Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 241417

Area Forecast Discussion
National Weather Service Pocatello ID
817 AM MDT Sat Mar 24 2018

.SHORT TERM...Saturday through Monday night. A quick update this
morning to boost precip potential across the Snake Plain and ERN
mountains based on current radar trends. See previous discussion
below. Huston

A strong low pressure disturbance continues to move southward just
off the Oregon coast. This morning there is a frontal band near
Jerome that will progress across our forecast area today, making for
some breezy winds in the snake Plain. Precipitation becomes focused
on the Eastern Highlands this afternoon with snow levels near 4500-
4600 feet elevation. some 1 to 3 inch accumulations expected in the
South Central and Eastern Highlands. Valley temperatures cooler
today, mostly lower to middle 40s. Sunday, the main low moves inland
with the southern end tracking a little further south into the Great
Basin. Everyone gets a chance at some precipitation, colder with
highs in the middle 30s to near 40F. Mostly snow showers with
accumulation of 1 to 3 inches in the mountains and less than an inch
in the lower valleys. There is a rather cold pool of air aloft with
the passing low, temperature lapse rates very steep that will
support some thunderstorm development sunday afternoon.
Thunderstorms with snow showers could locally produce some higher
snow amounts of around 5 inches. This disturbance moves east of the
state Monday night with precipitation ending. The colder
temperatures will serve to minimize additional snow melt and runoff.

.LONG TERM...Tuesday Through Next Saturday.
All indications point to next week being a period of benign, low-
impact weather across SE Idaho. On the large scale, GFS/ECMWF both
advertise a pronounced/amplified trough pushing east away from our
forecast area early in the week, with surface to mid-level high
pressure setting up over the eastern Pacific and some weak attendant
ridging trying to work into our area from the west. This will place
SE Idaho in a broad NW flow regime between these two features. By
the end of the week, the ridge breaks down and the trough over the
eastern US de-amplifies, flattening the country into a largely zonal
west-east flow. This pattern may open us up to more pronounced storm
systems trying to move in beyond the end of the forecast period, as
hinted at by the 8-14 day CPC precip outlook. Significant temp
advection appears unlikely, but we do expect a slow warming trend
all week with most areas improving a few degrees each day. We may
approach the 60 degree mark by Fri in the eastern Magic Valley!

Coming down to the surface and stepping through the details, Tues
should be dry and partly sunny on the backside of our weekend system
with that ridging nudging in from the west. Tues eve-Wed eve, the
long-range models continue to indicate a stream of moisture and
perhaps a very weak disturbance embedded in the NW flow pushing SE
into ID, MT, and WY. GFS/EC show good general agreement with this
activity and have been trending slightly wetter for the last couple
days. Massaged the PoP grids and nudged values slightly higher based
on a blend of the GFS/EC/WPC guidance, supporting (mostly) snow
shower activity in our mntns but keeping decent portions of the
Snake River Plain and eastern Magic Valley dry. This activity looks
very light at this time with QPF/snow totals of a nuisance variety.
Forecast confidence is moderate to high through Thurs AM.

Confidence remains low Thurs-Sat. Generally, the models push NW flow
moisture further east, drying us out. However, some hints of shower
activity remain in the guidance, especially in the Island Park
region and along the WY border. Highest model-generated PoPs barely
trigger slight chance showers in the grids, so we continue to keep
the forecast dry at this time. Either way, any activity would be
very light. Warm temps will likely invoke a snowmelt response, but a
lack of precip should help keep impacts in check. - KSmith/RS

Scattered rain and snow showers are expected today/tonight across
the region. The NAM and GFS are much wetter than the latest runs of
some of the higher resolution short-term models such as the RAP and
HRRR, so our forecast confidence is not great with regard to precip
coverage. Best timing appears to be through early afternoon at KSUN,
through eve at KBYI, late AM and afternoon at KPIH/KIDA, and
afternoon/tonight at KDIJ. Really not seeing indications in the
guidance of significantly lowering cigs or vsbys at KSUN or the
Snake Plain terminals, so expecting primarily VFR with brief MVFR
possible during active precip. KDIJ has a better chance of dropping
into MVFR or IFR conditions for both cig and vsbys this afternoon,
especially with temps supporting all snow. Afternoon winds will be
breezy at the Snake Plain terminals. - KSmith/RS



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