Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Nov 04 2024 19:53:54 FOUS30 KWBC 041953 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 253 PM EST Mon Nov 4 2024 Day 1 Valid 16Z Mon Nov 04 2024 - 12Z Tue Nov 05 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST OKLAHOMA THROUGH THE OZARKS OF ARKANSAS AND MISSOURI... ....Texas to the Great Lakes... 16Z Update: Highly amplified flow ahead of a potent closed low/upper trough axis moving through the Western CONUS will continue to usher in well-above normal moisture anomalies from the Gulf Coast up through the Great Lakes during the period. Heavy rains overnight across OK into the AR/MO Ozarks have crashed FFG indices regionally with cases of hourly FFG's now down <1"/hr necessary to garner flash flood concerns. A widespread areal average of 2-4" with locally as high as 7" has fallen over the above area, saturating top layer soils up into the 70-85th percentile as indicated via the latest NASA SPoRT output for the 0-10cm layer. Expectation is for the pattern to continue evolving as such that another wave of heavy precip will initiate upstream over OK as the nose of a 110kt 500mb jet streak rounds the base of the mean trough over NM and ejects into the Southern Plains. Large scale ascent will be maximized over within the time frame of 21z through the remainder of the period allowing for a large cluster of showers and storms to breakout and pivot east-northeast, aided by the mean flow. A theta-E tongue is well documented over Eastern TX through the Lower Mississippi Valley up through the the Missouri Ozarks lending to favorable instability continuing through the afternoon and evening time frames. CAMs have become aggressive with the QPF output given the aforementioned variables lending to a repeated pattern of heavy rainfall over areas that saw copious amounts of rain overnight. 12z HREF signals a high end probability for at least 3" within the neighborhood probability fields (70-90+%) for the Ozarks down into the Southeast corner of OK and is correlated well with a 25-50% EAS prob field for 3" with a max prob of 60% centered over South- Central MO to the east of Springfield. HREF blended mean output is elevated with 3-5" of rainfall over those areas with a max of up to 7", all of which is additional to what has transpired. Considering the expected additional rainfall with rates generally 1-2"/hr at peak intensity likely breaching the current FFG indicators for 1/3/6 hr periods, and in coordination with the TSA/LZK/SHV/SGF WFOs impacted, a Moderate Risk ERO was introduced to cover that area from Southeast OK up through North and Northwest AR into South- Central MO. Further south into TX, there is a growing consensus of heavy thunderstorm genesis on the southern edge of the flanking line along the axis of convergence settling within the western edge of the theta-E gradient later this evening, mainly after the development of the nocturnal LLJ. The jet streak on the base of the mean trough under the closed low will be juxtaposed to the northwest of Central TX allowing for increasing RER dynamics within the confines of the I-35 corridor extending from DFW down into the Austin/San Antonio metros. There is consensus on the development of the flanking line, but exactly where the heaviest will occur depends on the CAM output. Some place the heavy core of precip near New Braunfels to San Antonio. Others place it near Austin to Waco. Regardless, the instability fields will be prime for any convective initiation with PWAT anomalies sufficiently above 2 deviations above normal. These types of setups are sneaky with regards to potential, leading to an expansion of the SLGT risk to include much of that I-35 corridor and adjacent areas in the Hill Country to the west of the interstate. Consensus for locally heavy rainfall north of the Missouri Valley continues up into the Great Lakes as the low pressure progresses into the Midwest with the amplified flow ahead of the potent longwave trough. Lack of sufficient surface based instability caps the potential for significant rainfall totals to more of 1-2" with locally up to 3", but this would be enough to concern some urban flash flood threats for those areas extending through Illinois into Southeast WI into Western MI. The previous SLGT risk was maintained as a result with a MRGL risk encompassing. Kleebauer ....New Mexico... 16Z Update: Very little change necessary for the previous MRGL risk over New Mexico as CAMs deviated very little from the overnight output. Heaviest rainfall concerns will be located within that northern periphery of the closed upper low pivoting eastward through the area as ascent remains maximized within the diffluent area north of the closed reflection, textbook synoptic scale evolution. Higher terrain will actually resolve as snow, so the main areas of concern will be the valleys and locales along the I-40 corridor. Kleebauer ...Previous Forecast.. A leeside low currently over south central New Mexico will track northeast as several mesolows work to consolidate into a dominant single low over Texas and Oklahoma late today. With peak heating, some wraparound moisture and modest instability over an area with modestly depressed FFG values, steady rain with some convective cells ongoing over the area by this afternoon could result in an isolated flash flooding threat, particularly over any burn scars or other flood sensitive areas. Thus, a small Marginal Risk area was added with this update. Precipitation will be ongoing further north and into the mountains of southern Colorado but temperatures there will be cold enough for most if not all of the precip to fall as snow, precluding any flash flooding risk. Wegman Day 2 Valid 12Z Tue Nov 05 2024 - 12Z Wed Nov 06 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MISSISSIPPI AND OHIO VALLEYS... 