Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 04 2022 08:34:47 FOUS30 KWBC 040834 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 334 AM EST Sun Dec 04 2022 Day 1 Valid 12Z Sun Dec 04 2022 - 12Z Mon Dec 05 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST ARIZONA AND SOUTHWEST NEW MEXICO... Most of Arizona and New Mexico will remain beneath the right-entrance region of a ~100 knot 250mb jet streak while, simultaneously, an upper low off the Pacific Northwest coast continues to funnel rich subtropical moisture overhead. NAEFS climatological percentiles remain exceptionally high over southeast Arizona and much of New Mexico this morning and afternoon with values above the 99th climatological percentiles in some cases. The IVT also remains quite robust as the upper low off the Pacific Northwest coast and the ridge over the Gulf of Mexico work in tandem to generate IVT values of 400-500 kg/m/s, which is also above the 99th climatological percentile. This remarkable amount of moisture is more than 5 standard deviations above normal, a true anomaly for this time of year. Similar to Saturday, however, instability will continue to be hard to come by, keeping a lid on rainfall rates from becoming too heavy. This will be what largely saves this area from having much more serious flooding concerns, given how incredibly anomalous this amount of atmospheric moisture is for this region. The region has been picking up steady rainfall from Saturday and soil moisture is more saturated than it was entering the weekend. The Marginal Risk remains largely unchanged, save a small northward expansion to account for recent trends in the rainfall noted on radar. While no reports of flash flooding have yet been received, the expectation of continued rainfall in the Marginal Risk area through much of the day today before weakening and shifting south should be enough time for isolated flash flooding to remain a concern. Wegman/Mullinax Day 2 Valid 12Z Sun Dec 04 2022 - 12Z Mon Dec 05 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR FAR SOUTHERN TENNESSEE AND NORTHERN ALABAMA AND GEORGIA... A strong cold front pushing through the Midwest on Monday will slowly sink south as a warm front lifts north into the Mid-South. High pressure over the East coast will work in tandem with lower pressure in lee of the southern Rockies to generate a southwesterly 850mb moisture moisture fetch out of the western Gulf of Mexico underneath an expansive 120 kt southwesterly jet over much of the country. This slug of moisture will join the subtropical jet emerging from the Southwest and result in a robust IVT across the Southern Plains and into the Mid-South. 00Z NAEFS suggests IVT values of 400-750 kg/m/s are possible across the South Central U.S. on Monday, which is >97.5% climatological percentile in many cases. As the upper low off the California coast and deep upper low in south-central Canada continue to help generate a strong jet streak oriented WSW to ENE across the U.S., diffluent flow will be positioned over the South, setting the stage for a heavy precipitation event along the warm front in the Mid-South on Monday. The latest trends in the guidance have generally been falling as far as total forecasted rainfall across the Slight risk area, and that axis of heaviest rainfall has nudged a bit towards the east as well. Should this trend continue, the Slight Risk may need to be abandoned in favor of the larger Marginal in place, but since there should still be a sizable swath of more than 2 inches of rain for far southern Tennessee and northern Alabama primarily, think despite no more explicit 3 inch or more daily totals being forecast, that there certainly is potential for isolated to widely scattered amounts over 3 inches, which certainly could pose a flooding threat, especially should that occur over downtown Huntsville or Chattanooga. The eastward shift in the guidance resulted in all but the northeastern-most corner of MS being removed from the Slight, and essentially all of AR as well. While there remains some uncertainty where the heaviest rain will be seen, that uncertainty has diminished quite a bit from 24 hours ago. However across Georgia, in coordination with the FFC/Atlanta, GA forecast office, the Slight Risk was expanded to include metro Atlanta and points north and west. While Atlanta isn't yet expected to get the brunt of the rainfall, even the 1-2 inches expected could cause flash flooding concerns given the inherently lower FFG values due to urbanization. Any additional small southward shift in the guidance could bring downtown Birmingham, AL also into the Slight risk. Wegman/Mullinax 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!7hnFl6CPZNAJqRbEHrDVizwKpYfkWRDhJ3eQuEgRvihP= d8KohcvMME546xE_7nRSTaZtAEvFdwtZpASxi2CNBCd4dmI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7hnFl6CPZNAJqRbEHrDVizwKpYfkWRDhJ3eQuEgRvihP= d8KohcvMME546xE_7nRSTaZtAEvFdwtZpASxi2CNiyOiK3U$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7hnFl6CPZNAJqRbEHrDVizwKpYfkWRDhJ3eQuEgRvihP= d8KohcvMME546xE_7nRSTaZtAEvFdwtZpASxi2CNignNQU0$=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) .