Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 190154
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
754 PM MDT Thu May 18 2017

.UPDATE...
Issued at 746 PM MDT Thu May 18 2017

Latest radar/satellite loops were continuing to show the
precipitation bands moving towards the I-80 corridor. However,
these bands appear to be weakening as they encounter 600-400mb dry
slot over southern Wyoming. As a result, we are backing off a bit
on snow amounts this evening. However, this dry slot may fill in
overnight as the moisture continues to surge north.


&&

.SHORT TERM...(This evening through Friday)
Issued at 300 PM MDT Thu May 18 2017

Powerful late Spring storm system underway across southeast WY
and the western Nebraska Panhandle as a slow-moving area of low
pressure spins towards CO. Radar echoes from KCYS 88D have changed
very little since early this morning, with band of persistent
widespread precip fueled by favorable llvl upslope flow and upper
level diffluence. Impacts have been many, with numerous road
closures including I-80 Rawlins-Cheyenne and US-287 Laramie to
the CO stateline. The heavy loading of snow on trees has broken
limbs and produced sporadic power outages across Cheyenne. Per
webcam images snow-levels were hovering around the 4000-4500 foot
layer and will remain there during the overnight period. All of
the inherited highlights remain on track with many areas in the
5500-7000 feet range having already received 4-8 inches. Areas
above 7000 feet have received around a foot of new snow as of
midday. See little reason in the 12z model output to deviate from
the earlier expectations that this area of snow/rain will persist
overnight and into the morning on Friday. Snow totals from this
event will undoubtedly break the 2 day snow records for a snow
event to occur after May 15th for Cheyenne. Widespread 2 to 3 feet
expected in the Snowy/Sierra madre and Laramie Ranges, with
upwards of 1-2 feet in adjacent foothills. The Cheyenne and
Laramie areas will no doubt see 12-18 inches, perhaps locally
higher, before the storm winds down tomorrow. Despite the wet
content of the snow, wind gusts of 25-35 mph have contributed to
areas of blowing and drifting, especially across higher
elevations. Have opted to add Goshen and eastern Laramie Counties
into a Winter Weather Advisory to reflect higher snowfall amounts
from Carpenter-Burns-Gun Barrel and into the SW corner of Goshen
County. Heavy snow rates to continue through at least 15z Friday,
before tapering off late in the morning especially by the
afternoon. Snow has had some difficulty accumulating on pavement
especially in elevations closest to the snow level thus far, but
expect that to change with sunset overnight. So bottom line is
that road conditions will continue deteriorating in most areas
overnight. Even tho precip ends tomorrow, it`ll do little help
temperatures, with highs only reaching the 30s in most areas.

.LONG TERM...(Saturday through Wednesday)
Issued at 330 AM MDT Thu May 18 2017

The strong low pressure system will push out to the northeast on
Saturday with drier air filtering in from the northwest. Outside of
a few mountain showers, precip should end by the afternoon.
Temperatures will warm 10 to 15 degrees from Friday. Models keep us
within cyclonic northwest flow through the early part of next week
with showers and afternoon thunderstorm chances expected each day
especially for the mountains. A vigorous shortwave will dive south
from Canada on Monday bringing a more widespread shot of precip with
a few thunderstorms as well. Winds will turn to the northwest behind
a cold front progged to move southeast across our counties midday
Monday with gusts around 35 mph likely. Tuesday will be a few
degrees cooler and not as windy. Looking toward midweek, models are
showing a fair amount of disagreement with the next system in
line for Wednesday and Thursday. GFS keeps cyclonic flow over the
northern CONUS while the ECMWF develops a shortwave ridge over
this area. For now, trended the forecast toward the consensus
blends to account for stark differences in the models.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 546 PM MDT Thu May 18 2017

Widespread ifr/mvfr conditions will continue through the night at
all sites. Snow will occur across the southeast WY sites with rain
over the western Nebraska Panhandle. CYS could see occasional LIFR
conditions through 12Z with areas of moderate snow. North to
northeast winds will gust to 20-30 kts through the evening for
sites to the east of the Laramie Range.


&&

.FIRE WEATHER...
Issued at 134 PM MDT Thu May 18 2017

No fire weather concerns over the next as significant storm
system brings heavy precipitation and cool conditions persist
across the region.

&&

.HYDROLOGY...
The concern coming out of this heavy precipitation event will be
the influx of water into area creeks, streams, and rivers. Expect
high plains locations to see these rises first, through the
upcoming weekend. Although flooding is not anticipated at this
time, many of these drainages will be running full thru the
upcoming weekend. Fortunately, the cool conditions will continue
for the next 1-2 weeks, which should help slow the flows into the
mainstem rivers. Will continue to watch these levels very closely
over the coming few weeks.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Warning until 3 PM MDT Friday for WYZ102-103-
     105>107-110-112-114>118.

     Winter Weather Advisory until 3 PM MDT Friday for WYZ101-104-108-
     109-111-113-119.

NE...Winter Storm Warning until 3 PM MDT Friday for NEZ095.

&&

$$

UPDATE...REC
SHORT TERM...CAH
LONG TERM...RJM
AVIATION...ZF
FIRE WEATHER...CAH



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