Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 220005
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
505 PM MST Sun Jan 21 2018

.SHORT TERM...(Tonight through Monday night)
Issued at 336 PM MST Sun Jan 21 2018

Conditions are gradually improving from west to east as the latest
storm system shifts onto the Plains and begins to wreak havoc.
The surface low appears to be over southeast Colorado with the
upper level low spinning south of Goodland and Burlington near the
CO-KS border with lee side cyclogenesis taking place. The western
slope is under a northerly flow this afternoon, with snow
continuing over the northern and central divide mountains and
adjacent valleys, as well as the northwest San Juans. Expect snow
to continue periodically through this evening for most areas in
this convective unstable environment, before coming to an end
tonight as the low shifts further east and loses its hold over
western Colorado. One area in particular to watch for through
tonight is the Uncompahgre Gorge in the northwest San Juans, which
includes the Highway 550 corridor from Ouray to Red Mountain
Pass. The northerly flow and cold, moist air lends itself to
enhanced upslope snowfall in this region, so have the Winter Storm
Warning continuing through midnight tonight for the NW San Juans,
with particular focus on this Gorge area, with the other
mountains of western Colorado seeing advisories continuing through
this evening.

Most areas around eastern Utah and western Colorado, mountains and
valleys, received some snowfall with the latest storm and have
snow on the ground. This added snow cover draws concerns for
forecasted temperatures as valley inversions will likely set up,
making temperatures colder than guidance. Spent some time lowering
forecasted highs and lows to trend towards colder guidance and in
some cases, below guidance in some of those inversion prone
valleys. Just how much colder will it get remains to be seen, but
probably can expect some areas to be colder than forecast the next
few nights or even days. Northwest flow will continue Monday with
a weak shortwave clipping the northern Colorado mountains
beginning Monday evening, lending to some light orographic
snowfall as H7 temperatures range in the favored dendritic crystal
growth region (-12C to -16C). A couple inches could fall with
this shortwave for the Park and Elkheads, Flattops and potentially
down to Vail Pass, although models are trending further north
with this shortwave so confidence is low in Vail Pass seeing any
appreciable accumulations.

.LONG TERM...(Tuesday through Sunday)
Issued at 336 PM MST Sun Jan 21 2018

Light orographic snow will continue in northwest flow through
Tuesday morning for the northwest Colorado mountains as a weak
shortwave passes. A ridge of high pressure will gradually slide
over the region late Tuesday into Wednesday, leading to dry
conditions and a light westerly flow. Forecast temperatures are
expected to remain colder than model guidance as fresh snow cover
from the most recent storm allows valley inversions to
strengthen, keeping cold air trapped while the slopes see some
moderation. The flow will shift to southwest by Thursday ahead of
an approaching storm system moving into the west. Therefore,
lowered temperatures through Wednesday night to slow the warming
trend. The gradient tightens on Thursday with H7 winds increasing
to 40 to 50 kts from the southwest, so expecting most inversions
to break by Thursday with decent mixing and therefore, milder
temperatures despite snow cover. Some high clouds will start
moving in by afternoon, however, which could hinder mixing, so we
shall see.

This trough of low pressure will bring precipitation to the
region Thursday evening through Friday as the cold front passes
through Friday morning. The models are in better agreement in
regards to the timing and track of this system, which shows it
impacting more of the northern and central areas and leaving the
southern part of the forecast area drier. Regardless, the models
are also consistent on much colder air moving in behind this cold
front by Friday morning, with widespread H7 temps of -10 to -15C,
which means efficient dendritic type of snow - light and fluffy
instead of wet and heavy. The flow shifts to northwest behind this
front by Friday morning with another piece of energy moving
through Friday evening. With favored orographics, cold air and
enough moisture, this should keep mountain snow going through
Saturday morning. There is not much moisture with this system
still, with specific humidities of 2 to 3 g/kg with the cold front
and 1 to 2 g/kg behind the front. This system looks convective
with lots of instability and dynamical forcing with the front, so
expecting a burst of heavy snowfall and quick drop of
temperatures to accompany the front as it moves through, which is
where the valleys may end up seeing their best chance for snow as
well. There is still plenty of time for these details to be
worked out and model solutions have a tendency to change. The lack
of good moisture and the trend for this system to track further
north with each run is a bit concerning. A ridge of high pressure
will build in behind this system by the weekend, leading to drier
conditions.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 505 PM MST Sun Jan 21 2018

Scattered snow showers will continue across much of western
Colorado through this evening with northerly winds gusting up to
25 kts. Snow showers will drop CIGS/VSBYS to LIFR/IFR thresholds,
otherwise ILS CIGS are expected through 06Z. After 06Z, conditions
are expected to improve with snow showers ending and CIGS lifting
above ILS breakpoints at all sites. Patchy valley fog will be
possible late tonight and early Monday morning with low confidence
of this impacting terminal sites. After 16Z/Mon, VFR conditions
are expected at all sites.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Winter Weather Advisory until 9 PM MST this evening for COZ004-
     009-010-012-013.

     Winter Storm Warning until midnight MST tonight for COZ018.

UT...None.
&&

$$

SHORT TERM...MDA
LONG TERM...MDA
AVIATION...JDC



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