Avalanche in Idaho, April 8, 2019

Area Forecast Discussion
National Weather Service Pocatello ID
206 PM MDT Mon Apr 8 2019

.SHORT TERM...Tonight through Wednesday. Rain and high mountain
snow will increase in the next few hours across our area. There
are some subtle differences within the higher resolution models
such that we painted a bit broader picture on where precipitation
will fall outside of the steady stream across the Sawtooths. Snow
levels will peak overnight around 8500-9500ft across eastern Idaho
with them locally higher across the southern highlands.
Precipitation amounts will vary, even in the Sawtooths, thanks to
showers/thunderstorms and even some bands developing. There is a
risk of isolated storms through most of the night...with gusty
winds and small hail possible. Heading into Tuesday, the cold
front sweeps across...dropping snow levels especially in the
afternoon hours. At sunrise, snow levels across the central
mountains start at 6500-7500ft and closer to 9000ft around Bear
Lake. By 6pm tomorrow, they will be around 5500-6000ft in the
central mountains and around 7000ft along the Wyoming border. Even
with snow levels dropping, impacts for traveling over passes
should be minimal and/or short-lived...mainly occurring during
bursts of heavier snow since road/ground temperatures are quite
warm now. Snowfall on those highest passes/peaks/ridges will be
anywhere from a slushy trace to 4". Winds will be ramping up
across the area. We will see west to southwest winds over much of
the Snake Plain, Magic Valley and South Hills/Albion Mountains at
20-35 mph with higher gusts. We will be issuing a multi-day WIND
ADVISORY for those locations starting tomorrow afternoon. 20-35mph
winds (including gusts) from the northwest will develop later in
the day across the central mountains and Montana Divide behind the
front.

For Tuesday night and Wednesday, the storm itself drops southeast
and strengthens as it heads toward the Central Rockies. As cooler
air filters in, we will see the potential for snow showers all the
way to the valley floors areawide, with a mix Wednesday afternoon.
Things start to get tricky for a couple of reasons. The first is
the likelihood of a convergence band dropping southeast across a
good chunk of the Snake Plain and eventually turning into a weak
upslope event into Wednesday morning along the I-15/84/86
corridor. Right now, we are not expecting any widespread impacts
other than bursts of precipitation. That said, this could be a
situation where one of the bursts or lingering upslope is enough
to produce a little longer duration snowfall...impacting travel in
the morning. The second is in the mountains, where again we could
see localized, brief impacts from snow bursts. We will have to continue
evaluating this and see if we need any headlines, but for now will
opt out of that idea. It may be something we can cover with short-
duration SPSs, etc. Any appreciable snowfall should remain above
6000-6500ft. Winds will remain pretty gusty throughout. Winds
eventually become northwest everywhere on Wednesday, 15-30 mph in
many areas. We should see gusts of 30-40mph across the ridgetops
in the central mountains and across the southeast highlands.
35-45mph gusts are possible across the Magic Valley and South
Hills/Albion Mountains. Keyes

.LONG TERM...Wednesday night through Monday.
The general trend for the long term is cold with unsettled
conditions.  The period of a better chance of dry conditions will be
probably Wednesday night into Thursday morning and Saturday
with better chances of showers Friday then Sunday into Monday.
Although GFS and european models differ in details they are in
agreement in the below normal temperatures through all of the long
term period with periods of high winds likely as well.   Snow levels
will vary from valley floors to 6 thousand feet.
GK

&&

.AVIATION...Cold front moves through region on Tuesday with widespread rain and
snow expected at all sites during the day.  This afternoon and
evening expect isoalted thunderstorm development and have vicinity
at all sites after 23z through around sunset.  The  main impact is
expected to be winds Tuesday through Wednesday where sustained 20 to
30 knot winds are possible at BYI, PIH and IDA behind the cold
frontal passage with winds staying elevated for a couple of days.
Expect a good chance of MVFR ceilings even occasionally IFR ceilings
Tuesday through Tuesday night especially as snow reaches valley
floors by WEdnesday morning.
GK

&&

.HYDROLOGY...Rain and high elevation snow is increasing as we
speak across eastern Idaho. Snow levels through tonight remain
between 8500-10000ft, but dropping to 5000-6000ft in the central
mountains to near 7000ft along the Wyoming border Tuesday
afternoon. They eventually drop to 4500-5500ft Wednesday, helping
slow down snowmelt a bit. Rain/Melted Snow totals through
Wednesday are generally 0.30-0.70" across the Snake Plain and
Magic Valley. Those amounts may be a bit overdone as we are trying
to factor potential bursts of heavier precipitation, which can
"inflate" those numbers a bit. For the mountains, we are
maintaining 0.50-1.50" amounts...but again we are trying to factor
in bursts of precipitation and some bands of heavier rain or snow.
We are going to keep the FLOOD WATCH for the mountains EXCEPT the
South Hills/Albion Mountains area through tomorrow as we await how
rivers and streams respond to the expected event. Although levels
are dropping a bit, we will keep the FLOOD ADVISORY for Silver
Creek going for the same reason. Until we see further response,
the only rivers that appear to be possible problems are the Big
Wood around Hailey and the Portneuf. The Big Wood at Hailey may
approach bankful in the next 48 hours. The Portneuf is already at
bankful or will be throughout the week. We could see a brief peak
at Minor Flood Stage at the Pocatello gauge midweek.  Keyes

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 3 PM Tuesday to midnight MDT Wednesday night
for IDZ051-055>057.

Flood Watch through Tuesday evening for IDZ072-073-075.

Flood Watch from 6 PM MDT this evening through Tuesday evening
for IDZ058>071-074.

&&