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fxus65 kpih 140419 
afdpih

Area forecast discussion
National Weather Service Pocatello Idaho
919 PM MST Wed Dec 13 2017

Update...updates this evening include capturing current clouds
and fog trends. Snow this evening is quickly ending except for a
few pockets east of I-15. Roads are improving from what snow fell
earlier but still look for slick spots. As the clouds are
thinning, the stratus and fog is making a run up the snake plain
as expected. That trend continues overnight with some additional
development expected along the Utah border in such places as the
Cache Valley, as well as the Stanley Basin in the central
mountains. We did up lows a bit in some places based on current
dewpoint trends. For now, we will lay off issuing any fog-related
headlines but that could easily change. Keyes

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Previous discussion... /issued 140 PM MST Wed Dec 13 2017/

Short term...tonight through Saturday. Satellite imagery was
highlighting a modest disturbance shearing southeast through the region
this afternoon. Radar returns suggest the leading portion of the
disturbance was already working into the southeast mountains with a moist
northwest flow expected over the area through the early evening hours. Have
continued a high probability of snowfall over the southeast Highlands early
this evening allowing things to diminish rapidly after 8pm. As skies
clear partially tonight, I am a bit concerned that we may develop
dense fog within the valleys. Not seeing a huge injection of
moisture into the lower layers of the Snake River Plain at this
point so we will hold off on any advisories for now and advise
following shift of concerns. Upper ridge amplifies along the coast
late tonight and Thursday keeping valley inversions going. The next
Pacific disturbance shears inland Friday night and continues through
Saturday morning before the trough axis shifts southeast of the area
Saturday evening ushering in some drying. Storm total snowfall from
this system is expected to range less than an inch in the snake
plain, 1-3" in the central mountains, and upwards of 3-6" over the eastern
mountains. Not enough at this point to warrant any highlights. There
may be enough mixing with this system to temporarily break down the
inversion Saturday but the ridge rebounds along the coast Sunday and
we may very well find ourselves right back in the thick of it. Looks
like we are transitioning into an active flow regime with waves
breaking over the West Coast ridge and thus have begun to trend more
purposefully towards statistical guidance. Huston

Long term...Saturday evening through Wednesday...long term forecast
models have not been consistent run-to-run regarding the timing of
the next several weather systems. As it stands now, snow showers
will be tapering off Saturday evening in eastern Idaho behind the
passage of an upper trough with north to northwest flow behind it.
Much colder morning temperatures are expected for Sunday and Monday
mornings now, with single digit to temperatures in the teens each
morning. The shortwave that was progressing southeastward through
the state Tuesday is showing less and less emphasis attm, with
little impacts to southeast Idaho. Potentially another weather system will
arrive in the middle portion of the week. Np/mh

Aviation...some patchy areas of fog linger early this afternoon;
with some weak snow showers also possible through the early evening
hours as well at pih, Ida and dij. There remains potential for fog
redevelopment allowing for IFR/LIFR ceilings/visibility across byi,
pih and Ida tonight once again under light winds and the inversions
in place. Np/mh

Air stagnation...inversions continue to hold and reduce air
quality, particularly for Franklin County where an air quality alter
remains in effect. Degraded air quality is expected to continue
through early Friday with possible improvements late Friday
afternoon into Saturday. Huston

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