April 04, 2018

Monster Late-Season Atmospheric River Will Hit the West Coast With Heavy Rain and Strong Winds

An extraordinarily strong atmospheric river will hit the West Coast starting tomorrow in the Pacific Northwest and then moving southward into California on Friday and Saturday.

A monster that will be one the strongest on record, particularly this late in the season.  And to add to the fun, an extremely powerful storm will form off our coast, bringing very strong winds over the Oregon coast.  Again, very unusual this time of the year.

Just a reminder-- an atmospheric river is a narrow current of high moisture values,  originating in the tropics and subtropics, that is associated with warm air and heavy rain, particularly when into coastal terrain.  In our neck of the woods, atmospheric rivers are often termed a pineapple express.

Satellite-measured integrated water vapor product this morning shows large values in the tropics (orange, red, and purple) with high values starting to move northeastward towards the North American west coast.

Let's start by viewing the UW WRF model's forecast of column-integrated water vapor...the amount of water in vertical column of air (see below).  The dark blues indicate very, very high levels, white and red are high values, with lesser values shown in green and light blue.

At 5 PM today (Wed), a tongue of modestly high values reaches the Pacific Northwest coastline.

One day later (5 PM Thursday), a much more threatening situation has developed.  Huge values (blue colors) are now in a very wide atmospheric river that is directed right at California.

Friday at 5 PM is stunning.  An extreme atmospheric river (blue all the way) is hitting California.

 And during the next 12-h (5 AM on Saturday), the river moves southward, bring heavy (and very unusual) rain to southern California---with the threat of causing more landslides from the debris from the December fires.

By Sunday, the atmospheric river moves out, but there is little relief for the sodden California folks, with another (but weaker) event late Monday and Tuesday (see below).

How much precipitation will California and the Northwest get?  Here are the precipitation totals for the next 8 days from the European Center model (courtesy of weatherbell.com). For California, we are talking about as much as 4-8 inches in terrain and heavy amounts (over 2 inches) over much of central and northern CA.  Remember April is typically a month that California dries out as the jet stream usually moves north.

Here is the Northwest, we will get a piece of the action, with several inches everywhere, but particularly heavy precipitation on the Olympics and in southern BC.
This is really an historic event that is very unusual for this time of the year.  Let me prove this to you.  Consider the forecast for water vapor in a vertical column for Friday at 11 AM, and lets display it as the percentage of normal (below).  OMG!  The values approaching the coast are 300-400% of normal!

 For those who know some statistics, we can see how far from normal the values are in terms of standard deviations (see below).  Large area above 6 standard deviations, which means we are talking about unprecedented values.

The National Weather Service has a flood watch out for the Sierra Nevada mountains and a heavy snow warming for the Cascades.

But the extreme action is not just with precipitation.  An amazingly strong oceanic cyclone will develop and move up the Northwest coast.   Take a look at the sea level pressure map forecast for 5 AM Saturday (below).  A deep (972 hPa low) west of Washington State, with a huge pressure gradient to its south and west.  That means big winds.

Here are the forecast wind gusts at that time--above 70 knots to the south of the low, with the Oregon coast getting hit hard (50+ knot gusts).

The low will slowly move up the coast, crossing northern Vancouver Island late  Saturday (see map at 5 PM Saturday below).  Big pressure gradients along the Washington Coast...so major wind event there.  Expect coastal power outages.

Having such a strong storm in April is unusual, but not unprecedented.  Anyway, this is getting exhausting....too much weather.  But we are in for an interesting period.  But one good thing...one can forget about West Coast drought or any water issues for this summer.  Our tank is about to be filled...if not overfilled.
Announcement:  The Northwest Weather Workshop is on April 27-28

The NW Weather Workshop is the big annual meeting for those interested in Northwest meteorology.  This year we will have a major session on the meteorology of NW wildfires and others on other aspects of our regional weather.  The gathering takes place at the NOAA facility in Seattle.  To view the agenda and to register, go to the meeting website.  The workshop is open to everyone, but registration is required.


  1. Looks like an interesting week!

    But... did you really say 'OMG'? ;^}

  2. Like one year ago.

    "It is probably the most powerful April storm to approach the Northwest coast during the past half century"

  3. It doesn't look like much rain will make it into Southern California, where it is needed the most.It has become drier with each successive model run in that area.

  4. So 50 knot gusts - not winds but the gusts - now qualify as being "hit hard" on the Oregon Coast? They must get "hit hard" 30 times a year. And that's just November (rimshot).

    Seriously, early April is notorious for this. Try practically the whole first half of April 2013 here in Seattle. Trees down, power out, wet and cold. April 15th - I just checked - had sustained 35 mph winds and 45 mph gusts - in Seattle - not the coasts. The 6th and 7th were similar that year - wind over 20 mph, gusts up to 35 - again that is inland, not the coasts. That year work was delayed on Snoqualmie Pass due to the snow storms... on April 30th.

    I love statistics and I will grant you the 6 SD's in the model but honestly, for those of us who have lived here for decades none of the general forecast for Seattle stands out to me.

    And this: "our tank is about to be filled..." Our tank was filled in January. No wait, make that December. You've posted on it. All of this water from this storm is going straight back down our swollen rivers here. Probably in CA too, where the cumulative reservoirs sit at 106% of average.

    Seriously Cliff, is this your doppelganger dropping the OMG's here? Wait! It's your Dopplerganger!

    Stay safe everyone and if you are going skiing monitor this - https://www.nwac.us/

    1. CA reservoirs sit above average right now because of the good year we had last year. However, the snowpack is ~60% of normal, and this storm won't add much to it, so the reservoirs aren't going to look so good by the summer, because there isn't as much snow to keep filling them.

  5. Well sunsnow...the latest 4-km wrf has 75 knot gusts along the Oregon coast. And when you get hit by gusts over 50 knots, trees and branches start to fall. There have been storms before in April...but this is a particularly strong one (4 SDs)...cliff

  6. Perhaps Cliff is repeating his meme regarding the area's reservoirs in the expectation of the usual hand - wringing and predictions of imminent doom as soon as the region experiences an usuual dry spell over the coming months.

  7. Cliff, I appreciate your explanations about incoming weather and your thoughts on other issues.

    Hyperbole??? Just a great writing style. Readable. Enjoyable. Interesting.

    People should keep in mind that invisible reservoirs are in need of filling too. AQUIFERS. We live on a well, tapping into a water-bearing layer about 60 ft below the surface. A limited resource. Not enough to irrigate lawns etc.

    Keep it up!

    Thank you

  8. Unknown...the reservoirs will be fine. They are now well above normal...and keep in mind that California has multi-year storage capacity. And this late season storm will top them off and enhance the soil moisture. And there is considerable snowpack in place (about 2/3rds of normal). The bottom line: no water problems for CA.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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