Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 251813
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1213 PM MDT MON JUL 25 2016

.SHORT TERM...(Today through Wednesday)
Issued at 219 AM MDT Mon Jul 25 2016

With moist southerly llvl winds rising up the Cheyenne ridge,
getting some fog from Vedauwoo to the I-80 Summit.  Not seeing this
fog spreading eastward into Laramie county at this point, and winds
will become more southwesterly by sunrise.  Heights aloft will rise
today and the sfc trough will move into Laramie county by the aftn.
The models continue to disagree on the amount of convective
initiation along the trough this aftn with the GFS and NAM showing
really no tstms.  The cap will be stronger today with warmer temps
aloft.  However, other models like the HRRR and NCAR ensembles show
initiation and the potential for a few of these storms to become
stronger and have rotating updrafts.  This makes sense given the
favorable instability and deep layer shear to the east of the
trough. Expanded the area of slight chance pops this aftn/early
evening to encompass much of the Nebraska Panhandle and into Laramie
county.  Also added small hail and gusty winds with these storms
given the marginal risk from SPC.

Synoptic pattern does not change much for Tuesday with the sfc
trough yet again moving to around the WY-NE border. The best
instability may be just east of our eastern CWA border, however
this will depend upon how far eastward the dryline will push.
While flow aloft is less on Tuesday, there is still enough bulk
shear (around 35 kts) to warrant a marginal risk of svr storms yet
again. Midlvl ridge shifts into southern Utah by Wednesday with
flow aloft becoming more northwesterly. A front will push across
the plains with higher dewpoints spreading westward toward the
Laramie Range. It will be another day of isolated/scattered
convection (potentially strong to svr). Temps will be a few
degrees cooler on Wednesday.

.LONG TERM...(Wednesday night through Sunday night)
Issued at 305 AM MDT Mon Jul 25 2016

Main forecast concern will be the timing and location of a cold
front midweek...which will push across the area sometime
Wednesday or Thursday. Medium to long range models continue to
show the center of the upper level high pressure retrograding
westward into the Great Basin region for the mid to late week.
Northwest flow aloft will develop across Wyoming and Nebraska by
late Wednesday and Thursday with temperatures lowering closer to
normal. Although the high will block subtropical moisture from
advecting northward into the forecast area...upslope easterly flow
will develop by late Wednesday resulting in a good supply of low
to midlevel moisture. A frontal boundary will also be positioned
somewhere between the Laramie Range and the Nebraska/Wyoming
border. This boundary will fluctuate between these two positions
Thursday and Friday as a wave of low pressure will move southeast
along this front. Will need to closely monitor the timing of
potential severe thunderstorms over the next few days...mainly
Thursday but Wednesday evening and Friday are possible
considerations as well. There still remains some distinct timing
differences between models...with the ECMWF and GEM further west
with front earlier in the forecast period...as early as Wednesday
evening. The GFS is now a full 24 to 30 hours behind...showing the
front stalling across western Nebraska Wednesday night and then
eventually moving southwest into eastern Wyoming Thursday evening.
All models show this frontal boundary stalling near the Laramie
Range for Friday...so there is reasonable confidence with this
solution. Kept the highest POP for Thursday evening and Thursday
night for scattered and even numerous thunderstorms possible. High
temperatures in the upper 70`s to upper 80`s are expected Thursday
and Friday.

For next weekend...model solutions begin to diverge again as the
GFS shows drier air a ridge axis moving overhead. The ECMWF shows
this ridge axis much further west through Sunday. Expect
temperatures to increase somewhat over the weekend with highs back
into the 90`s across the eastern plains. Kept POP below 20 percent
due to considerable uncertainty in thunderstorm coverage.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1213 PM MDT Mon Jul 25 2016

Main concern to aviation ops will be the timing and coverage of
thunderstorm activity later this afternoon and into early this
evening. Latest forecast information suggests KCYS...KAIA and KCDR
will be best positioned to see thunderstorm activity 21-04z.
Coverage appears to be less compared to yesterday. Thunderstorms
will dissipate by late evening...with another round anticipated
Tuesday afternoon.

&&

.FIRE WEATHER...
Issued at 219 AM MDT Mon Jul 25 2016

Minimal fire weather concerns through Wednesday.  Minimum humidity
values will fall to 10-20 percent each afternoon for areas to the
west of the Laramie Range.  However, wind gusts will stay mostly
below 20 mph and thus no fire weather threat is anticipated.
There will be a slight chance of thunderstorms across the plains
during the afternoon and evening through Wednesday, however the
chance of widespread wetting rains will be small.

&&

.CYS Watches/Warnings/Advisories...
WY...None.
NE...None.

&&

$$

SHORT TERM...ZF
LONG TERM...TJT
AVIATION...CAH
FIRE WEATHER...ZF



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