Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 190341 AAB
AFDCYS

Area Forecast Discussion...UPDATED
National Weather Service Cheyenne WY
941 PM MDT Mon Sep 18 2017

.UPDATE...
Issued at 941 PM MDT Mon Sep 18 2017

For the evening update only minor changes to the hourly
temperature forecasts (trending warmer a few locations in the
east), otherwise still looking for a fairly windy day tomorrow for
areas west of Interstate 25 with breezy and warm conditions east
onto the High Plains. Still expecting patchy rains to affect the
western zones by tomorrow afternoon as the cold front moves
southeast across the area. We are advertising a higher fire danger
risk tomorrow along and east of I-25 to include Wheatland,
Torrington and Cheyenne areas due to the potential for low
afternoon humidity ahead of the front, and breezy west to
southwest winds 15 to 25 mph with gusts to near 40 mph at times.
Timing of the front and cloudcover critical to just how low the
humidity will go, but the wind forecast carries a higher
confidence level. Stay tuned by morning for a potential upgrade of
the Fire Weather Watch across parts of southeast Wyoming to a Red
Flag Warning.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 220 PM MDT Mon Sep 18 2017

Latest water vapor loop was showing a fairly distinct upper level
trof moving into the Pacific northwest. This feature is inducing
good pressure falls over southeast Wyoming with good downslope
flow. This is yielding to very mild temperatures this afternoon.
Latest visible imagery was showing some high clouds starting to
stream into the western portion of the forecast area. These clouds
will continue to be on the increase overnight, which may limit
radiational cooling especially over the gap areas. As a result,
wind speeds may remain rather high over the gap region at
Arlington tonight, and persist through tomorrow. Wind speeds are
expected to continue increase over southeast Wyoming Tuesday and
will may yield to more Red Flag conditions. We are most confident
in areas east of the Laramie Range in southeast Wyoming tomorrow,
since they will likely not see the cloud cover until late in the
day. Further west, we may see a band of rain showers develop under
the upper level trof and associated jet streak. However, the bulk
of this jet energy is expected to lift northeast out our area by
evening. So we are not real confident on how far east this band of
showers will advance. At this point, it appears the bulk of these
showers will stay west of the Laramie Range tomorrow.

Wednesday will be another breezy and mild day with afternoon highs
in the 60s and 70s.

.LONG TERM...(Wednesday night through Monday)
Issued at 220 PM MDT Mon Sep 18 2017

Attention this period will focus on the large upper trough that
will dig over the western CONUS Weds night and Thursday then move
very slowly eastward through the weekend. Temperature and pcpn
forecasts for late in the week will be tricky as there are some
differences in timing of the arrival of colder air to the area
with the GFS slower to bring in the cooler air as opposed to the
EC. It looks to be dry across the CWA through early Friday then
showers should begin spreading east across the area late Friday.
Saturday and Sunday should be rather cool and showery as periodic
disturbances lift north across the area and overrun the cooler
surface airmass. Bulk of pcpn looks to shift east of the CWA
Monday but some showers still possible mainly around the mtns.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 556 PM MDT Mon Sep 18 2017

VFR expected overnight across SE Wyoming and the western Nebraska
Panhandle, but changes are coming for tomorrow in the form of
windy conditions along and west of I-25 with gusts by mid morning
approaching 40 kts for Rawlins, Laramie and Cheyenne. We brought
the wind speeds up mid to late morning tomorrow across the
Nebraska Panhandle as models have trended up on west to
gusty southwesterly winds. MVFR ceilings were brought in by
tomorrow afternoon at Rawlins just behind the passing cold front.


&&

.FIRE WEATHER...
Issued at 220 PM MDT Mon Sep 18 2017

Latest surface analysis was showing going good pressure falls
taking place across southeast Wyoming in advance of the upper
level disturbance moving out of the Pacific northwest. These
pressure falls are causing wind speeds to increase across much of
southeast Wyoming. The higher gusts have been taking place near
Rawlins and Douglas where wind speeds are gusting over 35 mph. The
current humidity over Douglas is 15 percent. Therefore, we decided
to issue a Red Flag Warning for 301 this afternoon. Meanwhile,
Rawlins has relative humidities around 20 to 25 percent, due to
higher elevation and some high clouds. Zone 310 has also had some
pretty low humidities as well, but wind speeds have been quite
marginal for Red Flag Criteria. As a result, we are holding off
for zone 310 for now.

The upper level disturbance is expected to move into western
Wyoming tomorrow. Ahead of this disturbance, wind speeds are
expected to become quite gusty. We went ahead and issued a Fire
Weather Watch for tomorrow afternoon in zone 310 due to lack of
cloud cover most of the day, low humidities again, and gusty wind
speeds over 30 mph.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Fire Weather Watch Tuesday afternoon for WYZ310.

NE...None.
&&

$$

UPDATE...JG
SHORT TERM...REC
LONG TERM...RE
AVIATION...JG
FIRE WEATHER...REC


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