Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Feb 18 2023 18:06:58 FOUS30 KWBC 181806 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 103 PM EST Sat Feb 18 2023 Day 1 Valid 16Z Sat Feb 18 2023 - 12Z Sun Feb 19 2023 ....16Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 2 Valid 12Z Sun Feb 19 2023 - 12Z Mon Feb 20 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST ARIZONA AND SOUTHERN NEW MEXICO... ....2030Z Update... A few changes were made with this afternoon's update. Much of the 12Z CAMs guidance remains in agreement that the axis of heaviest rainfall will extend far enough north to impact portions of southeast AZ and southern NM. It's notable however that much of the global models keep the rainfall south into Mexico. Nonetheless, with more and more CAMs agreeing on locally heavy rainfall, especially on the west-facing windward side of the mountains. Instability remains largely absent, so topography will be the number one source of low level forcing. In the upper levels, the cutoff low near the northern Baja California Peninsula will circulate a strong shortwave eastward towards the Marginal Risk region. At the same time, the jet stream over the Intermountain West will also have a separate shortwave racing southeastward down the jet. These two shortwaves will fail to merge, however the two will be able to work in concert with each other and the upper level divergence east of the upper low to support lift of a plume of Pacific moisture moving northeastward into the desert Southwest. This moisture is highly anomalous for this time of year, as is the pattern of an upper level low stuck off the Baja California Peninsula. While more widespread rainfall totals should remain under an inch in most areas, the aforementioned upslope/windward sides of the mountains have the potential to see more than 2 inches of rain, especially the Sacramento Mountains of southern NM. Thus, in coordination with the TWC/Tucson, AZ and EPZ/El Paso, TX forecast offices, the Marginal Risk was expanded a bit north to include more of the mountains of far southeastern AZ, and extended east to include the Sacramento Mountains and metro El Paso, the latter included since FFGs are locally lower there due to urbanization. Wegman ....Previous Discussion... A compact, highly anomalous closed low will drop southwest of Baja California by late Saturday into Sunday with height anomalies around 4 to 4.5 standard deviations from monthly climatological norms. As such, sub-tropical moisture will stream northeastward out of the subtropical East Pacific with total PWat Values over 1.25" streaming northeastward across central Baja California directed toward the terrain of northeast Sonora and northwest Chihuahua, values are 2.5-3 Std Dev from mean, though total moisture flux with IVT values of 400-500 kg/m/s forecast into the terrain of Mexico, bleeding into far SE Arizona and far SW New Mexico. This high anomaly strongly suggests the potential for intense rainfall at times, with a solid duration throughout the day focused on the terrain which may result in localized flooding concerns with guidance suggesting 2-3" totals through 24hrs ending at 20.12z, particularly the 00z Hi-Res CAM solutions. However, the angle/orientation of the moisture flux relative to the Northwest Mexican coast and strength of downstream ridging will determine the placement of the plume. NAM, NAM-Conest, and ARW2 solutions suggest greater ridging and a more northward angle to the plume allowing for increased rainfall risk to SE AZ/SW NM, though GFS/ECMWF and ARW/HRRR suggest axis intersecting the terrain remains just south of the AZ/NM boarder in Mexico with these enhanced rain areas. This remains solid latitudinal spread, but given the near record moisture and flux potential and desert ground conditions, even modest rainfall totals may still pose a low-end risk for flooding particularly if the persistence on one or two ridges/ranges allows for those max totals near 2-3". In coordination with local forecast offices, have introduced a small Marginal Risk... which may need to be further expanded northward with subsequent model trends and increased hi-res CAM solutions.=20 Gallina/Wegman Day 3 Valid 12Z Mon Feb 20 2023 - 12Z Tue Feb 21 2023 ....2030Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Lingering light rain is likely to be ongoing at the start of the Day 3 period Monday morning across southeastern AZ/southern NM, where the Day 2/Sunday Marginal Risk is currently located, but the rain not expected to significantly worsen any isolated flash flooding started on Sunday. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-UVHzjW6x_7lIUn4RHKcJYzCzctgHeWzYA3OQGDODwhS= oLNMFzfPgS5pq20I4OC5ac0tMazohbeILO4h_jYUWerMivM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-UVHzjW6x_7lIUn4RHKcJYzCzctgHeWzYA3OQGDODwhS= oLNMFzfPgS5pq20I4OC5ac0tMazohbeILO4h_jYUGQevZxw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-UVHzjW6x_7lIUn4RHKcJYzCzctgHeWzYA3OQGDODwhS= oLNMFzfPgS5pq20I4OC5ac0tMazohbeILO4h_jYUQZJoQ4o$=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) .