Yet Another Atmospheric River Taking Aim at Northern California; Significant Precipitation Totals Expected

Home / Active / Yet Another Atmospheric River Taking Aim at Northern California; Significant Precipitation Totals Expected

A very stormy first full week of February is ahead for northern California after a historically wet January across the state, which will once again spike flood concerns as a variety of impacts from the chain of systems combine to create a messy situation. For the first half of the week (Monday – Wednesday), an upper-low will be nearly stationary well off the west coast in the eastern Pacific, driving a long fetch of subtropical moisture into northern California. This fetch of moisture will be lifted by nearly continuous warm air advection, until the low finally gets ejected northeastward into the Pacific Northwest and British Columbia Thursday into Friday, continuing precipitation across the state.

The first half of this event will be the primary atmospheric river event, in which a moisture plume remains directed at or stalled out across an area for at least 24 hours. During the day Monday, we’ll be technically between systems (Sunday night’s storm and the large stationary low off the western U.S. coast), although the moisture plume associated with the large low offshore will begin surge northward into northern California through the day. Cold air will be aloft behind Sunday night’s cold front, however warm air advection ahead of the next offshore low will begin to pick up later in the day. This combination of remnant cold air aloft providing instability, along with increasing moisture and lift through the day will help a band of showers and thunderstorms line up somewhere between San Francisco northeast up I-80 through Sacramento and up the Highway-50 & I-80 corridor into the mountains, extending as far south as Stockton and as far north as Yuba City at times. Snow levels will hang out around 6000 – 6500 feet in the sierra, and 4000 – 5000ft in the coastal & northern mountains, with potentially impactful snowfall rates before they rise Monday night.

GFS’s forecast precipitable water, a measure of total-atmosphere moisture content, valid early Tuesday morning. Easily notable is the long fetch of rich moisture being transported from the subtropics around the Hawaiian islands right into northern & central California.

Through the day Monday, precipitation will slowly convert from convective to stratiform, and likely be essentially all stratiform precipitation by Monday night. During the overnight hours Monday, 990 – 995mb mid-latitude cyclone will split off of the elongated low off the western U.S. coast, and impact the southern Oregon coast. This will swing its own cold front through northern California bringing widespread precipitation throughout the northern 2/3rds of the state, over the more focused band of rain/thunderstorms during the day Monday in north-central California. Rainfall rates will increase on a widespread scale late Monday night into early Tuesday as the cold front approaches as the low itself actually strengthens a bit off the northern California coast before moving inland into the Pacific northwest Tuesday morning. Given this low develops in a very moisture-rich, warm environment, as you can imagine snow levels will begin to skyrocket — winding up near 8000ft by late Monday night, and likely higher than that by Tuesday morning. Another thing you can imagine with a surface cyclone in such proximity is wind — and I expect widespread gusts to 45 – 50mph Monday night/early Tuesday across the lowlands, with mountain/sierra gusts in excess of 60mph possible.

NAM’s depiction of the surface cyclone off the northern California coast Monday night increasing precipitation rates. The 12km NAM suggests a ~988mb low, while global models suggest the depth of the low a bit weaker — around the aforementioned 990 – 995 millibar range. A deeper low, such as this solution, would lead to stronger potential surface winds Monday night/early Tuesday.

Post-mid-latitude cyclone, the moisture plume remains aimed at northern & central California through the day Tuesday with light to moderate intensity bands of showers continuing through the day. Tuesday night there could be a brief lull in precipitation, however offshore the quasi-stationary low begins to start moving west. Warm air advection once again picks up Wednesday, with rain becoming more and more likely throughout the day on a widespread scale as the huge eastern Pacific cyclone begins nudging northwest. By Wednesday night, the cold front slowly edges closer and closer to the coast — increasing lift further and thus precipitation rates, with likely widespread moderate to heavy precipitation occurring across a good chunk of northern California by late Wednesday night, continuing through the entire day Thursday. The cold front finally moves ashore and exists northern California by Thursday night — with the heaviest rain rates occurring just ahead of it. By Friday, the moist onshore flow continues and snow levels finally lower, although the majority of the precipitation will be showery.

Rainfall amounts through Friday look to be quite spectacular, once again for the second month in a row and within a 5 day period. In the lower elevations along the coast, inland into the delta, Sacramento & northern San Joaquin valley, anywhere from 2 – 5 inches is likely on a widespread scale, with some eastern valley locations perhaps getting 6 or 7 inches locally. In the higher terrain including coastal mountains and foothills of the sierra, 6 – 14″ of rain is likely — locally more along southwest facing slopes. Above 4000 – 5000ft in the northern mountains, sierra & its westerns slopes, rainfall amounts of 14 – 20″ are quite possible — potentially locally doubling amounts seen in the first half of January.

Generalized precipitation totals currently anticipated through Friday night.

Snow levels remain up around 7500 – 8500ft through much of this precipitation event aside from Friday when the entire eastern Pacific low (which during its most expansive phase, will likely be upwards of 2500 miles wide) moves ashore into the western U.S. & Canada, which will translate to a massive amount of runoff into mountain rivers & streams, and downstream into valley waterways. Any snow below those levels will once again face the fate of being melted and converted to runoff quite quickly. Additionally, rivers, creeks, and streams throughout northern California remain extra sensitive due to water being converted to runoff almost instantly after the recent rains saturating soils. In some places, the soil still isn’t totally saturated and can handle more absorption. However, when even these places experience moderate or high rain rates, water is sent to runoff quick as well. In short, I expect valley and coastal creaks, streams and rivers to experience the highest risk of flooding — with the Cosumnes river expected to flood from Michigan Bar westward until it meets the Mokelumne river. The Sacramento river is also expected to overshoot flood stage in the northern Sacramento valley.

We’ll finally dry out over the weekend into early next week, but there’s decent model agreement that we’ll see more rain later next week, with long-range guidance suggesting even more through much of the rest of the month. Stay tuned, more to come as winter unleashes everything its got on the west.

Showing 2 comments
  • Mike Leflar

    Thanks for the great blog. It’s very enjoyable reading. Keep up the good work!

    • wxtracker15

      Thanks, Mike! Always glad to hear it’s a worthy read after putting buckets of time into these posts.

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