Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 220958
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
358 AM MDT Wed Mar 22 2017

.SHORT TERM...(Today through Thursday)
Issued at 317 AM MDT Wed Mar 22 2017

Southwest flow at the surface and aloft will increase today
ahead of the next Pacific storm was moving onshore early this
morning. digging along the California coast. 700 mb winds will
increase to 25 kts in the NAM12 this afternoon and continue
increasing to 45 kts late this evening as a 100 kt jet goes
overhead. The model continue to keep the moisture west of the area
today, except for extreme NE UT and NW CO late this afternoon. The
HiRes and HRRR bring the main precip band into NE UT in a big way
this evening. Around 12Z Thursday morning the surface front will
cross the CO and UT border and move slowly into toward central CO.
A significant north to south oriented band of precipitation will
accompany the front.

With moisture steadily increasing today resulting in low RHs
generally above 20 percent, Fire weather concerns will be
minimized today.

.LONG TERM...(Thursday through Tuesday)
Issued at 340 PM MDT Tue Mar 21 2017

An early spring storm system will continue to impact the area
through Friday morning, bringing valley rain and mountain snow.
The latest 00Z model runs remain a bit inconsistent with the
exact track of this upper low which has implications on the
forecast snow amounts and temperatures, which has lowered
confidence. Regardless, this storm will bring a change from warm,
above normal near record warmth to cooler, more winter-like
conditions by Thursday afternoon and evening with temperatures
closer to normal values.

The timing with the cold front and precipitation onset has been
pushed back even later than previous runs have indicated, now
showing the best chances for precipitation to begin Thursday
morning and afternoon. The cold front looks to move through
Thursday evening as the flow shifts from southwest to north-
northwest behind it. Snow levels look to be high initially between
8000 and 9000 ft MSL Thursday afternoon and lowering to between
6000 and 7000 ft MSL by Thursday night into Friday morning.
Thunderstorms are possible Thursday afternoon in the mild, pre-
frontal southwest flow with steep lapse rates and marginal
instability and lift. Model snow and QPF output is lower than
previous models with somewhat of a dry slot working in Thursday
morning and afternoon. This could be reason for the increased
instability leading to thunderstorm potential. Lift does appear
strongest Thursday afternoon and evening when looking at time
heights over a few mountain locations, coinciding with pre-
frontal southwest flow and approximate frontal passage. Once the
flow shifts around to the north-northwest late Thursday evening
into Friday morning, the northwest and north facing slopes would
be favored for better snowfall accumulation.

The differences lie in the details, especially the exact track of
the upper low. The NAM12 has the upper low tracking across Arizona
Thursday and closing off over New Mexico Thursday evening before
lifting into the Texas panhandle by Friday morning and
strengthening. The GFS is further north than the NAM with the
upper low tracking across the Colorado-New Mexico border Thursday
and closing off by Thursday night into Friday morning over
southeast Colorado and the Oklahoma panhandle region. The EC is a
little further north than this. All models are showing this low to
be vertically stacked and strengthening once it gets over to the
Front Range. This tends to take all the energy out to the Plains
and Front Range, which the models indicate by Friday morning and
afternoon with drier and subsident air moving in on the back side
by late Friday morning into the afternoon. The exact track of this
low will determine which areas see the best snowfall. Currently,
it looks like the southern Colorado mountains and to a lesser
extent the central Colorado mountains would be favored for 5 to
10 inches possible with locally higher amounts above 10,000 feet.
There is potential for a Gorge event in the NW San Juans Thursday
night into Friday morning as the flow shifts around to the north-
northwest but at that point, moisture begins to erode and forcing
wanes. After coordinating with surrounding offices, decided to
wait before issuing any winter highlites to see more consistency
in the next couple model runs. H7 temps are warmer in the NAM than
the GFS but lower to about -4C to -6C on average by Thursday
night into Friday morning. Some lower valleys may see a change
over to snow or a rain/snow mix by this time, but accumulations in
the valleys look limited. Will continue the Special Weather
Statement to address the impacts and potential. Later shifts will
need to re-assess whether winter highlites will be needed for this
storm depending on model storm track and better consistency
between models.

A transitory shortwave ridge of high pressure quickly builds in by
Friday afternoon as the upper low moves into the Texas-Oklahoma
panhandle and into southern Oklahoma, taking all the energy with
it onto the Front Range and Plains east of the divide. An
unsettled pattern will commence through the rest of the weekend
into next week with temperatures closer to normal if not slightly
above. The next open wave moves across Saturday evening into
Sunday morning with another storm system impacting the area Monday
into Tuesday, bringing more precipitation.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 317 AM MDT Wed Mar 22 2017

VFR conditions with CIGS above ILS breakpoints will continue at
all TAF sites across eastern Utah and western Colorado through
the next 24 hours. Winds will be terrain driven, becoming breezy
from the southwest after 18Z and continuing through 02Z/Thursday.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...None.
UT...None.
&&

$$

SHORT TERM...CC
LONG TERM...MDA
AVIATION...CC


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