Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 251530

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
930 AM MDT Tue Apr 25 2017

Issued at 922 AM MDT Tue Apr 25 2017

Snow looks to have ended for the time being over the northern and
central mountain while some snow continues for the San Juans.
HRRR and other short range models do show some convection this
afternoon, from about noon onwards, and some heavier showers are
certainly possible but do not anticipate much in the way of
impacts. All roads are now wet and with temperatures warming,
significant accumulations are looking less likely. Of course, a
stronger cell may drop a quick inch but it will melt very quickly.
With that in mind, went ahead and cancelled the advisories for
those two areas. Will keep the San Juan advisories going for now
but may be pulling them down early as well.


.SHORT TERM...(Today through Wednesday)
Issued at 416 AM MDT Tue Apr 25 2017

A deep, cold Pacific storm system continued to track across the
region early this morning. The cold front associated with this
system had pushed south of Gunnison and appeared along a KBDG to
KTEX to KALS line as of 09Z. The 100KT jet over the front had
shifted to the east and as a result showers had decreased. Snow
levels fell to near 7500 feet behind the front as evidenced by
surface observations at both KASE and KGUC. Farther north, KCAG
at roughly 6200 feet saw a period of snow before midnight. Showers
are expected to develop over the Four Corners region with frontal
passage this morning, helped along by a vorticity lobe rotating
around the closed mid-level low centered over the northern High
Plains. Expect rain at Cortez and Durango, though Pagosa Springs
at roughly 500 feet higher could see snowfall, but accumulations
are unlikely.

Models suggest an increase in coverage and intensity of showers as
the vort lobe mentioned previously sweeps southeastward across
the area from late morning through the afternoon. Concurrently,
lapse rates will steepen in response to surface warming below the
cold core aloft. Understandably, this will result in some stronger
showers and some embedded thunderstorms as well. Decent northerly
flow behind the front will provide good orographic lift in this
unstable environment. Model snowfall accumulations suggest that
convective snowfall could pile up in favored areas like the
Uncompahgre Gorge, atop the Grand Mesa, and over the Elk Mountains.
However, rising temperatures will limit significant accumulations
to areas above 9000 feet consistent with current Winter Weather
Advisories. Confidence not sufficient to upgrade advisories at
this time but urge the day shift to keep a close eye on area
snotels as convective snowfall tends to bring some surprises.

Expect snowfall to persist into the evening hours, but will be
decreasing during the latter part of the evening and overnight as
the Pacific storm tracks farther east over the Plains. Light
showers are likely to continue into Wednesday morning over the
Continental Divide, however additional accumulations are expected
to be light and localized. Showers then increase, especially
across the northern mountains late Wednesday morning and into the
afternoon ahead of the next Pacific system approaching from the

Today`s highs will be chilly for this time of year given the
passage of the cold front discussed previously combined with
clouds and precipitation. Consequently, expect temperatures to
peak some 5 to 10 degrees below normal for this time of year. It
follows then, that overnight lows will also be unseasonably cold
and could approach freezing in the known cold spots in the lower
central valleys where sensitive vegetation resides. However, a
hard freeze is unlikely and therefore have no plans to hoist a
Freeze Warning with this package. Expect temperatures to moderate
Wednesday as warm air advection sets in ahead of the next Pacific

.LONG TERM...(Wednesday night through Monday)
Issued at 416 AM MDT Tue Apr 25 2017

Showers continue into Wednesday evening as the next system
approaches. The upper level jet stays oriented from NW to SE
across the Pacific Northwest towards the Four Corners during this
period, keeping our CWA in the left exit region of the upper level
jet. The strongest jet streak is over the Pacific Northwest of
170 kts which translates down towards 110 kts over the Four
Corners. The cold front associated with this system moves across
early Thursday morning with northwest flow lingering through
Thursday afternoon and evening, favoring the northern and central
Colorado mountains. Another round of decent snowfall is possible
Wednesday evening through Thursday afternoon, so will need to keep
an eye on the details as we near closer to the event. One concern
is that a ridge of high pressure will amplify over the Pacific
Ocean, pushing the upper level jet further northeastward, allowing
drier air to traverse from the northwest and effectively cut off
the moisture supply and put an end to most precipitation by
Thursday afternoon.

As this Pacific ridge amplifies and pushes further east, this also
amplifies the troughiness over the Intermountain West. Previous
forecasts were indicating the potential for a significant storm
system to develop by late in the week and impact the area through
this weekend. The latest 12Z guidance is now showing this low
developing further east than previous runs with the low developing
over the northern Utah- Colorado border Friday morning and
tracking southward towards the Four Corners by Friday evening.
This brings increasing chances of widespread precipitation across
the area and colder air advecting in with H7 temps lowering
towards -10C Friday morning through Saturday morning. There is
potential for below freezing temperatures in the lower valleys so
something to keep an eye on.

The amplified Pacific ridge moves further eastward into the
western states and draws concerns for the going forecast in
terms of significant snow potential for the mountains with this
system as this also draws much drier air into the region during
this period. This further eastward track also brings the focus for
best precipitation onto the Front Range and Plains in the
southwest flow ahead of the system on Saturday. PW values remain
less than 0.25 inches from Thursday afternoon through at least
Sunday, so this doesn`t bode well for significant precipitation.
The ridge helps to quickly push this system out of the region
towards the Plains by Saturday evening with an unsettled but drier
northwest flow over the area Sunday and Monday, keeping the
northern Colorado mountains favored for continued orographic
snowfall. Still kept the PoPs on the conservative side for late in
the week as even though the 12Z GFS and EC are in good agreement
with this further eastward and drier solution, run to run
consistency is lagging.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 532 AM MDT Tue Apr 25 2017

Widespread rain and snow showers will continue over the forecast
through this evening. Expect ILS CIGS at all sites with areas of
IFR/MVFR CIGS and VSBYs. Mountains will be obscured through
this evening. Conditions improve from west to east tonight.


CO...Winter Weather Advisory until 9 PM MDT this evening for COZ018-



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