Area Forecast Discussion
Issued by NWS Pocatello, ID
FXUS65 KPIH 232112
Area Forecast Discussion
National Weather Service Pocatello ID
212 PM MST Mon Jan 23 2017
.SHORT TERM...Tonight through Wednesday. Complicated system
overhead currently. As indicated by models and satellite imagery,
broad upper trough remains in place over western US. Main
circulation over Pacific Ocean appears to be filling in while
circulation over OR/CA/NV has strengthened. Expect the upper low
center to reform over ID this afternoon and evening. In addition,
surface low currently located over Great Salt Lake expected to
gradually shift E/NE. Combination of these features puts much of
SE ID in cross hairs for elevated snow through tonight.
Accumulations this morning were higher than originally expected,
with snowfall rates meeting or exceeding inch/hour. Short range
models, particularly HRRR and RAP continue to show redevelopment
of moderate to heavy band of snow this evening into the overnight.
The difference between the two is where the band will be located.
Both favor orographic enhancement along the I-86 corridor between
Pocatello and Burley, but RAP slightly further west than HRRR.
Regardless, model spread including GFS, NAM, NAM experimental,
NSSL-WRF, etc all favor higher snow amounts through tonight with
8-16 inch totals somewhere between Pocatello and Burley with a
slightly smaller bullseye between Idaho Falls and the INL.
Operational NAM and GFS slightly lower in the 6-10 inch range, and
have tempered forecast snow amounts with those values. The
positioning of the low is likely to have the greatest impact with
respect to direction of the winds and orographic forcing. Have
upgraded all advisories to warnings for tonight especially
regarding potential impact for evening and morning commutes
regardless of snow amounts. Confidence remains relatively high for
timing of the bands but moderate on placement and amounts.
Complicating factor is strengthening of the winds in the
Burley/Oakley area, and in the north along the MT border. Have
left blowing snow in the forecast overnight for these areas where
winds are expected to gust over 20kts.
Upper low shifts into Plains states during the day Tuesday.
Remaining pattern Tuesday through Wednesday includes northerly
flow stacking moisture into SE highlands. Gradually cooling
temperatures increase snow ratios but deep moisture shifts east
with the low. Have kept chance precip most higher elevations near
the WY and UT borders, otherwise chances decrease toward the NW
central mountains. Increasing snow ratios will complicate matters,
but QPF amounts decrease so each 24 hour period should have
relatively low snow amounts. DMH
.LONG TERM...Wednesday night through next Monday. Development of
upper ridge is consistent in long-term models through much of the
period. We will have enough upslope component in the 850-700mb layer
to trigger some light snowfall over the Caribou Highlands and Upper
Snake Highlands. An east-west oriented trough will slide down out of
Canada on Friday, cutting off the upslope component and producing
some drying. Thus will keep PoPs low Friday through Monday. With the
passage of the upper trough, and a northerly flow in its wake,
expect to see temperatures coming below normal. Hinsberger
.AVIATION...Pessimistic TAFs for the 18Z package for most of the
terminals. Radar trends show two distinct areas of snow in play for
southeast Idaho. The first is currently over KPIH and dropping
visibility as low as 1/2 mile. The second is still over KBOI and
models show this area surging eastward this evening. NAM and BUFKIT
soundings suggest heavy and prolonged QPF associated with the second
wave lasting through 18Z. Snow could become heavy enough to
temporarily drop KBYI, KPIH, and KIDA below minimums. Additionally,
winds are expected to increase this afternoon/evening as the second
band of snow approaches southeast Idaho. Hinsberger
.HYDROLOGY...After discussion with Custer Emergency Management,
the areal flood advisory will be continued for one more day. There
has been some improvement in conditions, but concerns have shifted
to future snow melt. Therefore it has been advised that we allow
the advisory to expire tomorrow. DMH
Winter Storm Warning until 6 AM MST Tuesday for IDZ017-020>025.