Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 18 2023 07:38:49 FOUS30 KWBC 180738 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 338 AM EDT Tue Jul 18 2023 Day 1 Valid 12Z 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 AND LOWER TENNESSEE VALLEY... ....Lower Missouri Valley, Mid Mississippi Valley into the Lower Ohio and Lower Tennessee Valleys... Models continue to show the potential for a narrow axis of very heavy rainfall amounts, especially in the 0000-1200 UTC Wednesday period from the Lower Missouri Valley/Mid Mississippi Valley/Lower Ohio Valley and Lower Tennessee Valley region along and to the north of the expected west to east oriented frontal boundary across these areas. There is model consensus on two rounds of precip to affect areas from the Lower Missouri Valley/Mid Mississippi Valley/Lower Ohio Valley into the Lower Tennessee Valley region during the upcoming day 1 period. An initial round of convection firing early day 1 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 day time 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 into the front attains a more westerly component, increasing the potential for back building and downstream training of convection=20 While there is general agreement in this scenario, there are some fairly significant differences in where the training episode will set up. The FV3LAM is the farthest north across southern IL/far southwest IN into west central KY and east central TN. In contrast the NAM NEST has the training axis from southeast MO, across far southern IL, western KY into north central TN. The ARW2 and HRRR from 0000 UTC are closer to the farther southern solutions. Given the spread in the locations of the training convection portion of the day 1 precip event, the slight and moderate risk areas were broadened and expanded to the south into central to western TN, while the northern portions of the risk areas were trimmed across northeast MO into south central IL. The new moderate risk area axis fits well with the latest max HREF neighborhood probabilities for 2 and 3"+ amounts and the Ensemble Agreement Scale (EAS) max axes for 1 and 2"+ amounts. Max precip amounts in the latest guidance continues to show 5-8"+ amounts.=20 These totals seem reasonable given the potential for two rounds of convection and the potential for the training episode in the early hours of Wednesday. This scenario is one where a high risk may be warranted, but given the spread in the area of best training early Wednesday morning, we will hold off for the time being and re-access with the 1200 UTC Tuesday hi res runs. ....Northeast... No significant changes 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 expected along and ahead of this front. These values will be lower than PW values with the heavy rainfall event on Sunday as that anomalous PW axis has moved well off the northeast coast. Additional convection likely to stretch along this front on Tuesday, 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 1 and 2" amounts. The 1"+ probabilities are high 60-90% in the slight risk area, with probabilities for 2"+ amounts in the 30-80% range but in a more narrow axis. There will be some overlap with heavy rains on Sunday and day 1 expected heavy axis from southeast NY State into northwest CT and western MA and east central NH. Much of the remainder of the heavy rains on Sunday farther to the southeast over southeast New England will remain to the southeast of the day 1 expected qpf axis. ....Southwest into the Southern Great Basin... Widespread monsoonal convection possible day 1 across portions of the Southwest, northward into the southern Great Basin. An axis of more anomalous PW values 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 40-60% probabilities of .50"+ amounts here and over portions of southeast AZ. This could produce some isolated runoff issues, especially across any burn scar regions or slot canyons. 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!_GIaQqoOsXgX2hXUO-XnU9j50rrTr-67mPnGkTfvfwiZ= Fiqdi2EsptZ9V4A4KQeJIwlsl-qLEAHg0i6ryfdUKMczIco$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_GIaQqoOsXgX2hXUO-XnU9j50rrTr-67mPnGkTfvfwiZ= Fiqdi2EsptZ9V4A4KQeJIwlsl-qLEAHg0i6ryfdUBR-SgZo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_GIaQqoOsXgX2hXUO-XnU9j50rrTr-67mPnGkTfvfwiZ= Fiqdi2EsptZ9V4A4KQeJIwlsl-qLEAHg0i6ryfdUBXPriTw$=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) .