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wy discuss


Office: CYS

FXUS65 KCYS 111133
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
433 AM MST Mon Dec 11 2017

.SHORT TERM...(Today through Wednesday)
Issued at 230 AM MST Mon Dec 11 2017

Winds and humidity continue to be the biggest forecast challenges
through the upcoming week with a strong winds event possible ahead
of a big pattern change next week.

Currently...We have a clipper system tracking across eastern South
Dakota this morning. A broad area of mid and high level cloudiness
over extreme northeastern Wyoming and southwestern South Dakota
this morning moving at a pretty fast rate and may be east of the
Panhandle by sunrise. A 1048mb surface high remains over southern
Idaho this morning.

Possibility of strong winds in the Panhandle today as a result of
the passing clipper system. GFS 850mb winds up to 40kts across the
Panhandle today after 18Z. Expecting wind gusts to 50mph out that
way this afternoon...especially over the southern Panhandle. 700mb
temperatures fall from +1C this morning to -7C over the northern
Panhandle by 18Z and 850mb temperatures falling 4-5 degrees C
through the morning hours. Will certainly help to keep humidity a
little higher today and have decided on holding off on any fire
headlines today.

Winds ease this evening after sunset with 850mb winds down to
25kts by 06Z tonight across the Panhandle. More westerly 700mb
winds will begin to warm up 700mb temperatures for Tuesday with
GFS 700mb temperatures back above freezing across the CWFA.

Next clipper system set to impact the Panhandle and areas east of
the Laramie Range Wednesday morning. Still dry, but increasing
winds again during the day Wednesday.

.LONG TERM...(Wednesday night through Sunday)
Issued at 230 AM MST Mon Dec 11 2017

Wednesday night/Thursday...Weak shortwave trough aloft sweeps across
our counties producing isolated to scattered snow showers over and
near our Northern Laramie, Snowy and Sierra Madre Ranges, dry
elsewhere Wednesday night. Windy and seasonably cool on Thursday
with 700 mb temperatures near -7 Celsius yielding maximum
temperatures from the mid 30s to mid 40s.

Friday...Marks the beginning of a long period of quite windy days as
the flow aloft backs to northwest and strengthens. Although
temperatures will be warmer due to strong downsloping winds and 700
mb temperatures near -2 Celsius, it will be quite windy based on
expected low and mid level gradients.

Saturday...Fast moving and low amplitude shortwave trough aloft
sweeps across our counties producing a chance of rain and snow
showers across our northern counties as well as over the Snowy and
Sierra Madre Ranges. Definitely cooler and quite windy based on
projected Craig to Casper 850/700 mb height gradients.

Sunday...Perhaps slightly warmer based on thicknesses, although
still quite windy per progged surface through 700 mb gradients and
the Craig to Casper 850/700 mb height gradients.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 431 AM MST Mon Dec 11 2017

Wyoming TAFS...VFR. Wind gusts to 33 knots through 00Z.

Nebraska TAFS...VFR. Wind gusts to 42 knots through 00Z.

&&

.FIRE WEATHER...
Issued at 230 AM MST Mon Dec 11 2017

We look to get a little reprieve in fire weather conditions today,
but winds will be strong across the Nebraska Panhandle. A Clipper
system, tracking into eastern South Dakota will drop a backdoor
cold front into areas east of the Laramie Range through the
morning hours. This front will bring mid and upper level clouds to
the Panhandle along with colder temperatures. So afternoon
humidity is expected to stay above critical levels today. Winds
ease some Tuesday before the next Clipper system moves into our
eastern zones Wednesday. A change in the overall weather pattern
expected next week as a wetter pattern looks to set up as early as
next weekend.

