Wednesday, January 27, 2016

An Atmospheric River Fiesta

Wednesday, January 27, 2016
1:41 am

Yesterday, I wrote about the Eastern third of the country and how much of that region was profoundly affected by a major blizzard that halted over 13,000 flights and caused 3 billion dollars in damage. Now, the weather action has shifted back to our neck of the woods, and while we won't see anything historic over the next week like what was witnessed in some states back east, the entire West Coast looks to get a pretty good soaking this week.

I've used the term "atmospheric river" a lot, and that's because we get a lot of them. This week will be an atmospheric river fiesta not only for Washington State but for the entire West Cost. The first of these rivers will impact us tonight into Thursday, and with snow levels above 7,000 feet, flooding is possible on many of our rivers. The Skokomish will certainly flood and could reach major flood stage. Models have trended a bit lower with precipitation amounts here in the lowlands mainly due to the atmospheric river staying trending further north in the models, but the Olympics and North Cascades are expected to get several inches of rain within a few hours. When you have that amount of rain in such a short time combined with high freezing levels and rivers that are already running high, you've got a recipe for flooding. Contrary to popular belief, a healthy snowpack like the one we have now actually decreases flooding concerns because it helps absorb water and prevents it from all flowing into rivers.

Valid 04:00 pm PST, Wed 27 Jan 2016 - 12hr Fcst
Credit: UW Atmospheric Sciences

This river will be remarkably persistent and will progress southward as the week goes on, re-energizing on Friday off the Central Californian coast. You can already see the beginning of the second river with that little ‘wave’ around 155 degrees west. In fact, there could still be some vestiges of this river on Sunday over SoCal! Even with this El Nino, they've been relatively dry, so finally getting some significant precipitation should bring a heavy sigh of relief.

Credit: National Center for Environmental Prediction (NCEP)

Here's one of my favorite model charts: the total accumulated precipitation for the 12z GFS run this morning. The picture below shows the total predicted accumulation over the next week. Tons of precipitation everywhere, and in the mountains, that precipitation will be in the form of snow.

If you think the East Coast got hit hard this past week, look at the charts below. The Sierra Nevada are a high mountain range that rises dramatically from the Central Valley and are very efficient at enhancing precipitation on their windward slopes. They often get extraordinarily heavy snowfall totals when there are moist winds perpendicular to them, and I would not be surprised if places like Mammoth Mountain and Mt. Shasta Ski Park picked up 3-4 feet of snow with this atmospheric river. If the Sierra Nevada and Washington Cascades switched places, the mountains here would get a lot more snow, Eastern Washington would be even drier, and traveling over the mountains would be a pain over the pass.

Here are the first 72 hours of snowfall. Not a blizzard, mainly because the warm atmospheric river over our area will send snow levels skyrocketing and prevent anything from accumulating here. BC gets clobbered though.

Valid 04:00 am PST, Sat 30 Jan 2016 - 72hr Fcst
Credit: UW Atmospheric Sciences

But look at the next 72 hours. A good amount of snow for the Cascades, but look at the Intermountain West. Utah gets buried. Colorado is off the chart (literally), but I'm sure they get gobs of snow too. Great news for skiers and reservoirs alike. 

Valid 04:00 am PST, Tue 02 Feb 2016 - 144hr Fcst
Credit: UW Atmospheric Sciences

We've had a somewhat atypical El Nino winter thus far, with more zonal flow into our area rather than split flow. However, long-range forecasts show the dreaded split flow returning, blocking anything from reaching the Pacific Northwest and unfortunately making it pretty difficult for that energy to spread southward to California as well. Although Northern California has gotten a lot of precipitation this year, it has been a letdown for SoCal.

Credit: National Center for Environmental Prediction

Enjoy the active weather this week!


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