Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 211528
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
928 AM MDT Mon Aug 21 2017

.UPDATE...
Issued at 924 AM MDT Mon Aug 21 2017

Dense Fog Advisory allowed to expire at 8 AM. Recent webcam, VIS
satellite and surface observations revealed gradual dissipation
of the fog and low stratus across portions of western Nebraska.
This trend will continue through late morning, with partly cloudy
skies forecast for the total solar eclipse late this morning and
early this afternoon. Adjusted sky grids to reflect current trends.

UPDATE Issued at 450 AM MDT Mon Aug 21 2017

Dense fog has been in place over eastern portions of the Nebraska
Panhandle over the past couple of hours, with observed
visibilities of 1/4mi. Expect this to continue through the early
morning, thus issued a Dense Fog Advisory. Fog expected to
dissipate between 8 & 9 am.

&&

.SHORT TERM...(Today through Tuesday night)
Issued at 232 AM MDT Mon Aug 21 2017

Eclipse Day has arrived! Stratus and patchy areas of fog are
prevalent over much of central Nebraska early this morning. Expect
this to spread west into the Panhandle through sunrise in moist
southeasterly flow east of a sfc front laid up along the Laramie
Range. May even see low clouds/fog creep into low lying river
valleys in southeast Wyoming as well. Visibilities could be
reduced to around one-half mile in spots. The front is expected
to shift east through the morning with post-frontal north-
northwest winds acting to scour out most of the stratus/fog by
late morning. However, a few pockets of low clouds could remain
across the Panhandle from Chadron to Sidney through noon. Early
morning water vapor imagery shows an upper level system over NM/CO
producing mid and high clouds over much of the forecast area at
this moment. A second stream of moisture ahead of a shortwave
trough extending from ND- Northern CA is expected to approach
from the northwest but should thin and remain out over northwest
portions of Wyoming until later this afternoon. Models continue to
show the southern stream of moisture shifting southeast as the
shortwave approaches from the northwest, with a dry slot moving
over northwest portions of our forecast area west of a line from
Laramie-Torrington-Chadron for eclipse time. Therefore, will
maintain mostly sunny skies (10-30% cloud cover) west of this
line, with partly to mostly cloudy (30-55%) further to the east
which may still be within the southern moisture stream. The main
concern for obstruction of the sun will be cloud ceilings around
10kft and higher, especially east of the aforementioned line.

If you`re heading out to observe this rare event, be safe!
Protect your eyes with proper eclipse viewing glasses, and be
mindful of the many people who will be out and about.

The shortwave trough will then move overhead this afternoon,
increasing cloud cover and sparking off a few showers and
thunderstorms over the mountains. Otherwise, the rest of today
looks to be a fairly quiet with breezy northerly winds gusting
20-25 mph looking to be the main impact for folks traveling across
the area. Stratus may redevelop late tonight as sfc flow returns
to the east-southeast in the wake of the departing system. Tuesday
looks mostly dry as an upper level ridge begins to build overhead,
but cannot rule out some mountain precip if the monsoonal
moisture plume stuck within the ridge makes it up to our area.

.LONG TERM...(Wednesday through Sunday)
Issued at 232 AM MDT Mon Aug 21 2017

Midlvl subtropical ridge will be located across New Mexico on
Wed/Thurs with west to southwest flow aloft across the CWA.  PW
values through Thursday will be 0.7-0.9 inches over southeast WY, so
expect scattered storms to develop during the aftns and evenings.  A
weak shortwave moving across northern Colorado on Thurs may provide
better destabilization for a slightly higher coverage of storms.  The
strong/severe storm threat looks low with relatively weak flow
aloft.  The upper level ridge will shift into the Great Basin by
next weekend with drier northwesterly flow and decreasing PW.
Although isolated tstms will still be possible over areas near the
Colorado Border on Friday/Saturday, the overall coverage of storms
will be diminishing.  Highs through the extended will be around
normal.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 536 AM MDT Mon Aug 21 2017

LIFR/IFR conditions in fog will continue over the next hour or two
across the Panhandle sites (SNY/AIA). The fog will be quick to
dissipate with visibilities improving after around 14Z. A front
will push across the plains by late morning with north/northeast
winds gusting to 20-25 kts through the afternoon.

&&

.FIRE WEATHER...
Issued at 232 AM MDT Mon Aug 21 2017

Afternoon humidities in the lower elevations of Carbon County may
drop to 15% today and Tuesday, but winds are expected to remain
relatively light at around 10 mph or less. An isolated t-storm is
possible in the mountains of southeast Wyoming both days as well.
Across the plains of southeast WY and NE Panhandle where fuels
are still critical, humidities today will drop to 20-35% from
west to east. Winds will be out of the north with gusts
potentially 20-25 mph from late morning through the early evening.
Moisture will increase Wednesday as monsoonal flow returns to the
region, and will persist through much of the week. This should
increase humidities over the district and produce a broader
coverage of showers and thunderstorms.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

UPDATE...MAJ
SHORT TERM...RJM
LONG TERM...ZF
AVIATION...ZF
FIRE WEATHER...RJM


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