Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Nov 24 2022 20:24:28 FOUS30 KWBC 242024 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 323 PM EST Thu Nov 24 2022 Day 1 Valid 16Z Thu Nov 24 2022 - 12Z Fri Nov 25 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN TEXAS INTO THE LOWER MISSISSIPPI VALLEY... ....16Z Update... The Slight Risk area across TX has been trimmed a bit towards the southeast with this update. The convection ongoing across northeast TX from north of San Antonio through to south and east of Dallas has thus far only been producing isolated instances of 1-1.5 inch/hour rainfall rates. Both the isolated and transient nature of these higher rainfall rates have thusfar not been enough to produce flash flooding. Therefore, the threat in these areas has been assessed to be better defined in the Marginal category. A little convection has been developing towards Houston and the Gulf Coast. There is good consensus in the HiRes guidance that this developing convection will merge with the ongoing convection further inland during peak heating with better PWATs closer to the Gulf Coast. Therefore, the reasoning for the ongoing Slight Risk in these areas remains unchanged. The higher rainfall rates described in the previous discussion below are far more likely to occur in the southern areas of the Slight Risk. Overall, however, the higher FFGs in this area would suggest that even here, the flash flooding risk is on the lower end of the risk category. Wegman ....Previous Discussion... A multi-day heavy rainfall event is expected to develop over the next 48 hours across portions of Texas and the Lower Mississippi Valley. Aloft, an active subtropical jet will remain positioned across eastern Texas to the Lower MS Valley, positioning the coastal regions in the favorable right entrance region of the 120-130 kt jet today. Meanwhile, southeasterly to southerly flow on the backside of the eastern U.S. high will allow anomalously high moisture to be lifted into the region as a warm front slowly moves northward. PWs between 1.7-1.9" with some isolated signal for 2"+ values will be near 3 standard deviations above normal for this time of year while upwards of 1500 J/kg of MUCAPE is likely along the coast and areas immediately inland (though will likely drop off quickly further north, limiting the intensity of the heavy rainfall). Convection is expected to develop later this morning or early afternoon across portions of the mid/upper Texas coast and track northeast/easterly across southern Louisiana into southern Mississippi later this evening and tonight. Some of this convection will have the potential to train and repeat over the same areas with the mean flow ending up near parallel to the expected storm motions. The environmental ingredients (sustained lift, anomalous moisture, and instability) supports intense rain rates approaching 2"/hr at times with an isolated signal for 2-3" hourly totals based on the 00Z HREF probabilities. An axis of 3-5" is likely across the mid/upper TX coast through southern LA and southern MS with some potential for isolated 5-6"+ totals where repeating rounds occur. This could lead to isolated to scattered instances of flash flooding today, particularly highly urbanized corridors and other sensitive locations. Further north across northeast Texas, moderate to locally heavy rainfall will likely repeat over the same area through early afternoon with some potential for hourly totals greater than 1" at times and totals of 2-3".=20 Taylor Day 2 Valid 12Z Fri Nov 25 2022 - 12Z Sat Nov 26 2022 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF CENTRAL AND UPPER TEXAS GULF COAST... ....2030Z Update... High resolution CAMs are beginning to key in on a potentially significant flash flooding event across the central and upper Texas Gulf Coast on Friday. Plentiful Gulf moisture and some instability along the TX Gulf Coast will support repeating thunderstorms. Most of the significant rainfall in the Moderate Risk area will be overnight Friday night into Saturday morning (after 00Z). While this will limit impacts as far as traffic is concerned, overnight flash flooding has its own dangers for those in flood-prone areas. Otherwise, the model discrepancies today are night-and-day better than 24 hours ago. That said, there remain some minor differences, particularly on where exactly the heaviest rainfall will be, with some models continuing to hint that the main forcing will be inland, closer to the upper level low. These areas will certainly get quite a bit of rain, but without the instability it's expected that there will be training showers, and very little in the way of convection. Thus, the 2-4" possible across north central TX are likely to fall for much of the day and the first part of the overnight...in other words, spread out over a long period of time. Thus, while the Moderate Risk was introduced along the coast, areas further inland were downgraded from the Slight to a Marginal risk. Due to the much better consistency (generally speaking) in the models with the overnight runs, much of the meteorology described in the previous discussion below remains unchanged. Wegman ....Previous Discussion... An anomalously deep closed low will approach and stall over Texas during the day 2 period which will be the driving factor for the second round of heavy rainfall potential in the region. East of the closed low, a broad area of forcing for ascent is expected across Texas in the highly divergent upper flow pattern. In the low levels, very moist flow aided by 20-40 kts at 850 mb will bring a renewed surge of moisture into Texas with the latest guidance showing precipitable water values exceeding 1.5" with some values near 2" likely along the immediate coast overlapping with an axis of higher instability. The near stationary closed low during the period will allow a slow moving axis of convection to set up from portions of central Texas toward the mid/upper Texas coast with the increasingly likely scenario that some areas could see a prolonged period of training heavy convection along the coast especially Friday afternoon through early Saturday morning. Model guidance however continues to struggle with the timing and speed of this system, with this forecast cycle seeing another significant slow down and shift south/southwest in the axis of heaviest precipitation. As such, it's a lower confidence set up which is typical with closed upper lows. However, the ingredients and conceptual model suggests a signal for heavy rainfall but exact location and details precluded anything higher than a Slight Risk at this time. It's possible a higher risk category may needed in future updates if the overlap of heaviest QPF between what materializes today and what is expected during day 2 is greater or if confidence in setup/location increases. The most concerning location right now looks to be along the mid/upper Texas coast where an additional 2-4"+ could fall during the day 2 period resulting in 48-hr (12Z Thursday-12Z Saturday) totals of 4-5" with localized 6-8"+ totals. Taylor 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!5WYhxKbMJPgQl0DAGt3HHdf2JVlfSCc6KijBviAF5-Je= xY2Y9HS03zcOUmEOEMuHwozPikk_1yC3P6vH47vxjO86XLE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5WYhxKbMJPgQl0DAGt3HHdf2JVlfSCc6KijBviAF5-Je= xY2Y9HS03zcOUmEOEMuHwozPikk_1yC3P6vH47vxJH5-SC0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5WYhxKbMJPgQl0DAGt3HHdf2JVlfSCc6KijBviAF5-Je= xY2Y9HS03zcOUmEOEMuHwozPikk_1yC3P6vH47vxoqmzf4A$=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) .