Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jan 27 2025 00:48:17 FOUS30 KWBC 270047 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 747 PM EST Sun Jan 26 2025 Day 1 Valid 01Z Mon Jan 27 2025 - 12Z Mon Jan 27 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND AROUND THE PALISADES, EATON, AND HURST BURN SCARS, GENERALLY JUST NORTH AND WEST OF THE CENTER OF LOS ANGELES, CALIFORNIA... ....Southern California... A deep layered low continues to drift south off the CA coast,=20 bringing showers and a few thunderstorms into coastal areas.=20 Hourly rainfall totals have generally been peaking around 0.3" with this shower activity, although we have seen amounts closer to 0.7" on a localized basis closer to Ventura. Recent HRRR runs and the=20 18z HREF both indicate an uptick in rainfall rates through the=20 evening hours as the low moves south and lapse rates steepen=20 allowing for a modest uptick in instability just offshore. HREF=20 probabilities of exceeding 0.5" in an hour increase into the=20 30-40% range tonight, and recent HRRR runs also show an uptick in=20 0.5" an hour totals. In fact the 23z run of the HRRR brings very=20 localized hourly rainfall over 0.75" just offshore. Given the=20 better instability should remain offshore, the higher=20 probabilities of 0.5" per hour or greater rainfall should indeed=20 stay just offshore, however it seems reasonable to expect some of=20 these higher rates to locally move onshore as well.=20 As of 00z, not seeing any observed rates to this level, however=20 both the HRRR and HREF would indicate an uptick in the potential as early as 02z and continuing to 12z. Any higher rates will likely=20 stay localized in nature, but at any point within that ~02z-12z=20 window we could see rates locally exceed 0.5" in an hour, and=20 potentially approach 0.75". If these higher totals happen to occur=20 within the Palisades, Eaton or Hurst burn scars then a significant=20 debris flow could result. Thus the Slight risk will remain for=20 these areas, with an increased debris flow risk this evening into=20 the overnight hours. ....Texas Gulf Coast into Lower Mississippi Valley... A Marginal risk remains across portions of southeast TX into=20 central LA and southwest MS. Recent HRRR runs and the 18z HREF=20 continue to support an additional 1-2" of rainfall over this=20 corridor, with an embedded narrow maximum axis of 2-3" (most=20 likely from far southeast TX into central LA). Rainfall rates will generally peak at or below 1.5" per hour, and so overall not=20 expecting this to pose much of a flash flood risk. However the=20 widespread moderate to locally heavy rain may continue to be=20 capable of producing mainly localized minor flooding of urban or=20 low lying areas. Chenard Day 2 Valid 12Z Mon Jan 27 2025 - 12Z Tue Jan 28 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE TRANSVERSE/PENINSULAR RANGES, LOS ANGELES BASIN, AND INLAND EMPIRE OF SOUTHERN CALIFORNIA... 20Z Update: Hi-res guidance has trended a little faster with the vacating convective potential across the more sensitive areas within the LOX/SGX domains, however the synoptic evolution with these upper lows can be tricky and take a little longer to progress out of areas leading to lingering shower/storm concerns despite other indications. Considering the circumstance with the burn scars and the very sensitive nature for flash flood prospects, decided to maintain continuity with the main time frame of interest likely the first 6 hour window, 12-18z Monday before conditions improve and the threat shifts eastward. The previous MRGL was not altered in any way from previous forecast issuance. Kleebauer ...Previous Discussion.. Shower activity in association with the deep closed low looks to continue into Monday morning over portions of Southern California, before the low pivots inland and brings an end to the potential for sub-hourly (15-min) rainfall rates of 0.25"+. This will continue the Marginal risk for excessive rainfall into Day 2 for both coastal and more inland low elevation areas (with particular concern for any lingering shower activity over aforementioned burn scars). The threat is expected to last for only the first half of Day 2 (prior to 00z Tuesday). Churchill Day 3 Valid 12Z Tue Jan 28 2025 - 12Z Wed Jan 29 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Churchill/Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!91N8nVdmr4LfybBZd19PEj0qkA6rxWH_zmb6IcSSiArZ= hhr9Lm6aQDJf-6JIQ7tcyZPpKNzQ1cWxn3Ye27qi5tDhaeU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!91N8nVdmr4LfybBZd19PEj0qkA6rxWH_zmb6IcSSiArZ= hhr9Lm6aQDJf-6JIQ7tcyZPpKNzQ1cWxn3Ye27qiWXdrH-Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!91N8nVdmr4LfybBZd19PEj0qkA6rxWH_zmb6IcSSiArZ= hhr9Lm6aQDJf-6JIQ7tcyZPpKNzQ1cWxn3Ye27qixF3dxwo$=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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