Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 180906

Area Forecast Discussion
National Weather Service Pocatello ID
306 AM MDT Wed Apr 18 2018

.SHORT TERM...Today through Fri night. Upper level ridge gets
pumped up today before the closed low that is feeding all the warm
air into the ridge makes its approach. The NAM guidance is very
dry during the first two days. However, the GFS has a faster and
closer approach, and brings more precipitation, mainly to the
mountains during this time. Unsure how much will really develop;
most of the moisture is based above 600mb, which is where the GFS
usually mis-places moisture, so the GFS may have unusually high
PoP for this situation. So have split the difference during this
time. By Fri, the NAM has more moisture, mainly in the southeast
corner, as it has a slower exit of the closed low from the Great
Basin. If either is more correct, the main thing is there is no
major front coming through at this time, and wind will not be a
problem, so there is something of a break from some of the
horrendous weather of late. Warming under the ridge will put
temperatures above normal by Fri. Messick

.LONG TERM...Saturday through next Wednesday night. A Trough of Low
Pressure will quickly sweep Eastward through Washington, the Idaho
Panhandle and Montana on Saturday, with an associated strong
Pressure gradient and attendant surface boundary expected to cross
SE Idaho. While the bulk of the Precipitation is expected to remain
confined to the Idaho Panhandle, Breezy conditions will once again
develop, strongest across the Eastern Magic Valley and Snake Plain
for Saturday Afternoon/early Evening. While the Wind is expected to
be less than the past couple of events, some difficult driving
conditions and chop on area waterways will develop. The next Trough
of Low Pressure is forecast to quickly develop across the PacNW on
Sunday followed by a trip into Idaho by late in the day. GFS and
Canadian forecast guidance is nearly in lock-step in terms of the
location and timing of associated Precipitation from Sun Evening
through Tue Morning, while the ECMWF lags by about 12 hours. Took
the middle of the road approach, which has a fair amount of support
from GFS Ensembles -- which boosted forecast confidence enough to
increase Shower and Isolated Thunderstorm potential across our area
beginning Sunday Evening. The Trough then shows signs of slowing as
it lumbers East into Wyoming, which would result in Shower chances
lingering into Monday, with the best chances along the Wyoming
border. Increased Precipitation potential and geographical coverage
as well Monday as a result of the slower trends.  Nudged
Temperatures downward on the back side of this system Tue-Wed and
increased Northerly Winds, particularly on Tuesday. Fair amount of
confidence in a drier and warmer trend developing for Wednesday
with a decrease in Wind. AD/Messick


.AVIATION...Aviation conditions forecast to continue to evolve
towards more tranquil conditions through the period with decreasing
winds and VFR conditions all areas, with the exception of DIJ.
Potential remains for MVFR Stratus through about 18Z this morning.
However, observational trends suggest this Stratus is a bit higher
than modeled, and have since amended the forecast in the direction
of observations. Feel fairly comfortable VFR Stratus will remain in
place, but do know there is that potential for at least a brief
period of MVFR Stratus -- as mentioned through about 18Z. AD/Messick



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