Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 261115

Area Forecast Discussion
National Weather Service Cheyenne WY
515 AM MDT Sun Mar 26 2017

.SHORT TERM...(Today through Monday Night)
Issued at 300 AM MDT Sun Mar 26 2017

Shortwave trough is currently moving eastward across southern
Colorado with weak upper level divergence producing scattered
rain/snow showers across the CWA.  Snow level across southeast WY
looks to be around 7500-8000 ft through the morning, so not going
see much accumulation across lower elevations.  Added areas of fog
to the southern Laramie Range through mid morning as WYDOT obs at
the I-80 Summit show vis below one half mile.   Weak front will push
across the entire CWA through 18Z, with highs struggling to get out
of the mid 40s to low 50s.  Kept at least a slight chance of showers
going through the afternoon, however QPF amounts will be light.

Shortwave ridging aloft will develop on Monday ahead of the upper
trough digging into the Great Basin.  Temps will warm some, with sfc
winds becoming southerly by the afternoon.  All the models show
precip developing by Monday evening for areas to the east of the
Laramie Range as llvl warm air advection / isentropic lift increase.
Did bump up PoPs across the plains of southeast WY on Monday
evening with the model agreement. With wet bulb zero heights
mostly around 7500-8500 ft, expecting mostly rain yet again
outside of the mtns.

.LONG TERM...(Tuesday through Saturday)
Issued at 300 AM MDT Sun Mar 26 2017

Tuesday through Wednesday...Robust upper trough will close off in
the vicinity of northern AZ by Tuesday morning and then slowly
drift eastward into the Texas Panhandle by Wednesday morning.
Models are in generally good agreement with the placement of the
upper features but there are some significant differences at lower
levels. Initially an inverted trough at 700 mb will extend north
into southern WY from the main low over northern NM. This trough
combined with ample upper diffluence will produce widespread
precip over much of the CWA Tuesday into Tuesday night. NAM and EC
are then quick to focus the low at 700 mb further south Tuesday
evening as the upper low becomes more cut off from the main flow.
The GFS is much further north with the 700 mb low and keeps
precip going through much of Tuesday night and into early
Wednesday morning. This is an important difference as all models
cool the boundary layer enough by late Tuesday night for snow over
much of the CWA. Looking at the ensemble spread, it seems that
the operational GFS is an outlier. It is a bit concerning however
and future trends will need to be watched. For now, think snow
will be confined primarily to the mountains with a quick shot of
snow over lower elevations of SE WY Tuesday evening prior to
precip ending as temperatures cool. Snow amounts may flirt with
criteria over the I-80 summit but amounts elsewhere should be 1-2
inches at most.

Thursday through Saturday...Brief ridging will bring dry and
warmer conditions on Thursday. Then the unsettled weather pattern
continues with models advertising yet another deep upper trough
impacting the area on Friday/Saturday. This system may have a bit
more cold air to work with and models have trended a bit further
north with the initial trough so this will also have to be
monitored for the potential for a more significant snowfall.


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 444 AM MDT Sun Mar 26 2017

West of the Laramie Range...areas of IFR ceilings with some light
rain/snow showers are persisting this morning. Latest satellite
imagery supports the HRRR idea of these conditions slowly eroding
from the west through the morning with most areas VFR by 18Z. East
of the Laramie Range...a period of MVFR ceilings with a few light
showers are possible for CYS and also for CDR in the northerly
flow behind the front. VFR elsewhere with light northerly winds
developing during the morning.


Issued at 300 AM MDT Sun Mar 26 2017

No fire weather concerns through early next week.  A weak cold front
will move across the region through this morning, bringing cooling
and higher humidity compared to yesterday.  Minimum afternoon
humidity values will stay above 25 percent through Wednesday.




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