Stormy Pattern Brewing for Western U.S.

Home / Active / Stormy Pattern Brewing for Western U.S.

Over at least the next 7 – 10 days, the stormiest pattern since last winter is expected to take over the northeastern Pacific and western United States as a barrage of powerful (parent) lows develop and drop out of the Gulf of Alaska, fueling further secondary (child) storms to develop along the fierce polar/Pacific jet streak driven by these strong systems. This pattern by itself would make for an eventful string of systems, however in this case we’ll have even more to bargain with: a bunch of subtropical moisture, thanks to a cutoff low & its associated subtropical jet hanging out east of Hawaii.

Storm #1: Monday/Tuesday

The first storm in this parade won’t be too big of a deal. If it was leading a real parade, it wouldn’t keep its job.

The reason this storm won’t be too big of a system is due to a lack of moisture. The aforementioned subtropical moisture won’t become available until mid-week, so this first system will be relying on moisture it picked up on the way to the door, with precipitable water values hovering slightly over an inch with the front. This front will be associated with a pretty fierce low slamming into the Pacific Northwest later Monday.

The front and its associated precipitation band will begin to move inland by late morning along the coast and north end of the Sacramento valley, and late afternoon for the Bay Area, southern Sacramento valley, and the mountains. Precipitation will continue along the I-80/Highway 50 corridor into the mid to late-evening, with snow levels hovering up around 7000 feet. Despite insulting this storm, it’ll still manage to drop a few tenths of an inch of rain throughout the valley, so don’t count it out completely. Higher up, orographic effects will help squeeze out up to an inch or so of liquid precipitation along the west slope of the sierra and around half an inch for the foothills. A few inches of snow is a good bet above that 7000 – 7500 foot level.

Storm #2: Wednesday/Thursday

A brief break is expected Tuesday as a westerly zonal flow develops ahead of the next system off the west coast. During the day Tuesday, a deep, cold low drops on out of the Gulf of Alaska, and its strong jet begins to vacuum up northward a stream of subtropical moisture & energy from the cutoff low near Hawaii. By early Wednesday, a piece of that energy from the cutoff low strengthens right off the northern California coast — intensifying the precipitation band associated with the cold front. Precipitation begins to overrun into northern California by Wednesday morning, well ahead of the front. Through the day, precipitation will continue to cover more area and slowly gain intensity as the cold front nears and lift strengthens.

Showers will continue through Thursday as a cold, moist westerly flow driven by the deep low in Pacific Northwest. Right now instability looks limited due to cloud cover and an abundance of shower activity Thursday afternoon, but if models start to suggest some patches of sun I wouldn’t be surprised if some thunderstorms become possible. That isn’t the case currently though.

Snow levels will start low (around 3000 – 4000 feet in the northern mountains & perhaps around 4000 – 5000 feet in the sierra) due to cold air getting trapped under the cloud layer Tuesday night/Wednesday morning. The subtropical moisture plume ahead of the cold front will get snow levels pumped up to around 7000 feet by late Wednesday. Snow levels begin to fall again early Thursday as the cold post-frontal environment takes over, dropping to 4500 – 5000 feet throughout the day.

During this event, around half an inch to an inch of rain is expected throughout the Sacramento & northern San Joaquin valley, with 1 – 3″ in the foothills and coastal mountains. In the sierra, especially the central sierra, 3 – 6″ of liquid precipitation is expected, with 1 – 2 feet of snow above around 6000 feet. Half an inch to an inch of rain is expected throughout the bay area, with 1 – 4″ in the northwest corner of the state.

Looks like this system should exit by Friday and allow some ridging to build into the west for a short time.

Storm #3: Questionable

Here’s where things get tricky. Models diverge and have been all over the place with the third storm in the parade. Occasionally, they’ve been suggesting our cutoff low near Hawaii getting phased with another very large, powerful Gulf of Alaskan low creating a slow-to-progress system with an atmospheric river getting placed somewhere in the west coast for a potentially long time. Other solutions do not phase it, and bring in a quick moving but decent-strength cold front from the Gulf of Alaskan low. At some point this subtropical low will be entrained and brought to the west, but where & when is hard to nail down currently. In the solution that doesn’t phase the two for storm number 3,  it phases it for a storm number 4 sometime during the week of Thanksgiving.

Because of this significant uncertainty, the situation will require reanalysis later this week in another update. Right now, I’d say perhaps anticipate some precipitation somewhere in California on Sunday — but things can and will change in regards to the forecast beyond Friday.

One of a handful of possible solutions in regards to the pattern late in the weekend into early next week. This graphic isn’t a forecast, but just an idea of what some models have suggested, and an idea some disagree with.

Stay tuned, and see you in the next post.

Leave a Reply

%d bloggers like this: