Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 221951

Area Forecast Discussion
National Weather Service Pocatello ID
151 PM MDT Fri Sep 22 2017

.DISCUSSION...A low pressure system positioned over Boise early this
afternoon will gradually fill and weaken as it shifts slowly SE into
NRN Utah Saturday and then ejects slowly NE into Wyoming Sunday. The
NAM and GFS models differ on the exact track of the ejecting low,
particularly Sunday, which will have a fairly dramatic impact on the
forecast across the SE highlands during that time frame. For now,
have extended the Winter Weather Advisory for the central mountains
until 11 PM this evening where the models indicate that we may pick
up an additional 1 to 3 inches above 6000 ft MSL. As the low drifts
SE into NE Nevada late tonight we expect to see at least partial
clearing while colder air infiltrates the region from the north.
Statistical model guidance is indicating freezing temperatures
throughout the Snake River Plain and Eastern Magic Valley by morning
and have thus issued a Freeze Warning for those areas. Due to the
discrepancy in the track of the low ejecting NE into Wyoming
Saturday night and Sunday, we have maintained the higher
precipitation probability and potential accumulations across the SE
highlands. We will have to take a look at a potential Winter Weather
Advisory for that region if we see the models consolidate toward the
northerly and wetter track of the NAM. More on that later. Beyond
Sunday, the models show a ridge building along the coast as a weak
Pacific disturbance tops the ridge and digs SE along the continental
divide Monday night/Tuesday producing a low end chance of showers
along the Montana divide through that time frame. By Wednesday, the
GFS has the ridge building inland with warmer and drier conditions
into the weekend. The ECMWF on the other hand wants to develop a
retrograding cut-off low from the passing wave Tuesday that sinks
into Nevada and later ejects NE into Wyoming Saturday providing at
least a low end chance of showers across the SE highlands. We are
not giving the ECMWF much weight at this time but will maintain some
"chicken-pops" across the SE mountains until we see some resolution
among the models either way. Daytime highs remain cool-ish through
Monday and then begin to gradually warm through the week. Huston

.AVIATION...Expect rain to continue to taper off this afternoon at
all sites with DIJ the most likely to continue this evening.  Expect
the MVFR ceilings to improve at SUN...PIH and IDA while continue the
marginal ceilings at DIJ.   BYI should remain VFR.  Expect mainly
dry conditions at all sites and remaining VFR but could see
afternoon showers and marginal ceilings at SUN and DIJ Saturday
afternoon.   Winds not expected to be strong but up valley 10 to 15
knot winds possible at IDA and PIH again on Saturday afternoon.  GK

.FIRE WEATHER...No fire weather concerns for several days with fuels
now non critical after several days of cold temperatures, high
humidity and abundant precipitation.   Could see a few showers
continuing through Sunday mainly in high elevations with a drying
and warming trend expected next week.  High temperatures will warm
to seasonal normal for late September by Tuesday and are expected to
rise to well above normal by next Thursday.  Expect mainly dry
conditions Monday through next week. GK

Winter Weather Advisory until 11 PM MDT this evening for IDZ018-019-

Freeze Warning late tonight through 9 AM MDT Saturday for IDZ017-020-

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