Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jan 17 2024 20:32:13 FOUS30 KWBC 172032 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 331 PM EST Wed Jan 17 2024 Day 1 Valid 16Z Wed Jan 17 2024 - 12Z Thu Jan 18 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF FAR NORTHWEST CALIFORNIA INTO WESTERN OREGON... ....16z update... The inherited day 1 Marginal risk area was extended northward into north/central Oregon. Much of the QPF guidance has trended wetter across that area for the period ending at 12z Thursday. Over an inch of rainfall has already fallen over the well saturated coasts of northern California up into central Oregon so any additional rain may lead to minor runoff/flash flooding. The greatest excessive rainfall threat remains over the southern Oregon into northern California coastline. The timing, synoptic pattern, and moisture/IVT setup remain the same as, if not similar, to what was previously forecast. Kebede ....Previous Discussion... No changes to the inherited Marginal risk across portions of far northwest CA and southwest OR. A strong mid level low is moving into western OR early this morning which is resulting in an uptick in rainfall intensity. After 12z onshore upslope flow will continue to be directed into this region to the south of a surface low expected to gradually move into western WA. This is not the strongest core of IVT, but it is persistent enough to result in up to a 12 hr period where 0.25"+/hr rainfall will be possible within the favorable western facing terrain. The consensus of current guidance still suggests now through 21z today as the most favored time for these heavier rainfall rates. Localized 0.5"/hr rainfall is still a possibility, with high res ensembles showing probabilities up to ~35%, but unlikely to see rates much higher than that in this setup. Given the prolonged nature of steady rain, event totals today may locally get as high as 4-7", with the 00z HREF neighborhood probabilities of exceeding 5" between 60-70%. Rainfall of this magnitude may begin to cause some mainly minor runoff concerns. Chenard Day 2 Valid 12Z Thu Jan 18 2024 - 12Z Fri Jan 19 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST OREGON INTO WESTERN WASHINGTON... The once removed Marginal Risk area was re-introduced with this afternoon's issuance in collaboration with local offices. The driving force for this change is the increased trends in our in-house 24hr QPF (2-3" with isolated 4"+), which was blended heavily toward the latest CAM's--also trended wetter (2-3"+). Mitigating factors include southerly flow running parallel to the terrain meaning less upslope enhancement of rainfall, a possibly more steady rain with manageable rates (0.1-0.25"/hr) and peak IVT values below 500Kg/ms. The strongest low level jet speeds will remain offshore with onshore speeds around 40kts. Heavy rain associated with the preliminary moisture plume is expected to ride up the Oregon coast tonight and arrive over the Marginal risk area around 12z Thursday. This heavy rain is then likely to be enhanced by a second fetch supported by the approaching East Pacific system. If trends continue then the heaviest axis of moisture/IVT/rainfall would make in onshore and cause more flooding impacts, so this will need to be monitored into the evening. Kebede Day 3 Valid 12Z Fri Jan 19 2024 - 12Z Sat Jan 20 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF CENTRAL AND NORTHERN CALIFORNIA... ....2030z update... Minor changes were made to this Marginal Risk are for day 3 (12z Friday -- 12z Saturday). QPF just north of the Bay area, where the areal 24hr maximum has been forecast, has trended slightly drier over the last several runs. That being said, the dynamic and moisture transport/setup across central to northern California remains in place from the overnight discussion. Kebede ....Previous Discussion... We will maintain a Marginal risk across portions of central and northern CA. A pretty strong mid level shortwave/low will help push a cold front towards the coast on Friday. The IVT plume offshore does begin to steadily weaken as it moves onshore, and the QPF signal in the guidance is borderline whether a Marginal is needed. However there will be pretty strong forcing associated with the mid level shortwave, and with falling heights we will have some weak instability approaching the coast. Thus this event will have the potential to produce pockets of heavier rainfall rates, with 0.5"/hr a possibility. There is still some model spread with the 00z GFS and NAM on the lower side of the QPF distribution, and the 00z ECMWF/UKMET/GEM wetter. If the latter solutions pan out, then a localized flood risk could evolve given the potential for briefly heavier rates and total rainfall of 1-2". With that said, the weakening nature of the IVT plume and the progressive nature of the system should keep the upper bound of any risk at the Marginal level with just localized flooding possible. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-eC-yeHNCFrjD0pHkONOG4iqPemXArptKUxnEATnGfxF= 0Tc_e0jCXcjXwj_jIt69d2s1-vQma1_WvRVvMS4FVyIRYSE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-eC-yeHNCFrjD0pHkONOG4iqPemXArptKUxnEATnGfxF= 0Tc_e0jCXcjXwj_jIt69d2s1-vQma1_WvRVvMS4Fd5HNEHI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-eC-yeHNCFrjD0pHkONOG4iqPemXArptKUxnEATnGfxF= 0Tc_e0jCXcjXwj_jIt69d2s1-vQma1_WvRVvMS4FyhjBjno$=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) .