Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Feb 19 2023 23:17:30 FOUS30 KWBC 192317 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 616 PM EST Sun Feb 19 2023 Day 1 Valid 16Z Sun Feb 19 2023 - 12Z Mon Feb 20 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN NEW MEXICO, FAR WEST TEXAS, AND THE SOUTHEAST CORNER OF ARIZONA... A compact, highly anomalous closed low has dropped west of Baja California with a standard deviation over 3.5 and well into 99th percentile for this time of the year. Strengthening west-southwesterly flow has brought mid-level moisture across northwest Mexico, with surface to 700 hPa moisture lifting east-northeast into southern NM and far western TX. Eventually, total PWat values will reach 1.25" along Baja CA into the lower elevations of Sonora, bringing 2-3 standard deviation moisture values into the region. The core of the rain will slide northeast into the southern California coast and strengthen flow along the atmospheric river with .75 to near 1" total PWat values into the higher plateau across northeast Sonora, southeast Arizona, southwest New Mexico. This is near record moisture for the date and even near monthly record with both GFS and ECMWF all-time period maxima in IVT (moisture transport) accounting for the acceleration of the low level winds. Winds in the 850-700 hPa layer will strengthen along 700 hPa vorticity stripe across Northern Old Mexico as the right entrance of the polar jet strengthens over the southern Rockies throughout the day (reaching 150kts along the CO/NM border shortly. The overall acceleration will support IVT values over 500 kg/m/s. While the moisture is solid, instability is limited, though slightly growing in SPC mesoanalyses which show a pool of 100 J/kg of MU CAPE building over southeast AZ which is leading to moderate rainfall moving into far western TX at the present time. While the forcing should be mainly due to orographic ascent, there is some overlapping of frontogenesis at 650 hPa and 750 hPa in the eastern portion of the Marginal Risk area noted around 0300 UTC within the 18z GFS guidance. Given that the axis of upper level right entrance ascent is narrow, the expected band of showers has been narrow and weakly anti-cyclonically curved. The moisture plume may be narrow but it will feature strong convergence along the westward facing north-south ranges of far Southeast AZ, southern NM, and Sonora. The persistence and record moisture will support consistent moderate rainfall within the orography with up to .1-.2"/hr rates occasionally peaking near ..25". This will support local amounts of 1.5-2" with perhaps isolated 2.5-3" along the highest peaks (8-10Kft) by early Monday morning where winds are stronger and the moisture depth remains sufficient. With these sort of maximum totals, the suite is driving HREF probability signals further north into SE AZ/SW NM indicative of Marginal Risk category for Excessive Rainfall. The plume should stay at the Mexico border and south, until around the Rio Grande Valley, where it lifts a bit to direct the plume into the Southern Sacramento Range in NM as well as intersecting the Franklin Range and Sacramento Mtns near Guadalupe Pass in TX.=20 Flash flooding is not expected given the lower rates; however, atypical flooding within washes emanating from the mountain ranges is probable. The best forcing will be diminishing around the 20/12z time frame, as the upper-level right exit region will start to dominate as the closed low approaches, the AR will also focus further south into higher terrain of S Sonora.=20=20 Roth/Gallina Day 2 Valid 12Z Mon Feb 20 2023 - 12Z Tue Feb 21 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 3 Valid 12Z Tue Feb 21 2023 - 12Z Wed Feb 22 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Lower Ohio Valley... The previous Marginal Risk over the Lower Ohio Valley was downgraded to remove the risk threat entirely with this update. The trend in all of the guidance for this period back to when this was Day 5-6 has been undeniably downward in total QPF. The lastest WPC QPF forecast now has a broad two tenths of an inch of rain or less across this region through 12Z Wednesday. Despite the recent rainfall and noting the potential for higher river levels along the Ohio with continued runoff from the rainfall a couple days ago, there simply is no longer enough rainfall forecast for this area to justify even a 5% chance of flash flooding. The slower trend in the guidance regarding the ejection of the upper level low near the Southwest has lead to most of the expected flooding impacts related to that feature to be delayed into the Day 4/Wednesday period, when a Slight Risk has been issued for much of central IL up to the Chicagoland region. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_5wQKwOk3zyFCTYS8bSCCKDCExu7gyeOdGgOwL3r8189= ZjiJv-9dO6oo2stEuWuTDbQClzIvbOgbDQm70kNp9NcZbFI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_5wQKwOk3zyFCTYS8bSCCKDCExu7gyeOdGgOwL3r8189= ZjiJv-9dO6oo2stEuWuTDbQClzIvbOgbDQm70kNpPMNyhkg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_5wQKwOk3zyFCTYS8bSCCKDCExu7gyeOdGgOwL3r8189= ZjiJv-9dO6oo2stEuWuTDbQClzIvbOgbDQm70kNpPbEray0$=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) .