Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 171225
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
525 AM MST Thu Jan 17 2019

.SHORT TERM...(Today through Friday)
Issued at 411 AM MST Thu Jan 17 2019

Near surface moisture left over from the last storm to move
through the region has resulted in areas of fog during the night.
The fog was dense in the Uncompahgre River Basin and the Grand
Flat of easter Utah and a Dense Fog Advisory remains in effect
for those areas. A transitory ridge has developed over the region
ahead of the next Pacific system to impact the area. High clouds
were streaming through the ridge and are likely to mitigate fog
density during the early morning hours.

Per the previous discussion, the next system moving into the area
will also be quite moist with 7h specific humidity ranging from 4
to 4.5 g/kg when the storm is at its peak. That moisture will
hold off during the early part of the day, reaching eastern Utah
this afternoon before overspreading the region tonight. Difluent
flow aloft and divergence associated with a coupled jet ahead of a
mid-level trough sweeping eastward across the western Great Basin
will provide the lift to get precipitation started, with showers
becoming likely for all but the southernmost portions of southeast
Utah and southwest Colorado by days end. 7h temperatures will be
mild ahead of the trough resulting in rain for valleys below 6500
to 7000 feet during the afternoon. The mountains will have all
snow however, and accumulations will begin for this event by
midday.

The heaviest snow will hold off until tonight as the mid-level
trough passes over the area. Mid-level negative div-q, orographics
and divergence in the left exit region of the 130 kt jet on the
flanks of the trough all combine to generate impressive lift.
A cold front associated with this system will cause rain to change
over to snow during the night. Favorable flow and colder
temperatures favor the northern valleys and the Upper Colorado and
Gunnison basins for advisory snow amounts with portions of the
Four Corners joining in later tonight.

Northwest flow and residual moisture will keep snow going into
Friday, with improving conditions spreading from eastern Utah in
the morning, to northwest corner and Four Corners region by
midday. By the time this storm is done, expect the northern
mountains will have received from 8 to 16 inches of new snow with
locally higher amounts while the plateaus and southwest San Juans
end up with around 6 to 12 inches. However, this system will need
to be closely monitored as the moisture currently forecast may be
underdone leaving the potential for greater snow accumulations.
From a drought and winter recreation perspective, we can only
hope.

Except for minor adjustments in timing, left highlights hoisted by
earlier shifts largely unchanged, though did upgrade the Winter
Weather Advisory for the Elkhead and Park Range to a Winter Storm
Warning.

Temperatures will be a bit milder today in southwest flow ahead of
the approaching storm, becoming cooler on Friday. Clouds and
precipitation will result in mild overnight lows tonight.

.LONG TERM...(Friday night through Wednesday)
Issued at 411 AM MST Thu Jan 17 2019

Our major storm system will come to a close on Friday night as drier
northwest flow takes over throughout the western slope. Weak ridging
will slide eastward through the southwestern CONUS on Saturday. Fast
flow will direct more Pacific moisture across the top of this ridge,
and high clouds will stream in over northern Utah and northern
Colorado throughout the day. Forecast guidance is in fair agreement
hinting at a bit of precipitation in the northern Colorado Mountains
on Saturday evening and overnight as the moist layer thickens and
lowers. Not much in the way of accumulations are expected south of
the Flat Tops, however from the Flat Tops northward through the Park
and Elkheads, a general 1-3 inch event appears more likely. The
southwest ridge will build and nudge this moist flow farther north
on Sunday, ending the snow in the northern mountains. Sunday will
end up being the warmest day in the forecast period as 700mb temps
rise, allowing surface temps to return to average or even above
average.

Medium range guidance has come into better agreement regarding the
next major storm system. A potent trough will dive into the western
CONUS on Sunday evening into Monday morning. This system will be
rather quick-moving, but will have an abundance of Pacific moisture
to work with as it passes through the central Rockies during the
day. As of the 0z model cycle tonight, the GFS still remains a bit
quicker than the ECMWF, however the two models have come into much
better agreement. Precipitation will begin in the northern mountains
around sunrise on Monday, spreading south and east throughout the
day. A rather strong cold front will accompany the onset of
precipitation, with 700mb temperatures dropping below -10 across
nearly all of the western slope by Monday afternoon. Snow ratios
will increase throughout this event as a result. Another notable
trait of this system will be the general direction of wind
throughout the event. While the most recent storm and the current
system have featured mainly southwesterly to westerly flow, the
Monday event will be dominated by west to northwesterly wind
vectors. Mountain locations that generally do better with these wind
vectors will likely overachieve in this event, especially given the
cold temperatures surging in from the north. A very early first call
supports the idea for at least 6 inches of snow in all of the
central and northern Colorado mountain ranges. The northern Grand
Mesa, Flat Tops, and Park Range may do particularly well with this
event. The San Juans as well as the southern valleys remain more of
a question mark with the southern extent of moisture with this
system ending up near the Four Corners.

Beyond this system, yet another opportunity for snow exists on
Wednesday as fast and moist Pacific flow continue to surge inland
from the west. This system will likely favor the northern mountains,
however forecast details have been highly variable in the day 6-7
range in the past few model cycles. Either way, powder-hungry winter
enthusiasts will have plenty of excitement over the next 7 days.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 525 AM MST Thu Jan 17 2019

Low stratus and fog will keep KGJT, KMTJ, KGUC and KCNY at
IFR/LIFR levels through midmorning. Meanwhile, strong winds aloft
will result in LLWS over KHDN through much of the day. LLWS
conditions will also impact KCNY, KASE, KGUC and KTEX later in the
day and overnight. As the next system comes in expect all TAF
sites to experience MVFR and/or periods of IFR/LIFR conditions in
rain, snow and fog beginning late this afternoon in eastern Utah,
then spreading across western Colorado this evening.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Dense Fog Advisory until 10 AM MST this morning for COZ011.

     Winter Weather Advisory from 3 PM this afternoon to 6 PM MST
     Friday for COZ002-003-005-010-017.

     Winter Weather Advisory from 6 PM this evening to 6 PM MST
     Friday for COZ008-018-019.

     Winter Storm Warning from 3 PM this afternoon to 6 PM MST Friday
     for COZ004-009-012-013.

     Winter Storm Warning from 6 PM this evening to 6 PM MST Friday
     for COZ014.

     Winter Weather Advisory from 9 PM this evening to noon MST
     Friday for COZ022-023.

     Winter Weather Advisory from 3 PM this afternoon to noon MST
     Friday for COZ001.

UT...Winter Weather Advisory from noon today to 9 AM MST Friday for
     UTZ023-025-028.

     Winter Weather Advisory from noon today to 6 AM MST Friday for
     UTZ024.

     Dense Fog Advisory until 10 AM MST this morning for UTZ027.

&&

$$

SHORT TERM...NL
LONG TERM...MAC
AVIATION...NL


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