Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jan 08 2023 08:07:27 FOUS30 KWBC 080807 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 306 AM EST Sun Jan 08 2023 Day 1 Valid 12Z Sun Jan 08 2023 - 12Z Mon Jan 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF NORTHERN AND CENTRAL CALIFORNIA... ....Southwest Oregon into Northern and Central California... The Excessive Rainfall Outlook inherited from yesterday's Day 2 needed no notable changes for the new Day 1 period, 12Z Sun-12Z Mon. The overall environment and synoptic pattern remained similar during this period and forecast reasoning was largely unchanged. There looks to be a relative "lull" for a few hours as one AR wanes and the next approaches. Additional rainfall on top of already saturated soils will keep the threat for excessive rainfall and flash flooding elevated despite the relative lull. As the front moves ashore and the upper shortwave trough pivots across northern-central CA this morning, precipitation coverage and rates from the current Atmospheric River (AR) will be on the wane. Upper level heights will then be on the rise later today ahead of an amplifying, negatively tilted longer wave trough, one with a much more robust and elongated Warm Conveyor Belt with deep-layer subtropical fetch from the central Pacific. Guidance continues to show PWs ~1.25" approaching the Central CA Coast Sunday night, along with even more anomalous low-mid layer moisture transport compared to the previous AR events. Both the GEFS and SREF in fact show 850-700 mb moisture flux ~5 standard deviations above normal directed toward the CA Central Coast by 12Z Monday. While the bulk of this particular event will be during the day Monday (current Day 2), deterministic and probabilistic QPF fields, along with analogs and ERO first guess fields applying the ARI of 24hr QPF, would support continuing with the Slight Risk that was inherited from yesterday's Day 2 ERO. This especially taking into account the anomalously wet soils (above the 90th percentile in the top 1 meter layer per the latest NASA SPoRT), owing to the onslaught of recent AR events. Hurley Day 2 Valid 12Z Mon Jan 09 2023 - 12Z Tue Jan 10 2023 ....A MODERATE RISK OF EXCESSIVE RAINFALL CONTINUES ACROSS A LARGE PORTION OF CALIFORNIA, INCLUDING THE BAY AREA AND MUCH OF THE CENTRAL VALLEY... ....NUMEROUS FLASH FLOODING EVENTS LIKELY, SOME POSSIBLY SIGNIFICANT, ESPECIALLY OVER BURN SCARS... ....California... The more significant Atmospheric River or AR event is still expect to impact much of CA ahead of a more amplified upper trough, where a more elongated AR is expected to tap into the subtropical regions of the south Pacific. Guidance continues to show PWs climbing between 1.25-1.5" (highest along the coast), with southwesterly 850 mb flow peaking between 60-70 kts. Low-mid layer moisture transport will be quite robust, as the 850-700 mb moisture flux peaks AT 5 standard deviation range above normal per the GEFS. Meanwhile, some elevated instability (at least 200-400 J/kg per the ECMWF and GFS), along with the degree of deep-layer forcing and moisture, would support peak hourly rainfall rates of 0.75-1.00+ inch, especially within the Moderate Risk area. Overall, guidance consensus remained quite good with respect to the QPF, with 24 hour totals between 2-4+ inches over most areas, along with pockets of 5-8+ inches along the coast-coastal ranges and across the western slopes of the Sierra. This will be a fairly significant event as far as ARs go, with the latest deterministic QPF indicating a 5 to 10 year average recurrence interval or ARI within portions of the Moderate Risk area. In addition, the latest HREF 40km neighborhood probabilities of 3 hourly QPF exceeding current 3 hour FFGs climb to >50% within the Moderate Risk area after 15Z Monday. Considering how wet the soils will be ahead of this event (over the 90th percentile within the top 100 cm across the majority of the outlook areas per the latest NASA SPoRT), impacts from flash flooding or more rapid inundation will become increasingly likely -- especially across the several burn scars since 2020. A High Risk may become necessary in subsequent ERO issuances, especially if the guidance shows another uptick with the QPF, along with consensus in terms of where the peak amounts or bullseyes will be. Hurley Day 3 Valid 12Z Tue Jan 10 2023 - 12Z Wed Jan 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE CALIFORNIA CENTRAL COAST AND CENTRAL VALLEY INTO SOUTHERN CALIFORNIA... ....California... The negatively tilted upper trough and associated surface front will traverse CA on Tuesday, thereby signaling an end to the more significant AR event on Day 2. Nevertheless, lingering widespread, pre-frontal rainfall on Tue, especially across the Central Coast-Central Valley and Southern CA, will maintain an enhanced (Slight) excessive rainfall risk with additional scattered flash flooding possible. Rainfall rates aren't expected to be as robust compared to Day 3, however even as PWs drop below 1" (closer to 0.75"), anticipate scattered areas of 0.50"/hr rainfall rates. An additional 1-2+ inches of rainfall is expected, with localized totals of 3+ inches possible across the Transverse Ranges in Southern CA along with the western slopes of the Sierra. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5c0Hdpxgz_YzS0VZpNNOFOCZj8Fuy1qyO-F_nO1YFQ7F= nMklOB9qsEn64D80y0CJGga0xvOgDGq-6u-TlFQORmn7Giw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5c0Hdpxgz_YzS0VZpNNOFOCZj8Fuy1qyO-F_nO1YFQ7F= nMklOB9qsEn64D80y0CJGga0xvOgDGq-6u-TlFQOmJdSlOY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5c0Hdpxgz_YzS0VZpNNOFOCZj8Fuy1qyO-F_nO1YFQ7F= nMklOB9qsEn64D80y0CJGga0xvOgDGq-6u-TlFQO6R9MN6k$=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. 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