Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 190921

Area Forecast Discussion
National Weather Service Pocatello ID
221 AM MST Tue Jan 19 2021

.SHORT TERM...Today through Thu night. Upper level ridge
strengthens over the southern Gem State with the axis directly
overhead by this evening. Northerly air flow with the west-east
oriented axis will continue the cold temperatures tonight. The
axis shifts south on Wed, shutting off the bitter cold air as
airflow becomes more west northwesterly. Expect a warming trend
from Wed to Thu, but both the magnitude of cooling from today to
Wed and Wed to Thu is under some debate, with not much consensus
in the changes. NAM is tending warmer than the GFS, and the NBM is
colder for highs, warmer for lows. This might be due to cloud
cover being overdone, so have trended towards the MOS guidance
with the NBM as the baseline. By Thu night, the next low is
pushing into southern Idaho and bringing precipitation, maybe
actually as early as Thu afternoon, but it is certainly stronger
by Thu evening. Airflow is more zonal, turning to southwesterly at
the end of the night and the forecast area still in the
overrunning portion of the low. Probabilities for precipitation
are still fairly low, staying slight chance and chance levels even
by the end of the night. Wind continues fairly light during this
period, and the very dry surface air should limit the amount of
fog and stratus the next couple of nights; as the cold air entered
tonight, the stratus was eliminated for all but the intersecting
drainages coming from the south and entering the Snake River, like
Goose Creek and the Portneuf River, and the Bear Lake basin.

.LONG TERM...Friday through next Tuesday. Models strongly support a
deepening split upper trough over the Western US late this week. A
low is expected to cut off in California Friday evening and begin
progressing eastward on Sunday. While the cutoff low in the Desert
Southwest may help usher in some mid-level moisture, the northern
branch of the upper trough will provide a source of lift. Best PoPs
still look to be in the Southeastern Highlands. Higher elevations
could see 5 to 8 inches of new snow with about 1 to 3 inches in the
valleys. The trough should push eastward slowly through the area by
Saturday night/early Sunday followed by a 24 to 36 hour quiet period
under transient ridging. Models bring in another Pacific trough into
the area by Monday night/Tuesday. Hinsberger


.AVIATION...A dry north/northeasterly flow should prevail throughout
the day today. Fog/stratus will not be nearly as widespread as the
previous two days. Satellite imagery this morning shows some fog in
the southern valleys. HREF flight category probabilities favor VFR
throughout the day, and only develop LIFR conditions early this
evening around KIDA. Hinsberger




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