Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 121732
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1132 AM MDT Sat Aug 12 2017

.UPDATE...
Issued at 1102 AM MDT Sat Aug 12 2017

Latest water vapor loop was showing shortwave energy moving
through southwest Wyoming late this morning. Starting to see the
cap weaken a bit across areas mainly along and west of the Laramie
Range with the weakest cap across the mountain ranges. This area
is only showing marginal cape values (<1000 j/kg) and weak shear
25-30kts. Dewpoints are also drying out a bit out a bit as well
due the shallower moisture in place. Would not be surprised to see
a few showers and thunderstorms develop in areas west of the
Laramie Range during the next couple of hours with the main threat
in this region being gusty wind speeds to 50 mph and small hail.

Meanwhile, things are finally starting to clear out across the
Nebraska Panhandle. This area has a very moist air mass in place
along with steep mid level lapse rates in place and better shear
(40-50kts) projected by the HRRR. With all that said, we should
be in for active afternoon in the Nebraska Panhandle and portions
of Goshen/Niobrara counties with the supercells containing large
hail and damaging winds. Tornado threat looks to be rather minimal
at this time, but we will have to keep an eye on the low level
flow to see if it backs ahead of the progressive shortwave.

&&

.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 12Z TAFS through 12Z Sunday morning)
Issued at 527 AM MDT Sat Aug 12 2017

Latest satellite trends show an expanding area of low clouds along
the Cheyenne Ridge now spreading northward through the Panhandle.
Expect MVFR with some IFR ceilings over the Cheyenne Ridge east
of KCYS to KSNY and northward to KBFF and KAIA through 15z before
slowly burning off. Still expect robust convection to develop as
early at 18-19Z over the Laramie Range and then spreading eastward
into the Panhandle by mid/late afternoon. Strongest storms will
likely produce hail and wind gusts to 50kt especially over the
Nebraska Panhandle.


&&

.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.
&&

$$

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



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