February 22, 2021

Amazing Precipitation Differences and Avalanche Threat

 The super rain shadow event of the past day resulted in startling precipitation contrasts over the region.

A picture worth one thousand words

The map below shows the 24h amounts ending at 11 AM this morning (click to enlarge, and a blow up is below).  A few hundredths of an inch over north Seattle and northern Kitsap, but as much as 6 inches in the Cascade foothills, tens of miles away.




The forecast of 24-h precipitation ending at 4 PM today, shows the rain shadow and the immense precipitation on the lower western slopes of the Cascades.  And look at eastern Washington: an immense dry zone in the lee of the Cascades.


This huge precipitation gradient was supercharged by strong westerly flow associated with a narrow, but potent atmospheric river.   And the air warmed considerably yesterday, resulting in rain hitting the lower passes.   Such heavy precipitation in the Cascades, on top of substantial snow of the past few weeks, has led to a serious avalanche risk, with the Northwest Avalanche Center going for high to EXTREME.   So serious that WSDOT closed down Stevens and Snoqualmie passes overnight.

The local snowpack currently is well above normal, as shown by the SNOTEL network this morning:


And the snow forecast this week is very aggressive, with 2-5 feet in the Cascades and Olympic by Friday evening.  Classic La Nina.


With the 8-14 day forecast from the Climate Prediction Center being colder than normal, the snow is not going anywhere soon (see below).   So both summer water resources and March snow fun look to be in good shape at this point.



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February 20, 2021

Super Rain Shadow

 There are rain shadows.... and then there are super rain shadows.  

On Sunday, a super shadow will produce an area of virtually no rain over northern Puget Sound country. And another super rain shadow will occur over the eastern slopes of the Cascades.

As I have discussed many times in this blog, when air approaches a terrain barrier, it is forced to rise on one side, producing bountiful precipitation, and when it descends the other side, the air is compressed, warms, and dries (see figure).  The wet side is the windward side, the dry side, the leeward one.


The effects of terrain--both windward enhancement and the lee rain shadow-- are enhanced when the wind is strong and headed straight up the mountain range on one side and straight down on the other, like the example shown below.


On Sunday, the situation will be perfect:  strong winds from the west will approach the north-south terrain barriers of our region.  And this wont be just any air...but a moist atmospheric river as well!

Here is the forecast 24-h total precipitation ending at 4 AM Monday. As much as 2-5 inches on the windward side of the Olympics and Cascades, but NOTHING east of the Olympics and over northern Puget Sound and NADA over the lower eastern slopes of the Cascades and nearby easterly Washington.


Here is a closer look over western Washington.  NOTHING from north Seattle to southern Snohomish County and back towards northern Kitsap.


The perfect day to visit  Richmond Beach Park on the Sound.    The folks in Sequim, normally smug in their rain shadow ascendency, will be jealous.

To show you how perfect the set up is, here are the winds and heights (like pressure) around 5000 ft (850 hPa pressure) at 4 PM Sunday.  Winds are parallel to the height lines....and you can see wind barbs if you look closely.

Just wow.  HUGE  north-south variation in heights (which means STRONG winds), with the winds coming out of the west.  Due west.   Winds are directed perpendicular to the north-south mountain ranges of our region, which means strong uplift (and precipitation) on one side, and extraordinary sinking on the lee side of the terrain crests.

And this westerly (from the west) flow will be very moist, as shown by this forecast of atmospheric moisture at 10 PM Sunday.  You see the  orange/red areas directed right into Washington?  THAT is a plume of moisture, commonly known as an atmospheric river.



But even with all that "juice", the strong sinking to  the lee of local barriers will make it  bone dry downstream of them.

A major issue will be heavy precipitation...including rain... in the lower passes on Sunday and Monday.  It might get unpleasant

So enjoy the dry rain shadows if  you are in a favored spot, and perhaps take a short trip to a local rain shadow if you are not.  

Finally, it turns out that there is a Super Rain Shadow action figure .  I will have to get one!


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Featured blog on the Decline of Public Radio Station KNKX: 

Amazing Precipitation Differences and Avalanche Threat

 The super rain shadow event of the past day resulted in startling precipitation contrasts over the region. A picture worth one thousand wor...