Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 23 2023 08:27:34 FOUS30 KWBC 230825 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 324 AM EST Sat Dec 23 2023 Day 1 Valid 12Z Sat Dec 23 2023 - 12Z Sun Dec 24 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER AREAS ALONG THE MIDDLE AND UPPER TEXAS COAST, AND ACROSS PORTIONS OF NORTH TEXAS INTO SOUTHERN OKLAHOMA... Longwave trough, with both northern and southern stream shortwaves, will gradually push east of the Rockies and into the High Plains late Sat and Sat night. Large-scale, robust deep-layer ascent will quickly develop downstream across the lower Central-Southern Plains and the western Gulf Coast, maximized within the ERO outlook areas within both the left-exit region of the upper level subtropical jet streak traversing northern Mexico, and the right exit region of the N-S oriented upper jet on the lee side of the upper trough. By Sat night, the thermodynamic profile quickly becomes conducive for heavy rainfall as the uptick in low-level southerly flow (30-40+ kts at 850 mb) draws in more favorable moisture/theta-e air off the western Gulf. The latest SREF and GEFS show moisture and moisture transport most anomalous farther north across the lower Central Plains (both PW and 850-700 mb moisture flux standard deviations between +3 and +4), however there is a trade-off as the degree of deep layer instability across northern OK into KS will be more limited (as then will be the short term rainfall rates). The Slight Risk from yesterday's Day 2 ERO -- from the central Red River Valley south-southwest through the DFW Metroplex and into central TX -- was unchanged based on the latest guidance. Still expect the more diffuse, widespread pre-frontal rainfall (largely stratiform) to ultimately give way to a quasi-linear, convective segments after 02-04Z. By that time, with the uptick in the LLJ and better alignment with the low-level flow with the 850-300 mb mean wind, expect an increase in upwind propagation and slower eastward progression of the convective segments, which would allow for a better chance of cell training. This along with the favorable thermodynamic profile (PWs 1.25-1.5" and MUCAPEs 500-1000+ J/Kg), hourly rainfall rates of at least 1-1.5" will become more likely within the strongest cores, possibly higher. Another Slight Risk was added along northern portions of the Middle TX Coast through the Upper TX Coast, including the Houston-Galveston Metros. Again, the LLJ and thus moisture flux anomalies over this region are not as anomalous compared to areas farther north; however, available deep-layer instability along the Gulf Coast will be more robust (1000-2000 J/Kg), as will the 1-3 hourly rainfall rates per the higher HREF probabilities. 00Z HREF probabilities of at least 3" are >50% over a large portion of this corridor, with 20-40% probs of at least 5". As such, localized totals between 3 and 5+ inches can be expected, as advertised by the 00Z CAMs. Hurley =20=20 Day 2 Valid 12Z Sun Dec 24 2023 - 12Z Mon Dec 25 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE WESTERN OZARKS AND ARKANSAS RIVER VALLEY SOUTH THROUGH THE ARKLATEX AND CENTRAL GULF COAST REGION... Aforementioned mid/upper trough is expected to close off at 500 mb by 12Z Christmas Day across the Plains. Strong southern stream enhancement (130-150 kt upper jet streak moving across South TX) will boost the deep-layer ascent and south-north ageostrophic low-level moisture/theta-e transport into eastern OK-TX as well as much of AR-LA and the MS-AL Gulf Coast regions. PWs >1.5" begin to creep into the Upper TX Coast region early in the period, then across southern LA and coastal MS later Sun-Sun night. The anomalous low-level transport, aligned low-mid level flow with an anomalous LLJ (favoring increased upwind propagation/backbuilding and thus risk for cell training), and at least modest deep-layer instability (at least 500-1000 J/Kg) will lead to a favorable thermodynamic and kinematic profile for heavy rainfall, especially given the active, robust southern stream which is indicative of the current strong warm-phase ENSO pattern. As such, modifications to both the Slight and Marginal Risks areas from yesterday's Day 3 ERO were relatively minor based on the latest spatial and temporal trends from the 00Z models. Hurley Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6oRci_meLqssJD3d3An9h4wZAjwaGD8Q28NArxCTTwCN= qPw9Vk5Ej7qT08XPFAUwk62ZGfevJe3PmjenPKG6eH2oswU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6oRci_meLqssJD3d3An9h4wZAjwaGD8Q28NArxCTTwCN= qPw9Vk5Ej7qT08XPFAUwk62ZGfevJe3PmjenPKG64maPvvc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6oRci_meLqssJD3d3An9h4wZAjwaGD8Q28NArxCTTwCN= qPw9Vk5Ej7qT08XPFAUwk62ZGfevJe3PmjenPKG6DmQWi7o$=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. 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