Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 182007

Area Forecast Discussion
National Weather Service Pocatello ID
207 PM MDT Thu Apr 18 2019

.SHORT TERM...Tonight through Sun afternoon. Upper level ridge
will last for just about another day, with two lows merging on the
west coast on Fri night to put an end to the sunny weather. The
first low is actually a merger of two lows in the eastern Pacific
off California, and the second is more a trough that will move
through southern Canada and northern Montana. They will then move
in synch into Idaho, bring a deep fetch of southerly air. This air
is subtropical and unstable, and as it clouds up for Sat, there is
not much cool-down in temperature. Snow levels start in excess of
10000 feet on Friday afternoon, and very little snow is expected
below 8500 feet elevation. Another 10 deg F of cooling occurs from
Sat to Sun. The colder air on the warm ground will become unstable
during the afternoons, with a thunderstorm threat at first in the
northwest half of the forecast area, then on Sun over almost the
entire forecast area. Outside of thunderstorms, no significant
wind is expected. The rainfall forecast has become heavier,
especially in the central Idaho mountains. So there still is a
threat for flooding due to rain and snow melt. See the hydrology
section for further details. Messick

.LONG TERM...Sunday night through next Thursday. Upper level trough
expected to lie over our region late Sunday with persistent showers
across most of the region Sunday evening. There is some uncertainty
as to how much wrap around moisture will make it into Central and
northern Idaho; therefore these areas have the best chance to dry
out by the evening and overnight hours going into Monday. Models are
still indicating the presense of moisture through Monday morning and
for Southeastern Idaho, through the afternoon. Gradually a high
pressure ridge will build in for the middle portion of the week with
temperatures back into the 70s. There is a potential new system
toward the end of the week, arriving by Thursday. NP


.AVIATION...Primary concern over the next 24 hours will be potential
fog development at IDA and PIH early tomorrow morning. It is looking
fairly likely at IDA with the possibility at PIH. The fog will be
close, but unsure if it will reach the terminal or not. For now we
will forecast a VCFG and still drop the CIGS out of precaution.
Reminder of the sites will be VFR throughout the period. NP


.HYDROLOGY...Runoff from snowmelt will be of concern later this week
as temperatures climb. Highs on Thursday will be in the 60s with
widespread 70 degree temperatures on Friday. The Portneuf River is
forecast to approach bankfull late this weekend with localized
flooding impacts possible. Impacts from flows from the Big Wood
River at Hailey are possible by the weekend. Increased flows on the
Little Wood River are anticipated as well.





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