Series of Storms, Including Another Atmospheric River, to Swamp Vulnerable Northern California

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This has been the wettest, stormiest winter and specifically past couple of months in decades — and we aren’t done. Compared to previous events in the late 90s and late 80s, the heavy precipitation events we’ve endured have thankfully been somewhat spread out overtime, and this is what has allowed our waterways (specifically rivers) to recede a bit between parades of storms. We’ve had several days of dry weather this week to allow this recession of rivers & streams once again just in time for this next parade to come marching through the state. Before we get into the specifics of each storm as typical, lets briefly go over the Oroville Dam situation. Due to the damage on the auxiliary spillway, it was used sparsely earlier this month between stormy periods — allowing the lake to fill up. The reservoir filled up to its highest level ever in it’s history: 902 feet, spilling over the emergency spillway which tops out at 900 feet itself. The emergency spillway wasn’t built to handle the erosion water pouring over it caused. This erosion essentially excavated the 30ft tall concrete emergency spillway, which sat atop dirt instead of a deeper concrete foundation. It never got pushed aside thankfully, as if it did the city of Oroville would’ve been inundated by a 30 foot tall wall of water rushing down along the Feather River. Over the last several days, water officials have been releasing the maximum amount of water the auxiliary spillway can handle, with the lake now down 30 feet since Sunday.

NAM’s forecast 6 hour precipitation (fill/pretty colors), MSLP (blue contours), and 500mb heights (orange contours), valid at 4pm Friday. Notice the focus of heavy precipitation is in southern California, although a band of heavier precipitation does setup on the northeast side of the low in our region.

Getting into the details of the incoming storms: storm number one, which opened the door for the next two primary storms, swept through northern & central California early Thursday bringing a quick but intense band of frontal precipitation. Storm number two is quickly approaching the coast, and will strengthen tonight offshore. This second storm will be a major precipitation producer for central & especially southern California, as a textbook atmospheric river event takes shape from Santa Barbara county southward. This event will be driven by a powerful mid-latitude cyclone off the central Californian coast, which will deepen to potentially near sub-980mb depth Friday. As the low approaches the coast, the northern end of the precipitation shield will begin to fill into northern California Friday morning — and stay locked into our region through the evening and locally early Saturday. While precipitation with this end of the precipitation shield won’t be as heavy as the southeastern section of the storm directing a very deep moisture plume into southern California for a relatively long period of time, strong lift and precipitable water values up around an inch will support widespread moderate precipitation, although some heavier embedded bands can’t be ruled out.

Friday’s storm will feature 5500 – 6500ft snow levels in our half of the state, though accumulations look relatively insignificant from this system staying under 6 – 10″ for the most part. Another impact from this storm we’ll likely miss out on due to the southern track of the low will be strong to potentially damaging winds, which is something we of course don’t need due to our over saturated soils. Dew east and southeast of the low along the southern Californian coast, gusts upwards of 60mph will be possible Friday, while winds in northern California remain under 40 – 45mph for the most part (generally from Sacramento south it seems). The trough as a whole including the surface low move inland overnight Friday into Saturday, keeping things showery across the state during the day, and dry out for the most part Saturday night as the trough departs into the Great Basin and Southwest.

Brief ridging will quickly break down between systems Sunday morning, as a weak front associated with a low off the Pacific Northwest coast scrapes northern California with some light precipitation north of the bay area for the most part. However, this system once again opens the door for a much stronger system to follow in its tracks, and by late Sunday it’ll quickly scoot up and be positioned off the northern California coast as a quickly strengthening wave cyclone forming along a band of deep moisture. This low will be associated with a large upper-low in the Gulf of Alaska, and begin to drive its 140 – 150 knot upper-level jet into California by late Sunday. This potent jet will have, as mentioned, hooked a fetch of subtropical moisture extending well south into the subtropical Pacific just east of Hawaii, and begin to focus it on northern California Sunday night.

GFS’s forecast precipitable water values, suggesting a narrow band of very high atmospheric moisture content aimed right at northern California. This image is valid at 4am Monday morning, with a band of 1.5″ values just offshore. These high values, in addition the strong lifting mechanisms provided via the low off the Pacific Northwest coast and friction from land setup a prime high-intensity precipitation event wherever this band of moisture lines up.

As this moisture plume begins to stream into northern California, the wave cyclone will have deepened/strengthened to somewhere in the 980 – 990mb ballpark, which is still a wide range of potential due to models having some strength and position differences still. Overall, there’s good agreement that the lows cold front and the potent moisture plume will line up and stall out somewhere from around I-80 north for around 6 – 12 hours by Monday, focusing a somewhat narrow but very intense band of precipitation on the region. Models show some variability run-to-run with this system, but the overall trend is for likely very heavy 24 hour precipitation totals to occur from Sacramento north in the valley, throughout the sierra & the foothills, and coastal hill from Santa Cruz north, with the heaviest precipitation occurring from late Sunday night, through the day Monday, and finally winding down late Monday night/early Tuesday. Liquid precipitation totals are currently modeled to be very significant, with amounts in excess of 5 inches across essentially all of the higher terrain from around Highway-50 northward, above 1000 – 2000ft in elevation, along with 3 – 5″ in the coastal mountains from San Francisco northward. In the valley, anywhere from 2 – 4.5″ also looks likely, although the wetter of the models (the ECMWF) suggests 5 – 6″ in parts of the Sacramento valley between Sacramento and Chico with higher amounts in the adjacent foothills and mountains, including lake Oroville’s watershed. Totals in the aforementioned region (mapped out below) would be troublesome on their own in such a quick time-frame, however given they’ll be falling on top of the already soaked state of the, well, state will amplify impacts and get water issues going quickly.

This event, if it comes into fruition as models currently simulate, would renew flood risk along the north end of the Sacramento river in the Sacramento valley, along the Cosumnes river from the foothills through Sacramento county and into the delta, and potentially other stretches of rivers and now widespread hydrologically sensitive streams and creeks throughout the region. Additionally, general urban flooding of poorly-drained or slowly drained areas could become a problem with this system or even the Friday system. Snow levels with this storm will rise to around 8000ft from the I-80/Highway-50 corridor southward (potentially higher in some spots along the sierra), and around 6500 – 7500ft in the northern sierra. These higher snow levels compared to Friday’s system will lead to another round of snowmelt, although in this case there probably won’t be a whole lot to melt (but there will be some), nevertheless it’ll add to watershed runoff. In addition to flood/heavy rain potential, this system will also move ashore into northern California with strong winds given the proximity of the surface low and its trajectory.

Models suggest we remain in a cool, unsettled pattern through much of the rest of the workweek, with a dryer pattern beginning to take over by next weekend. No major storms are expected after the late Sunday/Monday event, though, and the weaker systems after would dump snow down into the western slope of the sierra to restore the snowpack. I’ll have another update out on the late Sunday/Monday storm this weekend as models close in on a more defined solution in regards to precipitation amounts, where the front will stall out the longest, and a better idea of how strong winds could be Monday with the fronts passage. In the meantime, if you live in a flood prone area — take precautions once again and be prepared for anything.

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