Saturday, May 26, 2018

Believe it or Not! May is the Wettest Month in Many Parts of the U.S. and the Driest in Others!

To a Northwestener accustomed to November to January being the wettest time of the year, it may seem awfully strange, but for many locations May is the wettest month of the year.    And for other locations May is the driest month of the year.

To appreciate this interesting climatological quirk, here is a wonderful graphic by climatologist Brian Brettschneider  showing the month of most precipitation around the U.S. (below).  (Brian makes some of the most interesting climatological graphics and lives in Alaska.)

Sure enough, there are wide swaths of the U.S. for which May is the wettest month: most importantly, northern Nevada into Wyoming and from Oklahoma to Ohio.     In these locations, spring thunderstorms are more important than midwinter cyclones and fronts.  Spring can be an unstable time of the year, with a strong sun warming the surface, while the atmosphere aloft is still relatively cool.  The produces a large change of temperature with height (lapse rate).And there are still enough passing disturbances to help initiate the storms.

In contrast, some locations have their driest month in May.    Like southern Arizona.  Why?   Arizona gets some rain from winter weather disturbances and lots of rain from the summer "southwest" monsoon from late June to September.  May is kind of a dead period for precipitation.

Another dry May location includes Georgia and the panhandle of Florida.  Take Tallahassee (see below).  Tallahassee gets some winter rain from fronts and cyclones, but MUCH more in midsummer from convection (thunderstorms) due to the huge amount of moisture coming off the Gulf.  Thus, spring and fall have less precipitation, with May barely edging out October.

 May is also a dry month in Alaska, with the winter storms ended.    "Autumn" fronts and cyclones move in early there (starting in August).    Thus, May is a very good time for an Alaskan cruise.  And also fairly dry on the Big Island of Hawaii.  Go to both if you can afford it.

To put this all into context, here is the average precipitation for May for the lower 48 states.  Some places get 6-8 inches (Arkansas, Tennessee) from thunderstorms, while parts of the southwest get less than a tenth of an inch.



Think about how weird it would be to live in a place like the region northwest of Atlanta, right on the edge of a huge change in time of year of heaviest rainfall.  Drive a hour or so one way (SE) , May is the driest month.   Go the opposite direction (NW) a hundred miles or so, May is the wettest month.  Think of the weather tourism possibilities.


2 comments:

RLL said...

Another advantage to a May cruise to Alaska (or BC or Alaskan ferries) is the long days.

jimijr said...

Is that a Big Red Dot over Dry Tortugas? Here in KEYW we just set a new all-time record for May -- 13+ inches -- and it is still coming thanks to Alberto.