Long-Duration Atmospheric River Event This Weekend to Spike Flood Risk Across Northern California

Home / Active / Long-Duration Atmospheric River Event This Weekend to Spike Flood Risk Across Northern California

The first week of the year has got off to a wet start — but we’ve only just begun to dip into this very wet pattern, with a extraordinary atmospheric river event inbound for the weekend. Before, however, a weak ridge will build in across the western U.S. late Thursday into Friday, giving us a quick blow dry, set on cold given very cold air is locked into the Great Basin & Rockies — quickly intruding westward into California while we’re dry. A quick valley freeze looks likely Thursday night into Friday morning, with highs Friday likely not even make it out of the low to mid-40s in the valley and coast, with 20s and 30s in the mountains. The primary reason this ridge rises up out of the south into the west is the offshore trough strengthening, amplifying the upstream ridge. This trough gets essentially recharged by a strong, cold upper low diving out of Alaska and Canada into the Gulf of Alaska, replacing the ridge that’s currently setup in the gulf. As this low dives south, the strengthening jet associated with the entire trough scoops up a upper-low that’s been sitting around well north-northeast of Hawaii and begins to absorb it.

The low hanging out north of Hawaii has been generating plenty of convection in the tropics and subtropics east of Hawaii, which have been getting tapped into the subtropical jet ahead of the low. All of this subtropical convection — the moisture associated with it, will become entrained into the powerful jet rounding the base of the trough by Friday. The trough swings eastward through the day Friday as it continues to absorb that secondary low, and by Friday night the leading edge of the precipitating cloud shield associated with the low impacts California beginning the extended period of precipitation. Current model guidance suggest precipitation beginning along the coast after 9 – 10pm, spreading into the valley and sierra after 1 – 3am. The cloud cover and precipitation will trap cold air in mountain areas along with interior valleys, meaning snow levels will start low. In the northern mountains surrounding the Redding area, snow levels could begin between 500 and 1000ft, while along the west slope of the sierra, around 3500 – 4500ft. By late afternoon Saturday, snow levels rise to elevations in excess of 7000ft on a widespread scale, remaining high through Sunday.

The GFS’s forecast 850mb temperatures (fill) and heights (contour), valid 5am Sunday. Notice the bands of 10c+ (yellow/orange) being transported northward toward and into northern and central California, the epicenter of the subtropical moisture plumes impacted region. These are temperatures a few thousand feet above the surface, and 10c is equivalent to 50°F, blowing directly into the mountains at the perfect angle to ring out as much warm rain as physically possible onto any leftover snow.

Precipitation rates will be moderate through much of the day Saturday, with some breaks here and there possible as the next wave develops and strengthens offshore toward the second half of Saturday, though overall precipitation will be widespread. As the two lows merge Sunday, the mid-level jet strengthens with a jet max of 100-knots plus blasting northern California during the day, along with lift strengthening simultaneously thanks to strong convergence along the front. The focus of the moisture plume will greatly affect precipitation amounts, and models still flutter around with it. Currently, guidance suggests it’ll setup somewhere between San Francisco-Sacramento or the Monterey-Modesto region, either way increasing precipitation amounts within a couple hundred miles of the target.

Impacts

Surface cyclone strengthens off the OR/WA/northern CA coast Saturday night into Sunday, tightening pressure gradients and directing a strong low-level jet into northern & central California for a solid 12 – 18 hours on top of heavy ongoing precipitation. Still some uncertainty regarding the strength and position of the surface low affecting both wind strength and duration of high winds, but at the very least some gusts upwards of 35 – 45mph are a good possibility across a large section of the state north of Santa Barbara county. Gusty winds combined with heavy precipitation and saturated soils are certain to begin causing some tree & power problems, especially if the wind event carries on for a solid 24 hours.

500mb winds (fill + directional arrows) & heights valid early Sunday as the primary Gulf of Alaskan low absorbs the secondary low, strengthening the jet blasting California. Pictured is a 100kt+ jet max offshore, although these rapid speeds can also be noted in interior northern California. The strengthening jet streak is suggestive of strong convergence in the lower levels, helping aid precipitation intensification.

Precipitation Totals

Precipitation should finally begin to wane early Monday as the cold front and the moisture plume slide south into southern California, though showers will remain possible (especially in the mountains with the westerly flow continuing, where snow levels are able to lower to around 5000ft). By this time, anywhere from 1 – 3″ of rain will have fallen across plenty of the Sacramento and San Joaquin valley, delta, bay area, and immediate coast north of the bay — locally, amounts could be a couple inches higher due to terrain enhancement and placement of the moisture plume. South of the bay along the coast, amounts between 2 – 5″ are possible, and in the coastal mountains: 3 – 6″, with a hotspot of 6 – 9″ possible where the plume stalls along the central coast, perhaps in the Santa Cruz mountains. Inland along the foothills and sierra the highest precipitation totals are anticipated, with amounts over 6″ expected everywhere over about 1000ft, with upwards of 10 – 18″ possible above 4000 – 5000ft.

Snowmelt & Flooding

An issue I haven’t delved into yet with this event will be snowmelt. It’s been a while since we’ve had a significant warm storm after a cooler storm — and in this case, there’s upwards of several feet of snow above 4000 – 5000ft. With snow levels rising to elevations at or above 7000ft through at least Sunday evening and a significant amount of rain expected in the mountains with this system given the excellent parallel flow with the western slope, all of it should melt and runoff along with the excess water from rain itself. Mountain rivers and streams could reach flood stage over the weekend, along with plenty of valley waterways. Out along the coast, especially in the north bay/Napa county, flooding is expected as water rushes out of the coastal mountains. Santa Cruz and Monterey counties could also deal with river & stream flooding as the moisture plume stalls out here Sunday. Aside from creeks, streams, and major rivers, general urban flooding will be even more widespread was 48 hours or so of moderate to heavy rain overwhelm some drainage systems.

Additional Storms Likely

This has been a lot to talk about already, and this is just one system. We probably won’t be done. The upper-low off the Pacific Northwest coast remains kicking through much of the week, driving additional storms into the region. In fact, models are in decent agreement another powerful storm will impact primarily northern California Tuesday or Wednesday, with another to follow likely by Thursday. This could double our precipitation amounts from the weekend. I’ll have another update out by Monday on the additional storms. Stay tuned, stay dry, and most importantly, stay safe.

Leave a Reply

%d bloggers like this: