Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 14 2023 15:59:30 FOUS30 KWBC 141559 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1058 AM EST Thu Dec 14 2023 Day 1 Valid 16Z Thu Dec 14 2023 - 12Z Fri Dec 15 2023 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF WEST TEXAS AND SOUTHWEST OKLAHOMA... 16Z update... Radar is showing convection lifting north across the Southern Plains overriding a stalled boundary over the Texas panhandle and east of an approaching surface low pressure system and its accompanied fronts. The latest guidance noted a northward shift in the QPF footprint while continuing to favor West/northern Texas/Panhandle region. The hi-res guidance does show a few streaks of +0.50 in/hr rates into the Oklahoma Panhandle region, however the majority are moving across the Texas Panhandle into west/southwest Oklahoma during this period. Given observed recent rain across northern portions of the Texas panhandle the Marginal Risk area was expanded northward near the Texas/Oklahoma panhandle border. Campbell Closed upper low over NM will continue to slowly propagate eastward with an increased large scale ascent pattern focused across the TX Panhandle into the western half of TX. A strong mid-level vorticity maxima will rotate around the broad circulation, entering into west TX by the end of the Zulu day today. The period from 00-12z Friday will be the greatest time frame for heavy rainfall for locations along and east of I-27 in the Panhandle down into the eastern Permian Basin and adjacent rolling plains. Totals on the order of 1-2" will be common within the period with some locations potentially seeing over 2" pending the training echo pattern with the north-northeast storm motions before shifting more northeasterly when the vorticity maxima pivots overhead and shifts the precip pattern into more of an upscale growth regime. The lack of buoyancy limits the potential of the setup, but the magnitude of large scale ascent and mid-level vorticity advection within an anomalous moisture field will maintain a degree of elevated convective potential and localized flash flooding across the above areas. Primary concern will be urban-based flooding in towns within the confines of the MRGL risk, as well as notorious low-water crossing points that can flood easier due to limited drainage. Kleebauer Day 2 Valid 12Z Fri Dec 15 2023 - 12Z Sat Dec 16 2023 ...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS EAST TEXAS AND NORTHWEST LOUISIANA... Stout longwave trough will traverse across the southern plains with large scale ascent shifting eastward into east TX and the Lower Mississippi Valley. Moisture field present will be applicable for producing locally heavy rainfall as the primary vorticity maxima rotating around the broad circulation will generate a period of elevated convective chances over the I-20 corridor down into areas north of the Houston metro. As you get closer to the Gulf coast, better instability presence will yield a higher risk for thunderstorms rooted at the surface with better rainfall rates possible during the second half of the period. Widespread 1-1.5" totals are forecast within both the deterministic and ensemble means, but some indications of over 2" within the better unstable environment(s) are showing up in the latest guidance with the hi-res just now peeking into the time frame. Considering just the overall synoptic pattern and ample moisture available, decided to maintain the previous MRGL risk, but not expecting a further upgrade unless we see a better convective risk unfold within the CAMs in future runs. Flash flood threat is low and isolated with urbanized flooding the biggest risk over the course of Friday into Saturday morning. Kleebauer Day 3 Valid 12Z Sat Dec 16 2023 - 12Z Sun Dec 17 2023 ...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS MUCH OF FLORIDA... The powerful longwave trough attributed to much of the heavy rainfall in the prior periods will continue to eject eastward, but becomes more enhanced as energy on the backside of the mean trough digs southeast into the Lower Mississippi Valley and begins to interact with a remnant shortwave over the Gulf of Mexico. A more vigorous mid-level shortwave pattern will initiate surface cyclogenesis in the central Gulf leading to much more organized precip field rooted in tropical moisture. Broad ascent pattern on the difluent side of the trough will spawn a mass of deep moist convection with backed flow over much of the FL Peninsula and adjacent central Gulf regions. Totals on the order of 3-5" will be common with heavy rainfall rates exceeding 1"/hr increasingly likely from the FL Keys all the way into parts of the Deep South. The complexities of the forecast come down to the overall shortwave interaction and future storm motion after initial cyclogenesis, but probabilistic forecasts for QPF on both the ECENS and GEFS, as well as the NBM indicate a wide area of potential for 2+" of precipitation with some of the deterministic pointing at QPF maximums of up to 7" given the environmental moisture availability and expected evolution of the synoptic pattern. December 2009 continues to be a prominent analog via CIPS which ended with a large areal extent of heavy precipitation over the Southeast U.S, eventually making its way north up the Atlantic seaboard. This is a textbook scenario for flash flooding across Florida and adjacent areas of the Southeast. The previous forecast maintained a fair amount of continuity, but did expand the northern edge of the SLGT risk to include more of northeast FL near Jacksonville, as well as expanding the northern edge of the MRGL over GA to reflect some trends in a further north progression of the overall QPF footprint. The western edge of the MRGL was extended to include the central Gulf coast out to eastern LA where places like New Orleans could very well see some minor flood concerns if the convective pattern holds firm from upstream. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4ux0rKe8_qnE_rYJKajP6wbqgILj78hDtyfbAOJl5Ch2= BdHKkNcpkgy9xDL9kiGBnxKB4c4DC1OXku-soGk48fEBr8w$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4ux0rKe8_qnE_rYJKajP6wbqgILj78hDtyfbAOJl5Ch2= BdHKkNcpkgy9xDL9kiGBnxKB4c4DC1OXku-soGk4WYsl9sc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4ux0rKe8_qnE_rYJKajP6wbqgILj78hDtyfbAOJl5Ch2= BdHKkNcpkgy9xDL9kiGBnxKB4c4DC1OXku-soGk4FXz6HQg$=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) .