Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 271649
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1049 AM MDT TUE SEP 27 2016

.SHORT TERM...(Today through Wednesday)
Issued at 402 AM MDT Tue Sep 27 2016

The Rex block discussed by the previous shift maintained its hold
over the area during the night. A broad area of high pressure
over the Great Basin prevented moisture circulating around the
upper-level low centered over the central Baja Peninsula from
entering southern Colorado and Utah. As a result, except for wispy
cirrus drifting southward across northeast Utah and northwest
Colorado, skies were clear over the area overnight.

Models indicated that the low will move slightly northward during
the day, but only so far as to spread thin cirrus over the Four
Corners region. Therefore, expect sunny or mostly sunny skies to
rule the day. The atmosphere will provide little synoptic
influence on winds so look for light diurnal circulations across
the forecast area. Afternoon highs should continue to rise as
models indicated about 2 deg (celsius) of warming at the 7h level.
Numerical guidance appeared to reasonably capture this trend so
boosted highs for this afternoon for much of the area to above
normal levels.

The closed low over Baja begins to slide northward as a low
pressure system off British Columbia`s coast drifts to the south.
By 12Z/Wed models were consistent in moving the Baja low center
over the CA/Mexico/AZ triple point. Moisture circulating the low
center begins to just brush southeast Utah and may generate an
isolated storm over Navajo mountain toward sunrise. Otherwise,
the southernmost portions of SW Colorado and SE Utah will see an
increase in mid and high clouds late tonight but should remain
dry. Elsewhere, skies will be clear. Overnight lows are expected
to be at, or just a little above normal.

The pattern continues to shift on Wednesday with the high pressure
ridge axis driven east of the Continental Divide by the British
Columbia low. In response, the Baja low slides northward arriving
over Las Vegas late in the day and flow finally shifts to the
south allowing sub-tropical moisture to move over the area.
Precipitable water (PW) values are indicated to reach 0.80 of an
inch by late afternoon over the Four Corners region, with more
modest amounts to the north. Weak forcing and moisture favor the
south so kept higher PoP values there with slight chances to the
north. Slow moving storms and rising PW may yield isolated heavy
rain over southeast Utah Wednesday afternoon. Guidance suggests
temperatures will remain above normal at midweek.

.LONG TERM...(Wednesday night through Monday)
Issued at 402 AM MDT Tue Sep 27 2016

Precipitable Water (PW) values increase to 0.75 to 1 inch on
Thursday and Friday as moisture continues to surge northward over
the area with the highest values over the Four Corners region.
Specific humidity values also increase with values in the range of
6 to 8 g/kg. This low becomes an open wave and ejects across the
area Thursday into Friday, with moisture remnants of T.S. Roslyn
moving over the area on Friday. There are timing differences in
the models but overall trends seem to indicate the chance for
showers and storms increasing on Thursday into Friday for more
active weather. This system is fairly mild with snow levels
expected to be around 12,500 feet. The latest model trends seem to
be coming in slower with timing of the moisture and precipitation
chances so will be something to keep an eye on for future model
runs as timing may change.

Saturday looks drier but enough moisture still remains for
scattered storms to develop in the afternoon over higher terrain
in southwest flow. By Sunday, the Pacific Northwest trough
progresses inland across the northern Rockies, with precipitation
chances increasing Sunday afternoon into the evening. A cold front
associated with this system will also move through the region
Sunday evening, with H7 temperatures dropping down to the 4C to
7C range Sunday afternoon and down to 1C across the north by
Sunday night. This will in turn lower snow levels on Sunday, with
snow levels around 11,000 feet Sunday afternoon, dropping down to
9,000 feet by Sunday night as the cold front moves through. This
could have some impacts for the mountains and passes, as the
heaviest precip is expected along the cold front. However, this
is still about a week out, so models may change but will be
something to keep in mind for future shifts.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday noon)
Issued at 1045 AM MDT Tue Sep 27 2016

VFR conditions will prevail. High clouds will invade from south
to north this afternoon then thicken and lower through Wednesday
morning. Winds will be light and terrain driven.


&&

.GJT Watches/Warnings/Advisories...
CO...None.
UT...None.
&&

$$

SHORT TERM...NL
LONG TERM...MDA
AVIATION...Joe


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