Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jan 29 2024 19:57:16 FOUS30 KWBC 291957 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 256 PM EST Mon Jan 29 2024 Day 1 Valid 16Z Mon Jan 29 2024 - 12Z Tue Jan 30 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley/Kleebauer Day 2 Valid 12Z Tue Jan 30 2024 - 12Z Wed Jan 31 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley/Kleebauer Day 3 Valid 12Z Wed Jan 31 2024 - 12Z Thu Feb 01 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST CALIFORNIA, INCLUDING THE COASTAL RANGES, PARTS OF THE SACRAMENTO VALLEY, THE BAY AREA, AND COASTAL CENTRAL CALIFORNIA... ....20Z Update... Current trends on guidance maintain a stable AR presence with transient motion to the south through the period. Higher end QPF is focused over the King Range south of Eureka where totals between 5-7" are likely based on deterministic QPF spread and 13Z NBM percentiles quantifying a 50-60% chance of reaching 6". There's a couple other maxes in the QPF distribution that align well with the NAEFS +5 deviation of the IVT pulse during the unfolding AR. Couple of the areas of interest were the rise in QPF within the higher terrain near Santa Cruz, as well as Big Sur south of Monterey where 3-4" forecasts are now the norm within deterministic and the correlating probabilities are taking that into account within the ensembles. In coordination with the Monterey WFO, have expanded the Slight Risk south along the coastal plain in central CA, now including the metros of San Francisco/Oakland, San Jose, and Monterey. Urban flooding will be the primary flood concerns within those corridors, but QPF forecast is not highest in those zones at this time. Motion of the AR plume is the main factor in capping the event to a higher end SLGT as guidance remains steadfast with the progression of the plume as it heads down the CA coast. Higher end potential will be along the northwest CA coast from just south of Eureka down to the terrain northwest of Santa Rosa.=20 Kleebauer ....Previous Discussion... Mid-upper level flow becomes increasingly meridional along the West Coast by day 3 (Wed-Wed night), as the deep cyclone approaches British Columbia 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. By late Wed and Wed night, robust 850 mb SW-SSW flow will peak between 4 to 5 standard deviations above normal for late Jan per the 00Z GEFS and SREF. Meanwhile, 850-700 mb moisture flux anomalies are expected to reach +3 to +4 standard deviations for a little while as the 1.00-1.25" PW plume skirts the CA coast. Strength and duration of the IVT anomaly would suggest 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 Wed-Wed night as it becomes more dislodged from the main (deep) low approaching western BC. The latest CSU GEFS-based first-guess fields, particularly the UFVS-verified version, support the continuation of the Slight Risk that was noted in yesterday's Day 4 ERO, with minor adjustments. Signals for a 'high end' Slight are evident per the 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's CWA where the latest guidance ensemble mean QPF on Day 3 is between 3-5+ inches over most areas, with spotty 6-8+ inches along some of the more elevated coastal ranges. Cannot rule out the need for a targeted Moderate Risk in future outlooks, especially if we begin to see a slower AR progression (and thus uptick in QPF) in the guidance trends. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_8rO7iswQQgMyqd5lJsGdcQRyXC34ytjSLD24LJOGqFl= tcLfow6Q24QSA-UCn1VrXb9i9LgctkwmsNvvH4EUGiDTqbc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_8rO7iswQQgMyqd5lJsGdcQRyXC34ytjSLD24LJOGqFl= tcLfow6Q24QSA-UCn1VrXb9i9LgctkwmsNvvH4EUkT9yyOA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_8rO7iswQQgMyqd5lJsGdcQRyXC34ytjSLD24LJOGqFl= tcLfow6Q24QSA-UCn1VrXb9i9LgctkwmsNvvH4EUyrkmvZI$=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) .