Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 240515

Area Forecast Discussion
National Weather Service Cheyenne WY
1115 PM MDT Sat Sep 23 2017

Issued at 924 PM MDT Sat Sep 23 2017

Band of jet induced N-S oriented precipitation continues to march
east across the CWA this evening. Precipitation totals have
generally held in the 0.25-0.4" range thus far. Latest HRRR
depicts this band slowly marching east thru the panhandle
overnight. There is some suggestion that the SE Wyoming plains may
see an additional band developing before midnight. Have boosted
precipitation chances along this band during the overnight period.
Will be adjusting precipitation chances downward for areas west
of this band with the next update before 10A.

Dense fog advisory has been issued over the I-80 summit as recent
observations and webcams show very limited visibilities. Given the
earlier rain, precip coming to an end as well as heavier than
normal travel expected along this corridor over the next few
hours, felt a heightened level of messaging was warranted.


.SHORT TERM...(This evening through Monday)
Issued at 337 PM MDT Sat Sep 23 2017

Water vapor imagery shows the closed upper low spinning across
eastern Nevada with good upper level divergence spreading over the
CWA.  The short term models are in good agreement at showing deep
layer lift increasing over areas along and west of the Laramie Range
this aftn, and then spreading over the plains from late aftn through
the evening.  Raised PoPs through 06Z, with likely/categorical PoPs
over almost the entire CWA.  Wet bulb zero heights and snow levels
will remain around 10500 ft through the early evening, so any
accumulating snows should be confined to the higher peaks of the
Snowy Range through tonight. However, cannot rule out snow mixed
with rain down to 8500-9000 ft in pockets of moderate to heavy
precip. The best forcing will shift eastward into the western
Nebraska Panhandle after 06Z, with a diminishing coverage of precip
by late tonight especially over southeast WY.

Upper low will move into central Utah by Sunday morning, with colder
air aloft moving into areas west of the Laramie Range.  By Sunday
aftn, 700mb temps will be around -1C over Albany/Converse counties
with snow levels dropping to generally 9000 ft.  As precip chances
increase once again by late Sunday morning into the aftn over
western zones, will likely see a better chance of accumulating snow
on some of the mtn roads, especially in the Snowy Range.  Night
shift may have to consider a Winter Weather Advisory for some of the
mtn zones for Sunday aftn/evening.  It will be another very cool day
with highs a good 20-25 degrees below normal.  Upper trough will
weaken and lift across Wyoming on Monday.  Isolated/scattered
showers will be possible, mainly over southeast WY through the day.

.LONG TERM...(Monday Night through Friday)
Issued at 337 PM MDT Sat Sep 23 2017

Monday night...As the trough aloft slowly begins to exit our
counties, will see a slight chance of showers over our northern and
eastern counties in the evening, then mostly dry after midnight.

Tuesday...Will be dry and warmer as a drier airmass invades our
region along with warming temperatures aloft.

Wednesday...Warming trend continues as low level temperatures warm
under cyclonic flow aloft. Continued dry as the low and mid levels
remain relatively dry.

Thursday...Temperatures nearly the same or slightly warmer than
Wednesday as a weak cold front moves over our counties and the
airmass recovers quickly.

Friday...Nearly stationary weak upper low remains over the Four
Corners, with some mid level moisture working northward into our
southern counties, helping to produce a slight chance of afternoon
showers over our Snowy and Sierra Madre Ranges, dry elsewhere.
Temperatures nearly the same or slightly warmer than Thursday,
through warming limited by increasing cloud cover and low level
south winds.


.AVIATION...(For the 06Z TAFS through 06Z Sunday evening)
Issued at 1115 PM MDT Sat Sep 23 2017

Fairly widespread IFR cigs with some patchy fog affecting most
terminals through the day Sunday in moist upslope flow.  Some
improvement to MVFR cigs expected Sunday afternoon.


Issued at 337 PM MDT Sat Sep 23 2017

No fire weather concerns through early next week.  It will be cool
and cloudy through Monday with widespread wetting precipitation


WY...Dense Fog Advisory until 9 AM MDT Sunday for WYZ116-117.




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