Major, Once in Multi-Year Storm, to Slam Northern California Thursday

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For most of this season, as I’ve repeated many times, most of the storms that have affected northern California have been relatively weak… except for perhaps the last storm, though even it wasn’t all-too greatly held together. This is about to change, big-time. North of Hawaii lies a band of powerful westerlies (strong upper-level winds), jetting eastward toward the west coast. These westerlies are transporting a long fetch of very moist, subtropical air, some of which originating from some sheared off moisture from ex-super typhoon Hagupit in the western Pacific, that swept through the Philippines. This band of strong westerlies will be the breeding ground for multiple powerful mid-latitude cyclones to form, the first of which will develop off the Pacific Northwest’s coast Tuesday into Wednesday, and rotate into the Gulf of Alaska. This initial, very powerful cyclone will be the driver for additional powerful lows to develop beneath it, along the jet, which will begin blasting northern California by later Wednesday.

Infrared satellite imagery from 12:30pm Monday, focused on the east-central Pacific (north of Hawaii), where a long fetch of moisture along a strong westerly jet can be noted, as well as a developing low along the "nose" of the jet.

Infrared satellite imagery from 12:30pm Monday, focused on the east-central Pacific (north of Hawaii), where a long fetch of moisture along a strong westerly jet can be noted, as well as a developing low along the “nose” of the jet.

Off the coast on Wednesday, a buckle in surface pressure develops along the jet, and rapidly gains strength from a buckle of lower pressure into a full-blown secondary low by Wednesday night. Models aren’t in the best agreement from this point on… with essentially all models having different solutions as to how deep this low gets, where it goes, and what (if anything) develops behind it. For now, a blend of models shall suffice, which would bring a 985 – 990mb surface low into OR/WA Thursday morning. Band of heavy precipitation ahead of the cold front looks like it’ll begin sweeping inland by late Wednesday night along the coast, and by early Thursday morning further inland across northern California. Precipitation will be, as mentioned, quite heavy with this band given precipitable water values around 1.5″, strong jet dynamics, and a strong and very favorable low-level south to southwesterly flow crashing into the hills and higher elevations. The strong southerly flow will be mostly the culprit of the surface low impacting the Pacific Northwest coast, sending a band of tight pressure gradients through northern California through the morning and day Thursday.

December-8th-M2

Currently, and again, with a blend of model solutions, this pressure gradient will feature a band of powerful low/mid-level winds that will sweep from north to south along the intense precipitation band through the day Thursday. 925 – 850mb winds in the 65 – 75kt range. Currently looks like a good bet some 45 – 55kt (51 to 63mph) gusts will make it to the surface across much of the lower elevations and hills, however, if the low is any deeper, or takes a further southerly track, gust potential would increase. These winds combined with very heavy precipitation will take down trees & limbs, especially since drought as weakened some, and soil in some places has been fairly saturated from the previous system. Timing of the strongest wind currently appears to be from about 4am to 4pm, with winds weakening fast behind the front.

12z NAM's forecast 850mb winds, valid early Thursday afternoon. Note: these wind speeds will likely not make it to the surface, but somewhat weaker winds likely will.

12z NAM’s forecast 850mb winds, valid early Thursday afternoon. Note: these wind speeds will likely not make it to the surface, but somewhat weaker winds likely will.

Given the powerful (and very moist) upper-level flow and generally favorable environment for cyclogenesis, cannot rule out additional strong wave development through the day Thursday or even early Friday. The GEM, for instance, wants to bring in another deep low, this one directly into northern California Thursday night into Friday. While this model’s solution is currently an outlier from other models, it’s worth noting and monitoring. Wouldn’t even have to be a totally formed mid-latitude cyclone for it to significantly affect the overall setup, but even just a developing shortwave or “open-wave” type deal.

Another issue with models also lies with the strength of this low, which is how fast/slow the cold frontal precipitation band slides through. The GFS is much more progressive than the ECMWF, quite rapidly moving the front southward and out of most of northern California by late afternoon Thursday in the north and early to mid-evening from the Bay Area southward. The ECMWF stalls the front once it gets near or just past the San Francisco area, with heavy precipitation blasting for an additional 3 – 6 hours along the I-80 corridor from the Bay Area to Sacramento and points north and east, targeting the northern San Joaquin valley northward to the central Sacramento valley, the central/northern motherload, and coast around San Francisco. Can’t say at this point if this solution will continue to stick around in future runs, but this is very much so worth noting, as a stall for only a few hours with such heavy precipitation rates could cause major problems in regards to flooding.

Major-Storm-Impacts

Current estimate of QPF would generate 2 – 4″ of rain for much of the Sacramento and northern San Joaquin valley, with generally about the same for the lower elevations of the coast. 3 – 6″ appears to be a good bet for much of the foothills and mountains from the coast to the sierra, with locally higher amounts upwards of 8″ in orographically favored hotspots, such as some slopes of Butte, Plumas, Yuba/Sierra, and Shasta counties. Some of these amounts could be low or high depending if the front slows down/stalls, or just sweeps right through. Heavy rainfall atop already wet soil will likely lead to direct runoff fairly quickly, leading to rises in creeks, while most rivers should bode well. Some creeks and streams may not handle the extreme surge in runoff well, and some very well could near or rise over flood stage, mainly Thursday afternoon into the overnight hours, when runoff is at its max from the system.

December-8th-M1

Snow levels will start out fairly high… probably at or over 7000ft, but should lower to 4000 – 5000ft Thursday night and Friday as the cold front slides through and the cold pool moves overhead. As snow levels lower, precipitation will likely continue fairly steady Thursday night, dumping snow over the sierra. Showers are likely to continue through Friday as well, as the cold pool slides overhead and increases instability. Generally 1 – 2ft of snow looks to be a good bet at the highest elevations, with several inches perhaps down to 5000ft. The cold pool with the trough as it slides overhead Friday could be enough to get thunderstorm chances going for the lower elevations… but will hold off a couple more days to get finer detail with that threat.

Next update will be posted on Wednesday, unless something major changes during the day Tuesday. The next update will have even further narrowed-down impacts and affects, with hopefully a bit more model agreement to work with.

Comments
  • wxtracker15

    Next update will be posted by or before 3 – 4pm Wednesday.

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