Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 211035

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
435 AM MDT Wed Mar 21 2018

.SHORT TERM...(Today through Thursday)
Issued at 435 AM MDT Wed Mar 21 2018

Eastern Utah and western Colorado were under a low amplitude high
pressure ridge this morning and that feature will persist through
the remainder of the day. A fair amount of moisture was moving
through the ridge and warm air advection (WAA) appeared
responsible for generating an extensive shield of alto-stratus
clouds across northwest Colorado. Cirrus was also moving through
the ridge so the area can expect a mixed bag with regard to clouds
throughout the day. Models suggest some light showers will fall
over the mountains, especially across northeast Utah and northwest
Colorado. Orographic lift in WAA regime is unlikely to generate
more than 1 to 2 inches by sunset this evening.

High pressure amplifies a bit tonight and the ridge axis shifts
slightly east of the Continental Divide as the trough along the
West Coast deepens. Flow veers slightly to the southwest as this
occurs. Clouds will continue to stream overhead, though ECMWF and
NAM indicated that precipitation over the mountains will come to
an end. The GFS suggested otherwise, so held onto slight chance
for additional snowfall over the mountains for the latter part of
the night.

On Thursday, moisture associated with the atmospheric river (AR)
event impacting southern California will arrive over the region.
Little lift indicated and lapse rates rather unimpressive which
will hinder precipitation which will be generated largely from
orographic lift, despite 7h winds increasing to 25 to 40 knots
from the southwest. Temperatures will be very mild with 7h level
temperatures ranging from around 6 C in the south to around 1 C in
the north. As a result, snow levels will be close to 11 kft in
the south and near 10 kft in the north which will limit impacts
from snow. Furthermore, accumulations will be limited by low snow
to water ratios. The strong winds at ridge level mentioned
previously will mix down during the afternoon resulting in breezy
conditions. Cloud cover should limit depth of mixing and therefore
do not anticipate the need for any wind advisories.

Temperatures will continue to trend warmer today, tonight and
Thursday. However, cloudy conditions on Thursday should limit
afternoon highs and cut temperatures back a couple of degrees at
most locations for that reason.

.LONG TERM...(Thursday night through Tuesday)
Issued at 435 AM MDT Wed Mar 21 2018

Make no mistake this is a moderately strong atmospheric river (AR)
arriving on the Left Coast of California both in amount of
integrated water vapor transport and duration in time. By Thursday
evening the 310K Theta surface showed the moisture diverging over
the Great Basin with one branch bending eastward toward our area
as the western trough begins to push the ridge axis eastward.
23/00Z time series of PWAT (precipitable water) over GJT suggest
amounts in excess of 0.6 of an inch. The daily record for that
time is 0.54 though the mean maximum value is close to the 0.6
mark. These are just numbers so to put it plainly there will be an
impressive amount of moisture to work out. So it should pour,
dump, nuke precipitation by Friday...but will it? Large scale
ascent with this next system splits a few times upstream and the
bulk of the stronger dynamics stay well to our north where thermal
gradients are much better defined. All we are left with over our
CWA is an open wave moving quickly through on Friday under a
weakening jet aloft. There are likely to be showers around
Thursday night into Friday, even some heavier precipitation driven
by convection, but this system will lack the organization to
wring out all of the AR moisture aloft. Elevated snow levels may
limit the impacts as snow melts as it falls on roads resulting in
wet surfaces during the daylight hours. There may even be a rain
on snow event boosting runoff and reducing snowpack. The trough
out west reloads by Saturday as a colder Pacific jet digs on the
backside of the first system. Southwest flow remains in place over
the Rockies as this occurs and temperatures will stay mild and,
for the most part, the weekend into early next week will be on the
dry side. Both models want to split the energy in the trough as
it moves over the Great Basin early Monday. The southern split
then forms a closed low south of the 4 Corners early next week.
Interesting solution with good agreement for now but will not be
favorable for another big storm to help boost snowpack before the
normal peak arrives in early April.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 435 AM MDT Wed Mar 21 2018

Moisture moving through the ridge over the area during the next 24
hours will bring clouds and scattered mountain snow showers.
However, this shower activity is unlikely to impact TAF sites. In
fact, expect VFR conditions and relatively light, terrain driven
winds today and tonight at all TAF sites. However, an extensive
mid-level cloud deck may cause ceilings to periodically fall
below ILS breakpoints at KEGE and KASE.






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