I posted the following on Todd's Yahoo Group mail list. Though you will
find this interesting...
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When I got home last evening [in Ashland, MA], my diligent VantagePro had
plenty of excellent data regarding yesterday AM's warm frontal passage and
resulting ++RA and
winds...
(1) Temp/dewpoint: At 9 AM the temp dewpoint was 54/54, at 10 AM up to
63/63. Remember that dewpoint is an absolute measure of water vapor
content. Quantitatively, water vapor content increased by about 40% over
this hour.
(2) Wind: At 9 AM dominant wind direction was NE with gust max 8 m/h. At
10 AM, the dominant wind direction was SW (180 degree change) with gust max
18 m/h.
(3) Sea level pressu At 8 AM 29.60", 9 AM 29.58", 10 AM 29.52" and 11
AM 29.50". In other words, pressure fell rapidly (.06"/hour) between 9 and
10 AM. (Warmer and wetter air is lighter air...sea level pressure measures
the weight of the column of air atop the instrument)
Comments:
Warm frontal passage between 9 and 10 AM. With introduction of much warmer,
wetter, thus lighter and more buoyant air at/near surface, the airmass
quickly destabilized.* At focus of frontal boundary, NE winds collided with
SW winds and suddenly soppy air parcels had nowhere to go but up. Many
observers commented on calm then sudden heavy rain and winds. The calm was
at the boundary and the ensuing heavy rains were the result of the sudden
lifting of warm. moist air. I did not hear thunder. However, it did occur
in eastern MA.
A "bonus?" of all this was big time gusts; 40+ m/h in Framingham, 65+ at
Blue Hills and suddenly unstable air allowed winds several thousand feet
blowing at/near hurricane force to mix down. (low level jet)..
Here in Ashland, at my wind sheltered location, my peak gust was 32 m/h at
noon. It was no coincidence, that the maximum temp of 67F was reached at
that exact time. Put another way, airmasss was the most unstable at that
time and therefore, most able to allow jetmax winds to mix to the surface,
Jot Ross - Ashland, MA
*Keep in mind that way up there at perhaps 3 1/2 miles or so, it was much
colder and temp either not changing and change very slowly in contrast to
quickly rising temps at/near surface. Thus, rapid destabilizing of airmass!
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