Severe Thunderstorm Threat On Thursday

There is a significant chance of several thunderstorms tomorrow (Thursday) over eastern Washington, eastern Oregon, and northern Idaho, with the potential for heavy rain, lots of lightning, and strong winds.

The Spokane National Weather Service office has a severe thunderstorm warning out:

And the NOAA Storm Prediction Center has a moderate level of warning (yellow), which is very unusual for our region.

The predicted strong thunderstorms will move in during the afternoon.  The National Weather Service high-resolution HRRR model shows an intense line of thunderstorms (convection) moving through around 10 PM Thursday.

Three hours later, the line crosses the Canadian border with some showers reaching western Washington.

The line of thunderstorms will be associated with very strong winds in eastern Washington, with some gusts exceeding 60 mph (see wind gust forecast at PM tomorrow, below).  I suspect the wind farms will be "feathered" to reduce damage.

Heavy precipitation will also fall, with the HRRR model going for huge totals over the Oregon Cascades.  Substantial rainfall (over a half inch) over the Yakima River drainage.

The UW forecast model is also producing heavy rain over eastern Washington, which should be a great relief to those concerned about drought.

Localized flooding in river valleys is possible.

Why the potential for such an intense event?    

The "set-up" is illustrated in the upper-level map below (for 500 hPa pressure, about 18,000 ft).

An upper-level trough of low pressure is approaching (yellow arrow), producing uplift that can release atmospheric instability.  A second trough to the south (red arrow) is moving moist, unstable air in from the southwest northward into our region.   Upslope on the eastern slopes of the Cascades will contribute additional meteorological fuel.

In short, later tomorrow will be an exciting time from the Cascades and to the east.

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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