Another Powerful Storm to Slam Northern California Tuesday/Wednesday; Major Impacts Expected

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Northern and central California have been inundated by heavy precipitation and strong winds for the past 48 hours or more in some locations, with rain falling up to 9000ft elevations causing fairly rapid snowmelt. Snowmelt and already intense runoff from heavy rain caused mountain rivers to swell and flood in many locations. Farther west, creeks and streams across the valley saw widespread bank breaches with isolated instances of damage caused by flooding, while along the coast even more river & stream flooding occurred. Perhaps most notably the Carmel River in Monterey county, where it flowed over a levee and breached it, flooding upwards of ten homes with “several feet of water” overnight Sunday, with swiftwater rescues needed to evacuate some. Snow levels have dropped to a more typical January level, around 4000 – 5000ft in general across much of the higher terrain in northern California. This has begun to relieve mountain rivers and streams as runoff slows. Showers continue across the region as a moist and cool westerly flow dominates.

Offshore, a cold upper-low exists off the Oregon & Washington coast, while off the California coast, a long fetch of moisture continues to be directed westward under the upper-low off the Pacific Northwest coast. A wave cyclone developing along the atmospheric river given strong moisture and thermal convergence will strengthen quickly overnight Monday into Tuesday as a very strong ~115-knot mid-level jet streak works its way upstream. Precipitation will begin to fill in Monday evening, well ahead of the low as the moisture plume begins to become enhanced by strengthening mid-level divergent flow as the low strengthens. Precipitation will be nearly constant across much of the region north of Fresno overnight Monday through Tuesday, as moisture continues to be directed into the region by the incoming cyclone, and moderate to heavy at times — especially as we get into Tuesday.

NAM’s forecast 500mb wind (fill) & heights (orange contours), wind vector arrows, and MSLP (blue contours) valid at 4pm Tuesday. The strong mid-level jet streak isn’t hard to find, with a modeled max at nearly 120 knots in the core of the streak. This is one of the strongest mid-level jets I’ve seen this close to the California coast in quite a while, signifying some significant intensification of the trough and low it’s associated with impacting the southern Oregon and northern California coast.

Late Tuesday morning, the low will have made it to the northern California/Oregon coastline, with its shield of heavier precipitation associated with (and ahead of) the cold front beginning to move inland — increasing rainfall rates to a heavy rate across much of northern California. Moisture plume associated with the low will bring inland precipitable water values up around 1.2 – 1.3″, which is down from the weekend system’s 1.8 – 2″ values, thanks to the direct subtropical feed near Hawaii. This moisture plume will be modified by colder air and a stronger mid-level jet, making up for the lower moisture with instability, strong jet dynamics/divergence, and a favorable upslope flow for mountain areas. Rain will stay moderate to heavy through Tuesday evening until the cold front passes through.

The wave cyclone is forecast to deepen to somewhere around 994 – 990mb by Tuesday afternoon as it moves ashore into Oregon, tightening pressure gradients between southern Oregon and central California significantly — putting northern California right in-between. 850mb winds increase to upwards of 60 knots from late Tuesday morning through the early to mid-evening hours along the coast, parts of the delta, and into the Sacramento valley, with a period of 65 – 70 knot winds forecast from around 2pm to 9pm, and is expected to be the peak period of winds, which will coincide with the heaviest precipitation with this storm. Currently believe that surface sustained winds will be between 20 – 30mph — with potential for gusts upwards of 60mph on a isolated basis with widespread gusts between 40 and 50mph across the lower elevations and foothills. Sierra peaks could see sustained winds upwards of 35mph with gusts to 75mph. These winds combined with heavy precipitation on top of already very saturated soils could make for some widespread impacts along the lines of downed trees and even power poles themselves, as was seen along I-80 near Colfax Sunday, where a long stretch of poles were blown down purely via wind & saturated soil, with no help by downed trees.

Snow levels with this system will be significantly lower compared to the weekend storm, given colder upper-low sitting off the OR/WA coast and the overall colder nature of this storm. Snow levels will flutter around between 4000 – 5000ft through Wednesday, with very heavy accumulations expected through the event. Strong winds and heavy snow Tuesday into early Wednesday will cause near-blizzard conditions over the sierra, with mountain roads, highways, and other passes potentially becoming treacherous or impassable at times.

Another potential impact from the colder nature of this storm, in combination with the available moisture plume is convection: potentially some strong convection after around 4pm through approximately 11pm immediately along and behind the surface cold front from the coast, inland into the valley as a secondary surface low impacts the north coast near Eureka. This surface low will move in with some potentially robust convective bands in an environment featuring CAPE values between 100 and 500 j/kg, which in itself isn’t all that significant, but given this will be a strongly sheared environment with a powerful mid-level jet roaring overhead, it may not take much to get some robust thunderstorms going even after sunset. In the valley and delta, any strong thunderstorms could transport locally stronger to potentially damaging wind gusts to the surface, while along the coast I wouldn’t rule out waterspouts impacting along the coast close to the surface low north of Mendocino county (making them tornadoes). Hail is also a good bet with storms that are able to climb vertically enough to reach the freezing level without being sheared apart from the vigorous jet and shear.

Precipitation will become showery after the cold front slides through with that increase in thunderstorm potential Tuesday evening and remain showery into Wednesday. Some thunderstorms are possible again Wednesday afternoon as the sun is able to break out and warm up the lower elevations a bit, steepening low-level lapse rates boosting low-level instability. Showers could increase again Wednesday evening into Thursday morning as another final wave slides down the California coast along the back side of the trough. No heavy precipitation is expected, but another quarter to half inch of rain could fall across much of the region, along with a good shot of snow as low as 2500 – 3000ft along the west slope of the sierra, meaning some upper foothill communities could see some accumulation.

Through Thursday, precipitation totals will locally double what we saw with the long-anticipated weekend storm. The Tuesday/Wednesday system will dump anywhere from 2 – 3″ of rain in the southern Sacramento and northern San Joaquin valley into the bay area, with 3 – 4″ along much of the rest of the coast as well as some localized regions in or near the bay area. Mountainous terrain along the coast with favorable slopes could see upwards of 5 – 8″ from the Oregon border to Monterey county, with similar totals expected in the mountainous terrain of the sierra and the western slope of the sierra — locally upwards of 12″ of liquid. This liquid will fall as snow above around 5000ft through Wednesday, dumping massive totals — upwards of 7 feet above around 6000 – 7000ft, with up to 5 feet down to some 5000 foot elevations along the west slope. The northern mountains should see anywhere from 1 – 3 feet above similar elevations, locally higher amounts in favored terrain and where convection locally lingers or reoccurs. A quick 1 – 3″ of snow could also fall as low as Colfax, Grass Valley, Magalia, upper Camino, and Georgetown early Thursday morning.

Heavy rainfall in the lower elevations will cause flooding once again, especially now that stream/creek and river levels are already high after the weekend storm, and flooding could remain a concern through as late as Thursday if convection reoccurs in certain areas dumping locally heavier rainfall totals than forecast, although forecast rainfall totals are already enough to cause (more) problems. We’ll dry out this weekend, which will be quite needed for once. However, longer range models suggest a return to a potentially stormy pattern next week. Some solutions indicate some potentially major storms, while others and some ensemble solutions are a bit dryer. To ice the whole cake on the extended: there’s a higher chance of a stormy second half of January than a dry one, but either way they’re just that: chances. Keep it tuned here for more. If anything wild happens this week through Thursday, I’ll post an update to this post. In the meantime, check out thw Facebook page (@norcalweather.net) for short term updates.

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