Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 18 2023 15:55:54 FOUS30 KWBC 181555 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1154 AM 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 ACROSS THE TENNESSEE VALLEY INTO THE SOUTHERN APPALACHIANS... ....Tennessee Valley into the Southern Appalachians... Heavy rains may continue into the morning hours of Wednesday from the expected active training event during the later portion of the day 1 time period. Similar to the day 1 period, there are differences where the post 1200 UTC most organized convection may be, but still a strong signal that heavy totals may continue into early day 2. The previous slight risk area was trimmed on the northeast side from southern WV into eastern KY and far Southwest VA to reflect model consensus for heavy rains early day 1 centered farther to the west across far southern KY into central to eastern TN. While the potential heavy rains early day 1 will likely be weakening with time Wednesday morning, additional scattered convection possible in the vicinity of the frontal boundary stretching from 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 uvvs in the vicinity of the west to east oriented front and above average PW axis. This will support additional locally heavy rains day 2 across these areas. Model consensus is that coverage of precip areas will be scattered, leading to low confidence in where heavy amounts will be. 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... No changes overall expected to the large scale flow pattern from the day 1 period. An axis of above average PW values 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. Scattered monsoonal convection also possible again into southeast AZ and NM. Similar to the day 1 period,=20 isolated runoff issues, especially across any burn scar regions or slot canyons are possible from isolated .50-1" rainfall totals. ....Upper Mississippi Valley... The marginal risk from the previous issuance was also expanded south from the Upper Lakes/northern MN area, through the Upper MS Valley. This was for convective potential ahead of fairly strong northern stream height falls forecast to push from the Northern Plains into the Upper MS Valley during day 2. 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 Oravec Day 3 Valid 12Z Thu Jul 20 2023 - 12Z Fri Jul 21 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE LEE OF THE CENTRAL ROCKIES INTO THE CENTRAL TO SOUTHERN HIGH PLAINS AND ACROSS THE SOUTHERN TO CENTRAL APPALACHIANS INTO THE NORTHERN MID ATLANTIC... ....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. These height falls will be pushing along a surface front dropping southward from Central Plains into the Southern Plains during Thursday. Enhanced upper divergence/uvvs associated with these height falls in an axis of above average PW values in the vicinity of this front 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.=20 No significant changes were made to the previous slight risk across this area given the good model agreement and potential for organized convection, especially in the 0000 to 1200 UTC Friday period. ....Southern to Central Appalachians into the Northern Mid-Atlantic... A strong northern stream trof 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. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!65gPm4Pno2rZTMsoIxmg2c6fr2lV4Yz__k5LeIEbQuqA= 1zSYSEAh6xDF-elhvoAPw6olTrjoQNyuwW2MD9qhM4XndTY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!65gPm4Pno2rZTMsoIxmg2c6fr2lV4Yz__k5LeIEbQuqA= 1zSYSEAh6xDF-elhvoAPw6olTrjoQNyuwW2MD9qhLjcJvN8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!65gPm4Pno2rZTMsoIxmg2c6fr2lV4Yz__k5LeIEbQuqA= 1zSYSEAh6xDF-elhvoAPw6olTrjoQNyuwW2MD9qhpjce3NE$=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) .