&&

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

$$

SHORT TERM...GCC
LONG TERM...RUBIN
AVIATION...RUBIN
FIRE WEATHER...GCC



Office: RIW FXUS65 KRIW 110927 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 227 AM MST Mon Dec 11 2017 .SHORT TERM...(Today through Wednesday night) Issued at 155 AM MST Mon Dec 11 2017 Imagery shows large scale continuing stagnant pattern in place with amplified blocking ridge west of the Mississippi (axis centered roughly along the west coast), a deep trof east, and WY west of the inflection point but still under nwrly flow aloft. An embedded wave/disturbance now moving through nern WY last night/this morning...pushing a clipper front with some increased north/northwest wind pushing through Johnson County, but without precip. The SFC has strong high pressure still guarding most of the entire wrn CONUS and nearly centered under the the ridge aloft. No significant active precipitation falling anywhere across the wrn CONUS. Through the forecast period: As expected, a wind shift/dry clipper front is moving through nern WY this morning with no significant sensible weather other than a wind shift. A persistent ridge/trof pattern (west/east) will continue across the CONUS with a record high or two possible Tuesday (locations that can mix effectively with little or no snow on the ground). Another dry wind shift/clipper front will move across nern WY once again Tuesday night. The ridge over the western CONUS will be slowly migrating wwd through this time so that by later Wednesday afternoon/night some weak ridge topping moisture may actually be able to get into FA along with a fast moving modest upper level embedded shortwave. Models differ in the ability to actually produce precipitation on the ground with MR models trying to get things a little wet while the Meso models remain dry for the most part. Have some doubt on the ability of this fast moving system with little moisture to be able to saturate close enough to the surface to produce much in the way of measurable precip. Guessing flurries will the extent of the "event" for the most part. .LONG TERM...(Thursday through Monday) Issued at 155 AM MST Mon Dec 11 2017 Dry conditions with slightly above average temperatures are expected Thursday in wake of a shortwave trough Wed/Wed night. The northerly flow aloft backs to the northwest Thursday night then more westerly Friday ahead of a shortwave trough pushing into the Pacific Northwest late Thursday night/Friday time frame. Mainly dry conditions are expected through the day Friday, with the main concern being some windy to possibly high wind in the Absaroka Mountains and Cody Foothills Thursday night/Friday morning. The GFS and ECMWF are showing a temperature gradient of about 9C to 12C developing late Thursday night/Friday morning which should induce at least some strong gap flow winds, with models hinting at some mountain wave activity as well. The shortwave trough over the Pacific Northwest/N. Intermountain west is progged to push east southeast across the northern/central Rockies Friday night and Saturday. This will give the forecast area a quick shot of some snowfall with the best shot over far west/northwest Wyoming Friday night. Chances for snow are less east of the Divide, but will respect this trough and at least have a slight chance mention as the Pacific cold front and associated dynamics pull across the area. In wake of the system on Saturday, global models are showing a moist west/northwest flow developing for Saturday night through Sunday night with the potential for significant snowfall across the west/northwest sections of the forecast area. A cold front Sunday night/Monday could spill some of the snow east of the Divide. && .AVIATION...(For the 12Z TAFS through 12Z Tuesday morning) Issued at 155 AM MST Mon Dec 11 2017 VFR conditions will occur through the period. Patchy fog or low clouds are possible in the far west valleys and Yellowstone National Park area this morning. Areas of MVFR/IFR conditions are expected due to low ceilings and visibility until 17Z. Patchy fog or low clouds may reform after 10Z Tuesday in these same locations. && .FIRE WEATHER... Issued AT 155 AM MST Mon Dec 11 2017 Fuels remain at critical levels across over portions of Fremont, Johnson and Natrona Counties. Other than some patchy freezing fog or low stratus in the far western valleys and perhaps vcnty of Boysen Reservoir in Northeast Fremont County this morning, expect mostly clear, cool and dry conditions today. A temperature inversion will remain in place for most locations across the forecast area with the exception of an hour or two during the mid afternoon period. Winds will be relatively light by Wyoming standards for much of the area except for the Wind Corridor and Johnson County which could see winds gusting out of the southwest or northwest 15 to 25 mph. Tuesday night will see another dry clipper front move through northeastern WY with a small chance for some flurries east of the Continental Divide later Wednesday afternoon/night. && .RIW WATCHES/WARNINGS/ADVISORIES...NONE. && $$ SHORT TERM...Braun LONG TERM...Murrell AVIATION...Ross FIRE WEATHER...Braun