Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 051530

Area Forecast Discussion
National Weather Service Pocatello ID
830 AM MST Tue Dec 5 2017

.UPDATE...Have made a few minor tweaks to current forecast for Sky
and Wx for fog/stratus coverage today. Satellite fog imagery
shows spread of stratus quite well this morning. High clouds
spilling from Montana into the region but mainly clipping just the
northeast corner of the state. Area web cams showing just a bit
of fog across the central portion of the Snake River Plain,
impacting mainly from the INL toward I-15 north of Idaho Falls.
Expectations are for some burn off through the day so have
tempered areas of freezing fog down to patchy and reduced the size
of the stratus field. At this time, believe redevelopment
overnight is likely. DMH


.PREV DISCUSSION... /issued 335 AM MST Tue Dec 5 2017/

SHORT TERM...Today through Thu night. High pressure ridge to the
west is keeping eastern Idaho in a cold northerly airflow pattern,
suppressing the warm-up with continued cold air advection. The
main forecast issue will likely be fog and stratus in the valleys.
Right now, it appears today to be limited to the northeast corner
of the forecast area, less coverage on Wed night, then none on
Thu night. This is based on boundary layer humidity and NAM mos
guidance ceiling categories for available points in the Snake
River plain/eastern Magic Valley. Highs and lows will bottom out
in the next 24 hours, then the northerly flow weakens and a
warming trend to both highs and lows develops. Expect Wed highs to
be 4 to 7 deg F warmer than today. No wind and very little

LONG TERM...Friday Through Tuesday. All long-range models
continue to indicate a ridge of high pressure will remain anchored
over the Pacific Coast right into next week. This theme has not
changed for several days now, leading to moderate to high forecast
confidence in an extended period of dry weather with party-mostly
sunny skies across SE ID, and no significant or impactful weather
expected. We once again made only minor tweaks to temps, cloud
cover, and wind based on the latest blend of model guidance, which
did not result in any significant changes to the going forecast.
There is some suggestion on the 00Z ECMWF, less so on the 00Z GFS,
of a weak front brushing the region Sun afternoon or night.
However, this feature looks weak, and the models are not
advertising much in the way of moisture nor any significant temp
change. We increased cloud cover just a bit during this period,
particularly across nrn/ern portions of the area, but just no
justification to add any precip to the forecast. This blocking
pattern does not favor any progressive warming, so expect fairly
stagnant high temps throughout the period, perhaps improving just
a few degrees between Fri and Tues. This pattern is also conducive
to strong temperature inversions setting up between valleys and
higher terrain, which will be enhanced by any lingering low-
elevation snowpack. This indicates some potential for low stratus
clouds and/or fog in valley locales, which is difficult to
forecast this far out and will have to be watched. Glancing beyond
the end of our forecast period, there is nothing to suggest this
pattern breaking down very quickly, and our next chance of precip
may lie beyond December 14th! Stay tuned. - KSmith/Messick

AVIATION...Only aviation concern over the next 2 days will be
the potential for fog and/or low stratus, particularly at
KIDA/KPIH/KDIJ. KIDA has already seen a stratus deck develop at
about 600 feet. NAM and HRRR guidance is showing strong
indications of this persisting well into this afternoon, with a
strong inversion and surface near-saturation reflected on model
soundings. However, we are seeing a bit of low- level lift/rising
motion also reflected in time-height profiles, and this combined
with the fact that winds remain above 10 knots this morning may
favor low stratus over fog and is likely keeping cigs and vsbys in
check (dewpoint depressions are holding at 2-3 degrees). We have
based the forecast for KIDA on this expectation of continued low
status issues with LIFR conditions possible, but forecast
confidence is low. Have delayed improvement until 19Z, but it
should be noted that models do suggest low stratus could persist
throughout the day. For KPIH/KDIJ, we are also monitoring closely
for fog and low stratus (greatest threat from 13Z to 18Z),
although impacts should be slightly less than KIDA. Decks above
KPIH are holding above 2,000 feet for now...these could lower to
IFR, again with low confidence. No problems expected at KBYI/KSUN.
Winds are expected to be highest this AM across the region,
becoming light for the rest of the day and tonight. Fog/low
stratus potential returns Wed morning at KIDA/KPIH/KDIJ. -



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