Area Forecast Discussion
Issued by NWS Cheyenne, WY

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790
FXUS65 KCYS 171114
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
414 AM MST Wed Jan 17 2018

.SHORT TERM...(Today through Friday night)
Issued at 330 AM MST Wed Jan 17 2018

Overnight water vapor imagery showed a upper trough axis from the
eastern Great Lakes to the lower MS Valley. A upper ridge dominated
much of the western CONUS. The weekend weather system was located
over the eastern Pacific. A surface trough extended from central MT
south along the Laramie Range separating ridges to the east and west.
Skies were clear with temperatures 15 to 25 degrees warmer compared
to 24 hours ago. Winds were light, except breezy along the I-80 and
I-25 wind corridors with gusts to 35 mph.

There is good model/ensemble consensus progging the upper ridge to
gradually translate east across the Rockies into the Great Plains.
Consequently, the flow aloft will back from northwest to southwest
through the short term. Increasing heights and 700mb temperatures with
the downslope westerly flow will promote above normal temperatures
through the end of the week. Highs in the 30s and 40s today will
warm into the 40s and 50s by Friday. Highs in the lower 60s will
be possible over the southeast WY plains Friday. Lows will moderate
from the teens and 20s tonight into the 20s and 30s by Friday.

Winds will be on the increase beginning late tonight. Confidence is
improving for a high wind episode from Thursday night through Friday.
The GFS and NAM CAG-CPR 700/850mb gradients rise above 50m after 00Z
Friday, peaking near 60m around 12Z Friday. The GFS progs 50-55kt
700mb winds along and west of the Laramie Range between 06Z and 18Z
Friday. Later shifts may consider issuing High Wind Watches for the
I-25 and I-80 wind corridors.

The Pacific low pressure system and cold front approach the western
CWFA Friday night, with snow spreading as far east as a Lusk to Laramie
line by 12Z Saturday. Accumulations up to an inch can be expected.

.LONG TERM...(Saturday through Tuesday)
Issued at 300 AM MST Wed Jan 17 2018

This period of the forecast will begin with snow moving in from the
west as an upper level short wave trough approaches. Focus will
continue to be the potential for a winter storm this weekend.

Models have slowed the speed of this storm system down. The ECMWF
has the upper level low closing off over central Colorado, while
the GFS delays the low closing off until the KS/CO border. If the
trough does become a closed low as the ECMWF portrays, this will
allow a TROWAL to develop over the forecast area. If this were to
happen higher snowfall amounts would be much more likely to occur
with a TROWAL overhead.

The GFS, on the other hand, is not as deep with the trough as the
ECMWF, which would cause the trough to move east faster than the
ECMWF and close off further east. The GEFS mean and all the GEFS
members agree very well with the GFS. Despite its faster speed of
the trough, the GFS still produces widespread snowfall amounts
that are similar to the ECMWF. However based on the fairly reliable
track record of the ECMWF, would side more with its solution of
the low closing off earlier over Colorado. This track would put
the forecast area in a much better position and better odds for
heavy snow, with even higher amounts possible under the TROWAL
should one develop.

CIPS Analogs, which uses the GFS model, has median snowfall amounts
of zero to three inches for this weekend based on the top 15 previous
winter storm systems that are similar to this upcoming event. However
this is based off of the GFS which has the trough closing off further
east than the preferred ECMWF, which would yield less snowfall. Based
on this am thinking the CIPS data is on the low side, assuming the
ECMWF depiction portrays the correct path of the trough. Bottom line,
the forecast area still has the potential to see heavy snowfall
amounts, and the timing for the bulk of the snowfall has shifted to
Saturday afternoon through Sunday.

There is still a good deal of uncertainty regarding where the heavy
snowfall will develop and when. Confidence is high that the forecast
area will receive measurable snow, but still low regarding the potential
for heavy snowfall amounts. Will continue to monitor the latest information
in the coming days.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 408 AM MST Wed Jan 17 2018

VFR conditions forecast for the TAFs. Light west to northwest
winds will prevail until the overnight hours when winds closer to
15 kts will occur for the WY sites as the pressure gradient
tightens.


&&

.FIRE WEATHER...
Issued at 300 AM MST Wed Jan 17 2018

A upper level ridge of high pressure will dominate the weather pattern
with dry conditions, breezy to windy periods and above normal temperatures
through Friday. Minimum relative humidities will average 25 to 30 percent
each afternoon. A Pacific storm system and cold front will move across
the central Rockies this weekend bringing colder temperatures and
accumulating snowfall.

&&

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

$$

SHORT TERM...MAJ
LONG TERM...JTL
AVIATION...JTL
FIRE WEATHER...MAJ



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