Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 171110

Area Forecast Discussion
National Weather Service Pocatello ID
410 AM MST Sat Nov 17 2018

.SHORT TERM...Today Through Tuesday Afternoon.


A general east-to-west band of snow with embedded valley rain at the
onset will continue to sink south this morning, exiting most of the
Snake River Plain by shortly after daybreak, and continuing across
the Southern Highlands, Cache Valley, Bear River Range, Bear Lake,
and the Caribou Highlands over the course of the morning hours. A
dusting to 1.5 inches of snow accumulation is expected as this band
sinks south, greatest in mntn locales. Some patchy fog and low
stratus is possible in the wake of the main band, particularly this
morning. Guidance continues to be quite progressive with pushing
this activity SE out of our area, and radar trends seem to be
supporting this idea. By this afternoon precip should end across SE
Idaho with gradually clearing skies. The exception may be down over
the Bear Lake region, where we hold on to a chance of some light
additional nuisance snow showers through late afternoon.

From the US-20 corridor across the Upper Snake Highlands down into
the I-15 corridor, Snake Plain, Arco Desert/Craters of the Moon, and
eastern Magic Valley, road temps from ITD have been running about 28-
34 degrees F overnight as the snow has been moving through, and
webcams across the region confirm this has kept at least all of the
major highways predominantly wet. Working into the eastern
Highlands, however, lower road temps have supported development of
slick conditions, particularly in the Teton Valley, Palisades/Swan
Valley, and Caribou Highlands. Anyone traveling across this region
this morning will need to plan for slick roadways and take the usual
precautions. Colder air is also filtering into the region from the
north as a cold front works in. This may cause wet and snow-covered
roads, particularly untreated ones, to freeze up and become more icy
this morning, again with greatest impacts in the eastern Highlands.

That cold front will continue to push south across the forecast area
throughout the day, with high pressure quickly building back into SE
Idaho in its wake starting overnight tonight. This will signal a
return to clear/sunny skies and dry weather that will continue Mon
and Tues as well. Our main attention during this period turns to the
potential for very cold morning low temps. NAM soundings illustrate
strong inversion conditions both Sun AM and Mon AM, where surface
temps in the lower elevations/valleys drop significantly as the
lower atmosphere decouples from the regional environmental flow,
with relatively warmer air located just 500 feet off the surface.
Supporting the potential for significant surface radiational cooling
will be clear skies and very light surface winds, generally 5 MPH or
less. Model guidance including the blends continue to run far too
warm for low temps, especially considering the above anticipated
conditions. MAV/MET guidance seemed to be closer to reality over the
past week at most locations, so again nudged the forecast toward
this guidance. The MAV is coming in 5-10 degrees warmer than the MET
by Mon morning, but both were still below our current forecast, and
we pushed lows down generally in between the two. This results in
low temps approaching zero in normally colder locales in our mntn
ranges and across the Island Park/Teton Valley region, with teens
expected elsewhere, both Sun and Mon morning. Soundings also hint at
near-saturated surface conditions, and given the recent precip, went
ahead and added patchy fog to the grids for all of our valleys and
the Snake Plain both mornings as well. Some more widespread areas of
fog may develop, but hard to pin down locations that may be impacted
at this time. Temps will slowly moderate as we head into the
upcoming holiday week. - KSmith/RS

.LONG TERM...Tuesday Evening Through Next Friday.
The high pressure ridge will persist into Wednesday night. Models
are showing moisture off the Pacific increasing at a little
quicker pace by Thursday morning, leading to a chance of rain/snow
across the area during the day. Models are in fairly good
agreement that a busy pattern will result in a disturbance moving
through about every 18-24 hours beginning Thursday morning and
ending Sunday evening. This could well be the first good shot at
snow accumulation in the mountains for the season. It could add up
to a foot or more over 3-4 days. These are very preliminary
numbers. The valley floors will be more difficult to estimate
given some fluctuation in snow levels and daytime temps. - RS


Back door front moving quickly through southeastern Idaho this
morning, and will probably clear KPIH about 13Z and by 18Z be
south of the state. Patchy fog may be a problem Sunday morning,
otherwise high pressure ridge will again dominate with a few
clouds sliding along the Divide. - RS




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