Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 120631
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
Issued by National Weather Service RIVERTON WY
1131 PM MST Thu Jan 11 2018

.SHORT TERM...(This evening through Friday night)

The high wind watch for the Laramie Valley and from Cheyenne north
to Wheatland has been upgraded to a high wind warning. Synoptic
pattern as well as high resolution models continue to show strong
winds developing over these areas from early in the morning
through mid day Friday. An initial push of 40 kts with gusts over
60 kts will move through the Cheyenne area around 4 AM. Then,
the wind should shift slightly to the northwest by 8-9 AM in a
post-frontal type regime.

Previous discussion:
Issued at 316 MST Thu Jan 11 2018 - High winds
will be likely once again for much of southeast Wyoming from tonight
through Friday morning. A quite favorable pattern may be setting up
for not only the wind corridors, but portions of the adjacent
valleys and high plains as well. Short term models remain in
excellent agreement w/ a distinct area of low pressure tracking
across northeast Wyoming between 00z and 15z Fri. This is expected
to result in strong low-lvl gradients, with the H85 and H7 CAG-CPR
gradient potentially exceeding 70 meters through early Friday. The
models show these gradients weakening after sunrise, but remain at
or above 50 meters through 18z or so. In addition, anticipate good
subsidence w/ a well-positioned and potent upper jet to our south-
west. By 00z this evening, H7-H8 flow should be approaching 50 kts
over the wind corridors w/ a continued steady increase to 55 to 65
kts through 12z. The 06z GFS suggests upwards of 70 kts at 750 mb in
the vicinity of Bordeaux. In-house probability guidance for BRX
suggests a strong likelihood of exceeding warning criteria between
06z and 15z. High Wind Watches have been upgraded to warnings, ARL
begins first around 00z. Delayed Bordeaux and the I80 summit until
06z based on the position of the surface trough.

The GFS and NAM both show strong 700-800 mb flow spreading across
a larger area than just the wind corridors. There is potential
for 55-65 knot flow at H75 over both LAR & CYS in the presence of
what should be impressive subsidence. Only a very weak inversion
can be seen on at the surface per forecast soundings, and low-
level omega fields are progged to be favorable for good downward
momentum transfer by both the GFS/NAM. This pattern appears to be
consistent w/local studies for CYS, which is usually indicative of
a more wide- spread high wind episode. That said, we issued a
High Wind Watch for CYS/ LAR as well as eastern Platte county
outside of BRX. Thinking some of the strongest gusts will occur
between 5-10 AM before the winds aloft come down in response to an
approaching front. Douglas could be close as well in Converse
county, but decided to leave them out of the Watch for now as they
should be just on the northern fringe of stronger flow and better
subsidence. The day shift may consider adding Converse county to
the Watch.

Snow will develop in the mountains late tonight, and continue thru
much of the day on Friday. Strong dynamic support w/ the upper jet
should yield a solid 6 to 12 inches in both the Snowy/Sierra Madre
ranges, so we issued a Winter Wx Advisory for those zones beginning
at midnight tonight.

.LONG TERM...(Saturday through Thursday)
Issued at 345 AM MST Thu Jan 11 2018

Medium to long range models continue to show north to northwest
flow aloft across southeast Wyoming and western Nebraska during
the weekend as the persistent upper level ridge rebuilds over the
eastern Pacific and gradually drifts eastward into the Great Basin
region. The position of this ridge and how far east it drifts will
determine how far west the next arctic front moves Sunday and
Sunday night. Models are in reasonable agreement with this front
and associated broad upper level trough as both features quickly
move south through the area by early Monday morning. All models
show some snowfall mainly across the High Plains along and east of
the Interstate 25 corridor, so kept POP between 30 to 50 percent
further north, with slightly lower values along the I-80 corridor.
Monday will likely be the coldest day with highs in the low 20`s
to low 30`s across most of the area. Much colder temperatures will
be just to the east of the Nebraska panhandle, so will have to
monitor the position of this arctic airmass closely just in case
in shifts westward. Expect temperatures to increase by the middle
of the week with highs around 50 by Wednesday as the upper level
flow becomes more westerly, ahead of the strong storm system near the
Pacific coastline.

&&

.AVIATION...(For the 06Z TAFs through 06Z Saturday)
Issued at 1130 PM MST Thu Jan 11 2018

VFR conditions at all TAF sites expected to continue until after 09Z
Fri.  After that time, light snow expected in NW NE panhandle as
well as from KRWL to KLAR to produce MVFR conditions.  Heavier
amounts will occur over mountains of SE WY through 18Z Fri.  Breezy
to windy conditions to continue overnight in SE WY, with speeds
increasing from KLAR to KCYS to KEAN after 09Z Fri.  Some TAF sites
could see gusts of 45 knots.  A south wind will occur in Western
Nebraska through Friday morning, then a wind shift to the north to
northwest after the front passes through that area between 16Z and
19Z.

&&

.FIRE WEATHER...
Issued at 316 AM MST Thu Jan 11 2018

No fire weather concerns. Despite periods of strong/gusty winds in
some areas, RH values will remain well outside of critical levels.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory from midnight tonight to 5 PM MST Friday
     for WYZ112-114.

     High Wind Warning from 11 PM this evening to 11 AM MST Friday
     for WYZ106-116-117.

     High Wind Warning from 5 PM this afternoon to 11 AM MST Friday
     for WYZ110.

     High Wind Warning from 2 AM to 11 AM MST Friday for WYZ107-115-
     118.

NE...None.

&&

$$

SHORT TERM...CLH/BEM
LONG TERM...TJT
AVIATION...BEM
FIRE WEATHER...CLH



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