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Old 01-01-2014, 10:43 PM
 
Location: East coast
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I notice that somewhere like Boise, Idaho (Boise, Idaho - Wikipedia, the free encyclopedia) or Las Vegas (Las Vegas - Wikipedia, the free encyclopedia) still have a very "Mediterranean"-like rainfall pattern with most precipitation falling in the cooler half of the year. Inland and eastward you can get a transition into more and more spring/summer rainfall peaks like Butte, Montana (Butte, Montana - Wikipedia, the free encyclopedia) or Denver (Denver - Wikipedia, the free encyclopedia)

What accounts for this. Is it a shift from frontal precipitation being the dominant type in fall/winter/spring to more and more summer convection further east and inland? In the Southwest, I'm guessing the monsoon also comes into play?
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Old 01-02-2014, 07:26 AM
 
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I think it's because of the western mountains creating a rain shadow.
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Old 01-02-2014, 10:50 AM
 
Location: Western Massachusetts
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The Rockies block most moisture from the Gulf of Mexico (mainly a summer source) from reaching west, while it drains the last remaining Pacific Ocean moisture (mainly a winter source) which has already lost moisture several times from passing through a number of mountain ranges to the west.

Boise is also relatively mild winter-wise compared to east of the Rockies.
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Old 01-02-2014, 10:53 AM
 
Location: Western Massachusetts
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this map shows the extent of the winter precipitation max, with the Rockies roughly the boundary of 50% in Oct-Mar. California and western Oregon stand out in having an almost winter only precipitation season.



Precipitation Maps | Western Regional Climate Center
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Old 01-02-2014, 10:56 AM
 
Location: London, UK
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Doesn't an area of high pressure position itself in the SW United States during the summer?
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Old 01-02-2014, 12:32 PM
 
Location: Vernon, British Columbia
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Quote:
Originally Posted by P London View Post
Doesn't an area of high pressure position itself in the SW United States during the summer?
Bingo! The air to the east is more humid and unstable in the summer, and thus gets a lot more convective precipitation in the warm months. These thunder storms spin in a counter-clockwise direction, which means the rain can be coming from any direction.

In the cold months the precipitation comes from storms moving in off the Pacific Ocean (the west). As a result, rain shadows develop with each successive mountain range blocking what remains of the moisture from the previous range.

BTW, where I live just north of the eastern side of Washington State, we get even amounts of precipitation in the cold and warm months. Places to the west (like Vancouver on the wet side of the mountains) get most of their precipitation in the cold months, and locations to the east (like Calgary) get most of their precipitation in the warm months.
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Old 01-24-2015, 12:32 PM
 
Location: Orcutt, CA (Santa Maria Valley)
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http://en.wikipedia.org/wiki/Park_City,_Utah#Climate
http://en.wikipedia.org/wiki/Evansto...ming#Geography

Evanston,WY and Park City,UT are right at the transition zone with a spring and fall precip max.
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Old 04-05-2016, 10:50 AM
 
Location: Downtown Phoenix, AZ
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In the distance between Phoenix and Riverside, CA; summer gets noticeably drier by Blythe, CA; even drier by Indio/Palm Springs, then on the west side of San Gorgonio Pass/Banning Pass, generally rainless summers
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