Heavy Rain, Strong Winds, & Heavy Sierra Snow Through the Weekend

Heavy Rain, Strong Winds, & Heavy Sierra Snow Through the Weekend

 FEBRUARY 6, 2014

A train of potent shortwaves and weak to moderate strength surface lows setting up along a strong westerly jet across the eastern Pacific has, and will continue to tap into a long band of deep sub-tropical moisture from the Hawaiian Islands region, setting up what is called an “Atmospheric River”, aimed directly at northern California beginning Friday through this weekend. This strong westerly jet, handful of of systems along the jet, and sub-tropical moisture plume are all being launched east towards the west coast (specifically northern California & southern Oregon) by a couple of deep lows in the eastern Pacific; one between the Allusion Islands and Hawaii, the other in the Gulf of Alaska. The southernmost low is nearly stationary, directing a 120 – 140kt jet streak into the western U.S. along with associated shortwaves, including perhaps some weaker surface lows from energy that splits off from the main low.

The first system ejects towards the Oregon coast through the day Friday, with warm air advection precipitation developing as early as Friday morning across favored terrain. As the system nears the coast in the afternoon and evening, a massive precipitation shield enhanced by strong orographics and jet streak support slams into northern California, with the “atmospheric river” blasting northern California under the low. Moderate to heavy precipitation is basically a given Friday night through most of Saturday, before another shortwave impacts Saturday night into Sunday, with another somewhat weaker surface low slamming into the Oregon coast.

NAM’s forecast 700mb vertical velocities for Friday evening, indicating moderate to strong vertical motion in the deeper oranges, reds, and purples.

While the second system doesn’t have as strong of a low, the core of a 120kt jet streak rides right over northern California overtop the impressive sub-tropical moisture plume, enhancing precipitation potentially more than what Friday/Saturday’s system does given the jet core remains in OR/WA.

NAM’s forecast 300mb (essentially jet stream level) winds, indicating a 115 – 125kt jet streak plowing over northern California Saturday night/early Sunday with the final system.

Generally, precipitation across a good 3/4 of northern California will be nearly consistent from Friday evening through Sunday evening, before breaking down to showers overnight Sunday into Monday. During the Friday night to Sunday evening period, vertical motions (aka lift) are expected to remain quite strong to even extreme, with orographic/upslope enhancement overloading 700mb vertical velocities at times with high resolution models, indicating a great low to mid-level south to southwest flow parallel to S/SW facing slopes, such as the foothills/mountains/sierra of the following counties: Shasta, Tehama, Plumas, Butte, Yuba, Sierra, and Placer counties.

With each shortwave passage, especially Friday night into Saturday, and again Saturday night into Sunday, each system will bring gusty winds as pressure gradients tighten up with the systems. Friday night/Saturday’s system is currently modeled to be backed by 40 – 50kt 850mb winds, which in combination with heavy precipitation could very well bring down some 30 – 45mph gusts in the Sacramento valley, with 40 – 50mph gusts along the coast. 25 – 35mph gusts seem to be a good bet with the Saturday night/Sunday storm given it lacks a stronger low, but as mentioned before it should be packing some pretty decent precipitation rates. After we see a good several hours of soaking rain, gusty woulds could more easily bring down some trees across the region, increasing power outage potential.

NAM’s forecast 850mb winds with 10-meter wind arrows valid Friday night/Saturday morning.

Onto those impressive precipitation totals a lot of us have been hearing about.

U.S. models have pretty well locked onto amounts, while the ECMWF has still been lagging behind total wise. The NAM/GFS and even the GEM all indicate 1 – 3″ of rain for all of the Sacramento and northern San Joaquin valley, with amounts of 3 – 5″ along the east side of the valley and in the foothills. Higher up, above around 2000 – 3000ft, 4 – 6″ amounts could be more prominent north of Jackson, with 6 – 8″ in the northern sierra/western slope as highlighted in the orographicically enhanced region aforementioned… possible upwards of 10″ depending on the placement of each precipitation enhanced. Along the coast, 2 – 5″ is possible between Eureka and Santa Cruz, with 4 – 6″ in coastal mountainous terrain.

Snowfall will be tricky with these systems due to changing airmasses. Friday, leftover cold air will promote snow levels around 5000ft, with significant snowfall in the sierra (especially the northern sierra). It appears possible that 1 – 3ft of snow could fall Friday/very early Saturday alone in the sierra from Lake Tahoe northward above around 6000ft, but as we head into Saturday, the presence of the sub-tropical moisture/atmospheric river makes itself known by warming 850mb temperatures to as much as +2 – 4c, promoting snow levels to as much as 7500ft – 8000ft. Heavy rainfall below the snow level could make for a snow-meltingly slushy mess between 4000 and 8000ft (or wherever the snow level decides to setup shop, but shouldn’t be too far from 8000ft come Saturday afternoon into Sunday morning). Snow levels are able to fall Sunday morning as the last shortwave comes inland, bringing some somewhat cooler air… enough to lower snow levels to around 6000 – 7000ft.

In total, perhaps 4 – 5ft of snow could fall above 8000 – 9000ft through the weekend, but a good bit could melt below around 6000ft given the increasing snow level and heavy rain falling atop a deep coat of snow.

While we’re in a historic, extreme (to exceptional) drought, it doesn’t mean we’re immune to flooding. 2 – 5″ of rain falling in the lower elevations can very easily cause some urban flooding, possibly even some issues with smaller creeks and streams… however no major waterways should have any problems, but doesn’t mean the smaller ones can’t cause problems.

Something else to monitor is convection. While Friday and Saturday will have limited convective potential given a lack of a nearby cold pool, but Sunday could end up being a bit more interesting as the final system comes through with some somewhat cooler air. If anything notable begins showing up model wise, I’ll post something focusing on the last system and it’s convective potential by Saturday evening.

Our dominant super-ridge, ridiculously resilient ridge (RRR), mega ridge, death ridge, or whatever you’d like to call it returns by early next week, putting us back into a dry spell we just don’t need, ending our once active pattern. Hopefully it breaks down or moves promptly, but we won’t know until a ridge-breaking storm shows up.