Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jan 30 2024 15:25:37 FOUS30 KWBC 301525 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1024 AM EST Tue Jan 30 2024 Day 1 Valid 16Z Tue Jan 30 2024 - 12Z Wed Jan 31 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard Day 2 Valid 12Z Wed Jan 31 2024 - 12Z Thu Feb 01 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS FAR SOUTHWEST OREGON AND PARTS OF NORTHWEST AND WESTERN CALIFORNIA, INCLUDING THE COASTAL RANGES, MUCH OF THE SACRAMENTO VALLEY, BAY AREA, AND NORTHERN PORTIONS OF THE CENTRAL CALIFORNIA COAST... Mid-upper level flow becomes increasingly meridional along the West Coast by day 2 (Wed-Wed night), as the deep cyclone approaches Vancouver Island while a robust NW-SE oriented upper level jet streak (130-150kt at 250 mb) allows the longwave trough to dig into southern CA by Thursday. Strong deep-layer synoptic scale support will ensue within increasing difluence aloft, as both the upper level divergence and south-to-north transport of low-mid layer subtropical moisture become enhanced within the left-exit region of the aforementioned upper jet streak. On Wednesday, robust 850 mb S-SW flow will be around 5 standard deviations above normal for late Jan per the 00Z GEFS and SREF, peaking between 50-60kts long the coast. Meanwhile, 850-700 mb moisture flux anomalies are expected to reach +4 to +5 standard deviations for a little (Monday morning) while as the 1.00-1.25" PW plume skirts the CA coast. Later in the period (Wed night), low-mid layer moisture flux anomalies are expected to scale back a bit as the AR drops south into the Central CA Coast. The strength and duration of the IVT anomaly equates to a moderate AR based on the CW3E AR Scale per both the GEFS and ECMWF ensemble means, with peak IVT magnitudes of 600-800+ kg/m/s. However the relatively swift duration of the AR should limit the coverage of short-term runoff impacts across the outlook area, along with the weakening magnitude of the AR with time (especially late Wed-Wed night) as it becomes more dislodged from the main (deep) low approaching Vancouver Island. The latest CSU GEFS-based first-guess fields, particularly the UFVS-verified version, continue to support the continuation of the Slight Risk that was noted in yesterday's Day 3 ERO, though given the timing (faster progression down the coast), have included a bit more of the Central CA Coast in the Slight. Signals for a 'high end' Slight continue to exist per the latest guidance, i.e. one where the neighborhood probability of rainfall exceeding current FFGs is at least 25% yet still less than 40% (threshold for Moderate). This especially in WFO EKA and northern MTR's CWA, where the latest guidance ensemble mean QPF on Day 2 is between 3-5+ inches over most areas, with spotty 6-7+ inches noted by some of the higher-resolution models (including the NAM CONUS-Nest and FV3) along some of the more elevated coastal ranges. Overall, the QPF compared to the past couple of forecast cycles has come down a tad, likely based on the faster progression of the AR per the model trends. As such, given the current trends, an upgrade to Moderate Risk during the Wed-Wed night period appears unlikely. Hurley Day 3 Valid 12Z Thu Feb 01 2024 - 12Z Fri Feb 02 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF SOUTHERN CALIFORNIA, INCLUDING SOUTHERN PORTIONS OF THE CENTRAL CALIFORNIA COAST ALONG WITH THE LOS ANGELES AND SAN DIEGO METRO AREAS... 130-150+ kt 250 mb jet streak will nudge into SoCal and northern Baja Thu-Thu night, again helping to enhance the deep-layer forcing before the upper trough pivots through the outlook area. Low-mid layer moisture and moisture flux anomalies along the AR are projected to be similar to the end of Day 2 (Wed night), i.e. with 850-700 mb moisture flux averaging around 3 standard deviations above normal as the 1.00"+ PW plume remains just off the coast. Given the latest trends, it appears the AR should peak as a borderline Moderate on Day 2 per the latest CW3E scale, then drop to a Weak AR on Day 3 (Thu-Thu night) as it progresses fairly swiftly through SoCal. Model QPFs are therefore not as robust as compared to Day 2, even with the upslope enhancement along the southern Transverse and western Peninsular ranges. Peak areal-average totals from the 00Z guidance is between 1.5-2.5", the bulk of which should fall within a 8-10hr period given the current projected AR progression. Based on the latest (faster) trends, have removed the northern portion of the Slight Risk that was present in yesterday's Day 4 ERO (areas north of Point Conception). Otherwise, have maintained the Slight Risk elsewhere, including Santa Barbara and the LA to San Diego metros, where some (albeit marginal) elevated instability -- up to 200-400 J/Kg per the 00Z ECMWF -- would more likely allow for 0.50"/hr and/or 1.5"/3hr rainfall rates. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-QMiRJ1DlOAvZYhvNo7I9N6zQnjabIp15wg96YprINgL= 4nxXhx7reuBwu4UZuZ2WninfKv5YiNXXhQHuUy7tvOlLojg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-QMiRJ1DlOAvZYhvNo7I9N6zQnjabIp15wg96YprINgL= 4nxXhx7reuBwu4UZuZ2WninfKv5YiNXXhQHuUy7tgfoymlE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-QMiRJ1DlOAvZYhvNo7I9N6zQnjabIp15wg96YprINgL= 4nxXhx7reuBwu4UZuZ2WninfKv5YiNXXhQHuUy7tUkXZiU4$=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) .