Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 200941
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
341 AM MDT TUE SEP 20 2016

.SHORT TERM...(Today through Wednesday night)
Issued at 320 AM MDT Tue Sep 20 2016

Plenty of high level clouds are streaming up from the southwest as
expected thanks to Tropical Storm Paine with more clouds and
precipitation on the way. The precip and clouds are all thanks to
a shortwave which is also approaching from the southwest. Some
troubling factors with models this morning, however. NAM12
forecast PWATS indicate around an inch by midafternoon which is
abnormally high for this time of year. This forecaster would
expect to see plenty of precipitation with PWATs that high but the
NAM just not reflecting this. In fact, the NAM keeps just about
our entire CWA dry until the evening hours and that, just a small
area of extreme southeastern Utah. By Wednesday morning, the NAM
brings in precip to the San Juans and some of the higher terrain
up north but by and large, qpf values and coverage is anemic at
best. Compare this with the GFS which brings more pronounced
precip over the San Juans from about noon onwards becoming more
widespread in the evening hours and you have the dilemma. Both
models have stuck to their guns for the last several model
runs...kind of. The NAM12 has remained very dry while the GFS has
been slowly lowering qpf amounts so there is that. The short range
models? They are also very dry almost mirroring the NAM.

The conundrum with this package then, is when to start precip and
where it is expected to fall. Took out all mention of precip this
morning with isolated chances from noon onwards favoring areas
south of the I-70 corridor and even this might be a bit too much.
Expect some gusty winds in the afternoon which will raise fire
weather concerns, mainly up north.  Slightly more confident that
isolated to scattered showers and storms will move through
Wednesday as the shortwave moves overhead and available moisture
remains high.

.LONG TERM...(Thursday through Monday)
Issued at 335 PM MDT Mon Sep 19 2016

Thursday into Friday: Moisture will work its way in from the west
with showers and storms developing under a strong southwest flow.
The gradient tightens with H7 winds of 30 kts from the south and
H5 winds of 40 kts from the southwest increasing to 45 kts at H7
and 60 to 80 kts at H5 by Thursday evening. The potential exists
for wind highlites on Thursday due to the tightened gradient so
this will need to be monitored. Thursday is also looking like
potential exists for some strong to severe storms, as much of the
forecast area remains in the right entrance region of a 100 kt jet
that curves cyclonically around the base of the upper level
trough that strengthens and forms a closed low over the Great
Basin. Sufficient speed and directional shear exists as well as
enough instability and forcing for more organized activity. Gusty
outflow winds, hail and heavy rain will be possible with storms
that form. This is all ahead of a strong stand up cold front that
will move through Thursday evening into Friday morning west to
east across the forecast area. The winds begin backing with height
after the cold front moves through Thursday evening into Friday
morning, as strong CAA takes place. H7 temps lower by 8 to 12
degrees from Thursday afternoon to Friday afternoon for much
cooler temperatures across the area. H7 temps will continue to
lower throughout the day on Friday as the upper level low lifts
northeast across Wyoming. H7 temps will be in the 0C to -2C range
for areas north of I-70 with snow levels lowering to 8000-9000 ft
MSL. Accumulating snow is possible above 8000 ft MSL with the
mountains of northeast Utah and northwest Colorado being the most
affected as the flow shifts around to northwest orographics from
Friday morning through Saturday morning. This may cause impacts
for travelers across these higher mountain passes so stay tuned to
the latest forecast as details come into greater focus.

Saturday afternoon through Monday: the main upper level low will
track across the northern Rockies into the northern High Plains
while the upper level trough gets stretched out across our
forecast area with a closed low splitting off the main trough and
forming over Arizona. The upper level flow will turn more
northerly Saturday afternoon and shift to an easterly flow across
our forecast area as the low pressure closes off over Arizona
underneath an upper level ridge across the west, essentially
putting an end to precipitation. The flow shifts back around to
the south by Monday with some storms possible in the afternoon
across the southern mountains. Otherwise drier conditions are in
store for Saturday afternoon through Monday with below normal
temperatures.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 320 AM MDT Tue Sep 20 2016

High clouds will continue to thicken and slowly lower as the day
progresses. Ceilings and visibilities will remain VFR through the
day except where some showers and storms form but do not expect
any TAF sites to be affected. Coverage will likely increase after
00z but still do not anticipate any problems for TAF sites.
Unsettled weather continues through 12z Wednesday and beyond.

&&

.FIRE WEATHER...
Issued at 335 PM MDT Mon Sep 19 2016

Low RH and gusty winds will combine to produce areas of critical
Fire Weather conditions across northwest Colorado this afternoon.
However, conditions do not look very widespread and more localized
so did not issue any Fire Weather highlites at this time. Winds
are also marginal in many areas as well. Moisture increase from
the south late this afternoon may also limit the duration of
these Fire Weather conditions.

&&

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

&&

$$

SHORT TERM...TGR
LONG TERM...MDA
AVIATION...TGR
FIRE WEATHER...MDA



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