Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jan 07 2025 22:36:52 FOUS30 KWBC 072236 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 536 PM EST Tue Jan 7 2025 Day 1 Valid 01Z Wed Jan 08 2025 - 12Z Wed Jan 08 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Roth/Gallina Day 2 Valid 12Z Wed Jan 08 2025 - 12Z Thu Jan 09 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Far southern tip of Texas... A highly anomalous upper-level trof is digging across northwest Old Mexico in the day two period. Broad southwesterly flow and increasingly diffluent upper-level pattern is enhancing wind speed and directional convergence offshore along the Western Gulf surface trough throughout the day. Warm, moist Gulf air will allow for showers and convective activity to linger through much of the day, but given the placement of the upper-level trough, convergence is enhanced further south off the northeast coast of Tamulipas with backed northerly flow toward the Texas coast. By late in the period, the atmospheric river of low to mid-level sub-tropical moisture will arrive and winds will veer and support weak cyclogenesis along the surface trough southwest of the TX coast. Increasing shower activity may reach the tip of Cameron and Willacy counties by 06-12z but will likely be well offshore or weak but the wave will lift past the area near the end of the Day 2 forecast period. While there are some suggestions of 1-3" totals by 12z, the rates are limited to about .5"/hr maybe reaching near 1" for the western most solutions at 12z itself. So there remains a non- zero risk for excessive rainfall as the parameters for intense rainfall will be increasingly put into place as moisture streams overlap and PWats become 2 std. dev from the running mean and IVT values are over 500 kg/m/s or well into the 99th percentile. CSU First Guess fields suggest a Marginal Risk may be warranted based on those parameters but the forcing may be too little/too late (off-shore) to place one at this time. Gallina Day 3 Valid 12Z Thu Jan 09 2025 - 12Z Fri Jan 10 2025 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE MIDDLE AND UPPER TEXAS COAST AND WESTERN LOUISIANA GULF COAST... As noted above, a weak surface wave/inflection will be developing and lifting north along the Western Gulf surface trough at the start of the forecast period 09.12z. The core of the sub-tropical moisture plume/AR will have over-topped the region with high flux. IVT values of 750 kg/m/s will increase to 1000 kg/m/s from 12-18z into the 10.06-12z time period as the surface cyclone significantly develops in the northwest Gulf (central to upper TX coast). These values are well outside of the CFSR 30 year record for January and values are 6-8 Std. Anomaly values above normal; indicating the strength of the moisture flux/availability. Limiting factor will be two fold. Placement of the low and associated TROWAL/deformation zone for solid convergence and instability. The latter is likely the greater limiting factor as the deep layer profile is going to be quite warm and stabilizing with a colder surface. Still, proximity to the warm Gulf and some frictional convergence along the favorable steering of flow should allow for consistent/persistent showers/training activity along the middle to upper Texas coast. WAA showers will increase throughout the day into SW and south-central LA, similarly enhanced by the low level convergence in proximity to the warm Gulf waters. Global guidance still indicates a SW to NE band of 3-5" along or just offshore of coastal Texas into SW LA, with the longer higher resolution guidance (like the CMC-Reg) peaking over 5". So will be interesting with addition of Hi-Res CAMs in subsequent updates. As such, will maintain the placement of the Marginal Risk of Excessive Rainfall with small adjustments along the edges (particularly east where trends are a bit faster). Gallina ---Prior Discussion--- Highly anomalous upper trough pattern across the Western CONUS will help generate a distinct shortwave ejection evolution that will create a litany of issues across TX into the Lower Mississippi Valley. Upper low expected to be centered over Sonora the prior period will open as it advances eastward with a second shortwave digging around the backside of the mean trough creating a fairly dynamic setup across the Southern Plains. A strong diffluent axis of ascent ahead of the ejecting trough will create a blossoming of precipitation over TX, spreading eastward as the trough migrates through the Lone Star State. Surface low pressure will spawn in-of the western Gulf with the low center generally close to the TX coastal plain, opening the door for a modest instability gradient to align within the immediate TX coast up to southwest LA as the pattern progresses. Convective regime will be most prominent along the coast where forecast theta_E's will slowly climb just above seasonal norms allowing for a focused area of surface instability along with the already favorable mid and upper level dynamics. A cluster of thunderstorms will likely form over the western Gulf within the confines of the surface reflection lending a better potential for convection to move inland with a heavy rain footprint tied to any storm moving onshore. There will be a sharp northern extent of how long any convective regime can last as much colder to the north of the coast will limit surface based instability and thwart the opportunity for convection to march inland. As of now, the forecast MRGL risk inherited from D4 was relatively maintained, although the northern periphery was trimmed back given the lackluster instability fields and relatively progressive storm motion on the northern fringe of the best instability coupled with ascent. The most favorable locations for rates that could generate enough support for flash flooding potential is still relegated to the immediate Gulf coast with a small buffer further inland if any cells can motion past the immediate coastal plain and affect some of the urban areas extending from CRP up through HOU and LCH. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!60jYIViIdSnWVVpSWzIlQAmLLppb4WZiRXZmY_l_J_46= 6LZoRAD4yoU8FAYMLaqSuiBJAIL9kPyyeZsKOOV6pm98SyQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!60jYIViIdSnWVVpSWzIlQAmLLppb4WZiRXZmY_l_J_46= 6LZoRAD4yoU8FAYMLaqSuiBJAIL9kPyyeZsKOOV6Hn8nl-c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!60jYIViIdSnWVVpSWzIlQAmLLppb4WZiRXZmY_l_J_46= 6LZoRAD4yoU8FAYMLaqSuiBJAIL9kPyyeZsKOOV6P36Mgpc$=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) .