Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus65 kcys 221745 

Area forecast discussion
National Weather Service Cheyenne Wyoming
1145 am MDT Fri Sep 22 2017

Short term...(today through sunday)
issued at 400 am MDT Fri Sep 22 2017

A combination of satellite and upper atmospheric observations shows
an extensive upper-level trough encompassing much of the western US
early this am. This will be the dominant weather feature during the
next several days as the models suggest little eastward progression
through Sunday night. Moist southwesterly flow is increasing, along
with strong upper difluence in advance of this system. We expect to
start seeing the effects of this storm by afternoon, in the form of
scattered showers and isolated thunderstorms. The greatest coverage
is expected after 21z over east central Wyoming and the northern NE
Panhandle, within the exit region of the 110-120 knot h25 jet along
W/ several smaller scale disturbances embedded within the flow. The
forecast for Saturday still looks miserable to say the least, given
continued difluent flow aloft and northeasterly low-level upsloping
becoming better established. Widespread cloud cover / precipitation
will undoubtedly be the result, so expect little/no opportunity for
solar heating with highs well below guidance. High peaks within the
Snowy Range above 10,000 feet could see a change over to snow by 21
z or so if precip rates are high enough, but forecast soundings are
still showing temperatures of 37+ f invof Medicine Bow Peak and the
wbz heights remain above 11,500 through the day, so it appears this
will not be too much of a problem. Ncar ensembles also suggest pcpn
will stay as rain through at least 00z sun. GFS/NAM/ecm show colder
air finally spreading eastward through the day on Sunday, but more-
so on Sunday night, so again we are tending to favor higher terrain
for an accumulating snow threat, but even that is questionable with
700 mb temperatures barely below 0c unlike earlier runs. That said, the
snow accumulation forecast is low-confidence. However, there is the
possibility of significant, warning-level snow amounts above 10,000
feet or so. We contemplated a Winter Storm Watch for the snowy mtns
but several inches of our total accumulation forecast assumes a Sat
afternoon change-over, and that just doesn't seem likely enough for
the time being. Widespread precipitation will continue on Sunday W/
the continued dynamic support, and highs will most likely fall into
the 45-55 degree range, warmest far southeast near ksny.

Long term...(sunday night through thursday)
issued at 400 am MDT Fri Sep 22 2017

Good consensus from the 00z medium range models/ensembles progging
the closed upper low over southwest Wyoming to lift northeast into
eastern Montana Monday night, with a trough axis extending southwest
through The Four Corners and Baja California. A north-northwest flow aloft
develops by Thursday as a upper ridge builds from the Great Basin
into southwest Canada.

With the track of the upper low across central and northern Wyoming
Sunday night and Monday, overall snow accumulations will be lower.
Three to six inches will be possible above 8000 feet over the snowy
and northern Laramie ranges, with lesser amounts for the surrounding
valleys. Elevations below 6500 feet can expect scattered rain
showers. The precipitation will end Monday night with the departing
low. Monday will be chilly with highs 45-55f lower elevations and 35-
45f high country. Milder temperatures return Tuesday along with
drier conditions as flow aloft backs to the southwest. Highs will
range from 55-65f with 40s for the mountains. On Wednesday, some
moisture sneaks northward from Colorado ahead of the upper trough, with
isolated showers developing over the southeast Wyoming mountains, plains
and southern NE Panhandle. Expect highs in the 60s east of I-25,
with 40s and 50s west. Showers will taper off from north to south
Wednesday night as the lift and moisture shifts into Colorado.
Dry and seasonal mild temperatures are forecast for Thursday.


Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1137 am MDT Fri Sep 22 2017

The main concern with this taf issuance is the potential for low
ceilings (mvfr) moving in late tonight. Latest radar/satellite
loop was showing some showers continuing to move towards krwl.
These showers appear to be quite light with more showers starting
to move out of northern Colorado. These showers may creep into the
klar/kcys taf sites during the next 2 to 3 hours. Ceilings should
stay mainly above 8kft. Meanwhile, a frontal boundary continues
to move south through the northern part of the state. This
boundary is progged to push through much of the area by this
evening. The wind will then start to turn to a more easterly
direction late this evening and will promote the potential for low
cloud development mainly after midnight. Not real confident that
these clouds will be able to clear out tomorrow morning, due to a
strong inversion in place.


Fire weather...
issued at 400 am MDT Fri Sep 22 2017

Fire weather concerns will be very low over the next few days W/the
anticipation of colder temperatures and widespread rain & potential
mountain snow. One more dry day today W/relative humidity values around 20 percent
or so, but better chances for afternoon showers and thunderstorms.


Cys watches/warnings/advisories...


National Weather Service Glossary of Abbreviations