Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Oct 26 2023 20:01:31 FOUS30 KWBC 262001 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 PM EDT Thu Oct 26 2023 Day 1 Valid 16Z Thu Oct 26 2023 - 12Z Fri Oct 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF WISCONSIN TO MICHIGAN & CENTRAL TO EASTERN TEXAS... ....16Z Update... Minor adjustments overall to the D1 risk areas with the primary changes relegated to expansion of MRGL to the west over the Midwest and the orientation of the MRGL and SLGT over TX as we trim back the western extent where rainfall potential will decrease as we move through the period. Shortwave ejecting through the central plains will continue to make advancement into the central and northern Midwest this afternoon with widespread light to moderate rain translating northeast under the large scale forcing. The SLGT risk over WI into northern MI remains fairly unchanged as multi-guidance consensus indicates the outlined area as the primary region for higher QPF, as well as a more susceptible area for flooding given the recent rainfall the past few days. Instability is generally lacking within the outlined risk areas, but enough large scale ascent and steady rainfall will produce local flooding capabilities, especially within urbanized areas. 12z HREF probabilities indicate a 30-40% chance for 12 hr totals exceeding 3" with some non-zero indications of 5" within the 24 hr period. With hi-res deterministic still catching up to the overall QPF footprint from the primary shortwave, models have been under-doing the expanse of the moderate to locally heavy rainfall pushing up into IA, which is expected to translate downstream into WI/MI. This aligns better with higher QPF forecasts within the global and deterministic suite leading to a continuation of the previous SLGT issuance and placement, as well as expanding the MRGL across eastern IA where heavy rain risk is occurring leading to low-end MRGL type flash flood concerns. Across Texas, flash flood concerns have shifted eastward with the primary convective pattern centered over the Austin/San Antonio corridor up to the east of Dallas. Multiple FFW's have been issued over central TX in response to the locally heavy rainfall with totals ranging from 1.5-3" with the zone of highest impact. Slow progression of the convective field within a very anomalous moist environment (PWATs between 2-2.3") will lead to scattered flash flood concerns within a more prone area for flooding considering the soils and historically higher runoff concerns. 12z HREF continued a persistent signal of enhanced rates between 1-2"/hr with high probabilities of 2-3"/3-hr intervals with a max around 50% for the 3" area within the axis from Austin to College Station to the east of Waco. Overall, the setup is on track from previous forecast with only slight modifications to the orientation to the risk areas. Kleebauer=20 ....Previous Discussion...=20 =20 Our potent shortwave over TX will eject northeast into the Central Plains, eventually crossing into the Midwest Thursday night into Early Friday. Convective development is expected along and south of a front 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. Heavy rainfall is expected downwind of the pool of 500+ J/kg MU CAPE. The Marginal Risk concern in eastern IA and northwest IL would be if 1.5"+ totals seen on some of the guidance near the IA/IL border materialized within an hour, which could become an urban problem. HREF probabilities for 3"/12-hrs are modest, running between 20-30% within an area that has seen a fair amount of rainfall very recently. The combination of long duration locally heavy rainfall will provide an opportunity for flash flooding concerns, especially within the zone extending from central WI eastward into northwest MI where the lower FFGs exist from the previous convective event. Risk areas saw some rearrangement, but not major changes from continuity. =20 Across central and eastern 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. MU CAPE of 1000+ J/kg should exist in this area. 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. This environment should support hourly rain totals to 2.5", which would be more overwhelming in urban environments, given recent dryness. The guidance shows some variance, but enough agreement that the Slight Risk area was nudged south. =20 Roth 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 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!6K6n1hlYkpSTtOqoHX43kRxe7N8PJ6nSKt1IjKRCSWzQ= IQSyWkMdqfDy4zD9IN_2JR4jmIn7NDCOVCiBnhRUusJbBa0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6K6n1hlYkpSTtOqoHX43kRxe7N8PJ6nSKt1IjKRCSWzQ= IQSyWkMdqfDy4zD9IN_2JR4jmIn7NDCOVCiBnhRUVL94ZVc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6K6n1hlYkpSTtOqoHX43kRxe7N8PJ6nSKt1IjKRCSWzQ= IQSyWkMdqfDy4zD9IN_2JR4jmIn7NDCOVCiBnhRUzRzR3RQ$=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) .