Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 202230

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
330 PM MST Sat Jan 20 2018

.SHORT TERM...(Tonight through Sunday night)
Issued at 330 PM MST Sat Jan 20 2018

The front slung northeast to southwest across NW Colorado and NE
Utah will move mid to upper level energy eastward this evening
across western Colorado, bringing precipitation to much of the
area. Strong WAA ahead of the front has increased temperatures 10+
degrees in some spots over the last couple of hours as this
warmer air mixes to the surface. Temperatures behind the front
have been sitting in the 20s all day with snow and blowing snow
across NE Utah. A -11C at H7 upper level closed low will trek
across the Four Corners tonight, which will bring the strongest
forcing and a line of showers west to east across western
Colorado. Added a chance of thundersnow in for the San Juans
tonight, when rates of 1 to 2" of snow will be possible. Many
valley locations will see rain change over to snow tonight, but
with the fast-moving nature of this system across western
Colorado, snow accumulations are anticipated to be in the light
1-3" range. The 12z NAM came in today extremely bullish,
especially across the Grand Valley, however seeing the 18z NAM
now, the latest run is hitting the southwestern Colorado valleys
hard after the best forcing is available. The model is likely
getting convective feedback, and while an isolated spot in a
valley somewhere could possibly see up to 4 or 5" of snow,
pinpointing this is near impossible and will depend on convective
bands setting up. Anticipating that snow amounts will vary a lot
based on elevation and exact position of banding. Your neighbor a
mile down the road may end up with a lot more or a lot less snow
and/or rain than you do.

Have added several winter wx advisories for high confidence
areas...including Steamboat Springs to the Yampa area, De Beque
Canyon and eastward on I-70, and the Blue Mesa/Gunnison Valley.
The town of Gunnison itself will likely be shadowed for the
initial part of this event, so higher accumulations are expected
west of town along highway 50. I-70 will be tricky as this event
will start out as rain, then transition as the front moves

As the Front Range surface low strengthens and ejects eastward on
Sunday, our snow event on the West Slope will transition to a
short-lived orographic event for the northern Colorado mountains.
Snow will taper off Sunday morning and afternoon as we lose upper
level support. Temperatures will remain cool Sunday night with
fog possible as the atmosphere stabilizes.

.LONG TERM...(Monday through Saturday)
Issued at 330 PM MST Sat Jan 20 2018

Northwest flow will linger over the region Monday with enough cold
air in the -10 to -12C range at H7 to produce light orographic snow
for the northwest Colorado mountains down to Vail Pass. Snow will
pick up over these NW CO mountains Monday evening into Tuesday
morning as a weak shortwave moves through the flow with H7
temperatures dropping to as low as -14 to -16C with decent moisture.
Ridge of high pressure moves in Tuesday afternoon through Wednesday
providing drier weather and a return from near to slightly above
normal temperatures.

The next storm to watch for arrives Thursday evening and continues
through Saturday.  Southwest flow will increase on Thursday ahead of
this approaching trough of low pressure as it moves onto the west
coast and into the Pacific Northwest, stretching southeastward
through the Great Basin. Southwest flow will increase in this mild
pre-frontal air mass Thursday afternoon and evening with H7 winds in
the 30 to 40 kt range. Very spring-like for this time of year. This
negatively tilted trough will move through the region Thursday night
through the day on Friday, with the cold air also moving in as
precipitation begins over the mountains and spreads into the
valleys.  H7 temps quickly lower to -10 to -14C for efficient
dendritic snow by Friday morning. The flow shifts to northwest
Friday afternoon as trough passage lags a bit behind the front.
Anticipating some convective element at play with banding and
potential for isolated thunderstorms with the cold front due to the
sharp temperature gradient and strong vort maxes moving through the
region, indicating instability and forcing in place.

The models are in fairly good agreement with the timing of this
system now, which lends to higher confidence that some areas will
see much needed rain and snow with this system. Increased PoPs
somewhat during this period to show this increasing trend. However,
differences lie in the exact track with the EC a bit further north
than the GFS with drier air nosing into the Four Corners and across
the southern valleys. Specific humidities are in the 2 to 3 g/kg
range with precipitable water values in the 0.2 to 0.3 range with
this storm. While this is not an abundant amount of moisture, it is
near normal for this time of year...and with H7 temps that cold in
the dendritic range, the mountains may still get some good snowfall
with this system of the dry and fluffy powder variety. Northwest
flow lingers through Saturday morning and afternoon, keeping good
orographic snow going over the northwest facing slopes through the


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1102 AM MST Sat Jan 20 2018

Unstable...moist southerly flow allowing showers to increase late
this morning and trend should continue this afternoon. Hit or miss
nature over many sites makes for a difficult forecast with quickly
changing conditions possible due to precipitation type in some
areas. KVEL at IFR conditions and this expected to continue well
into the evening as the system to west lifts across the Rockies.
Expect downward trend over remainder of the TAF sites as this
occurs and this should be indicated in 18Z issuance. Expect many
amendments through the evening as snow increases with a frontal
passage then lets up behind it. Widespread lower flight criteria
expected through much of the night into Sunday morning.


CO...Winter Weather Advisory until 10 AM MST Sunday for COZ001-002.

     Winter Weather Advisory until 2 PM MST Sunday for COZ003-009-012-

     Winter Weather Advisory until 2 PM MST Sunday for COZ007-008-014.

     Winter Weather Advisory until 9 PM MST Sunday for COZ004-010-013.

     Winter Weather Advisory until 10 AM MST Sunday for COZ005.

UT...Winter Weather Advisory until 10 AM MST Sunday for UTZ025-028.

     Winter Storm Warning until 6 AM MST Sunday for UTZ023.



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