Friday, February 2, 2018

An Unusual Heavy Rain Event on Sunday

Heavy rain will hit portions of western Washington on Sunday, bringing flooding to some rivers on the west side of the Cascades.   But it will be an usual event for reasons I will describe below.

Normally, our heavy rains events are associated with atmospheric rivers bringing warmth and moisture northward from the subtropics on southerly or southwesterly flow.  The following image shows the total atmospheric water vapor content for such a typical event, with the plume of moisture heading from the south-southwest.

But the forecast moisture distribution on Sunday is very different, with water vapor streaming in from the west (see image)

The reason for this is that we have an area of high pressure south of us...a ridge.. that is pulling moisture northward in the central Pacific and then swinging it around the high...and then directly into us.


So you may ask...so who cares about the direction of the moisture?   It turns out it does make a difference, because the winds bringing in the moisture will be different.  Instead of the normal southerly or southwesterly flow, there will be westerly (from the west) flow. 

This is important.   The Cascades are roughly a north-south barrier and upslope flow, and resulting precipitation, can be greater for westerly than southerly flow.   In the business we call this more orographic enhancement.

Let me show you the forecast precipitation this weekend from the UW WRF model that is run in my department.

The 24h total ending 4 AM Saturday is wet enough, with 1-2 inches over the western slopes of the central and north Cascades.  That is orographic enhancement in action. Note the profound rain shadow in the lee (east) of the Cascades---this is also the result of the strong westerly flow.  And yes, a mini rainshdow to the SE of the Olympics.

 The next 24 hours has the same pattern, but weaker precipitation with unusual amounts of precipitation over NW Washington.

 But the next 24h is another story, with heavy precipitation (black color, 2-5 inches) over the western slopes of the Cascades.

 You want to be impressed?...here is the total of all these periods...the 72 precipitation ending 4 AM Monday.  Yikes!  5-10 inches of rain over the lower slopes of the central and northern Washington Cascades.

I am afraid that some flooding is inevitable from such rainfall, and the NOAA/NWS river forecast center is going for flood stage over several western WA rivers draining the central and northern
Cascades (see below)

Is this heavy precipitation good news for skiers?  Not necessarily.  The air mass is relatively warm, with a snow level around 7000 ft, so there will be rain at lower elevations, with snow over the high terrain.   Here is the snowfall prediction for the next 72 hr...you can see what I mean.


Last weekend, the Snoqualmie Summit snow report talked about "slop".  God knows what they will call it on Sunday.  Saturated slop, perhaps.

6 comments:

andrew said...

Interesting. This might be the first forecast flood event in my memory to not include the Skokomish.

Ricky Poole said...

If it’s gonba rain at least snow in the mountains. I live in Seattle not for warm rain or sunny weather. I live here for the cold mountain snow. 2 weeks of good snow is lame.

Rebecca Timson said...

After another foot + of snow in the past couple days, it is raining right now at Stevens Pass and rain is predicted for Saturday and Sunday.

I love NW skiing, but can't imagine moving here for the mountain weather in winter.

Ansel said...

Ricky, if you want cold mountain snow, the Rockies is where to find it. Not here, I can tell you, with 30 years of mostly so-so skiing. I went to school in Gunnison, Colorado, so I know.

My State immigration control disinformation campaign includes the following on my license plate frame (Washington): "Ten months of bad skiing and two months of chilly swimming."

But unfortunately, it isn't working!

BAMCIS said...

Mosquitoes are out in force already. Good guess other pests are having a free-for-all as well. Such as Ticks, Rats and Lawns that have to be mowed already. All the usual favorites that we would hope a good hard freeze might keep in check.

This winter = fail. Thanks, ridge.

Seeyah this summer for some triple digits too. Maybe some wildfire smoke. Its going to be awesome. At least the triple digits will kill the lawn. Beats me about the other pests.

Sulla said...

I hear you BAMCIS. Another unimpressive Winter (we'll be to NW spring within the month). But I'm sure that the summer heat will come back for a third season in a row.