Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 222037

Area Forecast Discussion
National Weather Service Cheyenne WY
237 PM MDT Sat Jul 22 2017

.SHORT TERM...(This evening through Monday)
Issued at 237 PM MDT Sat Jul 22 2017

Sfc trough is currently located from CYS to CDR with drier air to
the northwest and higher dewpoints to the southeast.  Cumulus and
some storms are developing in the convergence along this trough.
Kept mainly 20-30 percent PoPs in the fcst from Laramie county
eastward into the southern Panhandle through the early evening.  The
deep layer shear is better this aftn at around 30-35 kts, thus
cannot rule out an isolated stronger storm with small hail.  Any
storm activity should drop wind down by late evening or move into
northern Colorado.

Heights aloft rise on Sunday with the midlvl ridge axis located near
the Laramie Range by the aftn.  After slightly cooler temps today,
we will see around 5 degrees of warming at most locations.  With the
subsidence aloft and PW values at a dry 0.5-0.75 inches, convection
should remain quite limited.  The exception may be along the
southern Laramie Range where southeasterly sfc winds will result in
weak convergence.  Monday will likely be the warmest day with highs
rising into the 90s for areas to the east of the Laramie Range.
Midlvl flow will remain westerly with low PW values across the CWA.
A weak shortwave trough moving into central WY by late aftn and
evening may promoted isolated storm develop over the Sierra Madre
and Snowy Ranges.

.LONG TERM...(Monday night through Saturday)
Issued at 237 PM MDT Sat Jul 22 2017

Medium range models are in good agreement with overall pattern
progression in retrograding the upper high west from the southern
plains towards the four corners. As this occurs, southwest flow
around the periphery of the upper high will be effective in
advecting monsoonal moisture into the intermountain west. Have
added thunderstorm chances to Tuesday as a southward moving sfc
front and mid-level shortwave energy moving thru. The sfc front
looks to stall southwest of the CWA on Wednesday, which should
focus convective chances mainly to areas in the hier terrain of
southeast WY. Afternoon thunderstorm chances continue on Thursday
as low-level convergence is centered on the Laramie Range. May
actually see a drier air mass work in for Friday and Saturday as
the upper high settles into the four corners allowing for drier
northwest flow into the region. Instability parameters look fairly
meager through much of the week, so wouldn`t expect to see much
widespread severe potential. The one exception may be Thursday,
where the GFS progs SBCAPE`s around 1500-2000 j/kg along with 40
knots of 0-6 km bulk shear. Much can change by then, but trends
bear watching. After one last warm day on Tuesday, afternoon
highs will generally cool into the 80s for Tue-Sat (exception
being 90s across lower elevations of the Panhandle).


(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1139 AM MDT Sat Jul 22 2017

Main concern to aviation ops over the next 24 hours will be
convective potential 20-02z today. Latest forecast information
points to an area of shower/thunderstorm development in the
vicinity of KTOR-KBFF by around 19z, with expansion east and
movement south/southeast through 01z. Have kept VCTS mention at
KBFF, KAIA and KSNY as a result of this possibility. There is
still some uncertainty just how far west these thunderstorms will
be able to develop, but also left VCTS at KCYS for now. Will
continue to re-evaluate trends thru the afternoon. VFR conditions
anticipated once convection departs after 01-02Z.


Issued at 237 PM MDT Sat Jul 22 2017

Minimal fire weather concerns through early next week.  With warm
temperatures in place, minimum afternoon humidity values will drop
to 15-20 percent over portions of southeast Wyoming on Sunday and
Monday.  The lowest humidity of 10-15 percent will be found across
lower elevations of Carbon county.  Despite the dry airmass, winds
are expected to be relatively light so no critical fire weather
conditions are anticipated.





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