Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 12 2024 20:02:26 FOUS30 KWBC 122002 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 302 PM EST Thu Dec 12 2024 Day 1 Valid 16Z Thu Dec 12 2024 - 12Z Fri Dec 13 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hamrick Day 2 Valid 12Z Fri Dec 13 2024 - 12Z Sat Dec 14 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST CALIFORNIA... 20z Update: Upgraded to a Slight risk across portions of northwest CA with this update. Event total rainfall from Friday through=20 Saturday morning is forecast to be 3-5" across the Slight risk=20 area, with isolated amounts of 5-7" within the most favored=20 southerly facing slopes. Rainfall rates should increase Friday as the core of IVT moves into the coast. By Friday night into=20 Saturday morning the heavier rain will shift southward as a strong mid level shortwave approaches the region. This feature will act=20 to increase large scale ascent, while also cooling the mid levels=20 enough to allow for some weak instability to develop within the IVT axis. The 12z HREF neighborhood probabilities of rainfall=20 exceeding 0.5" in an hour are over 50% across most of the Slight=20 risk...both across the most favored upslope areas and also within a narrow southward shifting axis of stronger convergence. The=20 forecast rainfall combined with these locally higher rates indicate some flooding is probable. Still not expecting this to be a high=20 end event given the overall progressive nature of the system and IVT peaking just around 750 kg/ms, but some creek and stream=20 flooding, along with a few landslides, are possible. Chenard ....Previous Discussion... The overall forecast maintained a general continuity in the expected synoptic evolution of a weak atmospheric river transpiring across the Northwest CA coast, extending northward into Southwest OR. IVT signatures between 700-1000 kg/ms will advect into the coastal plain with some protrusion inland on the initial surge as the southwest to northeast alignment of the pulse relegates a majority of the moisture into the coast and adjacent coastal ranges that will garner the greatest QPF just given the upslope component of the wind field. A widespread 2-4" with local max of 5-5.5" is forecast over the period with the general maxima likely within the coastal King Range as is customary with IVT pulses exhibiting a southwestern angle of approach across Northern CA. Rainfall rates on the order of 0.25-0.5"/hr will be most common within these bands of precipitation, however a few instances of >0.5"/hr might be realized during the period between 00-12z Saturday when the primary surge is forecast amongst the 00z deterministic suite. This correlates well with the anticipated IVT maximum making headway into the coastal plain which could lead to localized flooding concerns in the expected zone(s) of impact. As of this juncture, the heaviest rainfall will lie north of Santa Rosa with the primary areas of interest likely positioned from the King Range up US101 to the southern half of the Siskiyou in southwestern OR. With this alignment, very little change was necessary from the previous MRGL risk issuance, thus the forecast was relatively unchanged as the MRGL risk was sufficient in consensus. Kleebauer Day 3 Valid 12Z Sat Dec 14 2024 - 12Z Sun Dec 15 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST CALIFORNIA... 20z Update: The atmospheric river described in the day 2=20 discussion will continue to impact portions of northern CA=20 Saturday morning. Thus the Slight risk was continued into this=20 period as well. The strong mid level shortwave mentioned in the=20 day 2 discussion will move onshore Saturday morning, with this=20 resulting in some weak instability and also likely a temporary=20 slowing of the IVT axis. These factors should result in some=20 higher rainfall rates from 12z-18z Sat over the Slight risk area,=20 with flooding of creeks, streams and low lying flood-prone areas=20 possible, along with the continued risk of smaller landslides. The=20 higher rates should quickly diminish as the shortwave moves east,=20 with the risk of heavy rain over by 18z. Thus this Slight risk is=20 really only for the first few hours of this day 3 period. Chenard ....Previous Discussion...=20 Atmospheric River setup from the previous period will translate into D3 with a greater inland protrusion of the IVT pulse as the upper level trough and associated surface low pivot onshore with an expected wind shift that will mute heavier rain prospects along the coast and eventually inland as we move into the second half of the period. The threat is currently very marginal in nature within the confines of the inland topography with the eastern foothills of the Sierra's being the furthest east expanse of any heavy rain threat, but even that is bordering on the lower end of a risk threshold. This period is more of the transition point in the pattern as the event will see a general decay in total rainfall and overall rates as the system moves inland. There is a further south push of the IVT pulse in the early portions of Saturday with some modest dynamical element involved as the 500mb vorticity evolution does bring some significant forcing towards the North- Central CA coastal plain with impacts to places like Santa Rosa down into the Bay area. Thankfully the pattern is progressive, but a quick 1-2" is within the realm of possibility for a mainly urban zone which could bring about some localized flooding prospects before the disturbance moves inland and we get a quick end to the chances. A MRGL risk continues for a good portion of Northwest CA with the risk area now extending down into the Bay area compared to the previous D2 alignment. A MRGL risk is also located at the base of the Sierra foothills for locations below 5500ft. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8CWvFuPq6jOpnDCz1UNGPlQFUOId-6C_FfGF4kGrTZR4= PbzHFKV5n-sqwp0NptC6pVaKcbKik6AQeSuqiTsQyC7XIJs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8CWvFuPq6jOpnDCz1UNGPlQFUOId-6C_FfGF4kGrTZR4= PbzHFKV5n-sqwp0NptC6pVaKcbKik6AQeSuqiTsQiFKcrxw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8CWvFuPq6jOpnDCz1UNGPlQFUOId-6C_FfGF4kGrTZR4= PbzHFKV5n-sqwp0NptC6pVaKcbKik6AQeSuqiTsQWiZ6oKk$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .