Narrow, High-Intensity Atmospheric River to Slam Flood-Prone North-Central California Monday

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Northern California is in it’s most hydrologically sensitive state it’s been sent into in well over a decade, as we’ve remained in an extraordinarily wet pattern since December. After just the first month of these wet, clingy storms in December problems started to arise, and now that we’re three months into this very wet period that has dumped far more than our normal water year’s worth of precipitation already, we have a yet another significant system preparing to unload on northern California. Much of northern California is 150 – 200% of “normal” as of the 17th of February with a good chunk of the western slope of the sierra and sierra itself approaching 300% of normal — with the Sacramento valley drainage being the wettest it’s been since 2006, over ten years ago. In fact, we’re going to top the 2005 – 2006 water year by the time we’re done with this coming week, and likely end up being the wettest year for the Sacramento valley as a whole so far in the 2000s, putting us on a run to begin nearing the 1995 – 1996 water year. When speaking on behalf of the entire state, the state has had the wettest season since the late 80s, when factoring in all the mountainous terrain and coast’s total precipitation. Additionally, the amount of high-intensity precipitation producing systems in the last two months has been astonishing, mostly due to the seemingly always available subtropical moisture plume for a low enough latitude reaching system to hook and reel in toward the western U.S., as these mid-latitude cyclones blow up in the eastern Pacific in the frequently prime eastern Pacific environment this winter season.

27 years worth of accumulated precipitation averaged out for the Sacramento (Valley) Drainage, from January 1980 through January 2017. Red lines are actual recordings, with the blue lines being the median of the wacky Californian precipitation years.

This next system will be a combination of lows including a deep cyclone north of Hawaii and a couple of older lows within a large upper-level low off the Pacific Northwest coast. Both of these sets will merge into one larger, more powerful mid-latitude cyclone off the western U.S. coast as Sunday rolls past — with the moisture source already having been secured by the low north of Hawaii in the central part of the Pacific earlier in the week, vacuuming deep moisture out of the subtropics west of Hawaii around 18N. As the two sets of lows begin to merge Sunday, the leading edge of the moisture plume begins to nose its way into northern and central California along the what you may call the “new” low’s warm front and help precipitation fill in throughout the region through the day. As the low begins to near the OR/WA coast late Sunday night into Monday, it slows up due to the strong jet streak becoming SW to NE oriented, instead of a more typical strong westerly flow you’d expect with deep mid-latitude cyclones and atmospheric river events. In this case, the SW-NE orientation helps slow the progression of the low and stall out the frontal precipitation band and associated deep subtropically enhanced moisture plume.

This continues through much of the day Monday, although as the jet becomes more and more anti-progression oriented, a new wave begins to form off the Californian coast and drag the precipitation band northward a bit during the day. Models are showing some disagreement in regards to where the band wobbles back and forth through the day as this new wave develops along the front, but a blend of solutions would suggest it gets held up the most from the Santa Cruz mountains northward to Sonoma county, then spreading inland and northeastward into the Feather River Basin and to the south into the I-80/Highway-50 corridor it’d appear. There are some solutions a bit further south and even a bit further north, but models appear to converge in this area as the median. Precipitation rates from late Sunday night through much, if not all of Monday into early Tuesday look to be quite heavy, primarily along that relatively narrow band — although precipitation will still be quite significant a decent distance around the intense/focused band of lift & moisture.

The second surface wave that develops along the jet, moisture plume, and front, will help tighten surface pressure gradients more than they already will be late Sunday/early Monday — helping drive a 60 – 80 knot low-level jet into northern California out of an excellent direction for orographic/terrain enhancement of precipitation along and near most mountain slopes: the south to southwest. In addition to being a significant player to precipitation intensity/enhancement, this low level jet will of course bring strong winds to the surface through the day Monday, peaking Monday night just ahead of and with the cold front. High resolution models suggest enough downward momentum and forcing to drive surface winds to potentially damaging status given already and rapidly increasingly soaked soils, with modeled gusts upwards of 45 – 55mph looking widespread & likely from Stockton to Redding in the valley, as well as in the mountains and coast from Montery to the Sonoma county coastline/elevated terrain.

Precipitation should finally let up to showers upon sunrise Tuesday, but that won’t be after 24 hours of potentially non-stop precipitation across many places in northern & central California. High resolution models that tend to handle the orographic/elevation-enduced friction & lift the mountains provide with these WSW-ENE oriented precipitation bands suggest amounts anywhere from 1.5 – 3.5″ of rain across essentially all of the Sacramento and northern San Joaquin valleys & Bay Area (lower elevations), with 3 – 6″ in the elevated terrain and coastal mountains. Inland, the northern and central sierra foothills & western slope could see anywhere from 4 – 8″ on a widespread scale, with similar amounts expected for the Shasta drainage and other favorable south/southwest facing slopes & canyons. I do anticipate the potential to see higher amounts that could be upwards of 10 – 12″ locally in the foothills & mountains where the band hangs out the longest during the late Sunday – late Monday time frame before the precipitation finally becomes progressive and moves southeast, with potential for valley amounts a bit higher as well (perhaps locally upwards of 4 – 5″).

Snow levels will of course be a big factor to this event in regards to runoff, and in this event they’ll fluctuate quite a bit. They’ll begin and hover around 7000 – 8500 feet in the sierra through much of event, as 850mb temperatures with the cloud/precipitation shield will stay at or above 5c, with the freezing level well above 9000ft. This means quite a bit of the high precipitation amounts that fall in the higher terrain will fall as just that: liquid (rain), and runoff quick converging into mountain streams and downstream into larger creeks/rivers. With rivers and many creeks already running high, this high ~24 hour precipitation event will cause considerable to dangerous rises in waters on a widespread scale from the mountains to the coast, with some waterways being effected nearly instantly upon the beginning of heavier precipitation, to a day or more of a delay as water runs downhill and downstream with more and more convergence. This rainfall event in itself has a 5 – 10 year return interval if forecasted correctly, or even just up to 80% of model guidance totals — although we’ve seen multiple storms with multi-year return intervals this season alone.

Snow levels will indeed begin to fall as the back edge of the cold front begins to works its way through late Monday, and especially on Tuesday as post-frontal showers and pieces of energy keep precipitation going across the west slope and the sierra. Heavy snow will fall above the snow level throughout the event, however once colder air punches south — very heavy snow will begin to fall, to as low as 5000 – 5500ft by Tuesday morning, and perhaps 4000 – 4500ft in the southern Cascades & interior northern coastal mountains. 1 – 2 feet is expected to fall just above the snow level, with anywhere from 3 – 5 feet above 7500 – 8000 feet through Tuesday.

With so much flooding ongoing already, this storm could cause flooding not seen in quite a while. Be sure to have an action plan if you live near a levee, creek, river, or other breachable waterway in the days ahead. Even if no flooding occurs on Monday itself, an unsettled pattern will continue through much of next week, and after the early week storm it will likely take very little to flood places already at the brim of capacity assuming they aren’t already over. More on the additional storminess soon.

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