Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 170925
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
325 AM MDT WED AUG 17 2016

.SHORT TERM...(Today through Friday)
Issued at 320 AM MDT Wed Aug 17 2016

Warm weather will continue today as weak upper-level ridging remains
intact over the central Rockies. Water vapor satellite imagery shows
a few weak disturbances undercutting the ridge, which combined w/low
level convergence along the Laramie Range should promote isolated to
widely scattered showers and thunderstorms this afternoon. Inverted-
V sounding profiles suggest potential for strong/gusty outflow winds
with any activity today. The ridge should begin to break down on Thu
as a weak trough meanders across CO and southern WY. Convection will
become more widespread on Thursday as instability increases owing to
cooling temperatures aloft. NAM CAPES around 1000-2000 J/kg combined
with 25-35 knots of 0-6 km shear would suggest a few stronger storms
are possible, so we are in agreement with the MRGL risk on the Day 2
Outlook from SPC. Strong cold frontal passage still on track for Thu
night and Friday. In fact, GFS/ECMWF/NAM are all considerably colder
with H7 temperatures with the 0C line encroaching our northern zones
by 00z Saturday. This cold advection, combined with strong northeast
low-level upslope and excellent convergence suggests very widespread
clouds/precipitation, especially along/east of the Laramie Range. We
would expect guidance to be much too warm in this scenario, so opted
to lower forecast highs around 5 degrees below consensus as 71 deg F
just seems way too high for Cheyenne in this pattern. There are good
indications that many areas could see at least a tenth of an inch of
rain on Friday afternoon. By late afternoon, could see a mix of rain
and snow over the higher mountains as thermal profiles cool.


.LONG TERM...(Friday night through next Tuesday)
Issued at 325 AM MDT Wed Aug 17 2016

Cool and unsettled weather appears to be on track into early
Saturday morning...but models are now in better agreement with a
more progressive solution as the strong upper level trough is
forecast to move east of the forecast area by Saturday afternoon.
Yesterday...several models were showing the trough stalling across
Wyoming resulting in the potential of a prolonged rainfall event
across southeast Wyoming and western Nebraska into Sunday. Will
continue to monitor this possibility...but the 00Z models and
ensembles now show the trough axis and the bulk of the cooler air
moving across the area Friday night. The GEM is the only minor
exception...but is only 6 hours behind the GFS and ECMWF which is
not too significant timing-wise anyway. Models are trending colder
however...with 700MB temperatures as low as 0C to -2C by early
Saturday. Still expect scattered to numerous showers/thunderstorms
Friday night...but most of the activity may just be through late
in the evening before conditions improve. Can not rule out some
snowfall in the mountains above 10000 feet due to the cold 700MB
temperatures. High temperatures on Saturday will be quite cool for
this time of the year and generally 10 to 20 degrees below
average. Highs in the upper 50`s to mid 60`s will be common...with
a few locations reaching 70 degrees during the day.
However...Breezy conditions will make most locations feel cooler.
Will also need to monitor the potential for frost or freeze
headlines for the more sheltered valleys...mainly for early Sunday
morning.

For early next week...all models point towards drier conditions
as the main trough will be well east of the area. Temperatures
will begin to moderate with highs returning to the 80`s or upper
70`s by Monday and Tuesday. A ridge axis will move into Wyoming
and Utah on Monday...so expect precipitation chances to be low
before some subtropical moisture begins to move back into the
central Rockys by the middle of next week.

&&

.AVIATION...(For the 12Z TAFS through Wednesday night)
Issued at 546 PM MDT Tue Aug 16 2016

VFR conditions expected today and into tonight with south to
southeast winds around 10 to 15 MPH. Showers and thunderstorms
will develop over the mountains with a few thunderstorms impacting
the adjacent valleys and high plains locations just east of I25.
Best chance for thunderstorms appears to be KLAR and KRWL this
afternoon...with a stray thundershower possible at KCYS this
evening. Otherwise...western NE terminals will remain dry through
most of today.

&&

.FIRE WEATHER...
Issued at 250 AM MDT Wed Aug 17 2016

Fire weather concerns will be minimal over the next few days w/light
winds expected across the area. Dry air will persist for those areas
west of the Laramie Range where humidities are expected to be in the
teens today. There will be daily chances for showers and storms with
gusty/erratic winds the main hazard. Widespread wetting rains likely
for Friday, along with cooler temperatures leading to much higher RH
values.

&&

.CYS Watches/Warnings/Advisories...

WY...None.
NE...None.
&&

$$

SHORT TERM...CLH
LONG TERM...TJT
AVIATION...TJT
FIRE WEATHER...CLH



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