April is the Month of Drying

In western Washington and Oregon, April is the month of drying.

And the same is true of much of the West Coast.

This may sound strange considering the popular mention of April showers in song and verse.

Consider the climatological probability of receiving at least 0.01 inch of precipitation at Seattle Tacoma Airport (below).  

A large drop between April 1 and May 1.  By far, this is the largest one-month decline of the year.  By the way,  .01 inch of liquid water is the criterion for measurable precipitation.

The probability of precipitation is relatively steady from mid-November to late March (50-60%), but declines to approximately 38% in late April.  

On the Washington coast (Clearwater), the situation is similar .... a big drop from the winter wet plateau to drier conditions in April (the probability of measurable rain dropping from  70% to 50%).

What about San Francisco further down the coast? 

Also a big drop in the probability of precipitation in April---but an equally large decline occurs during March.   The drying starts earlier in San Francisco.

To further illustrate the profound drying in April, below is a plot of the average precipitation along the West Coast in March and May.

Wow.   Huge decline everywhere.  Precipitation essentially goes away for Southern California during April.   LA beach season has begun!

So the question you must be asking is why?   Why is there a big drying in April over the West Coast? 

Significant changes occur in the atmosphere between the two months.  

Consider sea level pressure.  In March, there is strong low pressure over the Gulf of Alaska and high pressure west of Mexico.   This pattern is associated with strong westerly (from the west) winds in the lower atmosphere that bring storms and moisture to the West Coast.

The same map (sea-level pressure) for May is very different.  High pressure has pushed up the West Coast.  With sinking air to the east of highs, that means that much of the coastal region is under descending air...which is inherently dry.

By May, a thermal trough of low pressure has moved northward up the interior... the classic summertime pattern.    And with high pressure moving northward and low pressure to the north retreating, the jet stream is weakening and storms are deflected northward.  

Finally, a little irony.   Even though April is a drying month, this April will start with a LOT of precipitation (see the totals through Saturday morning).  Two to three inches of liquid water in some of the mountains!