Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 212140

Area Forecast Discussion
National Weather Service Grand Junction CO
340 PM MDT Tue Mar 21 2017

.SHORT TERM...(Tonight through Wednesday night)
Issued at 340 PM MDT Tue Mar 21 2017

A few isolated showers have developed this afternoon across
northwest Colorado with some lightning strikes evident across
Wyoming due to a passing weak shortwave in southwest flow. Expect
gusty winds in vicinity of these showers due to virga with little
if any precip reaching the ground due to dry low levels and high
cloud bases. Otherwise, mild temperatures remain with some breezy
conditions. Showers should die off after sunset. Overnight lows
should be milder due to increased cloud cover. Southwest flow will
increase on Wednesday ahead of the next Pacific storm system
digging along the California coast. H7 winds will be in the 20 to
30 kt range Wednesday afternoon but moisture will also be
increasing with specific humidity values climbing towards 4 g/kg
on average and precipitable water values approaching 0.5 to 0.6
inches by Wednesday evening. This is well above normal for this
time of year. The atmosphere Wednesday afternoon will be dry
initially until the better moisture works in. Fire weather
concerns on Wednesday are minimal due to this increase in
moisture. Some isolated showers are possible Wednesday afternoon
due to already present instability. 100 kt jet lays over Utah on
Wednesday with H7 winds increasing towards 40 to 50 kts by
Wednesday night as the upper level jet noses into the Four Corners
region by Thursday morning.

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

An early spring storm system will impact the area Thursday morning
through Friday morning, bringing valley rain and mountain snow.
The latest 12Z model runs, however, are 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 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1156 AM MDT Tue Mar 21 2017

VFR conditions will prevail over the next 24 hours, but winds will
pick up as a trough begins to interact with ridging in place.
Virga will develop over the higher mountains after 18Z, but not
expecting any measurable rain to the region. The virga will
produce erratic wind gusts late this afternoon and this evening,
so covered these in the tempos seen in the 18z tafs.


Issued at 340 PM MDT Tue Mar 21 2017

Well above normal high temperatures and dry weather will continue
today. Relative humidities are again expected to drop into the 15
to 25 percent range this afternoon. The gustiest winds will occur
after 3 pm with wind gusts of 20 to 25 mph at lower elevations.
The gusty winds combined with warm temperatures, low humidity and
still dormant or dry vegetation will require a heightened fire
weather awareness. Check the latest forecast before attempting any
burning through as conditions can change rapidly. The potential
for near critical fire weather conditions will be greatest for
elevations below 8000 feet.




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