A Wet Period Ahead for the U.S. West Coast

The first half of May has been generally dry and warm over the Pacific Northwest, but soon everything will change.

The second half of the month promises to be cool and wet in Washington and Oregon, with northern California swamped by highly unusual amounts of rain.   As we shall see, this wet period has highly positive implications for the upcoming wildfire/smoke season.

To give some perspective, below is the precipitation departure from normal for the past 60 days over the western U.S.   Much of the region has had above-normal or near-normal precipitation, with the big exception being western Washington, which has been drier than normal.  Importantly for wildfire concerns, southwestern Oregon and eastern Oregon have been moist and the area east of Cascade crest in Washington has been near normal. 

Much of coastal California has been on the dry side during the past two months, but that is following a very wet winter.  The concern for California is that abundant winter precipitation will lead to lots of grass growth, which will burn later in the summer as it inevitably dries out. 

The forecasts for the next two weeks for the West Coast are consistent and wet.  The UW WRF forecast for accumulated precipitation over the next week indicate amazingly wet conditions over northern CA, with 5-10 inches over the Sierra Nevada and northern CA mountains.  This is HUGE for late May.  Heavy precipitation extends northward across the Oregon Cascades and eastern Oregon, and western Washington gets 1-2 inches (normal total May rainfall at Seattle is a little under two inches).  Forget watering your lawn.

Turning to the vaunted European Center model totals for a similar period, one sees very similar amounts of accumulated precipitation.  Folks, this is really wet for Oregon and California.

Now, any good forecaster does not look at a single model prediction, but views an ensemble of many of them to understand the uncertainty of the forecast.    Here are the 51 forecasts of accumulated precipitation at Seattle from the European Model ensemble for the next ten days.  90% of the members are on the same page, with precipitation starting on late Tuesday and 1-3 inches by May 24th.

Even light rain in Yakima, on the eastern side of the Cascades.

So what are the implications of all this?   

First, the atmosphere is not locking up in a warm/dry pattern as it did in 2015.  Second, this is an unusually wet pattern for California/southern Oregon so late in the season, one that will top off their reservoirs and ensure that there will be plenty of water in the Golden State this year. 

It will also delay the wildfire season there.  Third, western Washington will get some needed precipitation, lessening the chances of any early grass fires and will help fill our reservoirs, which are actually not in bad shape anyway.  The wet period will also radically reduce local water usage, leaving more water in the reservoirs.

There is a lot of fear-inducing headlines in the media and among some politicians about a terrible fire season because of the recent dry period and the low snowpack at some sites in northern Washington.

These worries are not realistic.   The upcoming wet period, which has been forecast for days, should moisten up the whole area.  The eastern side of the state had plenty of precipitation during the past winter and streamflow/soil moisture is fine there. 

The snowpack in much of California, Oregon, and southern WA is fine.   And low percentages in parts of WA this time of the year can be very deceptive.  Let me demonstrate this.

Below are the values of snow water equivalent (SWE, amount of water in the snowpack) and precipitation accumulation of this year versus normal at Stevens Pass--a location with anomalous low snowpack (SWE) today.     Total precipitation (black--this year, gray--normal) is just a little below normal (that will change during the next week!). 

But what about snowpack (Red normal, blue this year)?  The snowpack peaked at about 75% of normal and will melt out around May 15th, rather than the normal June 1.   So the % of normal today is crazy low (maybe 5%), but clearly the situation is not that serious...just an early melt out from the warm temperatures of the past month.

The bottom line:  At this point, it is premature to predict an early or more severe fire season for our region.

from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/05/a-wet-period-ahead-for-us-west-coast.html