Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 091004

Area Forecast Discussion
National Weather Service Pocatello ID
304 AM MST Mon Dec 9 2019

Our latest winter storm has finally departed, with a synoptic-
scale view showing the split-feature trough axis pushing SE into
the Plains, and a narrow, almost shortwave-style ridge or tongue
of high pressure pressing east onshore across the Pacific coast
with rising heights into SE Idaho. The post-storm environment
leaves us with two concerns this AM: fog, and low stratus/snow.

LOW STRATUS/LINGERING PRECIP: GOES-17 channels reveal varying
degrees of low stratus clouds stretching from southern Lincoln
County/eastern Magic Valley, east into the southern/eastern Snake
Plain, and then across the southeast CWA east of Rockland and
south of Palisades. Area webcams do show some isolated instances
of LIGHT snow flurries lingering under this stratus deck in a few
spots south/east of McCammon down to Bear Lake and the UT border.
KPIH ASOS currently reflects some mid-level clouds up at around
6,500 feet, and while it is difficult to evaluate the extent of
this on the satellite products (although it is reflected on KPIH
NAM/NAMNest BUFKIT soundings), it stands to reason this lingering
light snow may be a combination of the atmosphere still "wringing
out" a bit of moisture as the storm system pulls away and perhaps
some seeder-feeder assistance where any mid-level clouds are
contributing ice crystals into the lower-level moisture to serve
as nuclei. In any event, most guidance sources including the high-
res ensembles are useless in trying to evaluate how long this may
last. We believe any lingering snow showers will generally
decline over the course of the AM, but we threw in some low PoPs
through 18z south of Inkom/Palisades to cover this. Any accums
should be spotty and low-impact, a dusting or less.

FOG: The presence of low stratus (and any mid-level clouds) tends
to take the edge off fog potential by reducing surface
radiational cooling, and early AM surface obs show winds are also
still A BIT breezy regionwide, especially in the Snake Plain.
However, area webcams and surface obs do show a few instances of
patchy fog, with the greatest coverage currently from just north
of Blackfoot up across Mud Lake, Idaho Falls, and Rexburg, toward
Dubois and Ashton. Visibilities are down to 1/4 mile or less,
including at the KIDA and KRXE ASOS stations. MOS guidance and
the HRRR ceiling/visibility products don`t seem to have a great
handle on this activity, on top of some of the potential
mitigating factors mentioned above. We have added patchy/areas of
dense fog into the grids through 11 AM, and have issued a Special
Weather Statement to cover ongoing impacts. Will continue to
monitor trends to see if a Dense Fog Advisory will be needed,
with duration and coverage of visibilities below 1/4 mile a key
consideration. Fog should generally dissipate by late morning.

REST OF TODAY: Some degree of low stratus should stick around
through the day outside of the Central Mntns, with also some
sunshine breaking through. High temps look pretty close to
seasonal norms, generally in the upper 20s to low 30s.

TONIGHT: High pressure continues to build into the area tonight,
with a likely reintensification of low stratus as supported by
BUFKIT soundings, MOS guidance, and the NBM, especially outside of
the Central Mntns. Have beefed up our sky forecast accordingly.
06z HRRR products favor the Snake Plain north of Blackfoot up
toward Fremont County again for both low stratus/fog potential.
Slightly elevated winds and that stratus again could again take
the edge off fog potential, but for now it seems prudent to
introduce patchy fog into the forecast for these areas and some
of our other valleys outside the Central Mntns. The other thing we
will be watching from early this eve into the overnight is the
potential for patchy freezing drizzle. NAM soundings seem at
least somewhat favorable in the Snake Plain and adjacent valleys
south and east, with sub-freezing saturation below about 750mb
(all between 0 and -10 C), abundant dry air in the mid-levels
with no seeder-feeder interference for ice crystal introduction,
and perhaps a hint of shear in the boundary layer. On the other
hand, ascent is in question, and statistically freezing drizzle
is rare in our area. At this time we certainly do NOT have the
confidence to add mention of FZDZ to the forecast, but will
mention potential to day shift for another evaluation as this
certainly would impact roads if it were to occur. Bottom line,
low stratus/fog are the greater concerns at this time.

TUESDAY/WEDNESDAY: Dry weather and some lingering stratus is again
expected Tues, with highs similar to today. Wed will feature
increasing clouds as our next progressive shortwave trough crosses
the region from west to east. The GFS/NAM continue to run more
amplified and pronounced (and thus wetter) with this feature,
while the ECMWF/Canadian are weaker/drier and also try to more
definitively split this piece of energy more into a sharper
component over MT and another over UT/AZ, sending SE Idaho right
through the "gap". The NBM has trended slightly wetter, but still
comes in at under 15% on PoPs for most areas. Would really like
to see a more definitive answer in the guidance before changing
our forecast from dry to showery...for now, we carry a few snow
showers near the immediate WY/MT borders during the day, and have
introduced a slight chance of snow showers across the southern and
eastern highlands in the eve. Any accumulations should remain
under 1 inch, and winds remain light, so we continue to expect no
major impacts Wed. - KSmith

The models have been consistent the last two nights with the idea
that the passing wave Wednesday will inject enough moisture into
the lower levels to support at least ongoing stratus and perhaps
some light snow/flurries across the SE mountains/Snake River Plain
Wednesday night despite the National Blend advertising zero PoPs
and no weather. The timing is a little more progressive, but the
models again remain fairly consistent with the idea of a moist
Pacific flow rolling over a shallow ridge into Idaho Thursday and
continuing into Saturday as a broad disorganized trough tries to
take shape over the NW by Saturday afternoon. Favored upslope
regions like the Sawtooth and Caribou/Big Hole Mountains should do
fairly well under the moist west flow while areas like Mud Lake
fall under the downstream shadow of the CNTRL mountains. The
three-day storm total snowfall for the CNTRL and ERN mountains
looks to fall between 1 and 2 feet of accumulation and I would
imagine that sometime in that time frame we would likely see some
Winter Weather highlights (most likely advisories) issued. The
disorganized trough translates through the region Sunday resulting
in scattered snow showers followed by a ridge of high pressure
and drier conditions Monday. - Huston


Early morning satellite imagery was showing a broad area of low
stratus from Jerome eastward to Pocatello and then a smaller area
of fog from Idaho Falls to Rexburg to Mud Lake. Mostly VFR CIGS
were noted across the SE mountains per observations although I
would imagine that there are certainly mountain obscurations and
ongoing light snow flurries in the supportive NW flow aloft. The
CNTRL mountains on the other hand were largely clear. Numerical
models suggest that the stratus will likely expand and impact at
least KBYI, KPIH, and KIDA throughout the day with some moderation
possible at KBYI and KPIH overnight. KDIJ will likely see impacts
from the stratus sometime today and continuing into the night.
KSUN is expected to remain under the drier NW flow aloft
throughout the period. However, the short term models suggest the
potential for the Snake Plain stratus field to advect up into the
Wood River Valley this afternoon as the normal terrain driven
winds become SE. We will play that one by ear for now as the
stratus field is a considerable distance to the south and east of
the terminal. - Huston




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