Area Forecast Discussion
Issued by NWS Cheyenne, WY

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023
FXUS65 KCYS 091713 AAA
AFDCYS

Area Forecast Discussion...UPDATED
National Weather Service Cheyenne WY
1013 AM MST Fri Dec 9 2016

.UPDATE...
Issued at 1007 AM MST Fri Dec 9 2016

Downsloping west winds aiding in rapid warming for the southeast
WY plains. It feels balmy with Cheyenne at 41 degrees and west
winds gusting to 25 mph at 10 am. Cancelled the high wind warning
for Arlington/Elk Mountain areas on I-80 a few hours ago as winds
have diminished, but still gusting to 40-45 mph. Raised high
temperatures to 40-45 degrees for Cheyenne this afternoon, and
lowered them across the northern/northeast CWA. Per SNOTEL data
from this morning, snow has tapered off over the Snowy/Sierra
Madres, but more moisture will move across the mountains this
afternoon.

&&

.SHORT TERM...(Today through Sunday)
Issued at 415 AM MST Fri Dec 9 2016

An active period upcoming for areas in and around the Medicine Bow
Mountains as the unrelenting progressive mid-upper level pattern
persists. First things firsts, lets discuss the ongoing high wind
event for the Arlington area. Wind gust speeds overnight have
hovered in the 40-55 mph range, with a few spots briefly touching 57
mph. The latest CAG-CPR gradients suggest two maxima, the first
which likely has already occurred around 09z, a brief lull to follow
thru around 12z, and another surge in the 15-18z timeframe. This
matches well with the 50-60 knot H7 progs thru this same time as
well. All-in-all a marginal event, but will let the High Wind
Warning continue through noon. The westward position of the lee-side
trof on the latest sfc analysis confirms that Arlington will be the
only area to be impacted by this first high wind event. Of course,
blowing snow is a concern as well however, latest webcam images
suggest visibilities have not been affected significantly. CAG-CPR
gradients relax some this afternoon and evening, before exceeding
the 60 meter threshold again 09-18z Saturday. The trend has been
slightly lower compared to previous model runs, but is still
sufficient to warrant another High Wind Warning for the Arlington
area for Saturday morning. Lee-side sfc trof position is pushed a
bit further east for the Saturday morning episode, but am not
confident to include SW Platte County for now. The day shift will
need to consider adding should 12z model output warrant. Breezy to
locally windy conditions will continue for areas along and west of
the Laramie Range into Sunday and Monday as well, but likely less
than these two events.

The next concern is snowfall over the Snowy/Sierra Madre Ranges. As
of 2 AM, observing sites have observed generally 5-10 inches of snow
with locally higher amounts at higher elevation sites. This, as
moist llvl orographics have been extremely favorable for west slope
locations. Snow will continue at least through the morning hours
with some lessening by afternoon. Given how much snow has already
fallen, the fact that models have been under-predicting precip
with this system, and how much add`l is expected, have increased
amounts and upgraded the Snowy/Sierra Madre Ranges to a Winter
Storm Warning this morning. This remains in effect until 5 PM
today.

Although some snow showers will continue Friday night and into
Saturday morning over the mountains, rates will not add much
accumulations. That will change by Saturday afternoon as the next
the next 150 knot H3 jet streak plows into the intermountain west.
Snow will develop once again over the Snowy/Sierra Madre mountains
Saturday afternoon and continue into Sunday morning. Another winter
highlight will undoubtedly be needed for the Snowy/Sierra Madre
range, but to prevent product confusion, will let the current system
run its course first. Latest runs have decreased impacts across
adjacent lower elevations with this second system. Even areas in and
around Arlington may not be all that impacted. Of course, an inch or
two with strong winds could prove otherwise. This system departs
Sunday morning, leaving dry conditions thru the rest of the day.

Temperatures will feel significantly warmer today and tomorrow with
highs climbing back into the 30s and 40s. Temps cool some west of
the Laramie Range by Sunday.

.LONG TERM...(Sunday night through Thursday night)
Issued at 415 AM MST Fri Dec 9 2016

Long range models all show progressive west to northwest flow
across the area with several Pacific systems moving across the
Rockys. However...models are also out of phase with substantial
timing differences between models and each operational run. This
is resulting in lower than average confidence with snowfall
chances in any particular 24 hour period. One thing we are
confident about is lighter winds once we get through the
weekend...and very cold temperatures expected to continue. It will
not be uncommon to see morning low temps in the single digits by
the middle of the week with highs struggling to reach 20 degrees.
It does appear to be quite unsettled across most of the forecast
area next week just due to the pattern alone...which suggests an
arctic airmass remaining over most of the Great Plains states and
several Pacific upper level troughs moving into the west
coast...and then eventually into the central or southern Rockys.
This pattern typically results in good warm air advection aloft
and overrunning precipitation over the very cold airmass near the
surface. Models show the first event potentially on Tuesday as the
arctic cold front will remain near the area across the high
plains. The ECMWF is slightly further south with the main area of
precipitation compared to the GFS...which shows it mainly over
Colorado or near the Colorado/Wyoming border. Kept POP between 30
to 45 percent across the valleys and high plains with precip
chances over 60 percent for the mountains through late Wednesday.
Not enough confidence to nail down the timing on this...since the
area of impact is still in question.

For late this week...models are hinting at the potential for a
major winter weather event across the forecast area...but are
completely different on timing with the GFS now a full 24 hours
later than the ECMWF. All ensembles show a potent upper level
trough/low off coastal California with deep moisture...warm air
advection...and a strong jet ahead of it. Meanwhile...the arctic
airmass will remain across the plains...with the position of the
arctic front fluctuating as far west as the Laramie Range and as
far east as central Nebraska/Kansas. Will have to closely monitor
how these features interact over the next several days...since
there is the potential for moderate to high amounts of
precipitation. For now...gradually trended POP higher late this
week. One thing to note is model consensus shows POP across the
whole area above 60 percent next Friday...even though it`s still
one week away.

&&

.AVIATION...(For the 12Z TAFS through Saturday morning)
Issued at 415 AM MST Fri Dec 9 2016

Periods of IFR visibilities in snow showers will continue around
KRWL until 14z this morning. Otherwise...VFR conditions are
expected to continue across area terminals today and into tonight.
CIGS should average between 5k-10k ft AGL today. Gusty winds will
gradually spread east into KLAR and KCYS late this morning through
this afternoon. Expect frequent gusts between 20 and 30 knots.


&&

.FIRE WEATHER...
Issued at 415 AM MST Fri Dec 9 2016

Windy and warmer conditions can be expected across area districts
through the weekend. Significant snow accumulations can be expected
in the Snowy and Sierra Madre Ranges where Winter Storm Warnings
have been issued. Otherwise, aside from some light snow chances in
areas around the mountains, expect most of the area to remain dry.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Warning until 6 PM MST this evening for WYZ112-114.

     High Wind Warning from midnight tonight to noon MST Saturday for
     WYZ110.

NE...None.
&&

$$

UPDATE...MAJ
SHORT TERM...CAH
LONG TERM...TJT
AVIATION...TJT
FIRE WEATHER...CAH



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