCV moving through the Great Lakes on Monday will make its way into the northern Mid-Atlantic on Tuesday. The upper trough will generate ascent atop the atmosphere while the environment sports MLCAPE up to 1,000 J/kg and PWs maxing between 1.8-2.0". While storm motions should generally range between 15-20 kts, ECMWF area averaged soundings in the Slight Risk area do show upshear Corfidi vectors <10 kts, suggesting the potential for back-building thunderstorms. This is particularly the case along mountain ranges that are oriented favorably to the 850-300mb mean flow out of the SW. 1-hr FFGs are lowest over the southern tier of NY where values are <1"/hr in some spots. For these reasons, a Slight Risk was introduced for parts of Northeast PA and the southern tier of NY given their more sensitive soils. ....Eastern IA & Northern IL... A 500mb disturbance tracking across the Northern Plains will track towards the western Corn Belt while a 50kt 500mb jet over the Upper MS Valley helps to provide favorable synoptic-scale lift over the region. At the same time, 850mb ridging over the Southeast and troughing over the northern Plains will cause an acceleration in the LLJ (925mb winds 2-3 standardized anomalies above normal in the Central Plains) to deliver a surge of 925-850mb moisture over the Midwest. This LLJ will also favorably intersect a stationary front draped WNW to ESE, which along with veering winds as air parcels rise will support organized thunderstorm activity. PWs up to 1.8", MUCAPE >2,000 J/kg, and hodographs that provide sufficient shear to sustain long duration thunderstorm activity could result in back building and training thunderstorms either along the stationary front and/or along outflow boundaries that are supported by deep cold pools. Latest 12Z HREF's 3-hr QPF > 3-hr FFGs product depict up to moderate chance probabilities (40-50%) for 3-hr FFG exceedance while low-to-moderate probabilities (20-40%) for 1-hr QPF > 1-hr FFGs were present over eastern IA. There remains some notable spread in guidance with regards to the position and orientation of where the heaviest QPF area will set up. That said, the region highlighted is also currently sporting 1-hr FFGs that in most areas are <2"/hr. Given the optimal atmospheric setup for storms generating >2"/hr rates and sensitive soils, a Slight Risk was introduced for portions of eastern IA and northern IL. ....Northern Rockies... A fast moving -1.5 to -2.0 standardized anomalies 500mb trough tracking through the Northern Rockies will also be accompanied by PWs that are hovering around 1.0", which are as much as +2 standardized anomalies over the region. In addition, MUCAPE up to 500 J/kg will support rainfall rates as high as 1.5"/hr in the most intense areas of convection. Storms should be progressive so that will help keep the flash flood threat to a very localized scale, but the available atmospheric moisture and instability could produce intense enough rainfall rates to cause rapid runoff along complex terrain and burn scars. A Marginal Risk was introduced to account for the very localized flash flood potential Tuesday afternoon and evening. Outside of minor tweaks to the Southwest's Marginal Risk, the remainder of the previous shift's forecast discussion is on track. Mullinax ---Previous Discussion--- ....Southwest Florida... The previous marginal risk area was decreased in size and concentrated over Southwest Florida. This corresponds to where the HREF neighborhood probabilities for the 12 hour period from 12Z Tue to 00Z Wed are highest for 2 and 3"+ amounts. Isolated runoff issues possible, especially over urbanized regions. ....Southwest... No changes expected to the large scale flow with the upper ridge remaining centered over the Southern Plains. This will again support shortwaves rotating northward on the western side of this ridge into the Southwest, supporting scattered convection, localized heavy rains and isolated runoff issues. Oravec Day 3 Valid 12Z Wed Jul 31 2024 - 12Z Thu Aug 01 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE UPPER MISSISSIPPI VALLEY INTO THE MID WEST... ....2030Z Update... Minor adjustments were made to the Slight Risk in the Midwest as there was not enough consensus on guidance to make significant adjustments at this moment. The only tweaks made were to take out the Marginal Risk in western NC and southern VA where QPF has dropped compared to the previous cycle and to expand the Marginal in the Northeast southward to include more of the southern tier of NY where soils are more sensitive. Otherwise, the previous forecast discussion remains on track. Mullinax --Previous Discussion-- ....Upper Mississippi Valley into the Midwest... The progressive northern stream flow will continue into day 3 with the next strong area of height falls expected to push eastward across the Northern Plains and into the Upper Mississippi Valley. There is potential for additional organized convection/heavy rainfall to push eastward with these height falls across eastern portions of the Northern Plains and into the Upper Mississippi Valley. There are some typical latitude differences with the max qpf axis. The day 3 slight risk area was drawn to favor the more southern solutions of the EC, NAM and CMC. The previous slight risk area over the Upper Mississippi Valley was suppressed more to the south and southwest to better match the more southern solutions with the next round of potentially organized convection. ....Eastern New York State into western New England... The strong height falls moving out of the Central Appalachians into the Northern Mid-Atlantic day 2, will progress across eastern NY State into New England day 3. There is a typical amount of qpf spread for the day 3 period, but general consensus for potential of moderate to locally heavy rainfall amounts in a region of much above average PW values ahead of these height falls. A broad marginal risk area was maintained from the previous issuance to capture the heavy rainfall spread in the models across this area day 3. ....Southwest... Similar to the days 1 and 2 period, the mid to upper level ridge will remain centered over the Southern Plains. Scattered convection again likely over portions of the Southwest as shortwave energy again rotates northward on the western side of this ridge and into the Southwest. Localized heavy rains and isolated runoff issues again possible for southeast Arizona into southwest to central New Mexico. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_56jXGyS4Xaz6m3_tLYVMPLCOWY8dbUxk-lnbygzY49z= BcZAr6ntgjb1DTm9lQ1op7v4Zfv3u9VvXDPIs3s1QZVPZWk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_56jXGyS4Xaz6m3_tLYVMPLCOWY8dbUxk-lnbygzY49z= BcZAr6ntgjb1DTm9lQ1op7v4Zfv3u9VvXDPIs3s1DV9fiF8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_56jXGyS4Xaz6m3_tLYVMPLCOWY8dbUxk-lnbygzY49z= BcZAr6ntgjb1DTm9lQ1op7v4Zfv3u9VvXDPIs3s1hCf_G5Q$=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) .