Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Oct 20 2024 00:35:30 FOUS30 KWBC 200035 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 835 PM EDT Sat Oct 19 2024 Day 1 Valid 01Z Sun Oct 20 2024 - 12Z Sun Oct 20 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN HIGH PLAINS... ....New Mexico and Texas Panhandle into Southeast Colorado... ....2130 UTC Update... Trimmed the Slight and Marginal Risk areas based on the latest observational and mesoanalysis trends as well as current RAP/HRRR trends. High pressure across south-central to southeast CO continues to drain more stable, anticyclonic low-level flow into northeast NM, with the low-level stability enhanced by the evap cooling from the rainfall. As a result we cut back on the northward extent of the Slight and Marginal Risk areas into southeast CO, again given the current and forecast instability trends. Hurley ....Previous Discussion... Several rounds of heavy to potentially excessive rainfall are expected across portions of northeast New Mexico into southeast Colorado through tonight given a highly diffluent upper flow pattern between a well defined closed low over northern Arizona and a southerly subtropical jet coming out of Mexico, this pattern has induced a 20 to 35 knot low level jet from the Gulf of Mexico into West Texas...resulting in the axis of a 1 inch precipitable water plume moving into place. Given short term CAM guidance...one round of convection that developed prior to the beginning of the Day 1 period on Saturday morning should gradually weaken by mid-morning followed by a relative lull in activity...followed by convection becoming reinvigorated later this afternoon/evening that persists into the overnight hours. With a variety of coarser-grid models and higher resolution CAM guidance pointing to the potential of 4 inches of rainfall in northeast New Mexico through 12Z Sunday...thinking is that this is a higher-end Slight risk. Also bolstering that idea is the consistency with which guidance has kept roughly the same area for the placement of moderate to heavy rainfall amounts. The greatest concern for excessive rainfall is near recent burn scars. ....Northwest... Maintained a Marginal risk from previous outlooks in this period as an atmospheric river slowly sags southward across parts of Washington state today. Model agreement remains good that the Olympic range and the far northern Cascades are in the best position for 3 to 5 inches of rain given the 1.00 to 1.25 inch precipitable water plume being directed normal to the terrain. Over time the axis will slowly sag southward with the magnitude of the moisture plume weakening with time. Bann Day 2 Valid 12Z Sun Oct 20 2024 - 12Z Mon Oct 21 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN AND WESTERN HIGH PLAINS AS WELL AS THE PACIFIC NORTHWEST... Plains... 21Z update... The latest guidance continues to favor a slower progression with the upper low therefore favoring a more westward placement of the QPF than the previous cycle and WPC forecast. The latest WPC QPF was adjusted and primarily focuses the highest totals over Northwest New Mexico. The inherited Marginal Risk area was offset with this change therefore the whole area was shifted westward about 2-3 tiers of counties; which removed portions of western Kansas and the Oklahoma and Texas Panhandles from the Marginal while now including more of northern and central Arizona and southern Colorado. Campbell Maintained the Marginal Risk area with much change needed from portions of New Mexico and the Texas panhandle northward into Colorado and Kansas given the model guidance which showed the threat of rain persisting into the Day 2 period. The overall threat of excessive rainfall should be diminishing once the upper low near the 4-corners region starts to weaken and lift northeast and once surface pressure east of the Front Range begin rising. Latest WPC deterministic QPF, the NBM and HREF generally produce an additional 1 to 1.5 inches of rainfall on Sunday into early Monday that has a considerable amount of overlap with the footprint of heaviest rainfall from Saturday. With rainfall rates likely to be lower than on Saturday and a smaller window of rainfall...felt a Marginal risk area should cover the risk. Northwest... 21Z update... No changes were necessary for this part of the country at this times. Campbell Maintained the Marginal risk area in parts of Washington state with only minor modification. The deepest moisture and heaviest rainfall rates should be shifting southward as a cold front makes its way inland from the eastern North Pacific. However...there looks to be a period in the Olympics and far northern Cascades of Washington that should get into rainfall associated with the surface and upper low location just north of the border...allowing for a continuation of the excessive rainfall threat into Sunday. The additional rainfall amounts only look to be under 1.5 inches...but it will be in addition to the 2 to 4 inch amounts on Saturday/Saturday night. Precipitable water values begin the period in the 1.00 to 1.25 inch range along the immediate coast and taper off to values closer to 0.50 to 0.75 inches in the northern Cascade...with values decreasing markedly after 00Z. Bann Day 3 Valid 12Z Mon Oct 21 2024 - 12Z Tue Oct 22 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!72rjJrym8dQJrcA-X52cdCl8TV2o6KrI6hosSNNtArtQ= o7CUJ62BCrtUyiB_zH3bFPy0frv9p6bTF2Y1DYX4uDV7vFc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!72rjJrym8dQJrcA-X52cdCl8TV2o6KrI6hosSNNtArtQ= o7CUJ62BCrtUyiB_zH3bFPy0frv9p6bTF2Y1DYX4HnYsSV4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!72rjJrym8dQJrcA-X52cdCl8TV2o6KrI6hosSNNtArtQ= o7CUJ62BCrtUyiB_zH3bFPy0frv9p6bTF2Y1DYX4Vefg3sg$=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) .