Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 19 2023 19:29:48 FOUS30 KWBC 191929 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 328 PM EDT Wed Jul 19 2023 Day 1 Valid 16Z Wed Jul 19 2023 - 12Z Thu Jul 20 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR WESTERN KENTUCKY & WESTERN TENNESSEE... ....Tennessee Valley... Heavy rains continue across portions of MO, western KY, and TN within an area of persistent 1000-500 hPa height/thickness diffluence which should perhaps drift a little east today. While the 12z HREF is too far northeast with ongoing activity, it could persist for a few more hours. The excessive rain areas were shifted westward to match radar trends. Otherwise, there is a signal in the mesoscale guidance for the initial convection to shift into eastern TN and becoming potentially problematic this afternoon, so re-extended the Slight Risk area eastward to account for the low but existing probabilities of 5"+. With the western portion of the Slight Risk and the Moderate Risk area, there's the potential for afternoon convection along a relatively stationary boundary being pinned by competing forces: current convective outflow trying to shift the boundary south and west, and some ridge expansion from west of the area which brings up 700 hPa towards 12z. Also, convection from the central Plains is expected to move into the western portion of the Slight and possibly the Moderate Risk area overnight into early Thursday. Therefore, there has been no downgrade to the risk, and a High Risk was considered. For the time being, the area that has seen 6-11" of rainfall is fairly small and considered too small for a High Risk upgrade. Should the situation this afternoon be worse than expected, an upgrade to a High Risk remains possible. ....Southern Mid-Atlantic/North Carolina... Locally heavy rains also possible Wednesday ahead of a surface front pushing east along the Mid-Atlantic coast. Kept the small slight risk area across far southeast Virginia in the urbanized Hampton Roads region. Farther to the north, the marginal risk was extended north into far southeast PA and northern NJ to cover heavy rainfall potential over areas that have relatively low ffg values.=20 ....Lower Missouri Valley toward the Lower Ohio Valley... Along the east to west oriented front, locally heavy rains are possible across much of Kansas into southeast Colorado given the favorable upper difluence and high PW axis along this front and sufficient instability. These areas, however, have been relatively drier than areas farther to the east along the front through the Lower MO Valley into the TN Valley. At the moment, the threat level was kept at marginal across these areas. ....Upper Mississippi Valley... Few changes were made to the marginal risk from the previous extending northeastward through the Upper MS Valley into the Upper Lakes region. Models remain consistent in showing a narrow axis of above average PW values and instability along and ahead of the associated cold front ahead of strong height falls pushing eastward from the Northern Plains into the Upper MS Valley. Model consensus continues to be for fairly progressive convection across this area. With precip mostly below average over the past few weeks across the Upper MS Valley and lower than average stream flows, believe the threat for excessive rainfall will remain marginal. ....Northern Sierra, Great Basin into portions of the Southwest... Only some slight changes made to the previous marginal risk areas the Great Basin and Southwest. PW values expected to remain above average, 1 to 2+ standard deviations above the mean, from the northern Sierra, eastward across portions of the Great Basin into the Central Rockies. Scattered diurnal convection likely in this above average PW axis, supporting isolated runoff issues. A small marginal risk area also maintained over southeast AZ into southwest NM where isolated slow moving cells again possible Wednesday afternoon. Roth/Oravec Day 2 Valid 12Z Thu Jul 20 2023 - 12Z Fri Jul 21 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL/SOUTHERN HIGH PLAINS & THE EASTERN UNITED STATES... ....Lee of the Central Rockies into the Central to Southern High Plains... Additional shortwave energy expected to rotate east southeastward on the northeast side of the Southwest upper high, pushing east southeast from the lee of the Central Rockies into the Central to Southern Plains. The shortwave will be progressive to the north of a surface front dropping southward from Central Plains into the Southern Plains during Thursday. Post frontal upslope flow and enhanced upper divergence will help initiate convection in the vicinity of southeast WY/western NE/northeast CO Thursday afternoon, but it is forecast to shift southward with time. This activity will likely enhance into organized convection pushing southeastward southeast CO/western to central KS/far northeast NM and the TX/OK Panhandle region Thursday night into the early hours of Friday in a region of above average PW values and CAPE values 1500 -3000 j/kg. Stream flows are still mostly above average across these areas, although much of the area has been dry over the past week. There may be some overlap to locally heavy rains amounts expected Wednesday afternoon and evening over portions of southeast CO into western KS, which could leave soils more saturated. No significant