The Wettest Month of the Year

Brian Brettschnieder is the state climatologist of Alaska.  He has created numerous outstanding graphics over the years. You can find his blog at https://us-climate.blogspot.com/ as well as on social media.

One of the more fascinating graphs that Brian has created is that of the wettest month of the year for locations across the U.S. based on climate normals.  The image below has so many interesting details that it would take a while to discuss them all.  But I'll point out a few.

• In general, the wettest month in the West is during winter, while east of the Rockies the wettest month is during the summer.  But there are several exceptions.
• The Deep South has a strip of area from Louisiana to Georgia where the wettest month is January or February (blue shades), the only area east of the Rockies with a winter maxima.
• Alaska's wettest time of year is late summer and fall.
• Eastern Utah and western Colorado have their wettest month in the fall, which is typically the driest time of year in most locations.

In the Northwest there are also interesting variations.

• Almost all of Washington, Oregon and north Idaho have their wettest month in either November or December.
• But just to the east, nearly the entire state of Montana has its wettest month in June.  Notice that the Idaho/Montana border has a change from November (north Idaho) to June (Montana) rather than having a gradual transition gradient.
• Where the Washington, Oregon and Idaho borders all meet, the wettest month is actually May.  Again, this area is surrounded locations with November or December as the wettest month.

The reason for these oddities in the Northwest are due to terrain.  The Bitterroot Mountains in the Idaho Panhandle keep much of western Montana in a precipitation shadow, as the wind generally comes from west to east.  So Montana's wettest month isn't until late spring, when the westerly winds are much weaker and often come from the east.

A similar topographic feature explains the spring maxima for the Lewiston-Clarkston area.  The Blue Mountains in northeast Oregon extend into southeast Washington.  These mountains cause a precipitation shadow for the L-C valley during the winter months when the predominate wind direction is from the southwest.

Despite being located in the rain-shadow lee of the Cascades, Wenatchee still has it's wettest months in the winter.  But it clearly has a secondary maxima in May.

Spokane used to have a similar looking graph.  But some very wet March's in the last 10 or so years have made their graph a bit messier.  One clear signal is still that November (not January) is the 2nd wettest month of the year, which surprises some folks.

In Lewiston, you can clearly see the wintertime precipitation shadow .  May is easily the wettest month of the year in the L-C valley, followed by April and then March.

While we're at it, is there any trend in the precipitation for the Inland Northwest?  The graph below shows the total annual precipitation at Spokane (blue line) and a smoothed average precipitation (green line).  You can clearly see that Spokane was rather wet in the late 1800s, which was then followed by the very dry period in the 1920s and 30s.  After recovering from this during the mid 20th century, precipitation remained fairly stable for the rest of the century.  But over the 21st century, a slight decline in precipitation has been noted.

For our upcoming weather, we're currently in a cold snap with widespread cold low pressure over the western US.

But warmer high pressure will build into our area by the middle of the week.

But it doesn't last long as another cool Pacific wave sweeps through the area by the end of the week.  The European models were the first to come up with this idea.  The U.S. models have just come in line with this thinking on their Saturday computer runs.

This pattern looks to repeat itself during the following week (Apr 14-20), with a few warm and dry days followed by a cooler, wetter period.

As I've said before, spring is the longest season in the Inland Northwest.  Summer lasts only a couple of months, from about the 4th of July to Labor Day.  Autumn is also a shorter season, lasting from Labor Day until about mid-November.  Winter, despite what some may insist, only really lasts from about Thanksgiving to President's Day.  Yes, some winters start in early November, and some last well into March.  But on average, it's only a 3 month season.  Spring, on the other hand, is a long slow climb out of winter, and lasts from late February all the way through June, more than 4 months.  Some June's can be rather summer-like, but most are late spring in the Inland NW.