Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 041045

Area Forecast Discussion
National Weather Service Pocatello ID
345 AM MST Mon Dec 4 2017

.SHORT TERM...Today through Wed night. Storm should wrap up this
morning with only a fraction of an inch left for areas east of the
Snake River/I-15. The skies aloft will clear, and the cold air
mass left in its wake means that temperatures will struggle to get
into the 30s for highs. And nearly clear skies aloft tonight means
a definite taste of winter--expect below zero in the alpine basins
in the mountains, and solidly teens in the Snake River plain. The
northerly airflow will continue to refresh the cold air mass, and
Tue will continue to be cold with temperatures trending cooler
from Today to Tue. A weaker flow on Wed will allow the limited
sunshine to start a warm-up, but with one caveat. The risk of fog
does not appear as bad as it does for tonight, but it is still a
risk. If it comes in more dense than forecast on Tue night, it
could remain a problem for Wed night as well, once it is trapped
under the stable and stagnant high pressure. Much depends on human
activity and the direction of the wind. The potential for low
visibility and black ice will be the main forecast concerns, and
it will be mainly limited to valleys. However, the snow blowers
can be put away for at least a little while as the blocking high
continues into the extended forecast period.


.LONG TERM...Thursday Through Monday. The long-term period will
be dominated by a ridge of high pressure setting up along the
Pacific Coast (what we call a Rex Block), providing an extended
run of dry weather across southeast Idaho. While there are some
differences in exact details, the GFS/ECMWF/Canadian models all
show good agreement at this time on this general setup and
associated dry weather, leading to at least moderate forecast
confidence in Thurs-Mon being dry and partly to mostly sunny for
everyone. Made some minor tweaks to cloud cover, temperatures, and
winds throughout the period based on the latest model blends, but
this did not result in any major changes to the going forecast.
In fact, long-range models are currently not showing another storm
system approaching our area until around Dec 13th! This pattern
does not favor progressive warming, so high temps should improve a
degree or two each day through next Mon, even by that time
generally remaining in the 30s. We may have periodic low
status/fog issues to contend with in some of our valley locales,
but difficult to pin this down so far out. - KSmith/Messick


.AVIATION...Snow from our departing storm system/associated Snake
Plain convergence event has cleared all of our TAF sites early
this AM except KDIJ, where reductions in vis to MVFR will remain
possible for the next few hours before all precip comes to an end.
Our attention then turns to low stratus/fog potential. KSUN
should fare fine unless the diurnal shift to southerly winds is
able to advect some fog/stratus north from the Snake Plain Mon
morning (if it develops), but computer guidance continues to offer
mixed signals pointing toward low stratus possibly impacting the
remainder of our terminals later this morning (MVFR possible), and
perhaps a slightly better chance of fog/low stratus late tonight
into Tues morning (with both cigs and vsbys possibly dropping to
IFR). Confidence in fog/stratus is quite low at this
time...continue to monitor the forecast. Gusty winds this morning
will gradually taper off throughout the day. - KSmith/Messick




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