Substantial Precipitation Has Returned to the Pacific Northwest: What Has Changed?

The western U.S. weather pattern that has dominated most of the winter is collapsing and will not be returning.

With this change, substantial moisture will spread over the region, particularly the eastern side.

Enough precipitation that some of the exaggerated drought worries by some local media, amateur YouTube channels, and others should "evaporate."

The big change is evident on the latest visible satellite image, which shows a potent front making landfall on the Northwest 

There has been a lot of problematic information provided by some, so let me describe what has and probably will happen.

Overall Wet and Warm

The essential aspect of this past winter season for our region is that it has been wet and warm, with ample water to fill the reservoirs but sufficiently warm to reduce the snowpack by roughly 50%.

To demonstrate this, there is the precipitation data from October through April for Washington State from 1900 to 2025.   This year was a bit wetter than normal.

The plot for temperature is found below.  This year was considerably warmer than normal, although not a record (which goes to the crazy warm year of 2015).

Interestingly, our precipitation this year has been episodic, with very wet atmospheric rivers and extended dry intervals, something I will explain below.

So what has been going on?

This year, we had a very persistent upper-level (500 hPa, about 18,000 ft) pattern (see below).  The yellow/red areas indicate enhanced high pressure--also known as ridging.

Such a high-pressure area is associated with warm, dry conditions over the western U.S.   But since we were on the edge of it, sometimes an atmospheric river from the southwest or west could push in, giving us substantial precipitation (such as in December and March).  

Regional reservoir managers were very wise and saved the water from these short-period atmospheric rivers--that is why we are in decent shape right now.  They should all be given a gold star.

It is important to note that this persistent upper-level pattern has little to do with global warming, a claim in some media accounts. 

 I have read the literature on this issue and have published on this exact topic, so I am not speculating about it.  On the other hand, some of the warming (around 2F of it) may be ascribed to the slow warming of the planet due to increasing greenhouse gases.

The upper-level pattern is changing now (see below).  

Over the next five days, low pressure (blue colors) will dominate the West Coast from Alaska to Baja, with the "evil" high-pressure ridge (red colors) moving well to the east.

10-15 days from now is shown below.  

OMG.  A deep trough over our region.  That means wet.  

The lastest 48-h precipitation forecast for our region (below) shows substantial precipitation.  

Look closely....very substantial precipitation over the Yakima Basin!  The weather gods have heard us!  Or perhaps the weather gods want the Seattle Times to cool the hype. 

The Yakima water managers were very wise to reduce river flow this week, since their region will get a thorough wetting.

The rain is not going to stop.  The totals over the next 15 days are substantial (see below).  

Good news for agriculture, good news for weather supply, and good news for wildfires.

This precipitation will ensure that the Yakima reservoirs will stay full and should keep the westside reservoirs in good shape (they will fill, and there will be no need for irrigation in the west with rain falling).

The Weather Gods are Not Happy With the Weather Coverage 

of a Certain Newspaper

The Heaviest Precipitation of the Year Ahead in Some Locations?

If you ask most Washington State residents when the heaviest precipitation of the year generally occurs, they will tell you during the late fall and winter.  

It turns out that this is true for many, but not all, Washington State locations.    For some locations in eastern Washington, JUNE is the wettest month or at least equal to December.   

For example, consider Northport, a town in northeast Washington on the Columbia River (red marker on the map)

At this location, June has as much precipitation as December!  At some locations in northwest Washington and southeast BC, June is clearly the wettest month

How can this be?

One reason is that thunderstorms and convective showers are increasing rapidly this time of the year, with maximum thunderstorm activity in late June (see map below)

But there is another reason.....and we will experience this feature this week.

During the cool season, the westerly (from the west) jet stream is strong, and eastern Washington is in the rainshadow (see map of annual precipitation below)

But during May and June, the westerly jet stream weakens, and sometimes a trough of low pressure slides south of Washington, with moist southeasterly flow moving northward up its eastern side into eastern Washington.  No rainshadow!

This is exactly what is predicted to occur on Monday and Tuesday (see upper level map (500 hPa, about 18,000 ft) for early Tuesday).

Eastern (and western) Washington needs to be prepared for substantial spring rains.

The total through Wednesday morning is substantial in the west, and much of eastern Washington gets moistened.

Another low-pressure trough will move in later in the week, with the accumulated precipitation total through Sunday at 5 PM shown below.  Very, very substantial amounts in eastern Washington and Oregon.  Less irrigation will be needed next week.

 Enough precipitation over the Olympics and eastern Washington that I suspect the drought talk should fade.