Scientific Forecaster Discussion
fxus65 kriw 130927
Area forecast discussion
National Weather Service Riverton Wyoming
227 am MST Wed Nov 13 2019
Short term...today through Friday night
The radar is showing some light precipitation across the area.
However, this appears to be more bark than bite, as much of it is
having trouble reaching the ground due to a dry atmosphere. There
could still be a few light showers around this morning, so we
maintained pops over northern and eastern areas but nothing of
consequence. All precipitation should end by around noon. The
European model (ecmwf) does have some light quantitative precipitation forecast this afternoon over northeastern
areas, but it is confined to the mountains and when this happens
it is usually not reality.
High pressure will then bring dry conditions through Friday. There
will be varying amounts of clouds from time to time, as well as a
gusty breeze in the favored areas, but nothing unusual for the
middle of November. In addition, with the wave bringing the showers
tonight, it has mixed the atmosphere a bit, so inversions should
not be as deep if there at all. The result will be a gradual
warming trend through Friday. The next Pacific system will
approach the area Friday night. The models may be a bit too fast,
like they usually are with Pacific systems more than 2 days out or
so. For now, we kept evening pops in the slight category and
confined to the far west with more substantial activity later at
Long term...Saturday through Wednesday
A shortwave trough that will be moving onshore over the
pacnw/British Columbia during the day Friday will begin to dig as it
makes its way to the region. Snow above 7500 feet will become
possible over the western mountains Friday night, as this system
approaches. 00z model guidance is indicating the base of the trough
digging over far southeast Idaho/SW Wyoming by 12z Saturday. This will allow for
snow to be more widespread, affecting both sides of The Divide.
Progression of the cold front (and the cold air) is faster now, with
snow levels quickly dropping to 5000 feet by 18z. Impacts from this
remain the same, with precipitation for areas east of The Divide
starting as rain or a rain/snow mix in the lower elevations. Icy
roads could form Saturday afternoon and evening, as temperatures
drop and snow begins to fall behind the cold front. The cold air
will impact areas west of The Divide after 12z, namely the Upper
Green river basin and the western valleys, with precipitation
starting out as snow. Icy roads will be more likely over the
Interstate 80 corridor Saturday afternoon and evening, before
changing over to snow. The speed of this system remains fast, with
precipitation quickly ending across the area after sunset. Icy
roads seem the more likely threat. Even if precipitation remains
rain or a rain/snow mix during the day Saturday, temperatures will
drop below freezing during the evening hours.
A Flat Ridge will build back over the epac, with a high center off
the central California coast. A 140+kt jet will be moving through this
ridge, indicative of a "pineapple express" over the Puget Sound and
southern British Columbia. This jet may end up over the region
Sunday and Monday, beginning to support the Saturday storm system,
that will be just off the Texas/la coasts. If this happens, strong
gusty winds could occur over the absaroka and Bighorn Mountains and
NE WY, as well as the Wind River basin. However, this will depend on
where the high center sets up. Currently, the models continue to
have issues on resolving the impacts from a cutoff low that will
form off the Baja California coast Saturday. The resolution of these
two features will dictate if the jet set up over Wyoming or not.
As far as the shortwave for Tuesday/Wednesday that has been talked
about for the last few shifts, the only thing that remains the same
is that the system will impact the region in some form or another.
There remains no run-to-run consistency and thus, confidence
continues to remain low at this point.
Aviation...for the 12z tafs through 12z Thursday
Taf sites are expected to be VFR through the period. The exception
will be the possibility of MVFR ceilings at the kcpr terminal
developing around 14z. However, right now ceilings are expected to
only drop to around 040-050 before lifting out by 18z.
Light snow will occur over the northern mountains through about
15z. Far northern mountains will be obscured through this time,
but will improve as the snow ends. Mainly sky clear skies will then be
in place through the rest of the taf period. Gusty winds will
continue once again at krks.
Any showers across northern Wyoming should end later this
morning. Other areas will have a dry and somewhat warmer day than
yesterday. A gusty breeze will blow across southern Wyoming.
Inversions will be weaker than yesterday. Relative humidity will
remain above critical levels. Mixing and smoke dispersal will be
generally fair to poor, except good across southern Wyoming. Dry
weather is expected from Wednesday night through Friday, before
the next chance of showers moves into the west on Friday night.