Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 21 2023 00:52:01 FOUS30 KWBC 210051 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 750 PM EST Wed Dec 20 2023 Day 1 Valid 01Z Thu Dec 21 2023 - 12Z Thu Dec 21 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... The forecast generally remains on track. Thus far we have seen hourly rainfall exceed 0.5" over portions of Santa Barbara and Las Angeles counties, with some 2"+ event totals being reported over the former. As we go through the overnight hours an axis of repeating showers, with embedded more intense convective elements, is expected to persist across portions of Santa Barbara and Ventura counties. The 18z HREF depicts 1"/hr neighborhood probabilities as high as 30-50% between 00z-06z, with some probabilities lingering through 12z. Recent HRRR runs show a similar outcome...with rates periodically exceeding 1" within the persistent axis of rainfall streaming northward across the region. The HREF and HRRR consensus is for an additional 1-3" of rainfall through 12z Thursday, peaking from portions of Santa Barbara county into western Ventura county. However even localized higher amounts appear likely, with HREF neighborhood probabilities of exceeding 3" over 70% and a 10-20% chance of exceeding 5". These isolated max values of 3-5" are also supported by the last several runs of the HRRR. Given the expected rainfall rates and totals, flash flooding should become increasingly likely as the night progresses..especially focused over southern/eastern Santa Barbara into western Ventura county. Some of this flooding is expected to be significant and potentially life threatening in nature...with extensive roadway flooding, rock and mudslides, debris flows near recent burn scars, and significant creek flooding all a possibility tonight into Thursday. Chenard Day 2 Valid 12Z Thu Dec 21 2023 - 12Z Fri Dec 22 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... ....20Z Update... Minimal changes to the D2 risk areas, including the Moderate risk in place over the Los Angeles metro and the terrain to the northwest. Models continue to signal a prolonged heavy rain event for the south-facing slopes of the Santa Ynez to the north of Santa Barbara proper where two-day totals are approaching 8-10" on hi-res deterministic guidance. 12Z HREF blended mean was exceptionally bullish for the western edge of the Santa Ynez with totals of 3-6" increasingly likely within the time frame of 12Z Thursday through 00z Friday before heavy rain focus shifts southeast into LA proper. Los Angeles will see its heaviest rainfall likely between 00-12Z Friday with some guidance indicating upwards of 2-3" within the time frame as 700mb VV panels depict a strong low to mid-level ascent field traversing over the metro area. This correlates with an extended period of heavy rainfall within the LA metro, as well as the hills just off the north as southeast flow over the terrain will enhance the upslope component and provide another local maximum just east of the Santa Ynez. Heavy rain will be possible down towards San Diego proper, but the forecast is for the heaviest rain to remain focused over upstream where flow will be more orthogonal to the coast leading to training. Overall, there is little change in the current forecasted evolution of the upper level disturbance and expected rainfall footprint within portions of Southern California, thus maintained the Moderate risk and will assess the need for any upgrades pending the result of what occurs during D1. Kleebauer ....Previous Discussion... A strong, anomalously deep closed upper low will slowly meander off the southern California coast Thursday into Thursday night, setting up a prolonged and potentially significant rainfall event for portions of Southern California including the Transverse Ranges and portions of the Los Angeles metro. The position of the upper low will put the area in the favored area of forcing for ascent, while embedded vort maxes rotating around the low will work to provide extra/enhanced lift at times, with the latest model guidance suggesting one wave coming through Thursday morning followed surge later in the evening/overnight Friday morning. In any case, the ingredients will feature fairly anomalous moisture, with PW values between 3-4 standard deviations above normal and IVT values 400-500 kg/m/s. Some instability of 500-750 J/kg is forecast to develop as well along the immediate coast, which will help to realize the higher rain rate potential, where hourly totals may exceed 1" at times, particularly for the favored south facing slopes of the Transverse Ranges. The other factor that increases the QPF and flood threat is the relatively slow motion of the storm, which may allow for some prolonged rain bands to set up. The favorable southerly flow nearly perpendicular to the terrain will enhance the orographic ascent and should enhance rainfall totals, which may reach 3-5" for the 24-hr period ending 12Z Friday. 