Area Forecast Discussion
Issued by NWS Cheyenne, WY

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326
FXUS65 KCYS 170448 CCA
AFDCYS

Area Forecast Discussion...Corrected Aviation
National Weather Service Cheyenne WY
850 PM MST Mon Jan 16 2017

.UPDATE...
Issued at 850 PM MST Mon Jan 16 2017

Skies have cleared early this evening. With lighter winds, expect
good radiational cooling later tonight. Lowered temperatures for
the valleys out west, as well as at Cheyenne and Alliance. Patchy
late night fog is still a possibility, especially for the Laramie
Valley.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 1235 PM MST Mon Jan 16 2017

Latest water vapor loop was showing the intense upper level low
continuing to spin to the northeast. This closed low is showing
signs of blocking the upper level ridge from building into the
Rockies and slowing down the erosion of the cloud cover and
subsidence. Meanwhile, another shortwave was spinning across the
desert southwest. This shortwave is expected to move northeast
towards to the southern Plains through Wednesday. What this means
for us is that we may struggle to get the upper level ridge to
build over our forecast area during the next couple of days until
the upper trof in the desert southwest moves northeast. The models
are trending a bit cooler than previous solutions.

Tonight:
We are a bit concerned that the Laramie Valley and may
experience some patchy fog tonight due to the potential for good
radiational cooling. Therefore, we went ahead and introduced the
fog in that zone group and lowered temperatures a couple of
degrees from guidance. We also lowered temperatures to the west
due to a deep snow pack in place.

Tuesday-Wednesday:
We did back off temperatures a couple of degrees from the previous
forecast, but the sunshine will be prevalent which will help not
feel as cold. The models are also showing the low level height
gradient (850mb) strengthening late Tuesday into Wednesday.
This may cause some impact at Bordeaux especially if the lee trof
strengthens in response to a weak shortwave riding along the
Canadian/Montana border. We did give that area a boost in wind
speeds on Tuesday night into Wednesday. Otherwise, current
forecast looks to be on track.

.LONG TERM...(Wednesday night through Monday)
Issued at 1235 PM MST Mon Jan 16 2017

Wednesday night/Thursday...A transitory ridge aloft will prevail
Wednesday night with increasing southerly flow aloft on Thursday
ahead of the next trough aloft with increasing cloudiness across our
western counties. Low and mid level moisture looks too limited for
any precipitation. Less cold Wednesday night as the airmass
moderates and continued relatively mild on Thursday based on
projected thicknesses and 700 mb temperatures.

Friday...Somewhat cooler in the wake of a weak cold front and with
increased cloudiness as a weak trough aloft moves across eastern
Colorado. Scattered snow showers over the Snowy and Sierra Madre
Ranges on moist flow aloft, dry elsewhere.

Saturday...A dry and seasonable day with weak flow aloft and some
cloudiness as indicated by the models.

Sunday...Ridging aloft again keeps our weather dry, aside from a few
snow showers over our Snowy and Sierra Madre Ranges.

Monday...Next progressive trough aloft progged to move across
Wyoming producing orographic snow showers over our Snowy and Sierra
Madre Ranges with dry weather elsewhere. Near seasonable high
temperatures expected based on projected thicknesses and 700 mb
temperatures near -8 Celsius.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday evening)
Issued at 850 PM MST Mon Jan 16 2017

VFR except for patchy late night fog for the western valleys, with a
brief period of MVFR-IFR cig/vis at Laramie. Light and variable
winds tonight will become southwesterly Tuesday. Sustained speeds
will average less than 15 kt.

&&

.FIRE WEATHER...
Issued at 1235 PM MST Mon Jan 16 2017

No real major fire weather concerns during the next couple of days
due to lighter wind speeds from the influence of the upper level
ridge. Temperatures are expected to climb into the 40s and 50s
which will yield in minimum relative humidities of 25 to 35
percent.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

UPDATE...MAJ
SHORT TERM...REC
LONG TERM...RUBIN
AVIATION...MAJ
FIRE WEATHER...REC



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