Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Oct 17 2024 00:39:54 FOUS30 KWBC 170039 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 839 PM EDT Wed Oct 16 2024 Day 1 Valid 01Z Thu Oct 17 2024 - 12Z Thu Oct 17 2024 ....16Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 2 Valid 12Z Thu Oct 17 2024 - 12Z Fri Oct 18 2024 ....2030Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 3 Valid 12Z Fri Oct 18 2024 - 12Z Sat Oct 19 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE FOUR CORNERS REGION... ....2030Z Update... The Marginal Risk area was expanded westward through the Four Corners and over to the Glen Canyon Recreation Area with this update. The Marginal Risk area was trimmed out of the highest peaks of the San Juan Mountains of southern Colorado as well. On the broad scale, the Marginal Risk covers the narrow corridor where the precipitation will be warm enough for predominantly rain, but also heavy enough to cause isolated flash flooding, especially in any slot canyons, burn scars, and low-lying flood prone areas. To the north (UT/CO), the air will be cold enough for freezing levels to be low enough to result in a significant amount of precipitation falling as snow. To the south (central/southern AZ/NM), the rainfall will be less heavy as the greatest forcing will be further north. Hence the broad but rather narrow stripe that the Marginal Risk encompasses. The westward expansion over extreme southern UT and extreme northern AZ was primarily focused on the canyons and other flashy and flood prone areas that could flash flood even with otherwise modest amounts of rain. The predominantly very dry conditions of the past few weeks in this area have likely caused the soils to become more hydrophobic, which supports some of the rain converting to runoff rather than saturating. This too would support an isolated flash flooding threat. Wegman ....Previous Discussion... Longwave pattern will evolve into a fairly prolific upper trough/closed-low over the Great Basin, slowly migrating eastward with sights on the Four Corners. Large scale ascent will become maximized downstream of the mean trough with a pronounced diffluent pattern in-of the Southern Rockies, including the Sangre de Cristo and San Juan mountain ranges where heavier precip is forecast. A cold front will slowly propagate eastward from the primary disturbance with a return flow setup pressing westward across the Eastern NM plains on the western flank of a sprawling surface high east of the Mississippi. A secondary cold front to the north will plunge southward thanks to a strong ridge of high pressure developing upstream in wake of the primary western low, along with a lee side trough developing over the Front Range. The tandem of boundaries and backed flow will converge over Northern NM with a strong isentropic ascent pattern initiating later Friday evening through Saturday AM and beyond. Convective signals are most pronounced in the initial stages of the pattern evolution mainly due to the return flow setup where more unstable air will advect northwestward into the Eastern NM plains before banking against the lee of the Southern Rockies. Both ensemble and deterministic output for heavy precip are locked in across the area encompassing much of Northern NM, but especially within the confines of places like Raton and points south where topographic enhancement within the convergent axis will generate local precip maxima when the convergence pattern truly solidifies late in the period. Totals of 1-2" of precip are forecast on the southern edge of the San Juans down into portions of the Sangre de Cristos, closing in on some of the prevalent burn scars in the region. A secondary maxima of 1-2" is forecast over that main axis of convergence over Northeastern NM in vicinity of the eastern flank of the Sangre de Cristos, including places like Taos and Angel Fire over into parts of the plains in the lee of the mountain chain. The period before will be more of a primer for the main event this period through the following D4 time frame. The combination of better moisture return and large scale forcing via diffluent mid- level flow and jet coupling provides ample support for a continuation of the previous MRGL risk, as well as an expansion of the risk area further southwest and east to match the ensemble QPF footprint in areas of highest convective potential. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6ejTgOV--2zs4eS8x0c3NZeixhhumG2nJ3LFXEHDhEfH= 6ICqrCCSvMdVRwgb10OqQRvEMkA6eXaQciSfMPcMutnosPQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6ejTgOV--2zs4eS8x0c3NZeixhhumG2nJ3LFXEHDhEfH= 6ICqrCCSvMdVRwgb10OqQRvEMkA6eXaQciSfMPcMJFf6P7c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6ejTgOV--2zs4eS8x0c3NZeixhhumG2nJ3LFXEHDhEfH= 6ICqrCCSvMdVRwgb10OqQRvEMkA6eXaQciSfMPcMK2zF6ZA$=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) .