changes to the previous outlook with only a slight southeast expansion of the slight risk into far northeast NM and far northwest TX. Widespread 1-2"+ amounts possible with localized values of 3-5"+. There remains some spread in the axis of heaviest amounts. Thus far, the mesoscale guidance shows a progressive convective system, so have kept the risk level as Slight. This could change should more rain than expected fall before now and then, or if the expected convective evolution changes to some degree. ....In and near KY and TN... An area of 1000-500 hPa thickness and height diffluence across the region implies the possibility of organized convection across the area. Recent rains across portions of KY and TN have left soils increasingly saturated. There is some signal, particularly amongst the 12z NAM and 12z Canadian Regional, which is seen somewhat in the 12z HREF probabilities of 5"+, of another potential convective episode in this region near and ahead of an invading cold front. Inflow at 850 hPa is convergence and high enough in magnitude to worry about organized convection should could train in bands. With ML CAPE rising to 3000+ J/kg interacting with PWs of 1.75-2", hourly rain totals to 2.5" are within the realm of possibility. The new Slight Risk area has greater than average potential for excessive rainfall/flash flooding considering recent rains.=20 ....Carolinas into the Northern Mid-Atlantic... The strong upper level trough will move from Upper MS Valley through the Great Lakes, OH Valley and Southern to Central Appalachians Thursday and Thursday night. Increasingly difluent upper flow, defined frontal convergence and southwest upslope into the Appalachians will support increasing convective coverage ahead of this front. The guidance is attempting to center in more on portions of PA and northern WV, so constrained the Slight Risk to those areas. Hourly rain totals to 2" with local 3" amounts are possible. Some of the region, particularly central PA, has seen a very wet week. Coordination with the CTP/State College PA forecast office helped lead to this reconfiguration. ....Eastern L.P. of Michigan... A Slight Risk remains across the eastern portions of the L.P. of Michigan where flash flood guidance values are relatively low in the wake of a wet week and an organized convective squall line along and ahead of the above mentioned cold front which will likely push across Thursday afternoon with hourly rainfall amounts up to 1.5", and local amounts of 3" where cells merge, manage to train for a couple hours, or a mesocyclones manage to form. Roth/Oravec Day 3 Valid 12Z Fri Jul 21 2023 - 12Z Sat Jul 22 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR THE MID-SOUTH & NORTHEAST NEW MEXICO... ....Mid-South/Southeast... An axis of above average PW values will stretch west to east from the Southern Plains into the Lower MS Valley/TN Valley and Southeast in the vicinity of the west to east oriented front forecast to be moving slowly south and southeast day 3.=20 Shortwaves moving west to east along the southern edge of the westerlies and this frontal zone will enhance lift in the above average PW axis and support potential for widespread scattered convection along and ahead of the boundary. Most of the guidance has shifted south in this region, so the Slight Risk follows suit better matching the intermediate 12z ECMWF and 12z Canadian Regional solutions, as the atmosphere ahead of the front is uncapped. The 12z NAM in particular shows a boundary moving ahead of the front which focuses convection farther south. Further adjustments to the south are possible as we get closer to the Friday. ....Southern High Plains... Low-level easterly upslope flow will strengthen Friday and Friday night as the cold front continues to sink southward through the Southern High Plains. To the south across Southern NM, an easterly wave moving south of the US/Mexican border at 700 hPa aids in the upslope and moisture surge. The small slight risk area added overnight across portions of central and northeast NM still generally works, when viewing the 12z guidance consensus of potentially heavy rains over northeast New Mexico. This potential precip area coincides with relatively low flash flood guidance values where rainfall over the past week has been 2 to 3 times average in places. ....Northeast Mid-Atlantic into New England... A strong upper level trough moves from the Appalachians through New England Friday. Precipitable water values ~1.5" and CAPE of 1000-2000 J/kg should promote convection containing heavy rainfall. Additional locally heavy precip totals possible from eastern New York State into central to northern New England, with hourly rain totals up to 1.75" and local amounts of 2-4" possible. Considering heavy rains over the past week, and the high stream flow across these areas, added a Slight Risk for northeast NY and much of VT. Roth/Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8uLtotM7QqMR1G2bqg5s55IuXShutF6OSRCN_eTgCZTT= rwGJ6j4kOYRu1NzTG1dG2uVm1bk1za9sWY5TqCw2KZ5bP6U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8uLtotM7QqMR1G2bqg5s55IuXShutF6OSRCN_eTgCZTT= rwGJ6j4kOYRu1NzTG1dG2uVm1bk1za9sWY5TqCw2lQ888Gs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8uLtotM7QqMR1G2bqg5s55IuXShutF6OSRCN_eTgCZTT= rwGJ6j4kOYRu1NzTG1dG2uVm1bk1za9sWY5TqCw2NyEZXCo$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .