Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 201806
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1206 PM MDT Thu Apr 20 2017

.SHORT TERM...(Today through Friday Night)
Issued at 359 AM MDT Thu Apr 20 2017

Main concerns in the short term revolve around the digging trough
currently seen just off the NW US coast early this morning.  As this
trough moves closer late this afternoon, upper diffluence along with
good PVA and height falls aloft will induce shower development over
the higher terrain of SE WY. Forecast soundings indicate weak CAPE
with moistening mid levels so can`t rule out a few thunderstorms as
well.

The upper trough will become negatively tilted as it moves overhead
tonight and eventually form a closed circulation of sorts over
northeast CO before it tracks quickly eastward into KS by Friday
afternoon. As this happens SE WY and western NE will be in an area
of favorable upslope SE to E flow in the low levels along with a mid
level deformation zone.  This would favor a period of widespread and
rather heavy precip especially along and east of the Laramie Range.
Snow levels gradually lower tonight to around 6 to 7 kft by early
Friday.  Think most snow accumulation will be confined to the Snowy
Range and Sierra Madres so went with an advisory in these areas. Not
the greatest wind direction for orographic enhancement, but there
should be sufficient moisture and lift with the mid level wave along
with some instability to maintain snow shower activity through
Friday morning.  The next area of concern is the Laramie Range
including the I-80 summit.  GFS and NAM show the heaviest precip
occurring between 06 and 12Z Friday and then shifting eastward
through Friday morning.  This is favorable diurnal timing for snow
accumulation.  Limiting factors will be recent warm temperatures
which have increased the sub surface temps significantly as well
just enough solar radiation Friday morning to keep snow from
accumulating significantly on the road surfaces.  Weighing all these
factors, decided to go with a Winter Weather Advisory for the South
Laramie Range as well.  Model qpf, should it verify and be all snow
would justify a warning but due to the reasons stated above, think
impacts to the roads will still be advisory criteria.  Over the
remainder of SE WY outside of the aforementioned areas, rain may mix
with or change to snow briefly early Friday morning.  Snow
accumulations should be 2 inches or less as both boundary layer and
ground temps will be marginal for much accumulation.

Additional weak shortwave energy will dive SE through the area on
Friday afternoon and evening which will probably keep shower
activity going especially over the mountains, but the steadier
precip should be coming to an end during the morning on Friday for
most areas.

.LONG TERM...(Saturday through Wednesday)
Issued at 359 AM MDT Thu Apr 20 2017

Saturday will be a cool and mostly cloudy day with weak troughing
overhead.  May see some isolated showers through the morning. Better
warming will occur by Sunday as the upper ridge axis moves overhead
and 700 mb temps increase to around 6C.  Highs will be back into the
 mid 60s to mid 70s for areas to the east of the Laramie Range.
Southwest winds will become breezy by Sunday aftn over Carbon county
as the gradient increases ahead of a shortwave moving into the Great
Basin.  The models are in rather good agreement at showing this
shortwave moving quickly across northern WY and then into the
northern Plains on Sunday night into Monday.  The main impact from
will be an increase in the llvl gradient (CAG-CPR gradient rises to
50-60 meters) and widespread breezy to windy conditions on Monday.
Progressive zonal flow aloft will prevail through midweek.  This
will bring periods of lower elevation showers and some snow across
the mtns. Temps will be seasonal from Monday through Wednesday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1151 AM MDT Thu Apr 20 2017

VFR conditions should prevail across much of southeast Wyoming and
the western Nebraska Panhandle through this afternoon, although we
expect rain showers to begin developing roughly along and south of
I-80 from KRWL to KCYS as early as 20z today. Ceilings will likely
lower through the afternoon/evening with a good potential for LIFR
or IFR conditions at KCYS/KLAR after 03z Fri. After 09z, pcpn will
become heavier and more widespread, especially between Laramie, WY
and Kimball, NE. A changeover to snow is expected at elevations at
or above about 6000 feet, so there is a risk for accumulating snow
at KCYS early in the AM with visibilities possibly reduced to one-
half mile. In the Nebraska Panhandle, precipitation will likely be
rain and overall aviation impacts should be lower. However, expect
MVFR to locally IFR ceilings to spread northeast across the region
between 12z-18z Friday.

&&

.FIRE WEATHER...
Issued at 359 AM MDT Thu Apr 20 2017

No fire weather concerns through Saturday.  A storm system will move
across the region this evening into Friday bringing cooler temperatures
and widespread precipitation.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory from 6 PM this evening to 6 PM MDT
     Friday for WYZ112-114-116.

NE...None.
&&

$$

SHORT TERM...DEL
LONG TERM...ZF
AVIATION...CLH
FIRE WEATHER...ZF



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