Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 04 2022 15:46:54 FOUS30 KWBC 041546 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1046 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 FAR SOUTHWEST NEW MEXICO... Most of Arizona and New Mexico remains beneath the divergent right-entrance region of a ~110 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 PW 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 around the 99th climatological percentile through 00Z this afternoon. However, similar to what was observed on Saturday, any sufficient instability to help garner heavier rainfall rates remains absent, thus keeping a lid on rainfall rates from becoming too heavy. The lack of instability is what keeps the Excessive Rainfall threat capped to a very localized area, which is surprising given how anomalous this amount of atmospheric moisture is over the region. Southern Arizona and far southwest New Mexico has been picking up steady rainfall since Saturday and soil moisture has been growing increasingly saturated. NASA SPoRT-LIS soil moisture percentiles in the 0-40cm layer have ballooned to as high as 90-98% in some cases and has a noticeably larger footprint across the region compared to this time yesterday. The Marginal Risk remains in place given the increasingly more sensitive soils, but any potential flash flood threat will be very localized. Areas that have received as much as 1-2" of rainfall in the last 24 hours, as well as typically dry washes and areas along complex terrain, are most prone to potential flash flooding through this afternoon. The bulk of today's heaviest rainfall is likely to occur before 03Z. It is possible that the Marginal Risk may be adjusted further or removed in the 01Z update later today as the axis of anomalous moisture presses east of the current Marginal Risk area overnight. Mullinax Day 2 Valid 12Z Mon Dec 05 2022 - 12Z Tue Dec 06 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 Valid 12Z Tue Dec 06 2022 - 12Z Wed Dec 07 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE TENNESSEE VALLEY... In response to the trough digging across the West on Tuesday, the jet stream and surface fronts will begin drifting northward across TN & KY. Generally laminar west-southwesterly flow from the jet level to the surface, an unfavorable right exit region of the jet, and the warm front beginning to win out over the colder air over the Great Lakes will all combine to reduce the overall precipitation along the front, despite continued highly anomalous atmospheric moisture and PWATs in the Marginal risk area. As such, the Marginal Risk area is largely a continuation of the rainfall expected from Monday, with the northward shift accounting for any rainfall seen across northern Tennessee from Monday. Soils are very dry across KY, so despite the likelihood of widely scattered to scattered daily rainfall totals around an inch, the soils and below normal river flows in this region should easily be able to handle this moisture. The southern end of the Marginal risk area is not expected to get as much rainfall as areas further north across Tennessee, but given the wet conditions from Monday along with some uncertainty as to how quickly the bulk of the rainfall drifts north, decided it was prudent to keep far northern AL & GA in the Marginal Risk, which was agreed upon with the HUN/Huntsville, AL, FFC/Atlanta, GA, and OHX/Nashville, TN forecast offices. It's worth noting that this very strong baroclinic boundary over the OH River Valley is unlikely to move much at all over the coming week, which certainly is a pattern favorable to eventual flooding concerns in KY, AR/eastern OK, and maybe portions of the Mid-Atlantic later in the week, but it will take a few days of rainfall to bring soils and rivers in this area to a level where flooding may become a concern. Changes from the inherited map included removing all of AR from the Marginal, considering the eastward shift on Monday of the bulk of the rainfall. This will keep all of AR at around 1" of total rainfall forecast for the combined Monday/Tuesday time frame...well shy of any flooding concern. There was also a Marginal risk area inherited for portions of the Northeast from Philadelphia through New York City and including the Catskills, Berkshires, and Green and White Mountains of New England. In coordination with the PHI/Philadelphia, PA, OKX/New York City, NY, ALY/Albany, NY, and BGM/Binghamton, NY forecast offices, that Marginal risk area was removed entirely. Rainfall totals will struggle to reach an inch for the day in most of New England, save any potential very localized upsloping. This is quite simply not enough rainfall to generate any flooding concerns. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7-I87umTTB31YJq6npypUv1-pbctsH9GJFyDAH5b5iOo= MS_EjZFEZdJMjSAvHHU5REQ_2TdnyBPD9DEPiQnGN1IQrc4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7-I87umTTB31YJq6npypUv1-pbctsH9GJFyDAH5b5iOo= MS_EjZFEZdJMjSAvHHU5REQ_2TdnyBPD9DEPiQnGbINfNcc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7-I87umTTB31YJq6npypUv1-pbctsH9GJFyDAH5b5iOo= MS_EjZFEZdJMjSAvHHU5REQ_2TdnyBPD9DEPiQnGgi8FS3k$=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) .