Area Forecast Discussion
Issued by NWS Cheyenne, WY

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926
FXUS65 KCYS 230028
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
628 PM MDT Fri Sep 22 2017

.SHORT TERM...(This evening through Sunday)
Issued at 145 PM MDT Fri Sep 22 2017

Currently a few weak showers and isolated thunderstorms are
moving quickly NE from NW Colorado into southeast WY mainly west
of the Laramie Range. These seem to be tied to ascent on the nose
of a 130kt upper jet extending from NW New Mexico into SE Wyoming.
These showers will continue to track NE and expand in coverage
over our northern counties this evening as more substantial
overrunning of a low level frontal zone develops. This front will
track quickly SE this evening and tonight bringing much cloudier
and cooler weather to the entire area through Sunday. For tomorrow
and tomorrow night, best forcing for ascent will lie in our
eastern counties in the diffluence within the entrance region of
the above mentioned jet There may perhaps be a secondary area of
upslope enhancement along the Laramie Range as well. Models have
continued the trend of keeping snow levels rather high through
early Sunday and with the main upper trough remaining to the west
before lifting out Sunday evening, this makes sense. Reduced QPF
amounts over the mountains through Sunday as well. The mid/upper
system becomes more vertically stacked and weakens as it tracks
across our mountains Sunday into Sunday evening. Model soundings
indicate precip will likely be intermittent and not heavy over the
mountains as periods of mid level drying rotate around the main
upper low and the best jet energy and forcing remains to the east.
This is also not a favorable low track for orographic enhancement
so while advisory level snow is likely, am not confident enough
at this time to in amounts to issue a watch.

For the remainder of the area, below 9000 ft expect generally rain,
with a good moderate soaking rain for the eastern plains especially
on Saturday evening.  Temperatures will be quite cool with the low
cloud cover and precipitation with highs struggling to make it out
of the 40s in SE Wyoming to low 50s over the Panhandle.

.LONG TERM...(Sunday night through Friday)
Issued at 145 PM MDT Fri Sep 22 2017

The main concern with this portion of the forecast is the timing
of the dry air moving into the forecast area Sunday night.

Latest model consensus is showing this closed upper level low
opening up on Sunday night and becoming more progressive as the
upper level ridge builds in from the Pacific northwest. As this
occurs, we will see drier air moving into southeast Wyoming. As a
result, we should begin to see the rain and snow (above 9000ft)
taper off from the southwest to northeast. However, the cloud
cover and scattered rain/snow showers will continue to hang on
Monday afternoon before the trof slowly moves off to the
northeast. We should see a drying out period Tuesday before a
clipper dives southeast from the Canada on Wednesday. This
feature may trigger a few showers/thunderstorms in the southern
half of the Nebraska Panhandle Wednesday afternoon/evening.
Otherwise, a slow warming trend is expected through Friday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 628 PM MDT Fri Sep 22 2017

Slow moving area of low pressure will ever so slowly move east
across the intermountain west through the weekend. VFR conditions
at all terminals early in in the period will lower to MVFR 04-07Z,
with even some IFR expected at KCYS 10-18Z. Although intermittent
showers will be a possibility overnight, don`t expect coverage or
precipitation rates to be all that impactful to TAF sites,
especially after 06Z. That changes as precipitation coverage
increases dramatically after 18Z. Have trended TAFs for area
airfields downward as a result.

&&

.FIRE WEATHER...
Issued at 145 PM MDT Fri Sep 22 2017

Cool temps and precipitation will limit any fire weather concerns
for the next three days.

&&

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

&&

$$

SHORT TERM...DEL
LONG TERM...REC
AVIATION...CAH
FIRE WEATHER...DEL



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