20Z Update: The previous MRGL risk was relatively maintained as the overall synoptic pattern if forecast to decay somewhat, limiting the magnitude of heavy rainfall with regards to both rates and totals. A core of heavy rainfall is still anticipated, but the limiting factor of rates generally 1-1.5"/hr at peak intensity over an area that has seen very little rainfall for close to a month will inhibit significant flash flood concerns and relegate the setup to more of the isolated flash flood variety. The current ensemble means indicate the heaviest QPF footprint within the confines of the Lower to Mid-Mississippi Valley inflection areas encompassing Northeastern AR through Southeast MO and Western TN. Some of the CAMs are a bit bullish over areas like Memphis which could garner some flash flood interest even with the lower rates just due to the urbanization factors. Even still, FFG markers for the 1-hour time frame are running between 2.5-3"/hr over the area, so the threat will be mainly within the MRGL threshold and perhaps just missing out on the Slight Risk prospects with the dry antecedent conditions being the deciding factor. ARI exceedance probabilities are pretty tame with a 5-10% output within the latest HREF. This isn't exactly one that moves the needle to an upgrade, in agreement with the impacted WFO's. As a result, only minor=20 adjustments were made to expand the risk area a touch further west=20 and north, and to nudge a bit more in the Southeast flank as=20 convective activity is likely over areas near Baton Rouge and=20 points west.=20 Kleebauer ...Previous Forecast..=20 The upper level trough responsible for the past few days of heavy rain across the southern and central Plains will finally retreat back into the broader jet into Ontario and Quebec. The upper level energy supporting the corridor of storms will continue supporting storms across the Mississippi and Ohio Valleys Tuesday and into Tuesday night respectively. As the energy lifts north, it will mark the end of persistent heavy rain and storms from south to north. Thus, the flooding threat will be more in the morning in Texas, and more towards the night for the Ohio Valley. Training lines of storms will move across the Mississippi Valley for much of the day. These storms will be capable of heavy rains which may cause isolated flash flooding, especially in any urban areas such as Memphis. By Tuesday night, weakening of the storms and the shift well away from any prior days' rain footprint will effectively end the flooding threat. Wegman Day 3 Valid 12Z Wed Nov 06 2024 - 12Z Thu Nov 07 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR CENTRAL AND EASTERN GEORGIA INTO PORTIONS OF WESTERN SOUTH CAROLINA... 20z Update: Guidance continues to trend upwards with regards to the evolving heavy rain forecast across GA into the Western portions of SC for Wednesday. Deep tropical moisture advection will funnel out ahead of now Tropical Depression 18 (TD18), which will be a hurricane likely by the time of this forecast evolution. The moisture flux is caused by the proxy of the disturbance within the western fringes of a broad surface ridge located over the Western Atlantic providing a "highway" for the enhanced tropical moisture plume to funnel northward into parts of the Southeastern U.S. To the west, our longwave trough and closed low pattern will become sheared out with the cold front slowing and eventually becoming a quasi-stationary boundary once it enters the Ohio and Tennessee=20 Valleys. A 130kt jet streak will be located over the Great Lakes=20 into the Northeastern U.S providing a broad upper forcing pattern=20 to transpire within the RER of the jet. This will help in=20 initiation of precip across parts of GA into SC with some modest=20 rainfall developing.=20 As we work into Wednesday evening the boundary to the north will slowly drift into the Southeast with a developing convergence axis located over Central and Eastern GA into parts of the SC Piedmont leading to a broadening axis of steady precip with some increasing convective elements thanks to the unstable airmass that is advecting into the area. Several members of the global ensemble suite along with the bias corrected ensemble forecast indicate a substantial area of rainfall with totals >4" becoming focused within the greatest convergence axis along the slow frontal progression. With the high pressure stable in the Atlantic, a persistent easterly component to the wind field will only add to the convergence regime providing a prime focal point for slow- moving convection over the course of the overnight hours through Thursday morning. This setup can be characterized as a Predecessor Rainfall Event (PRE) that can develop well out ahead of an approaching tropical disturbance as it moves closer to CONUS. The key in all this is the tropical moisture advection that enhances the environment for more effective warm rain processes that aren't common within more mid-latitude cyclones and patterns. The interaction of the front and surface ridging will make all the difference on the specific locations that see the most rainfall. In this case, there's a growing consensus over Central and Eastern GA extending to the GA/SC line near Augusta.=20 Considering the variables at play and the trend towards a more prolific rainfall event, a Slight Risk was introduced across the areas above with a MRGL risk extending through portions of the Southeast, including Florida where the proximity to the tropical disturbance will allow for some heavier convective prospects over the Peninsula. There is still some uncertainty on the exact location of the heaviest precip and PRE setup, so be sure to monitor the situation closely as the location of the higher risk could change, as well as an upgrade to a higher risk if the signal maintains magnitude and/or higher confidence in the potential. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6juNROXd3QOzlWNvKSYwIEU1cas0NltOfGffodxj6oiL= 5XlDBpIDUCxPqPTSLVX7BnnXHkIw0IL7TkaTnqNyq6Y8Ugs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6juNROXd3QOzlWNvKSYwIEU1cas0NltOfGffodxj6oiL= 5XlDBpIDUCxPqPTSLVX7BnnXHkIw0IL7TkaTnqNysafem4Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6juNROXd3QOzlWNvKSYwIEU1cas0NltOfGffodxj6oiL= 5XlDBpIDUCxPqPTSLVX7BnnXHkIw0IL7TkaTnqNyuAXeZqk$=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) .