Area Forecast Discussion
Issued by NWS Cheyenne, WY

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156
FXUS65 KCYS 190854
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
254 AM MDT TUE JUL 19 2016

.SHORT TERM...(Today through Thursday)
Issued at 300 AM MDT Tue Jul 19 2016

Forecast challenges deal with possible fire weather issues and
return of severe thunderstorms.

Currently...MCS currently over central Nebraska early this
morning. Surface low being analyzed on surface analysis over
eastern Montana with an outflow boundary from the huge MCS being
pushed southwest into central Colorado this morning. Mosaic radar
showing the Cheyenne CWFA echo free at the moment. Some
convection being painted over western Colorado moving north and
northeast on the western periphery of the upper level ridge being
analyzed over central Oklahoma. Still moist at the surface with
low 60 dewpoints east of the Laramie Range and mid 40 dewpoints
west.

Latest GFS and ECMWF showing another disturbance moving through
the monsoonal flow to bring convection back to the mountains to
our west this afternoon. Best chances for showers and
thunderstorms will be the Snowy and Sierra Madre Ranges this
afternoon. Latest HRRR guidance showing convection then moving off
the mountains towards the east. Continued 20-30 percent chance
PoPs for areas east of the Laramie Range for this afternoon with
best chances along the Interstate 80 Corridor. Not expecting
severe thunderstorms today as we have uni-directional flow from
the surface through upper levels from the southwest and fairly
low CAPE even out into the Panhandle.

Some indication of the upper level ridge building back to the west
for Wednesday that would shift monsoonal flow further west. Seen
both on the GFS and ECMWF...so confidence is there that areas east
of the Laramie Range should stay dry through the day. One concern
though is that we could see enhanced mid level winds over Converse
County. GFS soundings showing 20-25kts of winds at 500mbs that
could mix down to the surface. Afternoon humidities in western
Converse County to fall close to 10 percent Wednesday afternoon.
Should they get any stronger...we may need some fire headlines for
FWZ 301.

.LONG TERM...(Thursday night through Monday)
Issued at 300 AM MDT Tue Jul 19 2016

The main concern in this forecast package will be which days will
have the best chance of thunderstorms.

Thursday night-Friday night:
Models and ensembles are still depicting monsoonal flow affecting
the area Thursday evening. This may leave a few showers and
thunderstorms in our mountain ranges Thursday evening, with very
limited precip. in the plains. It appears as if the upper level
trof will start to move into Washington and Oregon on Thursday
night. This trof is progged to push northeast and skirt along the
Canadian/U.S. border on Friday and Saturday. This will tend to
flatten the upper level flow over our neck of the woods. This type
of flow will tend to increase the downslope potential on Friday
which may tend to suppress some of the convective development on
leeward side of the mountains. The models are still depicting some
precip over the plains on Friday afternoon/evening. This
convection may end up being quite high based, so precip.
measurements will still be quite low, but there could be virga
showers which could yield to some high wind gusts. Most of these
showers should dissipate after sunset on Friday. Another concern
on Friday will be the enhanced fire danger due to the drying
effect from the downslope flow and dry thunderstorms.

Saturday-Monday:
The models are still advertising the frontal boundary surging
south of the forecast area by early Saturday morning. As a result,
dry and mild conditions are expected. This trend should continue
on Saturday evening. The chance of thunderstorms will most likely
return on Sunday afternoon as the low level moisture surges back
into the area ahead of a potential vorticity anomaly moving into
northern Wyoming. A few of these storms may become strong or
perhaps severe depending on how much low level moisture returns.
Effective bulk shear should also be sufficient with values
possibly ranging from 35 to 40 kts. This potential vorticity
anomaly is progged to lift northeast into the Northern Plains on
Monday which may keep the risk of thunderstorms in the Nebraska
Panhandle on Monday afternoon.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1219 AM MDT Tue Jul 19 2016

Latest satellite loop was showing that the bulk of the convection
was east of the Nebraska Panhandle. We may see a few isolated
showers and thunderstorms develop overnight near the vicinity of
the low level jet in the Nebraska Panhandle behind the MCS. Latest
satellite loop was showing some accas developing near KTOR and
points to the northeast. Not confident it will affect any our TAF
sites except possibly KCDR. Otherwise, quiet conditions are
expected tonight with very limited chance for fog. May see a few
thunderstorms again tomorrow due to monsoonal moisture affecting
the area along with stronger southwest flow.

&&

.FIRE WEATHER...
Issued at 300 AM MDT Tue Jul 19 2016

Very dry conditions for areas across FWZ 301 today and Wednesday
with afternoon humidites falling into the mid teens today and low
teens Wednesday. Winds today should be light from the west but we
could see them a little stronger across western 301 for Wednesday
afternoon. For now...do believe wind gusts will be just below
critical levels...gusting to 20 to 25 MPH. For areas near the
Wyoming/Colorado border and out in the Panhandle...these areas
will be influenced more by the monsoonal moisture moving in from
the south and southwest. Afternoon humidities remain above
critical levels with daily chances for afternoon showers and
thunderstorms through Thursday.

&&

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

$$

SHORT TERM...GCC
LONG TERM...REC
AVIATION...REC
FIRE WEATHER...GCC



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