Heavy Rain and Potential Flooding (And a Warning about Parking)

I knew it was dangerous to do a blog on arid conditions on Friday the 13th.

Yesterday, a low pressure center to our southeast rotated moist, easterly flow into our region with substantial upslope precipitation leading to National Weather Service flood watches along the eastern slopes of the southern WA Cascades and over the Blue Mountains of NW Oregon (see figure below).

Here are the 24 h precipitation totals for southern WA.  Some locations near the Cascade crest received more than an inch, with many locations along the eastern slopes wetted by a half-inch.

With the system centered to the south, Oregon got even more rain, with some locations in the coastal mountains getting more than an inch.

Eastern slope rivers are surging from the combination of rainfall and snowmelt.  The critical Yakima river reservoirs are near capacity and still rising (blue line below).  Yakima agriculture should have plenty of water this summer.

As noted earlier, the cause of this wet bounty on the Cascade eastern slopes and Oregon has been a  low center positioned ever southern Idaho.  The 850 hPa (5000 ft) map below shows this feature clearly (wind barbs and temperatures at the level are also shown).

The slow-moving low will continue precipitation over eastern WA and Oregon for the next 24-h, as shown by the latest WRF model forecast below.  Go north, if you want drier weather.

This week is going to be interesting.   First, a weak ridge will develop over us, providing temporary improvement.  But then  a HUGEridge develops over the Pacific and a very deep low forms over our region (see upper level map for Friday at 8 AM).  WOW.

This a cool, wet pattern for us.  To show this, here is the 72h precipitation total ending 5AM Saturday.   Even California gets a piece of this.   You can expect the wildfire danger to be greatly reduced and all current fires should be controlled.   

One negative about rain--it is not good for maturing cherry crops.  California's harvest was radically reduced by recent, unseasonable rain.  I hope the same doesn't happen with WA cherries, which I love.
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Public Service Announcement: WARNING TO HIKERS ON CHUCKANUT DRIVE south of Bellingham.

If you are hiking near the trailhead of the wonderful Oyster Dome hike on Chuckanut Drive (SR 11), be VERY careful.  The State Patrol and their towing company buddies are towing lots of cars.  So be very careful if you park there.

Arid Spring

I have been doing a lot of gardening the past week, and I am startled by how dry the soil is.

What is particularly surprising is the aridity of the soil following the wettest winter in Northwest history.  The lack of rain was accompanied by very warm temperatures (April was the warmest on record at several western WA location, which has led to an earlier start of the fire season, with several wildfires west of the Cascades.  The picture below shows a fire in Snohomish County.

So what happened?  Let's start by looking at the percentage of normal precipitation for the past two weeks (below). Western WA and Oregon are very dry, with some locations at less than 5% of normal.

But percentages can be deceiving--here is the departure of observed precipitation from normal for the same period.  Less scary....but you will notice that some areas of western Washington are down a bit over two inches.  Much better situation east of the Cascades and over California/Nevada.

What about the entire spring?  Here are similar figures for the last 60 days.   For the percent of normal precipitation, much of the Northwest is well below normal--some locations 25-50% of normal.

Departure of accumulated precipitation?  2-6 inches in western Washington, 0-2 inches in eastern WA.  

So why have we been so dry and warm here in western Washington?   The key cause is persistent high pressure (ridging ) aloft.    Here is the difference from normal (anomaly) over the past 30 days for upper level  (500 hPa) heights (like pressure).  Above normal heights (yellow colors) over SW Canada and the northern portions of the Northwest).    Such ridging brings sinking air and dry/warm conditions.

This persistent pattern is rapidly evolving and next week an area of low pressure will move towards our coast.  The result will be clouds and rain.     Take a look at the latest 6-10 day precipitation forecast by the Climate Prediction Center.  Above normal precipitation over our region.  I suspect this will end the fire threat for a while.

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KPLU Event!

Meet an astronaut (and me) tomorrow (Saturday, May 14) at the Museum of Flight for a KPLU fundraiser (music at 6 PM) and presentation (7PM).  Information here: