Area Forecast Discussion

FXUS65 KPIH 212110
Area Forecast Discussion
National Weather Service Pocatello ID
210 PM MST Wed Feb 21 2018
.SHORT TERM...Tonight through Sat night. A brief break tonight and
Thu morning for most locations from the snow, but another weak
trough with some fair moisture is expected to arrive mid-day to
the forecast area. Only light snow, but getting to all elevations,
with widespread 1 to 3 inches, with 1 to 2 in the Snake River
plain from Thu mid-day through Fri mid-day. The hits keep on
coming with another storm, much stronger and more moisture,
arriving Sat morning, continuing through Sat night. This should 
at least be Winter Weather Advisory level for at least some
locations. The Thu storm is slightly warmer with its track coming
more from the west than storms have lately, so temperatures for
most will feature a warming trend. Eastern Magic Valley expected 
to warm to slightly above freezing. Both storms have windy 
conditions (20 to 30 mph) in the post-front time period, so that 
would be Fri afternoon and Sat night. Messick
.LONG TERM...Sunday Through Next Thursday.
The long term period features a persistent upper-level trough in the 
western US, w/ periodic shortwaves/disturbances tracking SE from the 
northern Pacific and rounding the base of the trough near the 4-
Corners region. This places SE Idaho in an active storm track that 
should continue beyond the end of the forecast period. Thus, the 
long term looks wet. High and low temps right through next week look 
to stay just cold enough to support all snow across the region, and 
in fact this afternoon's preferred model blend nudged high/low temps 
a degree or two colder every day from Sun onward. 
Our weekend storm system looks to continue into Sun. The GFS 
features a more pronounced trough that remains more progressive than 
the ECMWF, so there is some uncertainty as to how quickly precip 
will wind down. The Snake Plain and Central Mntns/Sawtooths have the 
best chance of drying out for a time Sun. Our next storm system 
arrives Sun eve/night. The GFS and EC show good agreement for now on 
the track of this fairly impressive trough and cold front, which may 
not fully cross the region until Tues afternoon. Early indications 
are that this system may bring yet another round of moderate to 
significant snow, especially to the western Sawtooths/Central Mntns 
(west of Challis and the Big Lost River valley), and the Upper Snake 
Highlands/mntns surrounding the Teton Valley and Palisades. Light 
accumulations cannot be ruled out at lower elevations as well. We 
will have to monitor model trends over the next several days. Windy 
conditions are also likely for the eastern Magic Valley, Snake 
Plain, and peaks/ridges region-wide Sun night into Mon eve. 
Mon night/Tues morning we attempt to dry out a bit, but with the 
slowly departing trough still in the vicinity, continued snow 
showers cannot be ruled out, especially in the mntns. Additionally, 
the GFS begins dumping moisture back into the region by Thurs 
morning. Thus, we carry generally low PoPs everywhere right through 
the end of the forecast period. - KSmith/Messick
.AVIATION...VFR continues this afternoon at all TAF sites this 
afternoon. We should see clouds between 2000-4000ft feet late 
tonight developing as the next storm approaches. Whether or not they 
qualify as MVFR or lower (scattered vs broken/overcast) will have to 
be watched closely.  We think that we should actually go MVFR/high 
IFR after midnight at KSUN as moisture arrives there first, with 
even light snow of flurries developing. By early morning (10-13z), 
latest forecast trends show a more likelihood of IFR/LIFR weather 
with low ceilings and/or visibility with snow. The trend will 
continue eastward during the day. KBYI most likely between 15-17z, 
18-20z at KPIH, and 20-23z for KIDA and KDIJ. Expect MVFR/IFR 
ceilings at these airports after these times.  Keyes