Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 18 2023 19:33:31 FOUS30 KWBC 181933 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 332 PM EDT Tue Jul 18 2023 Day 1 Valid 16Z Tue Jul 18 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... The potential remains for one or two narrow axes of very heavy rainfall amounts in two rounds, with the potentially more significant round expected Tuesday night into Wednesday morning period a hair farther east from the first across portions of the Lower Missouri Valley/Mid Mississippi Valley/Lower Ohio Valley and Lower Tennessee Valley region expected near a frontal boundary and within an area of diffluent 1000-500 hPa heights/thickness which shifts from MO into western KY with time. The inflow at 850 hPa should remain persistent out of the west-southwest, growing in magnitude with time. 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. The overall CAPE field is expected to move/shift eastward with time.=20 An initial round of convection firing late Tuesday morning ahead of a vort moving east southeast from the Central Plains into Missouri. Model consensus is that this initial organized area of convection will be fairly progressive to the southeast across the risk area during the afternoon and evening hours of Tuesday. This would then be followed by a second round of greater training potential convection sometime in the 0300 to 1200 UTC Wednesday period when the low level inflow increases partially due to the nocturnal low-level jet, increasing the potential for back building and downstream training of convection, which due to increasing instability from west to east could lead to a second heavy rain streak east of the earlier convective round. While there is general agreement in this scenario, there are some fairly differences in where the training episode will set up and whether or not we'll have one training band or two. Also complicating matters is the significant variance in rainfall over the past week across the region, with areas generally west, south, and southeast of St. Louis the wettest while the remainder of the region has been fairly dry. The changes in the Moderate Risk area fit well with the 12z HREF neighborhood probabilities for 5"+ and 8"+ amounts. These totals seem reasonable given the potential for two rounds of convection which may overlap and the potential for the training episode in the early hours of Wednesday when more significant dynamics are at play. Hourly rain totals to 2.5" 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. Changes in the Slight and Moderate risk areas were coordinated with NWS forecast offices in that region. The expansion made to the Marginal Risk across the Ohio Valley and WV was made based on the 12z HREF guidance. ....Northeast... Only slight changes were made to the marginal and slight risk areas from the previous outlook across the Northeast. The slow moving front currently over the eastern Great Lakes region will begin to push more quickly eastward day 1 across the Northeast. A narrow axis of above average PW values in the 1.5-1.75"+ range are expected along and ahead of this front. These values will be lower than PW values were with the heavy rainfall event on Sunday as that anomalous PW axis has moved well off the northeast coast, but still would support hourly rain totals to 2". Additional convection is likely to stretch along this front on Tuesday, perhaps very soon after this discussion's issuance, supporting locally heavy rainfall totals from in the vicinity of northern NJ/eastern NY State, northeast through western and northwestern New England. The slight risk area fits well with the HREF neighborhood probabilities for heavy rainfall amounts. There will be some overlap with heavy rains on Sunday and the expected heavy axis on Tuesday from southeast NY State into northwest CT and western MA and east central NH. As the 12z HREF has some signal for 5" amounts, this should be near the high bar for local amounts in this region.=20 ....Southwest into the Great Basin... Widespread monsoonal convection possible across portions of the Southwest and Great Basin as daytime heating interacts with PWs of 0.75-1", which would foster heavy rainfall particularly in areas of terrain. An axis of more anomalous PW values are expected to 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 do 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... Heavy rainfall over the Space Coast is forecast during daytime heating, which could have some organization as effective bulk shear approaches 25 kts, within an area of PWs of 2-2.25". Hourly rain totals up to 3" are possible, and there is a small signal for 5"+. Enough evidence exists for a new Marginal Risk area in that location. Down south, other than the usual disorganized but efficient pulse convection in an anomalously moist airmass during daytime heating today, an easterly wave approaches the southern portion of the peninsula and the Keys very late in the period which could lead to an early morning uptick on Wednesday. Enough ingredients were available -- particularly moisture and instability -- to extend the new Marginal Risk southward through the Keys. Roth/Oravec 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9wjPr-3vUDknWXiOqvFqUL6xzFTeykV5A1TeXmJEJ_tQ= Ueyi-gCu0neogboHETbS7ChP-ZBwDME5rcmRU5KwLRr5DY4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9wjPr-3vUDknWXiOqvFqUL6xzFTeykV5A1TeXmJEJ_tQ= Ueyi-gCu0neogboHETbS7ChP-ZBwDME5rcmRU5KwE83Dr8U$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9wjPr-3vUDknWXiOqvFqUL6xzFTeykV5A1TeXmJEJ_tQ= Ueyi-gCu0neogboHETbS7ChP-ZBwDME5rcmRU5KwWohks9o$=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) .