Seasonal Precipitation Surprises

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You ask any resident of Washington, Oregon, or California:   when is the wettest time of the year?   Without hesitation they will answer:  during the winter.  The Goretex season.

But ask a resident of Montana, particularly the eastern two-thirds of the state, and they would give you a different answer:  May and June!

Or talk to someone from southern Arizona or New Mexico and they will give you an even different answer:  July and August.

Why are there such differences?

First, lets explore the seasonal variation of precipitation a bit.

Here is a fascinating image that give the percentage of the annual rainfall falling from October through March...essentially winter.
Much of the West Coast gets 80% or more of their annual precipitation during this winter period, with the Bay area getting more than 90%.  Winter wet and thus summer dry.  This is what we term a Mediterranean climate.  Residents of the wet NW rarely think they have much in common with the Mediterranean climes, but it is true.  Rick Steves needs to know about this--why send so many folks to Italy and southern France when we have the same climate right here at home!

But look inland: the percentage of annual precipitation during winter drops continuously, and by the time you get to the eastern side of the Rockies much of that region only gets 20-30% of their precipitation during winter.  Winter is the dry time of the year there!
These folks in live in a weird climate!

Now here is the percentage of annual precipitation during Spring (April through June).
Mamma Mia!  Eastern Montana, Wyoming, and the Dakotas get roughly HALF of their precipitation during these three months...this is their wet season.  On the other hand, this is a time of EXTREME aridity in southern Arizona and far southwest CA.   A good time to enjoy some warmth in Tucson with little worry about a cloudburst.
And finally there is midsummer (July and August).   The West Coast gets LESS THAN TEN PERCENT of its annual precipitation then, while southern Arizona and much of New Mexico get roughly half of their annual precipitation.   This is the time of the Southwest Monsoon, when moisture streams up the Gulf of California into the Desert Southwest.  Lots of lightning and thunderstorms.

So how do we explain these differences in annual precipitation?

During the winter the jet stream..the current of westerly (from the west) winds in the midlatitudes, is strong, bringing wet Pacific systems into the West Coast.  But folks to the west of the Rockies are shielded by the mountains...they are on the downslope, rainshadow side of the barrier.

During the spring the jet stream weakens and fewer and weaker storms approach the West Coast.  We start to dry out (yes, there are still lots of clouds and sprinkles).  But the weaker jet stream is associated with increases undulations in the flow, including more cut-off lows and troughs the bring southerly flow, less rainshadowing, and more rain to Montana and vicinity.  Here is an example of such flow (this shows the flow at 500hPa, roughly 18,000 ft, the winds are parallel to the lines).


In midsummer, the jet stream and weather disturbances from off the Pacific are history, but monsoon moisture coming out of Mexico produces thunderstorms and convective showers in Arizona, New Mexico, and parts of Colorado.

Now over the eastern U.S. the situation is even stranger...they get nearly the same amount of precipitation EVERY MONTH OF THE YEAR.  Cyclones and fronts during winter, thunderstorms during the summer.

Think about it: We get our rain over during the winter, when days are short.  Our summers are dry and mild, when days are long.  We are blessed.