Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 222352

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
452 PM MST Thu Feb 22 2018

.SHORT TERM...(Tonight through Friday night)
Issued at 311 PM MST Thu Feb 22 2018

Weak instability under a modest southwest flow with lingering
moisture has lent itself to scattered light snow showers as
expected this afternoon. These showers will last through the
evening with minimal snow accumulations expected due to the light
nature of snowfall. The bigger storm system is on our doorstep,
set to dig into the Great Basin tonight and move across the
forecast area as an open wave trough Friday night. Snowfall will
begin after midnight with the leading edge of precipitation
entering eastern Utah early Friday morning. The coverage will
become more widespread across western Colorado by late Friday
morning into the afternoon. Snow will be heavy at times as
convective banding sets up. Lapse rates are steep in the realm of
8 to 9 degrees C/km so isolated thunderstorms are also possible.
Winds will pick up on Friday ahead of and with the cold front,
with H7 winds increasing to 30 to 40 kts, so blowing snow is also
a concern. Lift is also fairly modest and after looking at time
heights all across the region, the best lift looks to occur with
the cold front passage Friday evening. Moisture will also be
modest up to 2 g/kg specific humidity, which isn`t a lot but is
enough. Reason being that the temperatures remain cold throughout
this entire event with H7 temps in the -10C to -14C range and
lowering Friday evening to as cold as -16C up north. This
basically means that precipitation type is highly likely to remain
all snow, in the efficient dendritic layer, so dry and fluffy
powder with high snow ratios (low water content).

Largely, it looks like advisory amounts for all mountain areas
of 5 to 10 inches on average look likely with this event. Believe
the San Juans and West Elks and Sawatch Range will see the best
accumulations due to the favored southwest flow and better lift
with 6 to 12 inches on average and locally higher amounts up to 15
inches. Issued a Winter Weather Advisory for all mountain areas
due to the above reasons, from midnight tonight through noon
Saturday for most. The eastern Utah mountains drop off a little
earlier by Friday night and Saturday morning though as
significantly drier air moves in behind the cold front in
northwest flow, putting an earlier end to snowfall here. Thinking
most significant snowfall will come to an end by late Saturday
morning as the storm system moves onto the Front Range and into
the Plains, with lee side cyclogenesis taking place. A surface and
H7 low develop and intensify which tends to take all the energy
and snow east with it. So, decided to keep highlites through noon
Saturday to give some leeway in terms of timing if any snow
lingers a bit longer. As far as the valleys are concerned, decided
to not issue highlites at this time. Thinking most valleys will
see 1 to 3 inches, so the forecast keeps this fairly broadbrushed
and reflects this thinking. Which valleys see the best snowfall
will all depend on how and where the convective banding sets up,
which models have a tough time resolving until the event is
ongoing. This will need to be monitored throughout the event and
is something the evening shift can take a closer look at to see if
highlites are needed elsewhere. Models do indicate some
convergence across the northern valleys Friday morning and also
along the entire cold front Friday evening. The best lift and
upper level divergence looks to occur over southwest Colorado.
So, these may be areas that need watching.

.LONG TERM...(Saturday through Thursday)
Issued at 311 PM MST Thu Feb 22 2018

The storm will be moving onto the Front Range by Saturday morning
with some lingering snow over the western Colorado mountains
through about noon before a brief break is seen Saturday
afternoon. The next system is on the heels of the previous system
and overall looks much weaker. This system moves through in
northwest flow Saturday night into Sunday morning and looks to
favor the mountain areas for the most part with sub-advisory
amounts. Light snow is possible in the adjacent valleys as well
due to moisture overrunning the cold air in place, with minimal
accumulations expected. Sunday afternoon now looks dry with cold
air still in place. Temperatures take a turn upwards on Monday as
the flow shifts to the southwest, advecting in some warmer air
aloft. The warming may be tempered a bit due to any snow cover in
place. Breezy conditions will ensue Tuesday afternoon in southwest
flow as yet another system moves through the area Tuesday evening
through Wednesday. Differences exist in the models this far out,
but overall the flow remains unsettled through the end of the
week with a still progressive pattern in place, adding more
moisture to a needy snowpack.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 452 PM MST Thu Feb 22 2018

Snow bands continue to progress through western Colorado late this
afternoon and areas under the bands have experienced reduced
visibility and LIFR conditions, however only for short periods of
time. These convective bands will persist this evening as the next
storm moves into the region. Scattered snow showers will become
numerous after midnight with widespread precipitation expected by
daybreak Friday. Frequent mountain obscuration and periods of
lowered CIGs/reduced visibility can be expected throughout the day
on Friday during snow showers.


CO...Winter Weather Advisory from midnight tonight to noon MST
     Saturday for COZ003-004-009-010-012-013-017>019.

UT...Winter Weather Advisory from midnight tonight to midnight MST
     Friday night for UTZ023-025.

     Winter Weather Advisory from midnight tonight to 9 AM MST
     Saturday for UTZ028.



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