Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Oct 26 2023 00:41:46 FOUS30 KWBC 260041 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 841 PM EDT Wed Oct 25 2023 Day 1 Valid 16Z Wed Oct 25 2023 - 12Z Thu Oct 26 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF CENTRAL TEXAS... ....01Z Update... A few notable changes were made with this update. The Moderate Risk area across Texas is in pretty good shape, so the western edge was removed as the heaviest rains continue to slowly drift eastward, with the exception of the portion near the Rio Grande. Northern areas will have a bit more success at moving the rainfall east, but storms have been training and reforming over the same areas with very little movement further south towards the river. Thus, the Moderate remains largely the same there. CAMs guidance suggests that the heaviest rain across the northern TX may make it a bit further east before 12Z than the previous forecast, so in coordination with FWD/Dallas-Fort Worth and EWX/San Antonio, TX forecast offices, the Moderate was expanded a row of counties south and east for when the main line of storms reaches there in the predawn hours. Much further north across Kansas, the previous Moderate Risk area across eastern Kansas was double downgraded to a Marginal with this update. Only shower activity has been seen across the hard hit areas of the state, and only showers are likely through the overnight period. Thus, despite the very low FFGs, the lack of forecasted rainfall should keep any flash flooding concerns to a minimum. The rainfall up into IA/MN has also been fast moving and light in intensity, so the Marginal risk was trimmed to just the southern and eastern half of IA and out of MN altogether. Wegman Day 2 Valid 12Z Thu Oct 26 2023 - 12Z Fri Oct 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN MIDWEST AND CENTRAL TEXAS... ....20Z Update... The two areas from the previous D2 issuance remain fairly similar, but have expanded each MRGL area slightly to account with the recent short term trends and associated probabilistic data from the 12z HREF. Have also added a SLGT risk across east TX to account for a strong signal for locally higher totals reaching 2-4" over central TX. Our potent shortwave over TX will eject northeast into the central plains, eventually crossing into the Midwest by the second half of the D2 period. The frontal boundary drapped over the northern Midwest will provide a focal point for area convective development along and south of the boundary where the theta-E gradient will be present. Scattered showers and thunderstorms over IA/MN will advance northeastward through the period, but an enhancement in the afternoon and evening hours is anticipated due to the increasing ascent provided by the shortwave presence. 12z HREF probability for 3"/12-hrs is running between 20-30% within an area that has seen a fair amount of rainfall very recently. The combination of prolonged moderate to locally heavy rainfall over a long period stretching from Thursday AM through the early evening will provide an opportunity for flash flooding concerns, especially within the zone extending from La Crosse to Green Bay where the lower FFGs exist from the previous convective event. Further east into the Great Lakes, locally heavy rain threat will continue over northern MI with the HREF blended mean and ensemble bias-corrected QPF fields indicating a local forecast maximum oriented off Lake Michigan near Traverse City to Gaylord. This area saw some of the higher impact rainfall over the past 24-48 hrs leading to much lower FFG's compared to normal. This allowed for the SLGT risk to be maintained within the zone to accompany the western expansion of the SLGT into the MN/IA/WI border. Across central TX, very anomalous moisture presence will persist due to prevailing southerly flow ahead of the mean trough to the west and the western flank of our mid-level ridge east of the Mississippi. PWAT indices between 2-2.3" will be common along and east of I-35 and south of I-20. Weak mid-level forcing advecting up from MX and the adjacent Gulf will meet a convergence point created from the area convection that will be decaying to the west. A sharp theta-E gradient will likely develop in-of the area northeast of Austin where FFGs are much lower due to the soil type and higher runoff potential. This will be the focal point of the convergence signal with plentiful agreement among guidance, especially the probability of at least 5"/12-hrs between 