Scientific Forecaster Discussion
fxus65 kslc 181052
Area forecast discussion
National Weather Service Salt Lake City Utah
452 am MDT Wed Oct 18 2017
Synopsis...a warm airmass will remain in place through Thursday. A
weak disturbance will lift across Utah late tonight into tomorrow,
followed by a stronger and colder storm system on Friday.
Short term (through 12z saturday)...a mean zonal flow pattern is in
place this morning, as the ridge that had been over the area was
flattened by a trough which crossed the Canadian rockies last
evening. The airmass mixed a bit better yesterday ahead of this
trough, and with the zonal flow today, temperatures today should
remain fairly warm and above normal except perhaps in the Great Salt
Lake Desert where the airmass has remained more strongly inverted.
Upstream, a weak shortwave disturbance is noted off the
central/Southern California coast, and this is expected to come
inland today and then lift northeast through Utah late tonight
through tomorrow as the pattern amplifies. This wave will be
accompanied by some moisture which models indicate could produce
some light precipitation over Utah. Given that the dry airmass
currently in place would seem difficult to overcome, have made very
little changes to the pop forecast, leaving just some small pops
mainly over the higher terrain.
The previously mentioned amplification of the pattern is due to a
trough that will be deepening off the Pacific northwest coast.
Southwesterly flow aloft will begin to strengthen tomorrow afternoon
which would help bring increased mixing and further warming to many
valleys. The trough will then push a fast-moving cold front through
the area Friday, first through northwest Utah late Friday morning,
then into southern Utah late Friday afternoon and into the evening
hours. Precipitation is expected to be confined mostly to northern
Utah as the best energy will remain near and north of the Idaho
border. With the fast-moving nature of the front and thus the
frontal band, precipitation is not expected to be significant or
long-lived. The airmass should be cold enough to produce snow down
to some of the higher valleys such as the Wasatch back and southwest
Wyoming, but precipitation may end before the colder air arrives
Long term (after 12z saturday)...behind the departing upper level
trough feature, rising heights punch into the Great Basin from the
west late Saturday. Fairly strong warm air advection also takes
place, as the airmass modifies with 700mb temperatures near -8c
Saturday morning to near 0c Saturday evening. Northwest flow aloft
helps to usher in this warm air advection behind the front.
Temperatures will begin a warming trend Sunday as heights rise some
10-12 dam from Saturday to Sunday, returning back to seasonal normal
for most locations in the area. Extended models are in agreement
with a fairly dry airmass building over the Great Basin through
early next week, aside from some high clouds in the area
precipitation chances look very low through early next week.
Aviation...no operational weather concerns for the kslc terminal
through the valid taf period. High clouds increasing through the day
from the south should not pose any operational impacts. Southeast
winds expected to switch to the northwest between 17z and 18z.
Fire weather...a mostly dry, warm, and stable airmass will remain
in place today. A weak weather disturbance will lift northeast
across the area late tonight through tomorrow which will bring an
increase in moisture to the area. Humidity recoveries tonight are
expected to be much improved across many areas, and afternoon relative humidity
tomorrow will trend higher as well. Light rain showers with snow
above around 10000-11000 ft may be possible with the disturbance,
mainly over the mountains, but wetting rain is not likely.
Winds will start to increase again from the southwest tomorrow
afternoon ahead of the next approaching storm system. This storm
will push a cold front through the fire district late Friday morning
through Friday evening. Precipitation will be limited primarily to
the northern half or third of Utah with rain in the lower valleys
and snow in the mountains and some higher valleys. Precipitation
amounts are not expected to be significant, with the best chance of
wetting rain near the Idaho border. Much cooler temperatures and
higher humidities will be left in the storm's wake on Saturday. High
pressure will then rebound through at least the first half of next
week allowing for a warmer, drier, and more stable airmass once
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