Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Feb 19 2023 15:58:29 FOUS30 KWBC 191558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1057 AM 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... ....16Z Update... No changes to the previous forecast were made with this morning's update. The consensus of CAMs remain in lockstep that the Marginal Risk area highlighted in the previous forecast remains right where the heaviest rain potential today will be. Wegman ....Previous Discussion... 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. Already CIRA LPW PW values are starting to increase along the southeast side of the circulation with strengthening southwesterly flow bringing mid-level moisture across northwest Mexico initially, with sfc to 700mb moisture lifting north. 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.=20 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.=20 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 850-700mb layer will strengthen along 700mb 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 at 22.00z). The overall acceleration will support IVT values over 500 kg/m/s. While the moisture is solid, it will not be unstable with CAPE values expected below 25 J/kg throughout the length of the atmospheric river stream. In fact, the forcing is going to limited toward orographic ascent and given the axis of upper level right entrance ascent is narrow, the expected band of showers is expected to be similarly narrow and weakly anti-cyclonically curved. Now, the 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 24hr totals of 1.5-2" with perhaps isolated 2.5-3" along the highest peaks (8-10Kft) where winds are stronger and the moisture depth remains sufficient. There remains latitudinal spread in the placement of the narrow band, mainly driven by the amount of shortwave ridging along/ahead of the closed low. The NAM and its hi-res CAM family members are driving the axis further north to affect the Chiricahua and Guadalupe Mts in AZ, southern Black/Pintos Altos Ranges in NM. 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. However, global models (GFS/ECMWF) and other hi-res CAMs such as the FV3CAM, ARW and Canadian Regional further south focusing on the mountains of northeast Sonora. Satellite trends along with model to model comparisions, suggest a flatter solution shedding energy into the Southern Plains a bit faster. This results in the plume staying at the Mexico boarder 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. While this is favored, the northern solutions cannot be fully ignored and as such small adjustments were made to the Marginal Risk particularly with orographic ascent being the dominant forcing for prolonged anomalous rainfall totals throughout the day. 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 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. ....Southeast AZ/Southwest NM/Western Texas Panhandle... The northwest Mexico Atmospheric River from Day 1 period will have low potential to still be ongoing at the 20.12z end valid time.=20 Best forcing should be diminishing by 20.06z, as the favorable right entrance of the upper level jet streak over N NM/CO has shifted into the Southern Plains, concurrently the jet streak rounding the eastern periphery of the closed low near the California Bight will be drifting across Sonora providing=20 upper-level decent likely suppressing remaining shower activity.=20 Additionally, low level flow, moisture plume will be directed further south across S Sonora toward Sinaloa, and not likely to track through the higher terrain of the Sierra Madre Occidental to affect AZ/NM any further. Flooding may still be ongoing at 20.12z toward 18z accounting for this lingering light rain, as well as, water still washing out of the higher terrain that fell in the Day 1 period. Gallina Day 3 Valid 12Z Tue Feb 21 2023 - 12Z Wed Feb 22 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER AND CENTRAL OHIO RIVER VALLEY... At the start of the Day 3 period, 21.12z, a latitudinal flat flow regime will exist across the Tennessee Valley into the central Appalachian mountains with modest, above average moisture values near the frontal zone with a transitional period between the exiting wave and the deep wave starting to emerge from the Southwest. Weak warm advective regime in Lower MS River Valley may spark scattered weak cells across N MS/AL and TN, but given limit angle of isentropic ascent and very weak instability (~250 or less J/kg of MUCAPE). Additionally, spread in global guidance continues to be fairly broad in latitude with the frontal zone and therefore potential training cells. The concern here remains the soil conditions which are saturated across much of the Tennessee River Valley and southern Ohio River Valley into the lower slopes of the Appalachians including the Cumberland Plateau. NASA SPoRT LIS relative soil moisture depict well above normal ratios (65-80%) or about the 70-80th percentile for the season, so any stronger cells could have the potential for above normal run-off resulting in some flooding concerns. While FFG values in the region have likely risen faster than warranted in the ground conditions, there is an order of difference between potential hourly rates and 3hr totals that the risk of exceedance is less than 5%, with exception over the more complex terrain of the Cumberland Plateau into central Appalachian Range.=20 By midday, the closed low in the Southwest has buckled the mid to upper level flow pattern and stronger warm air advection will lift the warm front across the central MS valley and into Kentucky nearing the Ohio River Valley by early overnight period (22.00-06z). Broad LLJ will intersect the warm front at a much better angle (near perpendicular), while instability steadily increases over the warm sector. While best instability is remains further upstream nearer the height-falls in MO to OK, ample MUCAPE to 250-500 will over-top the best moisture convergence at/along the front. Elevated strong convection should become more likely in strength and coverage between 06-12z from the Tri-Rivers/S IL across N KY. Deeper layer steering flow will be veer more west-southwest to east-northeast allowing for some potential for scattered cells to train; though northward propagation of the warm front may limit some of the best training potential. While ground conditions are not as saturated as further south and east, there has been above average precipitation that did result in flooding conditions over the last week. As such, a Marginal Risk of Excessive Rainfall remains for much of the Lower Ohio River Valley toward southwest Ohio/northern & central Kentucky.=20 Gallina Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4KoQcEhcNfYH55Fzb1AhVK8BpRP8zn-i20Uj06Iaiw5E= 9IFd_7qG8QHPddYxTWZq6-W2G7IkIGnA3q2YZBmQ4GnmIxA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4KoQcEhcNfYH55Fzb1AhVK8BpRP8zn-i20Uj06Iaiw5E= 9IFd_7qG8QHPddYxTWZq6-W2G7IkIGnA3q2YZBmQnLeiB00$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4KoQcEhcNfYH55Fzb1AhVK8BpRP8zn-i20Uj06Iaiw5E= 9IFd_7qG8QHPddYxTWZq6-W2G7IkIGnA3q2YZBmQrmT380M$=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) .