Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 222044
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
244 PM MDT Fri Mar 22 2024

.SHORT TERM...Tonight through Saturday night. Satellite imagery
shows upper ridge over East Idaho, weakening ahead of shortwave
axis swinging northeast out of Nevada. Second shortwave located
along the Oregon coast early this afternoon. Showers spread into
the southern highlands beginning this evening, then spread north
overnight. By the time this feature exits early tomorrow, second
shortwave feature approaches. HREF favors best instability over
the eastern highlands during the afternoon, just ahead of weak
frontal boundary sliding through the region. Have left isolated
thunderstorms in place across the northeast quadrant of the
forecast area. Winds do pick up slightly during the afternoon
associated with the increased surface gradient and frontal
boundary, but mid level support is lacking with 700mb winds only
25-30kts in a narrow band behind the front. Thus kept winds below
Advisory criteria. Snow levels fall very close to valley floors
overnight Saturday night as temperatures fall to near freezing
for most of the Snake Plain. However, most of the precipitation
should be confined to the eastern highlands, generally east of
I-15 for Saturday night into early Sunday. Snow amounts generally
3-6" expected, which falls just shy of consideration for a Winter
Weather Advisory at this time. If, however, QPF looks to increase
or colder air raises snow-liquid-ratios, an Advisory may be
considered going forward. DMH

.LONG TERM...SUNDAY THROUGH NEXT FRIDAY...
The entire long-term period of the forecast will be characterized by
a train of multiple shortwave troughs and low pressure systems
working in off the Pacific, resulting in daily chances of rain and
snow along with cooler overall temps. Overnight low temps will
generally support snow to valley floors assuming precip is ongoing
(although accumulations should remain quite limited outside of some
Montana valleys), while snow levels rise as high as 4,500 to
5,500 feet some afternoons, allowing rain to mix in especially in
the Snake Plain corridor and some of the adjacent srn highland
valleys. Highs will generally be in the upper 30s and 40s for most
population centers, occasionally touching 50 especially at lower
elevations west of Pocatello. The 12z deterministic suite of long-
range models hint at a potential break in precip potential on Wed,
but the NBM is not yet convinced, and 500mb height cluster
analysis offers a wide range of scenarios with respect to the
amplitude of potential shortwave ridging that day with really only
1 cluster (31% of the ensemble member space) solidly drier than
the full multi-model ensemble mean. Wed may also feature the
warmest temps of the week (more 50s), but especially if the day
holds on the dry side. The heaviest/most widespread periods of
precip may favor Sun, and then Wed night and Thu, but this is
certainly subject to change at this range. Sadly, our early taste
of spring is gone. For now. - KSmith

&&

.AVIATION...
Our extended early taste of spring/period of dry and low-impact wx
is about to come to an end as a low pressure system moves onshore
along the Pacific coast and pushes inland. Expect gradually
increasing and lowering clouds over the course of today and tonight,
but the good news is our full suite of model guidance offers almost
no support at this time for cigs falling below VFR, at least through
Sat afternoon. KSUN and KDIJ currently stand the best potential to
see decks as low as 4,000 feet AGL by early Sat AM. Winds should
also hold at 10kts or less regionwide through Sat AM. Refined TAF
timing on initiation of VCSH (00-04z/6pm-10pm this eve)...followed
by a trend into -SHRA or -SHRASN (03-08z/9pm-2am tonight except
closer to sunrise Sat AM at KSUN) based on the 12z HREF suite of
CAMs. Once things get showery, there may actually be 1-3 hour period
breaks here and there, but at this time we didn`t have the
confidence to try to flesh these out in the TAFs...especially given
some differences in the details from model to model. Lower
confidence resides with vsbys for KSUN and KDIJ...temps will be
marginal for rain vs. snow at both terminals, and while model
guidance generally suggests little to no drop in vsby through the
TAF period, it won`t be hard to bring things down during any period
when steady precip can switch to snow. For what it`s worth, our
official NBM-led forecast and MOS guidance lean warm enough to favor
rain. For now, just hinted at reductions with 6SM attached to -SHRASN
periods, but did not go lower. Confidence is much higher at
KBYI/KPIH/KIDA that precip will remain all rain. Looking ahead
just past the end of the 18z TAF period...expect winds to increase
into the 10-15kt range Sat afternoon with this system, along with
cigs trending toward MVFR Sat eve, especially for KPIH, KIDA, and
KDIJ, as unsettled wx continues for the next several days.
- KSmith

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$


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