Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 300950

Area Forecast Discussion
National Weather Service Cheyenne WY
350 AM MDT Fri Sep 30 2016

.SHORT TERM...(Today through Sunday)
Issued at 300 AM MDT Fri Sep 30 2016

Early morning satellite pix indicate shortwave trough lifting
northeast through Wyoming and Colorado. Regional radar mosaics at
08z depict pockets of showers and weak thunderstorms across NW
Colorado, S Montana, and W South Dakota, with a general minimum in
between across Wyoming. Will see precipitation chances spill into
areas in and around the SE Wyoming mountains and adjacent plains
today as current NW Colorado precip spreads northeastward. Progd
SBCAPE values of 200-500 j/kg by afternoon suggest some convection
will be deep enough to produce a few lightning strikes. A weak
shear field will limit strength and coverage. Am not expecting
convection to make it any farther east than the North Platte
River. Another unseasonably warm day ahead albeit a few degrees
cooler compared to yesterday given the greater coverage of clouds
in association with sub- tropical moisture plume. Shortwave trough
departs this evening and with loss of instability, any convection
will collapse, yielding dry conditions tonight.

A similar story for Saturday minus an organized shortwave. De-
amplifying ridge overhead along with a developing lee-side
pressure trough will be enough for mainly mainly mountain
afternoon/evening weak convection. Did spill convection chances
into the Laramie Valley and in the vicinity of the Cheyenne Ridge
Saturday evening. Decreasing instability Saturday evening will
again bring an end to any chances of precip. High temperatures
Saturday will again be unseasonably warm reaching almost 70 west
and 70s to near 80 east.

Flow backs to the southwest on Sunday as next week`s weather maker
inches closer. Best convection chances will be in areas along and
west of the Laramie Range as the approaching jet combines with
mid-level destabilization in falling heights ahead of the trough.
The main limiting factor Sunday will be limited mid-level
moisture, yet some small chances are still warranted in the west.
Highs will again be unseasonably warm with upper 60s west and
mainly 70s east.

.LONG TERM...(Sunday Night through Thursday night)
Issued at 350 AM MDT Fri Sep 30 2016

The main forecast concern for the Long Range will be the upper
level low forecast to move east/northeast across the northern and
central Rockys Monday through Wednesday. Models indicate that this
system will stall as it moves into the Great Plains...resulting in
a prolonged period of cool and unsettled conditions. The big
question is evolution and position of the storm system as it moves
across the Rockys and into the Plains. Operational models and
ensembles vary greatly with regards to the position...which
ranges from southeast Montana to northeast Colorado by late
Tuesday/early Wednesday...and is roughly the time period when the
system stalls. The exact position of the upper level low as well
as the surface low will determine precipitation amounts...amount
of cold air...and downslope flow component of the llvl wind
through the middle of next week. Due to these confidence
issues...kept POP roughly the same as previous forecast with the
highest probabilities from Rawlins...northeast to Douglas and
Chadron Nebraska. The ECMWF is the forecast north with it`s track
of the low...even further north than the ensemble outliers. Kept
lower POP over the I80 corridor...but still kept the mention of
precipitation. In looks to be cold enough for snow
above 6000 feet include most of southeast Wyoming. So will have to
monitor the track of this low closely over the next 48 to 72
hours. A very windy period is possible Tuesday and Wednesday due
to rapid cyclogenesis across the northern Plains. Very confident
that Wyoming and western Nebraska will experience a sharp cool
down beginning Monday night/Tuesday as a strong cold front pushes
through the area. Highs may struggle to reach 50-55 degrees
Tuesday and Wednesday...especially across southeast Wyoming.

By Wednesday night and Thursday...all models show the upper level
low lifting northeast into the Dakotas...with improving conditions
across southeast Wyoming and most of western Nebraska. will remain cool and windy with highs in the 50`s to
mid 60`s.


.AVIATION...(For the 12Z TAFS through tonight)
Issued at 300 AM MDT Fri Sep 30 2016

VFR conditions expected to continue today and into
tonight...although some rain shower and thunderstorm activity is
possible over the next 12 to 18 hours...especially across
southeast Wyoming. CIGS will gradually lower down to 8k to 14k
feet AGL today as a weather disturbance moves across the area from
KRWL eastward to KCYS.


Issued at 300 AM MDT Fri Sep 30 2016

No fire weather concerns for the foreseeable future as minimum
relative humidities drop only to around 25-30 percent through the
weekend and colder/unsettled conditions develop next week. There
may be a few weak showers/storms mainly across higher elevations
of southeast Wyoming through the weekend. Accumulating snow will
be possible Monday Night through Tuesday Night in elevations above
6000 feet.




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