Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 180344

Area Forecast Discussion
National Weather Service Cheyenne WY
944 PM MDT Sat Mar 17 2018

Issued at 807 PM MDT Sat Mar 17 2018

At this point, the forecast still looks to be in decent shape, so
no further changes to the headlines at this point. Latest water
vapor loop was showing the snow showers developing around Unitah
and Sweetwater counties. This appears to be developing on the
leading edge of jet streak and orographic lift. Not seeing any
evidence of these showers pushing east anytime soon. Therefore, we
may end up remaining fairly dry tonight. However, cannot rule out
a stray shower or two across the western portion of the forecast
area. We did back off on pops a bit to reflect this. Still cannot
rule out some patchy fog in the North Platte River Valley.
However, wind speeds are still a bit gusty at Scottsbluff which
limit the fog development until closer to daybreak when things
decouple a bit more.

Latest water vapor loop was showing the bulk of the shortwave
energy moving through Arizona and Utah. This shortwave energy
should start to move the mid level trof axis eastward and cause
snowfall to develop across Rawlins by mid morning, Arlington, and
Laramie by early afternoon. We did back off on the timing a bit
for a more later arrival. Of interesting note, the latest NAM
tries to develop a meso low around 700mb in northern Niobrara
county tomorrow morning which breaks out snowfall in that region.
At this time, there is not enough evidence to support this.
Therefore, we have followed closer to the trends that are
occurringin the west.


.SHORT TERM...(This evening through Monday)
Issued at 245 PM MDT Sat Mar 17 2018

Main forecast concern will be the next snowfall event expected to
begin sometime around noon Sunday west of I-25. For this
afternoon, pleasant weather after a cold morning with a few areas
dipping into the single digits due to a remnant surface snowpack.
Mostly sunny skies will continue this afternoon with increasing
clouds tonight. Can not rule out some fog forming across the North
Platte River valley and areas north which still have a decent
snowpack still on the ground. Some of this fog could be dense
tonight, but do not expect it to linger into late Sunday morning.

For SUNDAY, upper level trough will slowly move east and become
negatively tilted as it moves across the central and southern
Rockies. With the lack of a substantial ridge across the Great
Plains, this storm is expected to take a track further south
compared to the Thursday/Friday storm. However, there will be more
cold air with this system over portions of the forecast area, so
impacts may be similar through Sunday night. All models show
decent upper level diffluence, although not as strong nor as long
compared to the previous storm, along with upslope flow and llvl
convergence. Before the front moves across the area, can not rule
out some thunder showers along the I-80 corridor before rain
changes to snow. Otherwise, any rainfall should quickly change to
all snow west of I-25 Sunday afternoon. With increasing north to
northeast winds becoming northwest in the evening, believe this is
a good set up for heavy snowfall along and near the Snowy Range,
so upgraded the Watch to a Warning. Issued Winter Wx Advisories
for the surrounding valleys and foothills for 3 to 6 inches of
snow through Sunday night. Did not include the Saratoga area, but
that may need to be added shortly depending on how much of a snow
shadow effect is observed in the upper North Platte River valley.

For Sunday night and Monday, will have to monitor the track and
evolution of the upper level low. Right now, models continue to
show it progressing eastward along the Kansas/Oklahoma border.
Further north, all models indicate a deformation axis with
convergence along and east of I-25 for a time as a shortwave
disturbance moves east behind the main storm. Will have to keep an
eye on snowfall rates, especially across Converse and Niobrara
county as well as the southern Nebraska panhandle, Sunday night.
for now, went with 1 to 3 inches of snow along and east of the
I-25 corridor. May see some locally higher amounts across the
southern panhandle and there is the potential for Winter Wx
Advisories to be extended eastward into this region as well for
Sunday night.

.LONG TERM...(Monday night through Friday)
Issued at 317 AM MDT Sat Mar 17 2018

A trend to drier and milder weather will occur by midweek as upper
ridging builds over the western CONUS Tuesday and moves east to
the plains states by late Thursday. A weak impulse ahead of the
building ridge may bring a few snow showers mainly to the mtns
Tuesday. Max temps a little above seasonal averages Weds and
Thursday. The next upper trough looks to affect the region late
Thursday through Friday with pcpn returning mainly over western
parts of the CWA then.


.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 543 PM MDT Sat Mar 17 2018

The main concern with this TAF issuance is the snow potential in
the Laramie and Rawlins TAF sites Sunday morning. Latest water
vapor loop was showing a lot of upper level energy across the
Intermountain west. This energy is expected to move east over
Utah tonight and push into Colorado tomorrow. As a result, we will
see an increase in upper level diffluence and precipitation
across western Wyoming tonight. This precipitation will spread
east and turn to mostly snow in the higher elevations of KRWL and
KLAR tomorrow morning. Ceilings will most likely start out around
10kft, but will quickly saturate and become IFR by midday. KCYS
may see some snowfall and lower ceilings late in the
day. Meanwhile, the Nebraska TAF sites may experience some patchy
fog and low MVFR/IFR ceilings with the best chance of fog in the
North Platte Valley. These ceilings should gradually climb by
midday as the boundary layer warms.


Issued at 207 PM MDT Sat Mar 17 2018

No Fire Weather concerns this weekend and into next week with
high relatively humidities and another chance for rain and snow
Sunday and Sunday night. Warmer and somewhat drier conditions are
expected Tuesday through Thursday.


WY...Winter Storm Warning from 9 AM Sunday to 6 AM MDT Monday for

     Winter Weather Advisory from 9 AM Sunday to 6 AM MDT Monday for

     Winter Weather Advisory from noon Sunday to 6 AM MDT Monday for

     Winter Storm Watch from Sunday afternoon through late Sunday
     night for WYZ116.



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