Scientific Forecaster Discussion
fxus65 kriw 180801
Area forecast discussion
National Weather Service Riverton Wyoming
201 am MDT Wed Oct 18 2017
Short term...Wednesday though Friday night
Again the main concerns over the next 48 hours will be of the non
precipitation variety. For early this morning, it will be wind. With
60 knot 700 millibar winds over the absarokas and mountain waves
breaking, there have been some very strong wind gusts near Clark,
some as high was 101 mph. Pressures are beginning to rise in the
area however so they should end soon. We will let the High Wind
Warning ride until expiration. Elsewhere, there have been some
strong wind gusts in the areas prone to southwest flow, like outer
drive in Casper. These should end around sunrise. And with rising
pressures and the pressure gradient lessening a bit, there should
be somewhat less wind today. This should keep fire conditions
below critical for the most part, although elevated fire
conditions will be still be common across many areas. Temperatures
will be warm once again, with some areas seeing highs in the 70s.
A stronger cold front will begin to approach the area for Thursday.
This will again increase the pressure gradient and also the wind
across areas favored by southwesterly flow. This is also expected to
be the warmest day of the week. As a result, we have a chance of
critical fire weather across some areas, especially Natrona County.
Otherwise, expect another mild to warm day with sunshine and some
passing high clouds.
The aforementioned cold front will then move into the west on
Friday. Precipitation will break out in the morning and continue
into the night. With the Pacific origin of the front, there is not a
ton of cold air with it. However, there is enough for snow levels
to fall to the valley floors during Friday afternoon. Amounts
look borderline for any highlights though. One concern could be
the timing of the heaviest snow with the trough passage and jet
dynamics, which would be Friday afternoon so it could snow fairly
hard for a couple of hours, especially in places like Teton Pass.
Some orographic showers will linger into the overnight, but snow
amounts at this time are expected to be light. Meanwhile, with
strong downsloping flow, most areas east of The Divide would see
little or no precipitation outside of the mountains. However, it
will be quite windy with some 40 knot mid level winds being forced
to the surface by the right front quadrant of a jet streak.
Temperatures will be tricky, as a lot depends on when the front
moves through. Cooler, but certainly not cold air will then move
into the area Friday night.
Long term...Saturday through Wednesday
Overview...cooler and breezy/windy conditions will prevail across
the area Saturday behind friday's cold front, with some lingering
snow showers over the northwest mountains. A strengthening westerly flow
will bring an increase in snowfall coverage and intensity to the northwest
mountains Saturday night-Sunday morning with some strong downslope
winds possible along the eastern foothills of The Divide. Milder
and breezy/windy conditions will prevail over much of the area
Sunday. A weak cold front will bring chances of rain and snow
showers across the north on Monday. A ridge of high pressure will
then strengthen over the western U.S. Tuesday and Wednesday bringing
dry weather with unseasonably warm temperatures across the area by
Discussion...upper air pattern on Saturday morning features split
trough across The Rockies moving into strong downstream ridge over
the eastern U.S. Upstream, strong 150-160 knot Pacific jetstreak
will be making landfall across southern British Columbia.
Medium range models in good overall agreement on this jetstreak
carving its way eastward along the U.S./Canadian border Sunday
before dipping southeast into the northern plains Sunday night and
Monday. Both GFS and European model (ecmwf) have trended a bit further north with
jet axis across the northern rockies Saturday night and Sunday,
placing heavier precipitation over western montana; lighter
amounts over northwest Wyoming. Any snow amounts over the northwest currently
look in sub- advisory territory. The main impacts will be strong
to potentially high wind down the east slopes of The Divide,
mainly from Clark/Cody to DuBois Saturday night and Sunday
morning; though this is not looking quite as favorable as earlier
runs with trend northward with the jet axis. Clipper system diving
into the northern plains on Sunday night-Monday will give
northern Wyoming a glancing blow with associated cold front
spreading some valley rain and mountain snow showers mainly across
the Bighorn range and along and north of I-90. Building ridge
over the western U.S. Will bring dry and mild weather Tuesday
through the end of the workweek. 700 mb temperatures near 8c by
Wednesday will result in highs in the 60s across most of the lower
basins with 50s in the western valleys.
VFR conditions will occur through the period at the terminal sites.
A gusty wind returns to the Cody foothills after 02z this evening.
..elevated fire conditions for portions of central Wyoming today...
An upper level disturbance will move across the area early this
morning. Following its passage, expect winds to decrease somewhat,
although relative humidity will still fall to around 10 percent in
many areas. Although red flag conditions are not expected today,
elevated conditions will be likely for many areas. Mixing conditions
will range from poor in the basins to good in some of the normally
breezier areas. Critical fire weather again looks likely for
Natrona County on Thursday afternoon.