Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 121055
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
355 AM MST Thu Jan 12 2017

.SHORT TERM...(Today through Friday)
Issued at 336 AM MST Thu Jan 12 2017

Ahead of a closed 500 mb low along the west coast a ridge is
pushing the old frontal surface over the forecast area as a warm
front. A line of showers will continue moving from sw to ne across
the northern half of the forecast area this morning, bringing a
brief uptick in precipitation. Later this afternoon warm advection
precip with spread across the area and continue through midnight.
The 500 mb along the west coast will close off and dive south to
the U.S./Mexico border. Through Friday the forecast area will
remain under the deformation zone between the southern low and the
westerly flow over the northern rockies. cloudiness and areas of
precip will also continue.

Tonight and Friday

.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 12Z TAFS through 12Z Friday morning)
Issued at 336 AM MST Thu Jan 12 2017

moist west flow continues and this will keep a threat of lower
cigs and showers pointed at the region through the next 24 hours. 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 9 AM MST this morning for COZ014.

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

UT...None.
&&

$$

SHORT TERM...CC
LONG TERM...MAC
AVIATION...CC



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