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.
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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.

from Cliff Mass Weather and Climate Blog https://ift.tt/2JjcPBt