I don't think jet streams have much to do with it. There are
two major jet streams which run east to west, the polar and
the subtropical. Both derive their power from the temperature
differentials between the continental region and the polar to
the north or the subtropical to the south. Both are at elevations
of 25,000 to 30,000 feet above the U.S. Both are relatively
inactive in the summer, with the subtropical jet stream being
barely discernible, according to this article from USA Today:
These jet streams are much too high (5-6 miles) to pick up
humidity from the ground, and are the least active in the
summertime, peaking in the winter, when the temperature
differential between the zones is the highest.
The main difference seems to be the temperature of the
oceans on the two coasts during summertime, as noted in
this question answered by Josh Larson of the USATODAY.com
weather team on the following page:
"...the average water temperature off Newport Beach, Calif.,
in the first part of August is 70 degrees while at the same
time the Atlantic Ocean off Myrtle Beach, S.C., averages 83
Now let's see what these big differences in water temperatures
mean for air temperatures. If you go to the our current
conditions and forecasts for Newport Beach and Myrtle Beach,
and then click on the link to 'Monthly Averages' at the bottom
of each city's page, you'll learn the average high and low in
Newport Beach in August are 73 and 64 degrees, while in Myrtle
Beach they are 89 and 70.
We said wind direction makes a big difference. In both places,
winds off the ocean are cooler than those from over the land.
During the summer both places often receive a sea breeze.
Obviously such a breeze will cool Newport Beach more than
Myrtle Beach since the California breeze is blowing over much
See about a quarter of the way down the page:
Think of it as the difference between standing next to a
bathtub full of hot water and cool water, and the difference
in humidity in the two locations becomes more obvious.
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A user's guide on this topic is on skermit-ga's site, here:
Additional information may be found from an exploration of
the links resulting from the Google searches outlined below.
Searches done, via Google:
"jet stream" humidity
humidity "east coast" "west coast" "jet stream"