Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 200859

Area Forecast Discussion
National Weather Service Pocatello ID
259 AM MDT Thu Apr 20 2017

.SHORT TERM...Tonight through Friday night.

A cold front will move through the region today. Most of the models
have a line of showers developing during the predawn/early morning
hours this morning across central Idaho and progressing east into
eastern Idaho by afternoon. Am a little hesitant to totally buy off
on this. Regional radar this morning doesn`t show much happening as
of 09z. NAM is definitely having some convective feedback issues in
the qpf fields and the GFS may be suffering from some of this as
well, although not quite as obvious. The HRRR is not nearly as
aggressive with the initial line and am siding the forecast in that
direction for this morning, which still brings a lot of showers
through the central Idaho mountains, the southern highlands, but not
as aggressive in the Magic Valley and Snake Plain. Still, expect
some showers through the Magic Valley this morning, but may be more
scattered vs numerous. Expect this line to eventually congeal by the
afternoon as it makes its way into the eastern highlands. Also,
expect scattered showers with isolated thunderstorms to develop
behind the main line this afternoon. Looks like it will be fairly
breezy as well but probably holding below wind advisory levels. Much
of the convection will dissipate this evening, but some will
continue to persist especially across the eastern highlands
overnight as an upslope flow component develops. Forecast looks
drier for Friday, but there`s still a chance for showers through the
day especially across the eastern highlands and maybe into the Snake
Plain as well. Expect skies to clear Friday night with very cold
temps likely as winds diminish as well. Strongly favored MAV/MET
guidance as opposed to in-house consensus data which is likely way
too warm.


.LONG TERM...Saturday through next Thursday...The start to the
weekend continues to look mild, with temperatures in the mid 60s and
partly cloudy skies across the Snake Plain. Gradually increasing sky
cover and a chance for snow/rain showers will accompany the Central
Mountains Sunday with the remainder of Southeast Idaho seeing rain
showers and a possible thunderstorm later in the day. The
progression of the cold front looks to have slowed slightly with the
European model solution not as boisterous in rainfall amounts
compared to the GFS model. For next week, a series of shortwaves
will pass eastward across the state and bring a sustained moist
period. For our forecast, this will mean on and off showers with a
thunderstorms mixed in occasionally throughout the week. Right now,
long term models generally agree early Monday should mostly be dry
and late Monday through Tuesday will be wet, but are not in the best
agreement after that which days will be dry and which days will be
wet. Snow showers are forecast above 7000 ft on Tuesday which will
accumulate between 1 to 3 inches. Additionally, each afternoon will
be breezy across the Interstate corridor with winds gusting to 30
mph. NP/DV


.AVIATION...As of 3:00 AM MDT...Approaching trough of low
pressure will send some robust precip across SE Idaho today.
NAM/GFS/HRRR/RAP model guidance all in very good agreement on
general progression of initial showers, reaching KSUN by 10-12Z,
KBYI by 11-13Z, KPIH and KIDA by 15-17Z, and KDIJ by 19-21Z,
continuing into this evening with some decent QPF totals. Last
night, some of the models were trying to break precip up into 2
dominant periods with a break in between. Ascent profiles on the NAM
still mildly alluding to this, as is RAP guidance. However, rest of
the models aren`t showing any distinct breaks, so not reading into
this. A few thunderstorms will be possible with decent shear
parameters and diurnal timing, however instability and LIs are not
very impressive. Bottom line, high forecast confidence in a wet and
showery day at the airports, with low confidence in storms and
possible breaks in the shower activity. Temps should be warm enough
to support all rain for most of the event, allowing for predominant
VFR with occasional MVFR possible in heavier showers and storms,
mostly due to low ceilings. By 06Z to 12Z Friday, clouds and precip
will be clearing from west to east as the system pulls out and a
ridge of high pressure begins to build in. Showers will be last to
clear KDIJ, with VCSH or -SHRASN possibly holding on into the day on
Friday with predominate VFR, MVFR possible in snow showers.


.HYDROLOGY...Another round of precipitation is expected Thursday
coupled with warming temperatures heading into the weekend will
support forecast rises on rivers through the latter half of the
week and into the weekend. The Portneuf at Pocatello will remain
in flood stage throughout the weekend. The Snake at Blackfoot is
expected to continue falling as releases from Palisades have been
scaled back and increases in flows coming off the Henry`s fork
will be modest. The Bear river at the Wyoming border is forecast
to remain below the official flood stage of 8.6 feet through the
week but we did notice issues near Dingle and Pegram when the
stage reached 7.38ft back on March 20th and it looks as if by
Thursday afternoon we may be running near that level again per the
latest river forecast. An advisory remains current for that area
and we will be making calls tomorrow to see how things are shaping
up. Huston



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