Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Oct 27 2023 00:55:34 FOUS30 KWBC 270055 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 854 PM EDT Thu Oct 26 2023 Day 1 Valid 01Z Fri Oct 27 2023 - 12Z Fri Oct 27 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....01Z Update... Much of the rainfall and all of the steady rain has ended across the upper Midwest. Current radar trends show a wide area of scattered showers across the region. Every once in a while a few of the showers get some convective elements, but none that are producing any rainfall capable of producing flash flooding. Given these trends, the threat of flash flooding through early Friday morning is determined to be less than 5 percent, so both the Slight and Marginal areas were removed with this update. Across eastern Texas, a previously impressive line of storms even with some embedded tornadic supercells is quickly weakening in intensity as it tracks eastward north and northeast of Houston. Rainfall rates have been steadily coming down, and the remaining line of rainfall is moving into an area of high FFGs/dry antecedent conditions. All CAMs guidance shows the entire line of rain fizzling out and dying over the next few hours. Given the rapid decrease in strongest storm strength, diminishing instability, and expected continued weakening of the line, the threat of flash flooding in this area was also determined to be less than 5 percent, so both the Slight and Marginal areas were removed with this update. Wegman Day 2 Valid 12Z Fri Oct 27 2023 - 12Z Sat Oct 28 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN TEXAS INTO THE ARKLATEX... ....20Z Update... Primary change to the previous forecast was the expansion of both the MRGL and SLGT risk areas based on the recent short term trends in guidance, as well as correlating with the previous rainfall footprint from the past 24 hrs. A strong convergence signal is still showing up across all numerical guidance as the upper-level pattern favors increasing large scale ascent across north TX into the Lower and Mid-Mississippi Valley. A stalled boundary in-of the Red River valley off to the northeast will be the focal point for convective development and propagation late-Friday into the end of the D2 period. A weak impulse rotating out of Mexico on the western flank of the mid-level ridge over the Southeastern U.S will aid in the initiation of area convection across north and northeast TX, increasing in coverage through the overnight period as forcing is maximized from both the surface convergence and mid-level vorticity maxima. 12z HREF probabilities are indicating a higher chance of rates between 1-2"/hr with 40-50% for at least 1"/hr and 20-30% for 2"/hr within the SLGT location. There is also a non-zero indication of at least 3" and 5" within the probabilistic guidance with much of the precip falling within a 6-12 hr window beginning early Friday evening. This also intersects with the area that has been hit fairly hard with heavy rainfall recently with local FFG's over north-central TX into southern OK responding accordingly. Current forecast shows local maximums within the primary zone of QPF indicating the convective nature of the precip field. Thus, the forecast was altered to adjust for where probabilities coincide with the general overlap of previous heavy rainfall, as well as the area where low-mid level convergence signatures are strongest for the 00-12z Saturday period. Kleebauer =20 ....North Texas into the Arklatex...=20 The anomalous PW axis across eastern Texas will begin to lift northeastward into the ArkLaTex ahead of the strong cold front sinking southeastward through the Central to Southern Plains.=20 This front is expected to slow its southeastward push towards Saturday morning 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 upward vertical motions in the anomalous PW axis. The model consensus is for a narrow axis of potentially moderate to heavy totals along this front from north Texas into southeast Oklahoma and northwest Arkansas which should occur late Friday into early Saturday, aided by the nocturnal low-level jet. MU CAPE of 1000-2000 J/kg should exist, along with increasing effective bulk shear to organize the activity and lead to 2.5" in an hour, roughly the maximum forecast for the guidance during the period. A Slight Risk was added where the forecast heavy rainfall best overlaps with the heavy rainfall occurred over the past 24 hours.=20 =20 Roth Day 3 Valid 12Z Sat Oct 28 2023 - 12Z Sun Oct 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS EASTERN OKLAHOMA THROUGH THE OZARKS... ....20Z Update... Marginal and Slight risks were expanded to encompass the area within the zone of best convergence along the stalled frontal boundary that will drape across northeast TX into the Ohio Valley. Weak impulses will rotate parallel to the surface front following the persistent flow pattern out of the southwest. Beginning to see indications of a fairly robust QPF footprint within the means as GEFS and NBM probabilistic indices for at least 1" has ramped up to near 80% with 20-30% chance of at least 2" showing up within the ensemble. Ensemble-bias corrected QPF is now running between 2-3" over a large span of the Ozarks into eastern OK with an expansion to the northeast. CIPS analog pattern for D3 is also robust with the top 3 analogs all featuring prolific rains across portions of the Lower Mississippi Valley and adjacent areas. Considering the pattern, have expanded the risk categories to mimic the proposals within the First Guess fields and correlating within the bounds of the 0.5" (MRGL risk) and 1.25" (SLGT) contours. Kleebauer ....Northeast TX into the Lower Ohio Valley... Precipitable water values of 1.5-2.2" are expected near a slow-moving front draped across the region. The degree of low-level inflow and effective bulk shear available should organize convection into training bands as the flow is fairly uniformly out of the west-southwest with height. MU CAPE of 500-1000 J/kg should be sufficient to achieve hourly rain totals to 2.5" at times, which would be a problem across area terrain.=20 Local amounts in the 3-5" range are indicated by the available guidance overall. Despite recent dryness, the existing Slight Risk area appeared best which was shifted minimally based upon the most recent model guidance. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9CnbJ0bIgC04RLLW0Lq4hYntcwts8HPEjVeE03LyJFYB= GUvZd6gaBjOwyFILJ_LahwezWSXG6iFZ92c9SyX-1U0Dips$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9CnbJ0bIgC04RLLW0Lq4hYntcwts8HPEjVeE03LyJFYB= GUvZd6gaBjOwyFILJ_LahwezWSXG6iFZ92c9SyX-gxxrAJg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9CnbJ0bIgC04RLLW0Lq4hYntcwts8HPEjVeE03LyJFYB= GUvZd6gaBjOwyFILJ_LahwezWSXG6iFZ92c9SyX-_jW6rM8$=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) .