Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 212135

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
335 PM MDT Wed Mar 21 2018

.SHORT TERM...(Tonight through Thursday night)
Issued at 335 PM MDT Wed Mar 21 2018

A weak shortwave moving through the ridge of high pressure today
has resulted in broken to overcast mid and high level clouds this
morning through the early afternoon, with light snow showers over
some of the higher terrain. Precipitation even occurred as low as
Craig and Hayden, with most valleys staying dry today. This has
also kept our high temperatures down a few degrees from forecast
and closer to the cooler guidance. Clouds are beginning to break
up this afternoon with some areas of sun returning as this weak
shortwave moves out of the region, allowing temps to warm up a few
degrees. Light snow will continue over the higher peaks through
this evening but very little accumulation is anticipated.

Southwest flow returns Thursday morning and increases by Thursday
afternoon as the next in a series of weather disturbances
approaches the area and moves into the Great Basin. This southwest
flow will tap into some sub-tropical moisture with the remnants of
an atmospheric river moving overhead. Specific humidities will
rise to between 5 and 6 g/kg on Thursday, with precipitable water
values rising to near 0.6 inches by Thursday afternoon and 0.75
inches by Thursday evening. Temperatures will be mild with H7
temps in the +3C to +6C range, meaning at least 60s for most
valleys with temps approaching low 70s for some lower valleys
further south. Did cut back on the high temperatures somewhat as
plenty of mid and high level cloud cover is expected to follow
suit along with the deeper moisture. This cloud cover could
hamper the deeper mixing and also prevent the stronger winds from
reaching the surface. Winds will be breezy, no doubt, but looks
like the best wind gusts will be in the southeast Utah and
northwest Colorado valleys where gusts in the 30s are more likely.
Decided not to issue any wind highlites as winds look to stay
below advisory thresholds due to expected cloud cover.

Some isolated to scattered showers are possible over the higher
terrain late Thursday afternoon in this warm advective prefrontal
flow, with possibility of isolated thunderstorms. These look to be
more of the dry variety with virga being the main concern due to
drier low levels. Precipitation increases in coverage Thursday
evening as top down saturation commences, with snow levels
starting high above 10,000 or 11,000 feet MSL due to milder
southwest flow.

.LONG TERM...(Friday through Wednesday)
Issued at 335 PM MDT Wed Mar 21 2018

This disturbance moves through the region as a quick open
shortwave trough, with the bulk of the forcing occurring with the
cold front and trough passage late Friday morning into early
Friday afternoon. Will snow fall in the high country? Sure, but
how high or how low remains the question. Let`s dissect it. The
positives: deep moisture remains elevated with specific humidity
values still in the 4 to 5 g/kg range by Friday. The negatives or
biggest concerns: first, the temperatures remain fairly mild
throughout with the coldest air at H7 being -2C (south) to -4C
(north) briefly on Friday morning with temps bouncing back to
between 0C and -2C on Friday afternoon. Second, the lift overall
is fairly weak with a narrow sliver of lift occurring with the
front and lapse rates not as steep as the last couple systems in
the realm of 6 to 7 degrees C/km. What does this mean? This means
snow levels may drop to around 9000 feet or 8500 feet by Friday
morning but stay fairly high as far as the best accumulations.
Convection will still be somewhat of a player with isolated
thunderstorms still possible late Friday morning into the
afternoon as the front and trough move through. Did not feel
confident enough to issue any winter highlites with this
afternoon package due to the milder temperatures, higher snow
levels and lack of strong dynamical forcing. It is hard to
pinpoint how much of this precipitation would be snow, rain, rain
on snow, or a mix. Thinking 3 to 6 inches seem reasonable above
10,000 feet MSL with snow favoring the southwest facing slopes as
the flow remains SW throughout much of the event. Some higher
amounts are possible with convection and cannot be ruled out, but
convection threat looks weaker than the previous couple systems.
Will let the evening and midnight shift re-evaluate and see if any
highlites might be needed as we are still a good 36 hours out.
That being said, some areas could see some decent rainfall with
this event with bursts of heavy precipitation mostly along the
front late Friday morning into early Friday afternoon. This system
moves out of the region by Friday evening with improving
conditions Friday night.

This weekend now looks to start off fairly dry with some higher
terrain showers still possible due to instability in the
atmosphere. The next weather maker will enter the Great Basin as
a closed low on Sunday and dive southward through Nevada by early
next week. This low wants to split from the main flow, keeping
our CWA in the col region between the closed low and the trough
it is splitting from. The closed low dives as far south as the
Arizona and Mexico border before trekking eastward across southern
New Mexico mid-week. Confidence in what will happen with this
system is low as models have been inconsistent run to run and
between solutions, even though they are in better agreement today
with the GFS seeming to be coming around to the ECMWF solution
from yesterday. The deeper cold air in the realm of -10C to -16C
seems to stay with the low center and at the base of this trough,
which makes sense as the low is splitting from the flow. So, our
eastern Utah and western Colorado zones are on the eastern fringe
of this cold air where we will see H7 temps wavering in and out of
the -6C to -8C each evening through early next week. There will
be scattered showers with rain in the valleys and snow in the
higher elevations, but there is no focus as to which areas will
see the better precipitation. Also, as the low moves further south
into Mexico, we tend to lose most of the better forcing and upper
level support for significant precipitation. Stuck with a blended
consensus between models until better agreement is reached.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1045 AM MDT Wed Mar 21 2018

Moisture and mid-level clouds will continue moving through the
ridge over the area for the next 24 hours bringing clouds and
scattered mountain snow showers. The only TAF sites where showers
should affect flight conditions should be KHDN, KTEX, KEGE and
KASE where CIGS will occasionally drop below ILS breakpoints.




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