Weather Service Report


652 
FXUS65 KGJT 180555
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1055 PM MST Tue Jan 17 2017

.UPDATE...
Issued at 1055 PM MST Tue Jan 17 2017

Visibilities at Steamboat Springs have dropped to half a mile in
fog. Been monitoring observations through the evening and some
sensors on the other side of the Park Range were briefly down to
quarter mile. With dense fog also still at Hayden, expect the
visibility along the Highway 40 corridor from east of Hayden
towards Steamboat Springs to drop to quarter mile or less at times
through the night. Issued a Dense Fog Advisory for COZ005 as well
and adjusted weather grids accordingly. Other areas of concern
include Vernal and Gunnison as they have been in low stratus deck
for most the evening with ceilings of 400 to 1000 feet with good
visibilities. Stratus deck is still remaining above the valley
floor but will continue to monitor for lowered visibilities if
this deck lowers.

UPDATE Issued at 551 PM MST Tue Jan 17 2017

Issued a Dense Fog Advisory for the Central Yampa River Basin
(COZ002), including the towns of Hayden, Craig and Meeker in
effect now until 11 am MST Wednesday. Visibilities have already
dropped to less than a quarter mile in the last hour at Hayden
with satellite showing this area of fog from east of Hayden back
towards Craig. Craig visibility has also dropped from 5 miles to
three-quarters of a mile in the last hour. Expect dense fog to
roll into Craig within the next hour as well. Patchy fog is
possible in most valleys tonight so will monitor other areas for
potential dense fog as well.

&&

.SHORT TERM...(Tonight through Wednesday night)
Issued at 235 PM MST Tue Jan 17 2017

Cloud cover in southwestern Colorado associated with lingering
low- level moisture has finally begun to break apart late this
afternoon. Additional cloud cover was noted on visible satellite
imagery over the Uinta Basin, where a strong inversion has kept a
low stratus deck locked in all day long from Vernal southward to
the East Tavaputs Plateau. This stratus deck is forecast to
linger overnight, and sky cover grids have been modified to show
this trend. Elsewhere across the region tonight, patchy fog is
expected to develop in valley locations across the north, with
some patchy fog also possible in central and southern valleys if
skies can sufficiently clear. Guidance has been a bit inconsistent
onlow temperatures tonight - understandable given the
opportunity for some fog and low stratus in many valley locations.
Have opted to lean toward the slightly warmer MET guidance
overnight considering the stubbornness of low level moisture
across the region.

The center of the weak upper- level low responsible for keeping
the cloud cover around will finally drift further to the east and
into the high plains early Wednesday morning. A dry day is
expected on Wednesday as the axis of the 500mb ridge crosses the
UT/CO border by early evening. As mentioned in previous
discussions, clear skies and higher RH values in the lowest levels
will make early morning fog likely by Thursday morning.

.LONG TERM...(Thursday through Tuesday)
Issued at 235 PM MST Tue Jan 17 2017

Cloud cover will increase on Thursday morning as the first in a
series of three storm systems approaches the Four Corners.
Flow will turn southwesterly Thursday afternoon, with high
temperatures about 5 degrees above average despite the thickening
clouds. Timing is still on track to bring precipitation into the
higher terrain of eastern Utah by noon, higher elevations in
western Colorado seeing light snow developing by 5pm. This storm
will bring mostly light snow accumulations in the mountains, with
rain changing over to light snow in the valleys overnight. Valley
locations will see a lull in precipitation on Friday afternoon
and evening before the next storm system moves early Saturday
morning. The higher elevations may not see precipitation come to
an end before the next storm moves in. 700mb temperatures will be
slightly colder with the second storm coming for Saturday, with
the center of the developing 500mb closed-low tracking just south
of the San Juan range. This would seem to indicate a better chance
for snow in the valleys, however the further south this low tracks
the less QPF models have been putting out along and to the north
of I-70. This will remain the primary forecast challenge in the
medium range for the next few days. Farther south in the San Juan
mountains, 12z guidance as well as WPC QPF forecasts have been
hinting at a decent snowfall event lasting through the end of the
Day-5 (Saturday evening) period.

Despite subtle differences in the strength and depth of the
shortwave on Saturday, both the GFS and ECMWF are in relatively
good agreement on a shortwave ridge passing over eastern Utah and
Western Colorado on Sunday. Precipitation should come to an end
in all but the highest elevations of the central Colorado
mountains on Sunday afternoon. The final act in the upcoming
three-part winter storm series will come during the Monday-
Tuesday-Wednesday timeframe. Naturally at this range there are
notable differences in forecast guidance, however both the GEFS
and EPS ensemble means develop a 1 to 2 sigma h5 trough over the
western US by Tuesday morning...increasing confidence in the
forecast at this time. Deterministic guidance suggests a robust
area of low pressure moving inland along the Canadian coast, with
a longwave trough spreading a stream of Pacific moisture into the
central Rockies by Tuesday. While exact precipitation totals are
unclear at this time, this early-week storm at the end of the
forecast period will most likely bring the most significant
snowfall totals to the mountains during the next week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1005 PM MST Tue Jan 17 2017

Low stratus will continue to have an impact on KVEL as low level
moisture remains trapped underneath a strong inversion. Patchy fog
will be possible among most valley locations with areas of dense
fog among valleys north of I-70 including KCAG and KHDN. Fog
should lift by 18Z. VFR conditions are expected Wednesday
afternoon with most of the area under clear skies, except valleys
where low stratus remains under inversions. Mid and high level
cloud cover are expected to increase from the west Wednesday
evening ahead of the next Pacific storm system.


&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Dense Fog Advisory until 11 AM MST Wednesday for COZ002-005.

UT...None.
&&

$$

UPDATE...MDA
SHORT TERM...MAC
LONG TERM...MAC
AVIATION...MDA

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Western Regional Climate Center, wrcc@dri.edu