Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 150255
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
755 PM MST Sat Jan 14 2017

.UPDATE...
Issued at 755 PM MST Sat Jan 14 2017

Visibilities are beginning to decrease across the same northern
valleys that have seen dense fog over the past several nights as
well as portions of the Central Gunnison River Basin. Areas of
dense fog will persist through at least 10 AM tomorrow morning.
Therefore, went ahead and hoisted a Dense Fog Advisory for these
areas and updated the grids accordingly. Visibilities are also
decreasing in and around Telluride, but will hold off on issuing
any highlights at this time and continue to monitor the situation.

UPDATE Issued at 454 PM MST Sat Jan 14 2017

All highlights will be allowed to expire at 5 pm this evening with
diminishing returns on radar fitting trends in HiRes RAP and HRRR
models. Still expect snow showers through the night, but
significant accumulations no longer expected in highlighted areas.
Fog has also lifted some across northwest Colorado with
visibilities generally above 3 miles, so the Dense Fog Advisory
will also be allowed to expire at 5 pm. Little more separation
between the T/Td at the moment, although another round of dense
fog is not out of the question later tonight as temperatures start
to cool back down.

&&

.SHORT TERM...(Tonight through Sunday night)
Issued at 338 PM MST Sat Jan 14 2017

The upper level low pressure system is currently situated just off
the Baja and making its way inland. During the late morning and
early afternoon hours, an enhanced band of precipitation moved
across southwest Colorado and southeast Utah producing heavy snow
in the mountains with rates of up to 3 inches per hour over the
San Juans, where winter highlites remain in place through 5 pm
today. The lower valleys south of I-70 were primarily affected
and precip type stayed rain for the most part with a few spots
mixing in briefly with snow. Current radar and satellite trends
show that this band has since weakened and is pretty much losing
its characteristics as it is moving into an area of more stable
air north of I-70. This area north of I-70 has been seeing areas
of fog, some of which has been dense throughout the morning and
early afternoon. Visibility has since improved as of 3 pm but
current guidance shows fog setting back in mainly across the
northern valleys overnight through Sunday morning. Later shifts
will have to look into the possibility of dense fog occurrence
once again given the moist low levels. Precipitation intensity
has also lessened this afternoon as well and expect a general
downward trend in shower activity this evening. Some convection
and thunderstorms are rotating around the low across Arizona and
southwest New Mexico, but do not expect this thunderstorm
activity to sustain itself by the time it makes its way northward.

Models appear to indicate an overall downturn in precipitation
this evening into the overnight hours for most areas. The
exception to this is a batch of showers left over from convective
activity passing through Arizona and New Mexico that will move
through southeast Utah. As the upper low shifts further east
across northern Mexico overnight into Sunday morning, some more
moisture looks to rotate up through southwest Colorado towards
Sunday morning. The H7 flow will shift around to the east which
tends to favor the eastern slopes of the eastern San Juans. It
appears this wave will clip the eastern portion of the western San
Juan mountains mainly up through Wolf Creek Pass and maybe Pagosa
Springs. It will be brief with weak forcing so maybe a couple
inches of snow for the western San Juans. A secondary shortwave
will move down the west coast on the back side of this low
pressure trough with some jet energy, helping to quickly push this
low further eastward as it lifts across Texas. This low is a bit
further south and east than previous models indicated which does
not favor our CWA for significant precipitation. Trended back the
pops by lowering them the further west and north you go during the
day on Sunday. Thinking that some light snowfall will continue but
additional accumulations appear minimal and gradual throughout
the day with no significant impacts. As said previously, the
eastern San Juans are favored in this upslope type of flow with
the western San Juans being quite shadowed. The orographic lift is
not overly strong enough to allow much significant snow to spill
over into our western San Juan mountains. Northerly flow takes
hold Sunday evening into Monday morning, bringing much drier air
into the region and putting an end to most precipitation.

Based on the above reasons, elected to let the current winter
highlites expire at 5 pm and will not continue or issue any for
the upcoming wraparound precip on Sunday. Snow will continue in
some areas, but accumulations are expected to be light.

.LONG TERM...(Monday through Saturday)
Issued at 338 PM MST Sat Jan 14 2017

Some light snow may linger over the San Juans Monday morning
favoring the north facing slopes but overall precipitation will be
coming to an end across the area as drier air advects in under
northerly flow. Some cooler air will be in place as well on Monday
following the final passage of this lingering storm system.

High pressure will build across the west and lay across the
region from west to east early this coming week, providing dry
conditions for most the region through Wednesday. A weak
shortwave looks to clip the northwest Colorado mountains on
Tuesday with mainly an increase in clouds and the slightest chance
of light snow. Temperatures should moderate by Tuesday into
Wednesday with values closer to normal if not slightly above as
warm air advection takes place ahead of the next series of storms
set to impact the region later this coming week. So before this
occurs, it looks like a dry and quiet period will be in store from
Monday through Wednesday.

Clouds will increase on Wednesday as the flow shifts to the west
ahead of the next series of Pacific storms making landfall onto
the Pacific Northwest and northern California coast. The first in
a series of storms moves across the region Thursday morning
through Friday morning with another one on its heels Friday
afternoon through the weekend. This second one looks stronger as
it deepens and forms a closed low on the lee side of the Rockies.
Details are a little fuzzy since this is a ways out and models
have a tendency to change, but overall it looks like an active and
unsettled pattern takes shape Thursday through the weekend with
more snow in the mountains and a rain/snow mix in the valleys
initially on Thursday, possibly changing to all snow by Friday as
H7 temps lower to around -8C by Friday morning and range from -10C
to -12C by the weekend. Moisture with this next series of storms
is modest but not overly as impressive as the previous atmospheric
river event that kept the weather active the first two weeks of
January. Specific humidity values with this next series of storms
range from 2.5 to 3.5 g/kg. In comparison, the two week long
atmospheric river event averaged about double this amount.
Stay tuned!

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 454 PM MST Sat Jan 14 2017

A Pacific storm passing to the south will continue to generate
precipitation south of the I-70 corridor through Sunday morning.
This will result in prolonged periods where CIGS are below ILS
breakpoints for all sites along and south of Interstate 70. In
addition, areas of MVFR and IFR CIGS and VSBYS will be common with
showers and fog moving through the region. Fog appears most likely
at KVEL, KCAG, KHDN and KSBS tonight with patchy fog in the
vicinity of KGJT, KRIL, KEGE and KMTJ. Conditions improve from
northwest to southeast after 18Z/SUN.


&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Dense Fog Advisory until 10 AM MST Sunday for COZ001-002-005-011.

UT...Dense Fog Advisory until 10 AM MST Sunday for UTZ024.

&&

$$

UPDATE...MMS
SHORT TERM...MDA
LONG TERM...MDA
AVIATION...JDC


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