Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 180529

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1129 PM MDT Sat Mar 17 2018

Issued at 845 PM MDT Sat Mar 17 2018

Made several timing adjustments to the winter weather advisories
in place. Short term models indicate an approximate 6 hour delay
in previous timing expectations, so many advisories have been
adjusted to not begin until early Sunday morning. Conditions are
quite clear across the region still, with the main baroclinic leaf
of this system only now reaching southwestern UT. Precipitation is
still expected to pick up in the morning hours and spread eastward
into most of western Colorado by noon Sunday as the cold front
gains momentum early on Sunday.


.SHORT TERM...(Tonight through Sunday night)
Issued at 259 PM MDT Sat Mar 17 2018

Satellite and observations showed increasing lift beginning to
move into western Colorado and eastern Utah. Mass response from
southwesterly flow aloft was increasing as evident by the changes
to the direction of the surface stations across eastern Utah.
Gusty winds will continue this afternoon as an upper trough and
attendant jet approach from the west. Temperatures in the southern
and central valleys were climbing through the upper 50s to near
60 in some spots. This relatively mild air mass will change late
tonight and early tomorrow as an approaching pacific front passes
through. Rapid lee cyclogenesis should take place Sunday morning
across south eastern Colorado. This will accelerate the front
through the mountains and stall an arctic front acorss northern
Colorado and Utah.

Snow will begin this evening for the high elevations continuing
through Sunday as a strong upper level storm system moves in from
the west. The dynamics aloft are impressive with a 110kt jet and
strong differential advection. Widespread precipitation is
expected to develop ahead of and along the cold front late tonight
into early Sunday. PoPs have been bumped up to match the latest
guidance on timing, which is just a hair slower than the last few
runs. The best dynamics appear to be shifted to the 12-18z time
frame. Hints at a trough of warm air aloft and some convective
instability from the cold advection aloft suggest there should be
some decent snowfall rate early to mid Sunday. the NAM nest and
GFS both support this with a few banded structures developing.

Some issues with this forecast include the timing of the cold air.
Mountains should be solid 5 to 10 inches with great forcing over
the southern San Juans, but the best chance for accumulating snow
in the valleys is early Sunday when the temperatures are most
conducive. That being said, confidence is low for the southern
valleys aka Cortez, Durango, and Pagosa. Lows are expected to hold
in the low 30s as opposed to the mid 20s of the Uinta and Yampa
Basins. As such, elected to hoist Advisories for the north as
extra forcing and cold air from the arctic front should help boost
totals to an easy 2 to 4 inches there. (see WSW update)

.Updates to WSW...

Winter weather Advisories have begun in UTZ023 for the Uinta
mountains. light snow is likely falling at elevation under the
cloud cover that has developed there. An update was made to
include the northern valley zones UTZ024 and COZ01 and 02. This
includes Vernal, Hayden Craig and Meeker. These zones will go into
effect at 03z 9pm MDT tonight along with UTZ025 at 6 pm MDT. The
rest of the zones remain unchanged and go into effect at 3 am

.LONG TERM...(Monday through Saturday)
Issued at 259 PM MDT Sat Mar 17 2018

Orographic snow in the northeastern mountains should continue
behind the system through mid day Tuesday as a few weak waves in
the flow aloft move through. Made updates to the forecast to
reflect that. Northwest flow ahead of shortwave ridging is expect
to move in Tuesday and Wednesday. This should allow much of the
area south of 70 to warm up through mid week. The north will lag
behind slightly as the Arctic front from the last system washes
out and drift north. Temperatures will climb back to near 60
degrees in some spots by Thursday.

Active weather is expected to return next weekend as another
series of troughs move through the area. Precip chances increase
Friday and continue through Sunday with mountain snow potential.
Given the relatively mild airmass expected, valley snow seems
unlikely at the time, but as the forecast gets closer details will
become more clear.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1129 PM MDT Sat Mar 17 2018

Cloud cover will increase from west to east, and conditions will
deteriorate overnight as a cold front moves in from the west.
Ceiling heights will lower into MVFR conditions and eventually IFR
conditions at numerous terminals as a cold front passes in the
late morning hours. Expect a period of rain turning to snow at
most terminals, which will temporarily reduce visibilities to 3
miles or less around sunrise in the northwest and through the
early afternoon in the central and southern mountains. Expect
obscurations to terrain and a wind shift from southerly to
westerly as the front passes on Saturday. Conditions will improve
across eastern Utah by 00z Sunday evening, then across most of
western Colorado by 06z Sunday night.


CO...Winter Weather Advisory from 3 AM Sunday to midnight MDT Sunday
     night for COZ001-002.

     Winter Weather Advisory from 6 AM Sunday to midnight MDT Sunday
     night for COZ003-017.

     Winter Weather Advisory from 9 AM Sunday to 6 AM MDT Monday for

     Winter Weather Advisory from noon Sunday to noon MDT Monday for

UT...Winter Weather Advisory until midnight MDT Sunday night for

     Winter Weather Advisory from 3 AM Sunday to midnight MDT Sunday
     night for UTZ024.

     Winter Weather Advisory until 6 PM MDT Sunday for UTZ023.

     Winter Weather Advisory from 6 AM Sunday to midnight MDT Sunday
     night for UTZ028.



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