Rainy Bloomsday

Since our previous blog, the computer forecasts (as expected) have been trending farther south with our upcoming storm, as has been the case for previous storms this spring.  One key difference that I pointed out in the last blog is that this storm is forecast to move much slower.  Why does that matter?

For fast-moving storms, the weather pretty much moves with the storm, from west to east.  But with a slow moving storm, clouds and rain have a chance to rotate around the storm.  So for folks (like us) who are north of the low, the rain actually moves into our area from southeast or east.  It's this pattern that makes May the wettest month of the year for Montana (see a previous blog on this).  This storm will bring heavy rain to all of Montana.

So even though the low is moving by to our south, we still stand a good chance of getting some rain from it.  But rather than coming from Seattle or Portland, our rain will be coming from Boise or perhaps even Missoula.

Saturday will be wet in southern Washington and the Cascades. A few showers may move into eastern Washington Saturday afternoon, but overall it should be rather mild with temperatures in the upper 60s.  Overnight and Sunday morning, rain is forecast to swing up into our area from the southeast.

Here's a sequence of the event from one computer model.  By Saturday evening, a large area of precipitation has developed from southeast Washington, down to Boise, and into Nevada.  But this front isn't moving to the east; it's moving north.

Around midnight Saturday, this rain has moved to the north and to the west, impacting Yakima and Spokane.

By sunrise on Sunday, this area of rain has moved into all of eastern Washington.

And by noon on Sunday, the front has completely turned upside down, oriented in a band from north-central Oregon into northeast Washington, moving east to west.

By Sunday evening, this band reverses its course, and moves to the east, giving everyone a second shot of rain.

Now this is just one computer forecast.  There are others with different timing and location.  But most are at least similar to what I've shown here.  So how much rain can we expect from this event?  I'm hesitant to even guess based on the last few storms.

The National Blend of Models takes all of the over 100 models and blends them together, along with adding some bias correction.  Here's what it has come up with for rainfall.  The blue shading represents at least 0.50" of rain.

As always, some individual models forecast more, some forecast less.  The European model is a little drier, giving northeast Washington about 0.25" of rain.

The US model is wetter, giving some locations in northeast Washington over an inch of precipitation.  But the GFS has overforecast most of the rainfall this spring.

Here's another way to look at the rainfall forecast from the National Blend of Models.  It shows the probabilities of getting certain amounts of rainfall at a selected point.  For Deer Park, there's an 88% chance of getting rain (yes, 12% of the computer forecasts don't give us anything). There's a 48% chance of getting more than 0.25", and a 17% chance of getting more than 0.50".  There's even a 2% chance of receiving more than an inch of rain.

One thing I did want to mention.  The US model has been consistently predicting some snow with this storm on Sunday (blue shading in the image below).  This is likely due to the heavier precipitation rate forecast by the model.  Now before you get upset, it should be noted that this is an outlier forecast.  I haven't been able to find any other computer forecasts of snow.  Also, even if it does happen, temperatures will be above freezing, so it's unlikely to accumulate. 

After the passage of this deep low pressure system, another weaker storm moves into the area for more showers on Monday and Tuesday.  After that, the weather pattern changes considerably.  We will be done with our cool and wet weather for the foreseeable future, replaced with dry weather and highs in the 70s.