Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 19 2024 00:27:08 FOUS30 KWBC 190026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 826 PM EDT Thu Apr 18 2024 Day 1 Valid 01Z Fri Apr 19 2024 - 12Z Fri Apr 19 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS, MISSISSIPPI VALLEY, AND WESTERN OHIO VALLEY... ....01Z Update... Only a few changes needed from the outlook issued earlier today. Those changes were largely based on trends in short-term radar imagery and the placement of a well-defined front making its way from the western Ohio Valley into the Southern Plains. Made somewhat of an eastward/southward expansion from the Ohio Valley to the mid-/southern Rio Grande Valley based on early evening instability fields...but the overall forecast reasoning has changed little. Bann ....16Z Update... Areal coverage of the inherited MRGL risk was expanded to account for the latest trends in guidance and radar/sat evolution this morning. Surface low is currently located over southern KS with expected northeastward motion through the period. A stationary boundary located over the northern Ozarks and points east will be a focal point for convection later this afternoon and evening with a shortwave dipping southeast into the Mid-Mississippi Valley. A secondary area of focus is unfolding over southern AR into northern LA as a weak shortwave ejects eastward out of TX and induces some modest large scale ascent within an increasingly unstable environment over the Deep South. Recent radar and IR satellite have indicated a quick growth of convection across the aforementioned area with radar estimates of rates approaching 2.5-3"/hr over far northeastern LA. This was a newer trend within the latest CAMs with persistence into the afternoon as the convection moves to the northeast in time (See MPD #0161 for more information). The primary area of interest will include that AR/LA border near the Mississippi River, eventually propagating into north-central MS this afternoon and dissipating prior to evening as the mid-level impulse degrades and we lose the upper forcing mechanism. This was still enough to warrant a push south of the MRGL risk with 1 and 3-hr FFG markers running right around the proposed rates of 2-3"/hr within those cells in question, backed up by 12z HREF probabilities of 20-30% potential for at least 2"/hr possible and 40-50% for at least 2"/3-hrs. Further north, the trend with the convective pattern over MO into the Mid-Mississippi Valley has been fairly consistent the past several deterministic runs and is in lock step with ML algorithms signaling a primary QPF maximum within the confines of eastern MO, southern IL, into far western KY. This was no different within this mornings CAMs and aligns perfectly with the current SPC D1 Outlook of a widespread Slight and Enhanced risk co-located within the northern extent of the MRGL risk area. An upgrade was entertained, but the progressive nature any convection was a deterrent for the update. It still bears watching as any storm that gets rooted at the surface along the stationary front could induce training potential that isn't being picked up by the latest CAMs. For now, maintained continuity based on the HREF blended mean QPF and lower end probabilities for exceeding 2"/hr within the hi- res ensemble (10-15% maximum). There are no changes within Texas as surface based convection will develop over central TX within an area of prime instability. Coverage is not expected to be widespread and primary interest will likely be one or two stronger storms that will produce some locally heavy rainfall over the northern Hill Country to about the I-35 corridor. Probabilities of at least 3" of total rainfall area generally 10-15% within central TX with a bullseye of 20-30% over the Hill Country between Brady and Junction. Kleebauer Day 2 Valid 12Z Fri Apr 19 2024 - 12Z Sat Apr 20 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE TEXAS BIG BEND AND WEST TEXAS UP THROUGH THE RED RIVER... ....20Z Update... A small break in the convective potential will ensue on Friday with a return mainly occurring Friday evening through the end of the period. A cold front will continue to motion south through TX Friday before stalling over the south-central portion of the state later in the afternoon. Out west, a shortwave trough will propagate eastward off the Pacific with increased large scale forcing occurring downstream over the Desert Southwest, eventually into TX by early Saturday morning. The combination of favorable upper forcing and cooler air advecting behind the cold front will enable a deepening isentropic environment in-of the southern plains from the Red River down through north TX. A secondary area of interest will be over the TX Big Bend where a weak shortwave impulse moving out of Mexico could spur a round of convection off the Serranias del Burro with southwestern steering flow taking it towards the Rio Grande by nightfall. There's a split in the potential within guidance with some sneaking the convective cluster over the border with locally heavy rainfall focused over the Rio Grande and southern Edwards Plateau. Others maintain a quiet outlook with the convection dissipating before crossing the border. This was reflected well within the HREF EAS probabilities remaining low into the 10-15% range with only a few guidance promoting the potential. Despite the low probabilities, moisture rich environment and higher end QPF within the upper quartile of outcomes allows for a maintenance of the MRGL risk area encompassing the Big Bend into the Edwards Plateau. Further north, the output from deterministic is very split with a 50/50 distribution on low end QPF development between 00-12z Saturday and much more favorable QPF for MRGL concerns to arise over the western rolling plains up through north TX and the Red River Valley. Considering the GFS/ECMWF ML output for the time frame being fairly similar and leaning towards the higher end of the QPF distribution, decided to maintain the MRGL from previous forecast and link the two areas together with some adjustments on the edges based on ensemble bias corrected data and HREF blended mean QPF footprint. Kleebauer Day 3 Valid 12Z Sat Apr 20 2024 - 12Z Sun Apr 21 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR CENTRAL AND EAST TEXAS THROUGH PORTIONS OF THE LOWER MISSISSIPPI VALLEY... ....20Z Update... The previous forecast remains on track with little deviation in both deterministic and ensemble based guidance leading to a higher confidence forecast at this juncture. The previous SLGT risk was expanded across the southern and western edges to account for some minor variance in the QPF distribution in the latest 12z ensemble output. Heaviest precip will be located over central and east TX with precip totals between 2-4" likely with locally higher positioning somewhere within the vicinity of the stalled frontal boundary. 1/3/6 hr FFG intervals are on the higher side for much of the area encompassed by the SLGT (2"/4"/5" respectively), however the area within central TX is most prone to flash flooding due to the composition of the soils and higher propensity for runoff. There's also been some heavier rain episodes recently that have primed the top layer of of soils with latest NASA SPoRT soil moisture composite indicating percentiles between 70-80% within a large area of east TX. Pending the convective output from future hi-res deterministic and correlated ensemble data, there's a chance an area within the SLGT could be upgraded further. For now, will lean more towards the higher end of SLGT with some time to monitor trends in the QPF forecasts, as well as assessment of where the stalled boundary will settle as that is the main feature for the potential. Kleebauer ....Previous Discussion... The front will remain stalled across the South meanwhile moisture will steadily increase. Precipitable water values of 1 to 1.5 inches (upwards of 2.5 sigma) will be in place across much of the Southern Plains and Lower Mississippi Valley also where there will be very strong mid-level ascent. This environment will be very conducive for widespread heavy rainfall. The highest QPF will focus over central and eastern Texas where areal averages of 2 to 4 inches will be common - a Slight Risk area covers much of this part of region. Isolated higher amounts will possible, particularly for locations east/northeast of the Hill Country. A Marginal Risk area spans from western Texas to eastern Mississippi. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4auTJi5JjTNq3qcQYVd_YiP9HcHw8Rc9dHY1sjGlno2n= 2STGc1mopDsWDDo5LCB6pvofSkbCCv3d6hNEEkcSUSiLgVM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4auTJi5JjTNq3qcQYVd_YiP9HcHw8Rc9dHY1sjGlno2n= 2STGc1mopDsWDDo5LCB6pvofSkbCCv3d6hNEEkcS9vlVzwc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4auTJi5JjTNq3qcQYVd_YiP9HcHw8Rc9dHY1sjGlno2n= 2STGc1mopDsWDDo5LCB6pvofSkbCCv3d6hNEEkcSjT-uFzM$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .