Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 112341
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
441 PM MST Wed Jan 11 2017

.UPDATE...
Issued at 205 PM MST Wed Jan 11 2017

Another update to temperature grids this afternoon the Grand
Valley has continued to warm as of the 2pm hour. For the record
it is worth noting that an 800mb temperature of +5 degrees C as
seen in the 12z GJT sounding, when fully mixed dry adiabatically,
would yield a high between 50 and 52 degrees. It appears as though
we have squeezed every last degree out of our temperature profile
today, despite this morning`s strong inversion.

Thunder also has been added to the short term weather grids
through 0z. Lightning has been detected around 2pm to the south of
Vernal, with additional thunder possible in eastern Utah and far
western Colorado.

&&

.SHORT TERM...(Tonight through Thursday night)
Issued at 205 PM MST Wed Jan 11 2017

Snowfall across the central and northern mountains will continue
this evening as moisture streams westward across the central
Rockies. A stronger band of snowfall has developed just to the
north of Interstate 70 across the higher terrain, and will move
east into the Flat Tops, Parks and all central mountain ranges
this evening. It is within this band that the heaviest snowfall
rates are expected and the bulk of the accumulation in the
mountains will fall. Farther south across the San Juans, only
light to moderate mountain snowfall is expected with the best
dynamics as well as afternoon convective elements remaining farther
north. As mentioned in updates throughout the day, valley
locations from Interstate 70 southward have warmed dramatically as
low level profiles achieved full mixing this afternoon. With such
mild air in place in the low- levels, it is unlikely that valley
locations will see any frozen precipitation this evening before
rain ends. The exception would be in heavier showers, where soft
hail has been reported in a few locations this afternoon.

Snow will continue to fall in the northern and central mountains
of Colorado early Thursday morning. Valleys will be dry, perhaps
as early as the midnight to 3am timeframe, with a minimum in PWATs
and precipitation area-wide during the early daylight hours
Thursday. As has been the case for the past several events in the
central Rockies, snow may not completely end in the higher terrain
before the next piece of energy and associated moisture moves in
by Thursday afternoon.

The next storm system affecting the Four Corners region will come
courtesy of a sharp trough moving southward along the California
coast Thursday morning. At 500mb, this system will cut off as it
moves almost perpendicular down the coastline Thursday and
Thursday night. This southerly movement will allow winds aloft
over eastern Utah and Western Colorado to turn southwesterly and
intensify on Thursday afternoon. 700mb wind speeds of around 30
knots or so will result in a breezy day in southeast Utah and
southwest Colorado. A renewed stream of Pacific moisture will head
toward the four corners by Thursday afternoon as well, however
PWATs will not be quite as impressive over the region...running
only about two sigma or so for the late-week timeframe. The
challenge for Thursday will be whether any precipitation can make
it to the surface of the valleys...and what form that
precipitation will be in. At the moment, guidance has trended
warmer with afternoon temperatures and it appears as though
valleys will see rainfall by 8pm or so, with snow in the mountains
above 7000 feet.

.LONG TERM...(Friday through Wednesday)
Issued at 205 PM MST Wed Jan 11 2017

The 500mb closed low along the southern California coast will
continue southward on Friday. As it does so, a subtle mid-level
divergence signature develops at 500mb. Despite the best Pacific
moisture stream slowly moving east of the area, this dynamic
support for lift combined with southwesterly upslope flow will
keep snow showers going in the central and southern Colorado
mountains. Cooler mid-level temperatures will support slightly
higher snowfall ratios than the most recent event for the
mountains. Western and southern facing slopes that typically do
well in this flow regime should expect to see decent accumulations
Friday, despite the relatively lackluster QPF totals during this
time frame shown on today`s 12z guidance.

After the 0z Saturday timeframe, the 500mb closed low will
meander eastward along the Arizona/Mexico border toward the Rio
Grand Valley. Guidance has consolidated a bit regarding evolution
of this system today, and this forecast package reflects a nearly
50/50 blend of the GFS and ECMWF through early next week. Saturday
will feature drying weather to the north of the Interstate 70
corridor, with temperatures right around average for mid January.
South of the interstate the forecast is much more tricky. Models
keep the 500mb closed low close enough to the Four Corners to
allow for snow to continue in the San Juan Mountains. The mean
500mb trough in the western United States will attempt to re-
capture the closed low, driving this system back north of the
border and into New Mexico by Saturday night into Sunday morning.
If this solution materializes, significant snows will result in
the southern Colorado Mountains. Impressive mid and upper- level
divergence signatures combined with a surge of moisture from the
Gulf of Mexico (PWAT anomalies around 3 to 4 sigma) hint at the
potential for a fairly major snowfall event in the southern
Rockies late this weekend. 700mb temperatures in southern Colorado
will average in the 3 to 6 degree C range, resulting in snow
levels around 7000 to 8000 feet as of today`s guidance. It should
be noted that predictability regarding cutoff low- pressure
systems in the Day-5 timeframe is usually poor. However, run-to-
run consensus in the GEFS and EPS ensemble guidance lends
increasing confidence to the forecast at this time.

As the upper level jet moves east of the Rockies, lee
cyclogenesis will take place late Sunday into early Monday in the
New Mexico high plains. Upslope snows will continue in the
front ranges of the southern Rockies, with moisture spilling west
into the San Juans and central Colorado ranges. It may not be
until late in the day on Monday before light snow ends our central
and southern mountain zones. The remainder of the forecast period
through Day 7 looks tranquil with seasonable temperatures as a
weak ridge moves overhead for Tuesday and Wednesday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 441 PM MST Wed Jan 11 2017

Line of showers with embedded thunderstorms containing gusty winds
and small hail and a mixture of rain and snow is moving across
central portions of Utah and western Colorado. This should
loose some momentum after sunset but impacts to TAF sites near
I-70 look possible over the first hour or so of the new forecast.
Otherwise the moist west flow continues and this should keep a
threat of lower cigs and weather pointed at the region through the
end of this TAF forecast. Mix of VFR to MVFR conditions will exist
through much of this forecast period with localized IFR after 15Z
on Thursday as the next system approaches.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Winter Weather Advisory until midnight MST tonight for
     COZ001>003-017>019.

     Winter Weather Advisory until 9 AM MST Thursday for COZ005-014.

     Winter Storm Warning until 9 AM MST Thursday for COZ004-009-010-
     012-013.

UT...Winter Weather Advisory until midnight MST tonight for UTZ023-
     025.

&&

$$

UPDATE...MAC
SHORT TERM...MAC
LONG TERM...MAC
AVIATION...TGJT



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