Finishing Off April

Here’s an Inland Northwest weather fact that you may not be aware of. On average, April is drier than March and May. We all know that in the Northwest, winter is the wet season while summer is the dry season. But the transition from the wet to dry season isn’t a smooth one. 

The bar chart below shows the average precipitation each month in Spokane. You can see that the height of the green bar for April is shorter than March or May.  In fact, June is almost as wet as April, on average.  

The reason for this is that there are actually two wet seasons: winter and late spring.  Winter is the season for strong Pacific storms.  By late spring (May and June), instability showers and thunderstorms become more frequent.  But the big rain events in late spring are often due to a weakening jet stream that is migrating northward, allowing slow-moving storms to dump rain on us.  

It’s worth noting that this pattern extends to other places east of the Cascades, including Pendleton and Boise.  Here’s the same graph for Pendleton:

West of the Cascades, the drying pattern of spring is more linear. Here’s what Seattle’s monthly precipitation looks like. And yes, February is drier because it has fewer days.

Of course, these are only averages. In any one year, we can have wet Aprils and/or dry Mays. But on average, May will generally be the wetter month. 

We still have a few more rain chances before the month of April finishes up. Here’s the expected rainfall for Easter Sunday afternoon’s showers. No heavy rain in our area.  Monday will be similar.

Tuesday through Thursday looks dry with highs in the 60s.  Good days to be outside on a tractor.  Not much wind on Tuesday or Wednesday either.

The weather pattern then shifts a bit by the end of next week, leading to a chance of rain starting Friday and continuing for the next few days to finish out the month.  That's not to say that it's going to rain every day.  It's just too far out to pin down what days it will rain.  But it doesn't look like a real wet pattern either. Here's a 10 day summary of our chances of rain.

Things become even more uncertain when looking at the first few days of May.  The most recent European models run last night would indicate a warming trend to start off May.  But that is a divergence from yesterday's forecast of continuing the showery period.  Too much uncertainty to give us any guidance at this point.  We'll have to wait a few days until the computers can lock in on a predictable weather pattern.