Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 182335

Area Forecast Discussion
National Weather Service Grand Junction CO
435 PM MST Wed Jan 18 2017

.SHORT TERM...(Tonight through Thursday night)
Issued at 320 PM MST Wed Jan 18 2017

Complex pattern setting up late this week into early next week
with at least a series of three storms passing through the Great
Basin and Central Rockies. After this burst of activity a more
prolonged dry period looks to be setting up with a return of the
amplified ridge to the left coast. With another atmospheric river
event on tap...mountain snowpack will continue the steep upward
trend...and the southern mountains appear to have the best odds
for the deepest snow amounts attm. As for the valleys...most folks
here will have to get out the snow shovels as these storms will
start out much colder than the past several...meaning snow should
be the dominant form of precipitation. Stratus and areas of fog
will be the near term challenges and went ahead with persistence
and expect the moisture to slosh back into the basins and over the
snow pack of the Yampa River Basin one more evening. Fog may be
more of a concern as the moisture profile is shrinking with drier
air moving in aloft. This dry air begins to slide eastward after
sunrise as the ridge axis overhead is gradually replaced with the
more moist southwest flow of the leading system. Again there will
be a top down saturation process needed most areas to get the
better precipitation to the ground. This process will be slow on
Thursday with mainly light snow over the higher peaks of SE Utah
into SW Colorado. A boost in ascent arrives with the cyclonically
curved jet Thursday evening to the 4 Corners region. This jet
ushers in the colder air aloft with this first system and as a
result the atmosphere becomes more unstable and this will help
boost snowfall rates overnight. Through sunrise on Friday the
Abajos to the southern San Juans and portions of the Uncompahgre
and Grand Mesa look to be in the favored storm track. The southern
valleys will also see their first shot of accumulating snow.

.LONG TERM...(Friday through Wednesday)
Issued at 320 PM MST Wed Jan 18 2017

Friday the jet shifts eastward and affect the overall large scale
ascent. However this will be replaced by stronger instability
aloft and this will keep the southerly to westerly orographics
going on the favored ranges. It should be hard for the
precipitation to switch back over to rain in most areas except the
low deserts near the 4 Corners. The difficult part of forecasting
snow amounts during this time will be the scattered nature of the
precipitation...but where the snow bands set could be
quite heavy at times. There will be a noticeable lull in the
action over other places which always appears to be the end of the
storm. However the next piece of energy arrives by early Friday
evening and is phased much more with the northern stream and again
has some colder air aloft keeping the static stability low across
the southern half of the CWA. would not be surprised to see some
isolated lightning strikes both Friday and especially Saturday
afternoons. Models are suggesting a mid level circulation crossing
from SE Utah into NCentral Colorado early Saturday morning through
the afternoon. This would be the focus of a heavier snow band but
the models have not been consistent with this. All the models are
also hinting at a surface low forming ahead of this trough in
northeast AZ and lifting a warm front into the San Juan foothills.
This shouldn`t be enough warm air to change the snow to rain...but
rather be a good focus for heavier precipitation Saturday morning
across the south. This looks to be when the southern valleys pick
up some decent snow and have such put a watch down in this region.
Early guess will be 5 to 10 inches though much of the output at
KDRO is closer to the 5. Snow rates from the Abajos to the San
Juan also look good during the period through Saturday afternoon
as this system wraps up to the east. Expect a gradual downturn in
the widespread significant accumulations from west to east
Saturday evening and we will need a break before the next stronger
system arrives by late Sunday. Will need more time to lock in on
this storm but it will have much stronger dynamics as it will be
a larger closed system tracking across the Northern Great Basin in
to the Central Plains. This gives us the opportunity to get snow
on the front side and again in the wrap around region. Cold
unsettled conditions should begin to abate on Wednesday with
ridging then dominating beyond.


.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 435 PM MST Wed Jan 18 2017

The main forecast challenge this period is whether low clouds will
redevelop over lower valley TAF sites. Satellite imagery and
observations are clearer than yesterday at this time. However, the
Uinta basin and southern CO foothills still have stratocu hanging
around. Given the proximity of those clouds will forecast KVEL and
KDRO to have lower ceilings at least for the next few hours...and
possibly longer at KVEL. Model guidance is quite a bit more
optimistic...not even bringing low ceilings back into the forecast
area until late Thursday with onset of precipitation moving in.
Low confidence forecast overnight for the lower valleys...but
higher confidence for all areas Thursday. Will likely see -SN or
-RA developing after 21Z in eastern UT and southwest CO with
coincident MVFR to IFR conditions.


CO...Winter Weather Advisory from 5 PM Thursday to 8 PM MST Saturday
     for COZ009-017-018.

     Winter Storm Warning from 2 PM Thursday to 8 PM MST Saturday for

     Winter Storm Watch from Thursday evening through Saturday
     afternoon for COZ021>023.

UT...Winter Storm Watch from Thursday evening through Saturday
     afternoon for UTZ022.

     Winter Weather Advisory from 2 PM Thursday to 8 PM MST Saturday
     for UTZ028.



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