Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 190908

Area Forecast Discussion
National Weather Service Pocatello ID
208 AM MST Mon Feb 19 2018

.SHORT TERM...Today through Wednesday night. An upper trough
continues to deepen over the West. The strongest dynamics have
shifted to the south into Utah, but models are bringing a
disturbance into southeast Idaho this afternoon/evening. This
upper disturbance will couple with surface convergence south and
west of a line from Mackay to Pocatello. Models are in agreement
on the development of this feature, but show a bit of uncertainty
in placement. GFS is placing this a bit further west than the NAM.
The convergence will be the focus of the heaviest snowfall, which
may impact the city of Pocatello, American Falls, and the
Interstate 86 corridor towards Raft River. Snowfall amounts should
range from 2 to 4 inches tomorrow afternoon/evening with this
system. The upper forcing mechanism will shift eastward by 06Z
tomorrow night.

Another concern for this morning will be the cold wind chill
temperatures expected over the Upper Snake Highlands. Wind chills
will drop to about -20 to -25 this morning, but should be confined
to areas north of Dubois including Monida pass and the Island Park
area. Cold temperatures will continue through much of the week,
though the lighter winds will keep wind chill extremes out of the
picture. Models bring in an upper trough to the area on Wednesday,
but precip amounts will be light. Hinsberger

.LONG TERM...Thursday through next Monday. The GFS, ECMWF and
ensembles were in fair agreement through Friday with increasing
variability among the solution thereafter resulting in decreasing
confidence for next weekend and early next week. For now, The models
showed a poorly organized trough advancing through the region
Thursday and Thursday night resulting in a chance mention of
snowfall across the region with no real changes made to the going
forecast. By Friday afternoon, the trough axis advanced through the
region with both models showing drier conditions across the area
through Friday night and thus nudged the forecast drier as a result.
By Saturday the operational GFS and ECMWF were showing a fairly well
organized trough and associated snowfall advancing through the NW
states into SE Idaho although the GFS was a little more progressive
in this regard. Thus bumped the precipitation potential upwards as a
result of the modest agreement between the operational models. By
Sunday, the GFS and ECMWF were showing differences that continued to
amplify heading into Monday. The ECMWF primarily featuring a trough
settling south across the region and the GFS favoring a low off the
coast with SW flow over the area. Split the difference here,
maintaining a chance mention of snow across the region until the
models come into alignment. Regarding temperatures, the models were
trying to recover from the colder temperatures early in the week
with a general warming trend approaching climatology towards the end
of the forecast period. Huston

.AVIATION...A broad upper trough was gradually advancing through the
region this morning with mid-level VFR CIGS streaming NE across the
SE mountains and SCT conditions within a large portion of the Snake
River Plain and CNTRL mountains. The models appear to be keying on a
back-door arctic front spilling over the divide and down the Snake
River Plain to KBYI this afternoon and/or evening as the upper
trough continues to advance through the region. The models appear to
be struggling on the timing, strength and progression of the
resulting low level wind convergence zone and associated snowfall
and lower CIGS (IFR/LIFR). Expect the timing issues to firm up later
this morning as the area of convergence takes shape within the Snake
Plain. For now, the model time sections show the lower CIGS starting
at KIDA and drifting south to KPIH by mid-morning and then short of
KBYI late this afternoon with lower CIG impacts nonetheless. KSUN may
see some light snow but otherwise remain mostly VFR throughout the
period. Model time sections at KDIJ show a mixed bag with mid and
upper CIGS continuing throughout the forecast period with occasional
MVFR CIGS in showers today. Huston



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