48-hr totals (including rain expected today/tonight) may exceed 6-8" (localized 10") in the areas northwest of Los Angeles in the Transverse Ranges, and this is likely to lead to significant flooding, including flash and urban flooding as well as potential for rock slides, mud slides, and debris flows over sensitive burn scars like the South Fire burn scar in Ventura County. Very few changes were made to the existing Moderate Risk area, which covers both the terrain flood risk as well as the potential for flash flooding for western LA metro areas.=20 Taylor Day 3 Valid 12Z Fri Dec 22 2023 - 12Z Sat Dec 23 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA AND ARIZONA... ....20Z Update... Latest guidance for the period continues to peg a localized heavy rainfall signature across southern and central AZ with a secondary max within the AZ/CA border as increased ascent on the north side of the closed low to eventual open trough will lead to enhanced rainfall for a period of time as the disturbance pivots into the Southwestern U.S. There's some minor discrepancy in the overall magnitude of the QPF with the GFS fairly bullish over the southern tier of AZ with the CMC a step down in magnitude and the ECMWF a further step away from the GFS deterministic. Comparing the latest ensemble spread, there's not much of a deviation between the GEFS/ECENS/GEPS with the evolution of the upper level low, so the distinction comes down to the handling of the mid-level vorticity maxima as the cyclone moves onshore, as well as the magnitude of the low-level moisture advection pattern pushing inland from the Pacific. The general consensus is a solid swath of up to 1" of QPF within the confines of south-central and central AZ with the best chance for upwards of 2+" located along the southern AZ border given the tongue of theta-E advection providing some modest instability which would favor better rainfall rates. The antecedent conditions over the area also leave a lot to be desired as an extended drought over the desert southwest will likely welcome the rainfall, even a plentiful amount at this juncture. Rainfall will be generally over a 6-12 hr time frame on Friday afternoon into Saturday morning which leads to average rainfall rates being less than formidable for widespread flooding. Localized flash flood threat will be possible, especially for those over the lower desert of Pima county up through Maricopa/Pinal into Gila. North of the Mogollon Rim will be cold enough for multiple precip types, so flood threat will cut off abruptly into the terrain of northern AZ. The previous Marginal Risk was maintained after collaboration with the local AZ WFOs, but will monitor for any upgrade potential as we move into the hi-res guidance cycles as we get a better picture on the convective pattern that may evolve.=20 Kleebauer ....Previous Discussion... The strong/deep closed upper level low is expected to be centered just offshore southern California early Friday morning then begins to pick up speed as it moves eastward toward central Arizona by early Saturday morning, opening up as it does so. This will put much of southern California, far southern Nevada, and Arizona in a favored region of forcing/lift while southerly low level flow positions a plume of higher moisture initially into southern California then across Arizona during the afternoon/evening hours. Moisture anomalies with this system are expected to remain well above normal, near 3 standard deviations above normal for this time of year, and precipitable water values are forecast to reach or exceed 1" at times, especially across southwest to south-central Arizona. The GFS and ECMWF IVT forecasts from CW3E show an impressive surge approaching 500-700 kg/m/s into central Arizona. During the afternoon, some limited instability may develop across the Desert regions of CA and eastward into southern Arizona, which may allow for some localized higher rain rates approaching 1"/hr at times. Overall, a broad area of 0.75-1.5" totals are expected for the 24-hr period ending 12Z Saturday, with some localized/embedded higher amounts up to 2.5" possible in any thunderstorms. Some localized flash flooding may develop, especially across southern Arizona if higher rates do materialize, but for now until confidence in ingredients and rain rate potential increases, a Marginal Risk reasonably covers the flood threat at this time though if trends continue, an upgrade to a higher risk category may be needed for parts of Arizona. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9I1Mux0vrUdujwcI2h60AOR9zkGmj6Ak398F2tql-9f6= vVCcqR8obGaJlPf5757o39KXtOEQR_4RoqfMTJUImFCTFbw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9I1Mux0vrUdujwcI2h60AOR9zkGmj6Ak398F2tql-9f6= vVCcqR8obGaJlPf5757o39KXtOEQR_4RoqfMTJUI6jjSQGA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9I1Mux0vrUdujwcI2h60AOR9zkGmj6Ak398F2tql-9f6= vVCcqR8obGaJlPf5757o39KXtOEQR_4RoqfMTJUI5DzSDYk$=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) .