Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 19 2023 00:58:06 FOUS30 KWBC 190057 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Tue Jul 18 2023 Day 1 Valid 01Z Wed Jul 19 2023 - 12Z Wed Jul 19 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID MISSISSIPPI VALLEY, LOWER OHIO VALLEY, & LOWER TENNESSEE VALLEY... ....Lower Missouri Valley, Mid Mississippi Valley into the Lower Ohio and Lower Tennessee Valleys... One round of moderate to heavy rainfall has moved through portions of the Mid-Mississippi Valley during the day...with the next round still on tap overnight from parts of Missouri into western Kentucky and northwest Tennessee as increasingly diffulent 1000-500 mb heights and thickness values spread over a frontal boundary draped over the area. 00Z sounding showed 850 mb winds already in the 25 to 35 kt range from southwest Missouri into Oklahoma...which should make their way into the Moderate Risk area later tonight. Precipitable water values should range in the 1.75-2.25" range. which implies tall, skinny CAPE and sufficient wet bulb zero height for warm rain processes. Concern has been for overnight convection to have a greater potential for training sometime in the 0300 to 1200 UTC Wednesday period when the low level inflow increases partially due to the nocturnal low-level jet. As previously mentioned, a complicating factor is the significant variance in rainfall over the past week across the region. Areas generally west, south, and southeast of St. Louis have been the wettest while the remainder of the region has been fairly dry. Changes made this outlook were to expand the Slight Risk towards the south and west a bit...now covering more of far southeast Missouri and to realign the Moderate a bit more to the northwest/southeast...closer to the orientation shown by the HRRR/ARW and close to the 12Z HREF neighborhood probabilities for 5"+ and 8"+ amounts. Hourly rain approaching 3" are anticipated, which would be more than enough for issues in any urban areas within the various Risk areas. Cell training and embedded mesocyclones appear to be the bigger players in heavy rain production in this pattern. ....Northeast... Only slight changes were made to the marginal and slight risk areas from the previous outlook across the Northeast based on the radar and satellite imagery trends through early this evening. A slow moving front currently making its way eastward over parts of western New England will begin to push more quickly eastward and slice into an narrow corridor with above average PW values in the 1.5-1.75"+ range. The combination could still support maximum hourly rain totals of an inch...with HREF from 12Z indicating a 20 pct chance of of an inch of rain in 3-hours before 06Z which tapers off after that. However...much of the area remains highly sensitive after several rounds of excessive rainfall recently so a Slight risk area still appears warranted. ....Southwest into the Great Basin... Scattered to embedded areas of numerous showers and thunderstorms over parts of the Great Basin into the Southwest U.S. will gradually fade with the loss of daytime heating. Until then, an airmass with precipitable water values of 0.75-1", which will continue to foster isolated heavy rainfall amounts...particularly in areas of terrain. An axis of more anomalous PW values will rotate eastward on the north side of the persistent southwest upper ridge across portions of NV into western CO, supporting scattered slow moving convection across these areas. HREF probabilities still show 20-30% probabilities of .50"+ amounts in an hour. This could produce some isolated runoff issues, especially across any burn scar regions, arroyos/dry washes, or slot canyons. ....Florida... With an easterly wave still slated to approach the southern Florida peninsula and the Keys late tonight/early Wednesday morning...maintained the Marginal Risk area there but trimmed out the Marginal Risk area farther north where the low-levels should be stabilizing fairly quickly with the loss of daytime heating.