Wednesday, December 18, 2019

A Moderate Atmospheric River is Aimed at the Northwest

Our region has been dry the past month, but that is going to change quickly during the next 24-h.  A modest atmospheric river will make landfall over northern Oregon/southwest Washington, bringing substantial precipitation to the mountains, with plenty over the lowlands.

The latest water vapor satellite imagery (which displays the amount of vapor from roughly 15,000 to 35,000 ft) shows a plume of high water vapor values moving out of the subtropics north of Hawaii and moving northeastward towards the West Coast (the blue arrows indicate the atmospheric river).  This feature has our name on it.


The model forecast of the total amount of water vapor in a column (known as IWV--Integrated Water Vapor-- in the biz) for 4 PM tomorrow (Thursday), shows a plume of high values extending to the WA/OR border (see below). 

You notice that values decrease over land.  You know why?  Because of a lot of water vapor is condensing out and falling as rain as the atmospheric river is forced to rise by our terrain


Why is there an atmospheric river you ask?  Good question.  They occur when there is a certain configuration with low pressure over the northeast Pacific and high pressure west of California.  To illustrate, below are heights of the 850 hPa pressure surface at 4 PM Thursday (you can think of it as pressure at 5000 ft).

You will note low pressure  over the Gulf of Alaska and high pressure off of California.  Between them is a region of large pressure change, which results in strong southwesterly winds.  Such winds move water vapor out of the tropics and subtropics.  Such winds also bring warmer air (which can hold more moisture)--note, the shading is temperature at that level with reddish colors being warmer.


So how much precipitation will we get?  Plenty.

Here are the forecast totals through 4 PM Saturday.   5 inches and more over the windward (western) side of terrain, including the Olympics, Cascades, and coastal mountains of Oregon.


But plenty even here in Seattle.  The UW high-resolution ensemble system (many high resolution forecasts) indicates around 1.5 inches here in Seattle by 4 PM Friday.  The black line is the mean of the many forecasts--usually a very skillful forecast.

There will be plenty of precipitation...but what about snow in the mountains?  There we have an issue--the atmospheric river will be warm, with rising freezing levels.  A small shift in the atmospheric river northward could greatly reduce any snowfall in Washington.

The forecast snow total through 4 PM Saturday is substantial over the central and northern WA Cascades. Two things will help.  First, cold air over eastern WA will hold out for a while, with cool air moving westward into gaps in the mountains.  Second, with the atmospheric river core mainly south of Washington, the warmest air will be over us for only about 12h, with cooler air moving in over the weekend.  It is going to be close though.


And there is more...it is going to get quite windy around our area later today, and a minor windstorm may pass through on Saturday. 

Lots of action--just what meteorologists and weather lovers enjoy.

15 comments:

  1. cliff, this is just annoying to me i love the blog i waited for this to happen its like the lowlands 4 weeks ago when we were realy cold it showed snow but as we got closer the snow diseard fast and went from highs in the 30s to uper 40s and stevens pass needs this snowfall my birthday was 2 days ago on the 16th of december i got a ticket to go up from dad Reminiscent of the 1998-1999 winter, copious amounts of snow have fallen across the Cascades this winter due in part to the moderate/strong La NiƱa in the equatorial Pacific. State transportation crews have been busy working to keep the mountain passes open, which have been closed numerous times due to avalanches and traffic problems. As of February 1, snowpack across much of the state is greater than 140% of normal. The biggest departures from average are in the central Cascades, particularly Stampede Pass, Snoqualmie Pass, and Stevens Pass, which have not had this much snow on the ground since 1974. Further, White Pass has a record February 1 snow depth of 110", surpassing the previous record of 88", and is also tied for the 3rd highest snowdepth ever at White Pass.

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  2. Is there a flow rate for atmospheric rivers that can be calculated in cfs or gpm? How similar to, say, the Columbia is this?

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    1. If the average rainfall for the 142 square miles of seattle is 1.5" for this storm, that amount of water is roughly equivalent to 30 minutes of the flow of the columbia at the mouth.

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  3. Cliff do you think this will help the shallow blob at all or has that fully dissolved on its own?

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    1. likely not. it may help a little it some what, but it will take a whole lot more than that..

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  4. I observed some ragged mammatus clouds in the vicinity of Ferndale this afternoon. They were not as well-defined as the formation over western Skagit County yesterday but were unmistakable nonetheless.

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  5. Dark day today in NW Bellingham. Max irradiance was 33W/m^2 with only 6hr40min of measurable (≥1W/m^2) daylight.

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  6. I do wonder at characterizations that the region has been "dry". Maybe this season hasn't been as wet as some other years, but dry? Maybe I'm alone in this but the rainfall in Seattle and Sea-Tac (and Bellingham) are lower than what the region sees overall.

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  7. Blake,

    There's been 3.84" of precip at BLI since 11/1/19. Average for the period (11/1/19-12/18/19) is 8.25". I'm no alarmist but I think it's fair to characterize a 7-week period, during the climatologically wettest part of the year, in which precip is 47% of average as "dry".

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    1. How about "dryer" not totally "dry". Just saying. You are about to get yours.

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    2. To answer your question, "dryer" would be inappropriate because it's a noun. You're looking for the adjective, "drier". Semantics is maybe not your thing.

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  8. I can also say that the typically sodden Upper Snoqualmie Valley has been startlingly dry so far this fall. I work outside for a living and we have yet to have a day this fall here that requires true rain gear (i.e. commercial fishing outerwear).

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  9. I don't see the minor windstorm you were seeing for Saturday? ECMWF is calling for S 3-4 knots. Very calm day. Wind forecasts have seemed to be fickle and rapidly shifting lately. I do see wind for today (Thurs) - ECMWF says 13 gusting 29 this evening (huge divergence - low confidence?) and NOAA says 25-35 (gale warning).

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  10. Is the “atmospheric river” from Hawaii, the same as the “pineapple express“?

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  11. Impressive wind speeds @ Camp Muir tonight, 99 mph gust @ 20:00 and sustained winds of 87 mph. Cliff, NOAA predicted that snow would change over to rain at the passes on Friday but it appears the cold air gave out quickly around 10pm on Thursday night, was this due to the high winds causing warm air to mix down to lower levels ? Pretty cool to see the 'battle of the masses' play out on the radar!

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