Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Grand Junction Colorado
1030 PM MST Tuesday Nov 25 2014
issued at 908 PM MST Tuesday Nov 25 2014
Extensive cloud cover across the northestern half of the forecast area
will keep overnight minimums relatively mild. Raised mins at a few
locales by a few degrees.
Update issued at 619 PM MST Tuesday Nov 25 2014
Moisture flux rises during the next 12 hours over northwest Colorado as
a short wave slide southeast in the Lee of the Rocky Mountains.
West to east gradient along The Divide is prominent during the
next 12 hours as well and this will help orographic snowfall.
Cold freezing air trapped in the Yampa River basin definitely
helps and keeps the air mass saturated. Nam12/rap13 showing hefty
quantitative precipitation forecast amounts during the overnight period over the Park Range...and
to a lesser extent The Gore range. Boosted quantitative precipitation forecast/snow amounts and
that pushed zone 4 into warning range. Web cams in the vicinity
indicate significant snow accumulations have already occurred.
Regarding zone 12...web cams in this zone showed obscured
mountains. Remote sensors indicate accumulating snowfall as well
and elected to hoist the advisory through Wednesday morning.
Short term...(tonight through Wednesday night)
issued at 249 PM MST Tuesday Nov 25 2014
In brisk northwest flow the last in a series of open waves is bringing
snow to northwest Colorado. A 120kt jet over northwest Colorado will weaken this
evening and shift east by Wednesday. Still northwest Colorado snowfall
intensity is forecast to increase this evening. Ideally these
overrunning events would have colder air pooled below and the
isothermal saturated layer somewhere near mountain top level. Though
the profile shows deep saturation... neither other conditions
Light snow has been reported as far south as Gunnison but the
brunt of snowfall will be over the Park Range especially the
zirkels. The latest snotel reports show around 6 inches of new
accumulation NE of Steamboat Springs. Around a foot of snow is
expected there by Wednesday noon diminishing to around 5 inches in
the Elk Mountains drier air works in from the top down Wednesday
morning bringing an end to snowfall.
Then dry conditions with modifying temperatures expected on
Wednesday afternoon as a Delaware-amplifying ridge builds in from the
west. High terrain winds remain breezy from the west but valley
winds will be mainly terrain driven.
Long term...(thursday through tuesday)
issued at 249 PM MST Tuesday Nov 25 2014
Flow aloft becomes westerly after midweek as the amplified West Coast
ridge builds inland and flattens...with the forecast area ending up
on the southern fringe of the upper jet/storm track. Warming aloft
will contribute to more stable conditions and tend to favor
inversions in valleys especially where snow cover persists.
Subsequently...have cut back on high temperatures a few degrees for many
central and northern valleys Thursday and Friday. Looks like a small embedded
wave clips the north late on Thursday with a bit more substantial
embedded wave moving across the forecast area on Sunday. But with
models short on moisture...will bring some clouds to the north but
keep any mention of precipitation out of the forecast for now on Thursday and
Models show a NE Pacific trough deepening off the West Coast early
next week and forming a closed low along the northern calif coast Monday.
12z GFS shows this low/trough kicking inland and across our area on
Tuesday...while the 12z European model (ecmwf) is about 12-24 hours slower. Both show
increasing moisture Monday night and Tuesday in SW flow ahead of the
trough. Will carry chance probability of precipitation for the mountains on Tuesday due to the
timing differences...but with the potential for a decent shot
moisture hovering in the background for Tuesday-Tuesday night should
Aviation...(for the 06z tafs through 06z Wednesday night)
issued at 951 PM MST Tuesday Nov 25 2014
Pockets of low level wind shear are expected to continue at kvel...kase and kege
through the early morning hours as strong northwest winds gradually let
Residual showers over the north central mountains
of western Colorado will continue but are becoming more isolated
as the night GOES on. A few showers will continue mountain obscurations
across north central Colorado through about noon tomorrow before
clearing out. Stratus across northern valleys of Utah and Colorado
tonight will clear toward daybreak Wednesday.
Colorado...Winter Weather Advisory until noon MST Wednesday for coz005-010-
Winter Storm Warning until noon MST Wednesday for coz004.