=20 With precipitable water values at or above 2 inches already in place and the approach of shortwave energy...there are at least some ingredients that will support isolated intense rainfall rates and 1 to 3 inches of rainfall totals. Bann Day 2 Valid 12Z Wed Jul 19 2023 - 12Z Thu Jul 20 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR TENNESSEE & SOUTHEAST VIRGINIA... ....Tennessee Valley into the Southern Appalachians and southern Mid-Atlantic states... Heavy rains should continue into the morning hours of Wednesday across portions of TN and KY. The previous slight risk area was reconfigured based on the latest guidance, with a new Slight Risk area added to southeast VA. An MCV from morning activity could be the catalyst for the heavy rainfall signal showing up near Southeast VA. While initial heavy rainfall should be fading with time on Wednesday morning, additional scattered convection possible in the vicinity of the frontal boundary stretching from Southeast VA through the the Tennessee Valley across the Mid Mississippi Valley and into the Southern Plains. PW values in the vicinity of this front expected to remain above average, 1.75-2"+. Additional shortwave energy moving along the southern edge of the westerlies will enhance upward vertical motion near the front and above average PW axis. This will support locally heavy rains across these areas. Model consensus is not cohesive on where the heavy amounts will be, but enough of a signal exists within the two Slight Risk areas. Subsequently, just a marginal risk depicted in the vicinity of the front to the west of the TN Valley heavy early day 2 rains. ....Southwest into the Great Basin... An axis of above average PW values -- generally 0.75-1" -- will continue on the north side of the Southwest upper ridge, supporting additional scattered convection across northern NV into central to northern UT and CO, with emphasis likely in area terrain. Scattered monsoonal convection also possible again into southeast AZ and NM. Isolated runoff issues, especially across any burn scar regions, dry washes/arroyos, or slot canyons are possible from isolated .50-1" rainfall totals. ....Upper Mississippi Valley... The marginal risk from the previous issuance was slightly reconfigured but remains more or less the same. This area describes convective potential ahead of fairly strong northern stream trough forecast to push from the Northern Plains into the Upper MS Valley. A narrow axis of above average PW values and instability along and ahead of the associated cold front will support potential for scattered convection Wednesday into early Thursday. At the moment, it appears convection ahead of this front should be fairly progressive, keeping the risk level at marginal.=20 Roth/Oravec Day 3 Valid 12Z Thu Jul 20 2023 - 12Z Fri Jul 21 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE CENTRAL/SOUTHERN HIGH PLAINS AND IN AND NEAR THE SOUTHERN & CENTRAL APPALACHIANS... ....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. A surface front dropping southward from Central Plains into the Southern Plains during Thursday with induce a period of upslope flow, which brings a significant amount of moisture, with PWs exceeding 1.5" in easternmost CO, which is on the high end of climatology for any month. Enhanced upper divergence associated with mid- to upper-level shortwaves and CAPE values of 2000-3000 J/kg will support increasing convection Thursday afternoon into Thursday night/early Friday along and north of this front. There is fairly good model agreement on an axis of moderate to heavy rains across southeast WY, southwest NE, eastern CO, western to central KS and northern OK. Stream flows are still mostly above average across these areas, although much of the area has been dry over the past week. Hourly rain totals to 2.5" with local amounts of 4-6" are possible based on the above ingredients and the available guidance. There remains spread in the guidance on the exact location, which left the risk level Slight. A Moderate Risk cannot be ruled out should the guidance become more agreeable or a little wetter. =20 ....Southern to Central Appalachians into the Northern Mid-Atlantic... A strong northern stream upper level trough will be pushing east through the Great Lakes, OH Valley, Southern to Central Appalachians region day 3, becoming increasingly negatively tilted with time. Strong upper difluence and defined frontal convergence in an axis PW values 1.5-1.75" and Cape values 1000-2500 j/kg will support widespread scattered convection along and ahead of the associated cold front. Expanded the slight risk to cover where potentially heavy rains coincide with the relatively low FFG values stretching from the Southern to Central Appalachians into the Northern Mid Atlantic. Roth/Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9g0XD_jL_Xky8mPjZypKzhgjj1ygDLQMgpc7SP0rBUi6= tUsPxu5cHycC6RjmXKE-NimOmnbzTbX3JwRB8U0EcS8r_5E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9g0XD_jL_Xky8mPjZypKzhgjj1ygDLQMgpc7SP0rBUi6= tUsPxu5cHycC6RjmXKE-NimOmnbzTbX3JwRB8U0EzbCrWS0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9g0XD_jL_Xky8mPjZypKzhgjj1ygDLQMgpc7SP0rBUi6= tUsPxu5cHycC6RjmXKE-NimOmnbzTbX3JwRB8U0E9GUB7yg$=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) .