Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 181713
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1113 AM MDT Mon Sep 18 2017

.UPDATE...
Issued at 1037 AM MDT Mon Sep 18 2017

Latest satellite loop was showing some high clouds moving through
western Wyoming and should continue to push east through the
remainder of the day. As a result, we increased cloud cover
percentages a bit this afternoon. Wind speeds are also starting to
pick up especially in Rawlins where they are starting to see gusts
to 30 mph due to surface pressure falls in advance of the upper
level trof moving through the northwest. The next concern is how
deep the low level moisture is this afternoon. Latest look at the
low level water vapor loop was showing a gradual increase in the
low level moisture across the western half of the state while the
eastern half of the state is showing fairly dry conditions aloft.
We are a bit concerned that this drier air aloft may be enough to
promote Red Flag Criteria in Converse and Niobrara counties where
the winds aloft appear to be a bit stronger. We will keep an eye
on the dewpoints in that area for now, to see if they start
dropping soon. If they do drop in the next hour or so, we may be
issuing a Red Flag Warning. Not as concerned about the west at
this point, since they are a bit cooler due to higher elevation
which is also favoring higher humidity values.

&&

.SHORT TERM...(Today through Wednesday)
Issued at 420 AM MDT Mon Sep 18 2017

Current Watervapor satellite loop shows the upper level
disturbance, which brought some showers and thunderstorms to the
High Plains yesterday, continuing to rapidly move eastward into
the central plains this morning. In its wake, fog and low stratus
have formed due to upslope southerly winds and rainfall late last
night, mainly across the southern NE panhandle. Drier weather is
expected to start off this week with warm southwest flow at all
levels of the atmosphere. Temperatures will respond this afternoon
as highs return to the 70`s and low/mid 80`s across the region,
with warmest temperatures mainly below 5000 feet. Expect windy
conditions this afternoon west of the Laramie Range, with breezy
winds as far east as the I-25 corridor, mainly due to an
increasing pressure gradient ahead of strong Pacific cold front
moves southeast towards southern Wyoming. This cold front will
push across the area on Tuesday as early morning highs around 60
will rapidly drop into the 50`s and even into the 40`s during the
day west of I-25. Further south and east, high temperatures will
remain in the upper 70`s to mid 80`s ahead of the front, mainly
southeast of a line from Laramie to Chadron Nebraska. Very windy
conditions are possible across southeast Wyoming, mainly along and
west of I-25 where gusts between 40 to 50 mph will be possible.

As the base of the upper level trough slides along the
Wyoming/Colorado border on Tuesday, some precipitation is expected
even though the main story will be the cooler temperatures and
gusty winds. Kept POP between 40 to 60 percent from the Sierra
Madre Range northeast towards the Douglas area for periods of rain
showers. Rain is expected to change over to snow above 9000 feet
with some snow possible down as low as 8000 feet. One to three
inches of wet snow is possible across the higher peaks late
Tuesday and Tuesday evening. East of this area, little or no
precipitation is expected due to dry downslope flow. Wednesday
morning lows will be quite chilly across the area with readings
generally in the 30`s to low 40`s. With some wind expected
overnight, wind chills will likely be in the 20`s around sunrise
Wednesday.

Models show the upper level trough quickly lifting northeast on
Wednesday with anticyclonic flow in its wake. Expect moderating
temperatures Wednesday afternoon, back to near average for this
time of the year. It will be dry with gusty winds through the day.

.LONG TERM...(Tuesday through Sunday)
Issued at 312 AM MDT Mon Sep 18 2017

The overall weather pattern through the extended forecast period is
likely to be quite unsettled, with models in excellent agreement w/
general upper-level troughing over the western CONUS. The GFS/ECMWF
agree that a significant disturbance will dig into the Four Corners
region by Fri/Sat, w/ 700 mb temperatures possibly falling below 0C
at some point during the weekend. We have high confidence in precip
becoming fairly widespread across southeast Wyoming and the western
Nebraska Panhandle, given notable dynamic support with a 100-115 kt
H25 jet bisecting the CWA. It is still too early to determine storm
impacts as models differ considerably with the overall evolution of
the upper trough, but there is some potential for rain to mix with,
or change entirely to snow at elevations as low as 5500 ft. We will
need to closely monitor this system in the coming days. We kept PoP
values in the 40-60 percent range for now, but based on the pattern
we continued to trend colder on temperatures. Highs Sunday may very
well struggle to get out of the 40s for many areas, particularly in
southeast Wyoming.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday morning)
Issued at 1113 AM MDT Mon Sep 18 2017

VFR expected for all terminals through the period. Some high clouds
will increase over the area this afternoon and tonight. Gusty winds
affecting KRWL with gusts over 30kts at times.

&&

.FIRE WEATHER...
Issued at 312 AM MDT Mon Sep 18 2017

Windy conditions will develop from west to east beginning today
as a strong cold front moves towards the area. Strong gusty winds
and low relative humidities may result in localized critical
conditions over portions of southeast Wyoming and possibly the
southwestern Nebraska panhandle. Conditions may be too localized
or brief to warrant a Red Flag Warning, but will continue to
monitor trends in humidity today. Cooler temperatures are expected
late this week and into the weekend as a strong Pacific storm
system brings the potential of widespread rain and mountain snow.

&&

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

$$

UPDATE...REC
SHORT TERM...TJT
LONG TERM...CLH
AVIATION...RE
FIRE WEATHER...TJT



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