Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 120927
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
327 AM MDT Sat Aug 12 2017

.SHORT TERM...(Today through Sunday night)
Issued at 327 AM MDT Sat Aug 12 2017

Stratus and fog are becoming more widespread this morning along
the Cheyenne Ridge and in the river valleys of the Nebraska
Panhandle. HRRR shows the continued expansion of fog through the
morning so followed these trends for this morning`s forecast.
Increased PoPs as well for over our northwest locations as radar
shows some rain showers headed this way. Otherwise, should remain
a pretty quite morning.

The rest of the weekend looks pretty active with the potential for
a few strong to severe storms in the afternoon and evening. For
today, a leading impulse of Pacific origin will move over the
forecast area midday ahead of the main upper low diving southeast
over Montana and the Dakotas. Timing of both features between the
models seems to be pretty close, however the associated sfc
features vary somewhat in the models. The main sfc trough
developing over our plains this morning looks to shift east quite
a bit faster in the GFS than other models, which would push the
main plume of moisture and instability east of our counties. The
NAM, ECMWF, and other high res models agree on keeping the sfc
trough further west during the day, oriented northeast to
southwest from Sioux County down to Laramie County. Will side with
the consensus of models. Ahead of the trough, SBCapes are progged
to be 2000-3000 J/kg in spots with a weak cap that should erode
through the afternoon hours.

Ample midlevel forcing and sfc convergence along the trough will
initiate storms by the mid afternoon, and bulk shear around 40 kts
combined with rather large instability will support supercellular
mode at least initially. However, increasing northwest flow in the
wake of the upper impulse looks to contribute to an upscale in
development as the storms push east across the Panhandle through
the late  afternoon and evening. Llvl shear progs are not all
that impressive with sfc flow mainly out of the south, but there
could be pockets of backed winds along terrain features which
could increase llvl shear. Therefore, think the main threats from
supercells will large hail (>1") and strong outflow winds
(>60mph), but any areas that see those backed sfc winds certainly
have the potential for a brief tornado as well. The main upper
trough will move across the Dakotas tonight and will push a cold
front across the northern half of the CWA. Models show it stalling
from the northern Laramie Range to the southeast for tonight. A
second impulse looks to approach from the southwest and trek
across northern Colorado tonight as well, so we could see a few
showers and weak storms continue overnight. Potential for fog will
be much more sporadic across the region as convection disrupts
the boundary layer somewhat.

The upper ridge will move over the Rockies on Sunday with yet
another midlevel impulse expected to move across the forecast area
in the afternoon. The cold front is progged to remain quasi-
stationary across our plains counties through the day, with
southeasterly return flow bringing back the higher dewpoints
especially in the Panhandle. Forecast soundings indicate the
postfrontal airmass will modify rather quickly through the day so
it`s looking a good bet we`ll be dealing with another day of
severe thunderstorms along the WY/NE border and points east in the
afternoon/evening. Further west, expect weaker thunderstorms to
develop as the wave moves overhead along with breezy westerly sfc
winds within unidirectional westerly flow.

.LONG TERM...(Monday through Friday)
Issued at 327 AM MDT Sat Aug 12 2017

Robust shortwave will move from the Great Basin into western WY
on Monday afternoon. Surface trough will be in place over the
plains with convective development likely in the afternoon
especially along and north of a Douglas to Chadron line. Could see
one or two strong storms again on Monday afternoon with the
expected shear/instability in place.

Pretty strong Pacific cold front passes across the area on Tuesday
with showers and thunderstorms developing ahead of it in the
afternoon.  Strength of the forcing and tightening low level
gradients may allow for strong winds with some of the more robust
convection.

Zonal flow sets up for Wednesday trending to more NW flow by Friday
as ridging begins to try to rebuild over the western US.  Looks like
much of the low level moisture gets pushed east of our CWA by
Wednesday with lessening chances for convection and seasonable temps.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1127 PM MDT Fri Aug 11 2017

Hi-res guidance continues to show a consistent signal for fog
formation primarily along the Cheyenne Ridge from KCYS to KSNY
between 08Z and 13Z. Brought in IFR visibility with the potential
for LIFR near KSNY where a more moist low level airmass is in
place. IFR also likely at least briefly near KBFF and KAIA as
winds will be very light and temperatures cool early this morning.
Any fog will burn off quickly Saturday with scattered
thunderstorms developing off the high terrain by 18-19Z and
spreading into the Panhandle by late afternoon. A few of these
storms may contain hail and wind gusts to 50kt, with the highest
threat for the Nebraska terminals.

&&

.FIRE WEATHER...
Issued at 327 AM MDT Sat Aug 12 2017

Fire weather concerns will remain minimal through middle of next
week with near normal temperatures and chances for showers and
thunderstorms expected through this time. High pressure looks to
build in from the west later in the week, bringing warmer and
drier conditions back to the region.

&&

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

$$

SHORT TERM...RJM
LONG TERM...DEL
AVIATION...DEL
FIRE WEATHER...RJM



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