Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 221720

Area Forecast Discussion
National Weather Service Cheyenne WY
1120 AM MDT Mon May 22 2017

.SHORT TERM...(Today - Wednesday Night)
Issued at 350 AM MDT Mon May 22 2017

Locally dense fog has developed over the I-80 summit early this AM
in response to ample boundary layer moisture and fairly light wind
across the area. Do not believe this will last all that long given
model guidance showing a drying trend over the next few hours, and
satellite signals already hinting at some erosion. This should not
require an Advisory, but will continue to monitor trends.

A vigorous upper-level shortwave was evident over eastern MT early
this AM per morning WV satellite imagery. This feature is expected
to move south across the area today, along w/ a surface cold front
which was analyzed roughly along the Canadian border at 9z. Shower
activity will increase through the day with dynamic support in the
left exit region of a 100 knot H25 jet streak oriented from north-
to-south across central WY and pronounced mid-level PVA. We should
see a few thunderstorms as well, but the main question is just how
many lightning strikes we will get in what may be a some-what weak
CAPE environment. GFS soundings show less than 200 J/kg CAPE while
the NAM suggests 500-1000 J/kg over most of the CWA. We will favor
the NAM solution as higher instability values make more sense with
cyclonic flow aloft and falling 700 mb temperatures with northerly
flow post-fropa. We also decided to add a mention of small hail to
the entire CWA for this afternoon/early evening as any given storm
could produce pea sized hail w/ this cold and energetic system. We
expect this most widespread activity between 21-03z with excellent
dynamics interacting w/ low-level convergence along the cold front
in the mid to late afternoon. The HRRR also suggests scattered-to-
numerous coverage of showers/storms, so we have increase PoPs into
the likely category based on HRRR output. Showers will decrease in
coverage into the night as instability wanes and subsidence begins
to increase behind the shortwave.

A trend toward warmer and drier weather will commence on Tue, with
the warmest day being on Wednesday as H7 temperatures climb to +10
to +12 deg C in response to upper-level ridging. One more weak mid
level disturbance is still evident in the models on Tuesday, so we
may see a few showers over eastern areas w/the lingering influence
of cyclonic flow, but would not expect to see much.

.LONG TERM...(Thursday - Sunday)
Issued at 350 AM MDT Mon May 22 2017

The end of the week will feature a positively tilted upper trough
moving slowly across the northwestern CONUS, and eventually across
the central Rockies and High Plains through the weekend. A couple of
embedded shortwaves will move over the forecast area in southwest
flow Thursday-Saturday with a cold front dropping south across the
area on Thursday morning. The front looks to hang up along the
Laramie Range Thursday afternoon, so after an active morning due to
an approaching shortwave and frontal passage, anticipate additional
activity early afternoon as well along and near the lee of the
Laramie Range. Instability looks to decrease quickly after frontal
passage, but would not be surprised to see a strong storm or two
late morning through early afternoon before the stable high pressure
settles in. Otherwise, expect showers to continue across much of the
forecast area in the later afternoon as energetic flow remains
overhead, quickly diminishing by evening. Friday looks to be a
typical post-frontal return flow day, so chances for showers and
thunderstorms will be best along and west of I-25 through the day,
with perhaps an expansion eastward into the panhandle in the evening
depending on strength of llvl moisture advection and development of
the llvl jet. The main trough and attendant cold front will sweep
southeast across the area on Saturday, bringing another wet and
stormy day. Conditions on Sunday are somewhat questionable at this
stage due to differences in the speed of this trough between the GFS
and ECMWF. The GFS is much slower than the EC, with another
shortwave bringing a chance for afternoon showers. The EC is dry and
mostly clear with high pressure building overhead. For now, split
the difference and kept some PoPs in the forecast. Sunday night
should clear out.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday morning)
Issued at 1120 AM MDT Mon May 22 2017

A cold front will move southeast across the area through this
afternoon. Scattered showers and a few thunderstorms will develop
with the front and move southeast with associated brief periods of
MVFR cigs and vsbys. Pcpn should end early this evening.


Issued at 350 AM MDT Mon May 22 2017

No fire weather concerns due to recent, widespread heavy rain/snow
and cooler temperatures.


Issued at 225 PM MDT Sun May 21 2017

Concerns through the middle of this week will focus on runoff from
the recent heavy precipitation into area creeks and streams east of
the Laramie Range. Although flooding is not anticipated, many of the
creeks and streams may reach bankfull by the middle of the week.
Fortunately, cooler temperatures over higher elevations will continue
this week, which should slow the snow melt and resultant runoff into
the mainstem rivers. Will continue to monitor these water levels
closely over the next few weeks.




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