12-00z on the latest neighborhood HREF probabilistic signal (30-40%). The EAS probability is also fairly robust for at least 2"/24-hrs (25-30%) located over the aforementioned inflection point. The combination of probabilistic signals and favorable synoptic scale setup was good enough to add a new SLGT risk to the along and east of I-35 near Austin/New Braunfels. Kleebauer ....Upper Mississippi Valley into the Upper Lakes... The well defined front stretching from the Upper Mississippi Valley into the Upper Great Lakes will continue to be the focus for additional heavy rains day 2. The next round of convection moving along and to the north of this front that is expected to begin late day 1, will continue into the day 2 time period as surface low pressure moving along this front helps to maintain strong west southwesterly flow into this front. This will support potential for additional heavy rains from northern IA, across far southeast MN, central to southern WI into the northern L.P. of MI and the southern U.P. of MI. The model consensus for the day 2 qpf axis has shifted southward from previous runs and has resulted in less overlap of areas that have received heavy rains over the past 24 hours and the day 2 qpf axis. ,...Central Texas... Large scale dynamics will be weakening across the Southern Plains day 2 as the closed low ejecting northeastward day 1 continues to weaken as it presses into the Central Plains and Mid Mississippi Valley region. However, an axis of anomalous PW values ..2 to 3+ standard deviations above the mean... will persist across eastern TX. Model consensus is for moderate to heavy amounts in this anomalous PW axis. This expected precip will be mostly to the southeast of the expected day 1 heavy amounts. Oravec Day 3 Valid 12Z Fri Oct 27 2023 - 12Z Sat Oct 28 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS INTO THE LOWER ARKANSAS RIVER VALLEY... ....20Z Update... Only a minor change necessary to the D3 MRGL from previous issuance. Guidance is still keying on the frontal convergence axis extending from north-central TX into the Arklatex by the end of the period. PWAT anomalies beginning at 2 deviations above normal rising to 3 deviations is forecast through the period allowing for locally heavy rainfall to be the norm within any area convection. There is some indications that the northeastern extent of the QPF field could trend higher as the recent guidance shows an uptick of the precip footprint as you head into Arkansas. This is well-defined within the D4 ERO where a SLGT risk is now forecast across the Ozarks. There is potential for there to be an upgrade to a SLGT somewhere within the forecast MRGL with some hints being given now within the First-Guess fields. There is enough spread in the ensembles for where the heaviest rain will occur, so have maintained continuity for the time being. Kleebauer ....Northeast Texas, southeast Oklahoma into northwest Arkansas... The anomalous PW axis across eastern Texas on day 2 will begin to lift northeastward ahead of the strong cold front sinking southeastward through the Central to Southern Plains during day 3. This front is expected to slow its southeastward push toward the end of the day 3 period becoming nearly stationary from northeast Texas into northwestern Arkansas. Precipitation should become more active along and ahead of this front late Friday afternoon into the early hours of Saturday when well defined frontal convergence and favorable right entrance region jet dynamics enhance uvvs in the anomalous PW axis. The model consensus is for a narrow axis of potentially moderate to heavy totals along this front from northeast Texas into southeast Oklahoma and northwest Arkansas. There may be some overlap of day 1 heavy precip and day 3 amounts across portions of northeast Texas into southeast Oklahoma. The threat level was maintained as marginal from the old day 4, but may need to be upgraded to slight in later issuances depending upon where the observed heavy amounts on day 1 occur. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4e17l2T7OHK-06kp9uTwbl2OMhf0pRdsjDAUs8wa1_83= eMQM0JH-4By0tIyJpCQat2Gzd9dPlc_qrGw2S3ALzHOrDOA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4e17l2T7OHK-06kp9uTwbl2OMhf0pRdsjDAUs8wa1_83= eMQM0JH-4By0tIyJpCQat2Gzd9dPlc_qrGw2S3ALp628FNY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4e17l2T7OHK-06kp9uTwbl2OMhf0pRdsjDAUs8wa1_83= eMQM0JH-4By0tIyJpCQat2Gzd9dPlc_qrGw2S3ALY09_B2A$=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) .