Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 211952

Area Forecast Discussion
National Weather Service Pocatello ID
152 PM MDT Sat Oct 21 2017

.SHORT TERM...Tonight through Tuesday. Early afternoon satellite
imagery was showing a low pressure system migrating east toward the
NW Canadian coast as a moist Pacific flow rotated into the NW states
and over a shallow ridge creating a broad area of overrunning
precipitation across Washington and NE Oregon. The numerical models
spread this area of warm overrunning precipitation EWRD into the
CNTRL mountains tonight as a modest disturbance ejects from the low
into SRN Canada. The models struggle from time to time on the
placement and strength of these overrunning events so I will try to
avoid swallowing the hook. Model trends from yesterday to today have
shown lighter precipitation amounts and a slight shift northward of
the overall pattern. The aforementioned disturbance shears east
along the Canadian border Sunday resulting in enhanced winds across
the region and prompting a wind advisory for the Snake River Plain.
The moist westerly flow continues over the CNTRL and NE mountains
through the day before gradually tapering off Sunday night through
Monday morning. Snowfall totals across the CNTRL and NE mountains
tonight through Sunday fall below Advisory levels and we have
elected to continue the Special Weather Statement to highlight the
light to modest accumulations in those areas through the Sunday. As
for Monday afternoon and Tuesday, a ridge of high pressure builds
NWRD across California driving the Pacific storm track north into
Canada leaving SE Idaho dry with warming daytime temperatures. Huston

.LONG TERM...Tuesday night through next Saturday night. 500mb long
wave forecast is for a high amplitude eastern Pacific Ridge/northern
mid-west trough pattern. This should tend to keep the forecast area
in dry/cold northwesterly to northerly airflow. And this is seen
even when factoring in the shortwave activity. There is one
shortwave early, Wed night/Thu, that moves from north to south
across central and eastern Idaho. The low comes from central Canada,
so PoPs are minuscule. With Mon and Tue previous to dry out from any
major precipitation, so not see a fog threat. So, except for wind
associated with the passing trough on Wed night/Thu, see very little
potential for a wind event, either. The northerly flow kicks in
strong enough on Fri to actually cool temperatures in the wake of
the low, where the low temperatures continue into Sat. Temperatures
will be closer to normal on the west side of the forecast area,
which is much closer to the upper level ridge axis and thus not so
much in this northerly airflow. Messick

.AVIATION...Incoming front should create quite a mess tonight before
its arrival. Gusty southerly wind will bring warming overnight
temperatures, so for KSUN and KDIJ this may mean a switch from snow
to rain during the overnight hours. The most difficult forecast is
for KDIJ; not sure if the warm air will get there soon enough, since
it is the farthest east. Also, it has the best chance of going IFR,
with KSUN a close second.  At this time, do not believe there should
be any problem with airport closure.  The three valley airdromes
will see rain, with a potential for some marginal VFR, although this
is only certain at KIDA; southerly downslope should help KPIH and
KBYI stay good VFR. Gusty south to southwest wind should peak this
afternoon and become more southwest overnight, with KDIJ the last to
lose the gusty wind. Messick

.FIRE WEATHER...A storm system will bring a threat of rain and snow
to the region for Saturday and Sunday. Expect cooler conditions for
Saturday with breezy conditions continuing through the weekend.
Temperatures will be a little warmer on Sunday as the system
gradually departs the area. Warmer and dry air will return to the
region for the upcoming week. Valle

Wind Advisory from 11 AM to 9 PM Sunday IDZ020-021.

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