Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 181945

Area Forecast Discussion
National Weather Service Pocatello ID
145 PM MDT Thu May 18 2017

.SHORT TERM...Tonight through Sunday. Early afternoon satellite
imagery was showing an unseasonably deep low pressure system
positioned over east-central Utah gradually spreading east into
Colorado with wrap around moisture extending across Wyoming into SE
Idaho. Numerical models continue to show an area of showers rotating
cyclonically (counter clock-wise) around the low into the SE
mountains tonight. A drier NNW flow edges into the region Friday
resulting in a few isolated afternoon showers favoring the mountains
per the NAM model whereas the GFS appears to be a little more
bullish on convection across the SE mountains. Tried to split the
difference and match up with neighbors here. The next upstream PAC
disturbance shears into WRN Canada Friday afternoon and then SE
along the continental divide Saturday and Sunday. The GFS is a bit
slower with the progression and also much more bullish with the
precipitation chances whereas the NAM is more progressive and dry-
ish. Favor the NAM in this regard as these overland waves are
typically dry but there may be enough residual moisture to work with
so once again, I have tried to split the difference and match up
with neighbors. Overnight lows in the Snake River Plain should be a
bit warmer than last night but still cold enough to support a frost
advisory. Huston

.LONG TERM...Sunday night through Thursday. Northwest flow will
continue through early next week. Showers and isolated storms are
still possible mainly along the Divide and across the eastern
highlands Monday. The current trend for Tuesday is to be pretty dry
although we can`t rule out a stray shower or storm to develop at the
moment. From Wednesday onward, the model trends are starting to vary
greatly. The GFS wants to bring the ridge back over us before
flattening it out as a storm develops offshore. The ECMWF is several
hundred miles eastward with the overall pattern, which basically
breaks down the ridge and develops southwest flow. This will bring
back a better chance of showers and storms. We kept with the
previous idea (a blend if you will) that did have isolated activity
mainly in the mountains. Either trend will bring warmer weather to
eastern Idaho. The GFS and the stronger ridge would be much warmer
that the current forecast.  Keyes

.AVIATION...MVFR ceilings expect this afternoon for PIH and DIJ with
gradual improvement through by the early evening hours. Ceilings may
drop again early tomorrow morning for PIH, IDA, DIJ and BYI with
potential stratus or fog/mist. Winds are not expected to be a
concern throughout this forecast period for any site. A slight
chance for evening showers will exist for PIH and BYI with minimal
and brief reduction in visibility and/or ceilings if any reach the
sites. NP/MH

.HYDROLOGY...The stage on area rivers is expected to continue to
drop over the next couple of days due to recently cooler
temperatures. As temperatures warm over the weekend and into the
middle of next week, look for a resumption of snowmelt production
which will drive the stage up on most area rivers. The Big Wood at
Hailey is expected to remain below action stage through the weekend
but return to bankful perhaps as early as Monday night and flood
stage by Wednesday night. The Portneuf at Pocatello will remain in
minor flood stage while continuing to fall over the coming week as
we gradually deplete the upper level snow pack in this low elevation
reach. The Bear River at the Wyoming Border will remain at minor
flood stage through the weekend and into early next week as water
rolls over the spillway at Woodruff Reservoir in Wyoming. No
problems are anticipated on the Henry`s Fork near Rexburg as levels
remain below bankful through the middle of next week. Huston

Frost Advisory late tonight through Friday morning for IDZ020-021.

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