Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 210018

Area Forecast Discussion
National Weather Service Grand Junction CO
618 PM MDT Fri Apr 20 2018

Issued at 618 PM MDT Fri Apr 20 2018

Made some adjustments to ongoing winter weather highlights over
the mountains of western Colorado. First, drier air wrapping into
the San Juans has significantly impacted snow production over the
area, and this is not expected to resume in any organized sense
over the southwest San Juans tonight. Snotels indicated only light
accumulations so far, so decided to drop the Winter Storm Warning
for CO zone 19, and downgrade CO zone 18 (northwest San Juans) to
an advisory. The north side should see an increase in snowfall as
the northerly flow becomes established. Similarly, downgraded the
Winter Storm Warnings to Winter Weather Advisories for the
central Colorado mountains where again, accumulations have been
less than forecast. Concurrently, added the Grand Mesa to the
advisory as Skyway and Mesa Lakes have received moderate
accumulations and should get more in the north flow tonight. As
temperatures fall, Hwy 65 should become snowpacked and icy.


.SHORT TERM...(Tonight through Saturday night)
Issued at 206 PM MDT Fri Apr 20 2018

The upper-level low pressure system responsible for today`s
unsettled weather continues to slowly drift to the south and east
this afternoon. As of 2pm, the center of this low pressure was
located in far southeast Utah. As a result, northerly flow has yet
to really set in over much of eastern Utah and western Colorado,
resulting in limited snowfall accumulations at this hour. As this
low pressure system moves over the Four Corners and onward
through northern New Mexico, winds are expected to shift
northeasterly to northerly, bringing colder air and moisture
southward. Short-term guidance does continue to indicate a slight
uptick in areal coverage of light snow and rain shower activity
this evening and early overnight, before the low pressure moves
far enough east of the area to bring precipitation to an end on
Saturday morning. Have nudged precipitation totals downward
slightly for the remainder of this event from a mix of model

On Saturday, dry air will begin working westward into eastern Utah
and the western slope of Colorado. However, flow will still remain
mostly northerly over the Divide...and guidance does show a bit
of lingering moisture in the lower and mid levels on the back side
of our storm system. Additionally with colder air aloft and
strong surface heating under the late-April sun, some convective
snow shower activity is possible in the mountains. Have kept some
chance PoPs in the forecast in the higher terrain along the
Divide all the way through 0z to account for these factors. Flow
aloft turns northwesterly to westerly after midnight on Sunday
morning, allowing for a mostly clear night area-wide.

.LONG TERM...(Sunday through Friday)
Issued at 206 PM MDT Fri Apr 20 2018

In contrast to the more active storm pattern over the past week
consisting of strong areas of low pressure, this long term
forecast looks to be dominated by weaker passing waves and only
limited chances for moisture through the end of the week. As
heights rise on Sunday and Monday, expect high temperatures to
follow suit, running around 5 degrees above average on Sunday, and
over 10 degrees average by Monday. Guidance has consistently shown
a weak trough passing through the northern Rockies during the
early-week timeframe. Warming temperatures with just a touch of
moisture to work with will result in the chance of some isolated
shower activity in the mountains on both Sunday late afternoon and
on Monday afternoon. On Tuesday, forecast confidence decreases
somewhat as the GFS indicates this trough will drag southward and
funnel more clouds and precipitation through western Colorado. In
contrast, the ECMWF pushes this system east of the Divide and
keeps the Four Corners region completely dry through midweek.
Current thinking favors the drier solution, and only lower-end
PoPs are shown in this forecast package during the Tuesday-
Wednesday timeframe.

Beyond this system, guidance comes back into agreement by Thursday
and Friday as a more robust (but dry) front dives southward from
Canada and clears any lingering moisture out of the northern and
eastern Rockies. For now, it appears as though both Thursday and
Friday of this coming week will turn out dry with afternoon highs
around 5 degrees above normal for late-April.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 533 PM MDT Fri Apr 20 2018

Storm system is tracking over Colorado this evening spreading
mositure and lift to the region. Unfortunately there is also some
dry air mixed into the equation. This is making for a difficult
forecast so there may a constant stream of amendments through the
evening. However it appears the western TAF sites should generally
stay VFR while the eastern sites should be impacted by low clouds
and passing precipitation as winds turn more northerly aloft.
Conditions should generally improve most areas through the mid
morning hours on Saturday.



CO...Winter Weather Advisory until 6 AM MDT Saturday for COZ004-009-

UT...Winter Weather Advisory until 6 AM MDT Saturday for UTZ023.



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