Avalanche in Idaho, April 3, 2020

Area Forecast Discussion
National Weather Service Pocatello ID
130 PM MDT Fri Apr 3 2020

.SHORT TERM...Tonight through Sunday night. Early afternoon
satellite imagery was showing a broad upper trough over WRN Canada
and the NRN Rockies with an embedded disturbance shearing ESE along
the NRN Washington border a bit further north than yesterday's
progs. Further west, two well defined systems were also noted digging
south through the NE Pacific/Gulf of Alaska which will impact the
region Sunday night through Monday at least (more on that below).
Initially, the models (particularly the HiRes models) favor isolated
to scattered shower development this afternoon favoring the CNTRL
and SE mountains and to a lesser extent the Snake River Plain.
Satellite imagery appears to be supportive of this development as we
are already witnessing increased convective activity in the
aforementioned areas. By 8PM-ish, the models shut down the
convection with the loss of daytime solar heating and trend rapidly
drier overnight as the Canadian trough lifts NE of the region. The
lead disturbance located well off the NW coast continues to drop
south overnight and begins to rotate into the SRN Oregon coastal
waters by morning. As a result, a transient ridge of high pressure
will spread into SRN Idaho promoting dry and stable conditions
throughout the day Saturday with daytime highs approaching normal
readings for a welcome change. Late Saturday afternoon, the second
trailing disturbance is seen digging rapidly south off the
Washington coast acting as a "kicker" for the coastal Oregon low
which begins to lift NNE through Oregon and into the Idaho panhandle
Saturday evening in a much weakened state. The track of this ejecting
system has trended further north and west with each new model run
and the accompanying precipitation impacts have trended weaker. Thus
I have undercut the National Blend of Models as a hedge. By Sunday
and continuing into Sunday night, the second low digs south along
the Oregon/NRN California coast with and increasingly moist SW flow
aloft setting up across SRN Idaho. This pattern should be supportive
of ongoing shower activity with rising snow levels. Thus have stayed
fairly close to the National Blend of Models solution throughout
the time frame. Huston

.LONG TERM...Monday through next Friday. The beginning of the week
starts off rather active under the influence of low pressure to the
southwest. The morning starts out with likely snow showers in our
mountains, above 5500 feet. Warm air advection will be underway
throughout the day though with increasing temperatures and
increasing snow levels. Forecast models, at this time, develop a
500mb vort max over western Idaho tracking it northeastward through
the day. An unstable air mass in place and sufficient moisture
already in place would allow for a probable day for thunderstorm
development by the afternoon or evening hours. Some forecast models
are already developing an organized line of convection passing
through our area. The greatest threat we are seeing at this time is
the threat for large hail.

Snow levels will rise between 6500 and 7500 feet, allowing for a
rain/snow mix in the mountains or purely all rain in the valleys
before gradually dropping again overnight. Look for the chances for
any precipitation (rain or snow) to end during the overnight hours.

The forecast for the middle portion of the week would appear
dependent on the track of the area of low pressure responsible for
Monday's storms. A slow progression eastward is favored at this
time, which would keep much of the bulk of the precip south of us.
However, moisture from its northern counterpart may very well come
over especially for areas along and south of I-86. This would be on
Tuesday and Wednesday's forecast. Areas to the north would be dry
and under the influence of weak higher pressure. By Thursday, the
low is looking to make its way into Colorado while high pressure
builds over Idaho. Temperatures warming actually above normal for
this time of year remain in the forecast. ND
&&

.AVIATION...Snow showers are possible this afternoon at DIJ,
especially after 22Z. Lowered CIG and VIS to IFR or MVFR are
therefore possible for a few hours, ending approx 03Z. Light snow
showers can't be ruled out until around midnight with lesser
impacts. A snow shower may be in the vicinity of IDA or PIH, or
briefly reach the terminal, which would not last more than an hour.
Winds are expected to have minimal impacts this forecast period. ND
&&

.PIH WATCHES/WARNINGS/ADVISORIES...None.
&&