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Area forecast discussion
National Weather Service Grand Junction Colorado
532 PM MDT Thu Sep 19 2019

Short term...(tonight through Friday night)
issued at 324 PM MDT Thu Sep 19 2019

The midlevel trough responsible for the windy conditions today is
stretched across the Great Basin. Southwesterly flow ahead of this
system is strong and causing gusts upwards of 50 mph in spots and at
least 25 mph elsewhere. No changes are planned for the red flag
warning currently in effect in eastern Utah until 9 PM this evening.
The relative humidity values across portions of far southeast Utah have stayed
above criteria partly due to increasing moisture from the south.
This push of moisture is beginning to spread into portions of
Colorado. Showers and storms will likely struggle to form until
later this evening when the slug of moisture is maximized over the
area. The one exception is far southern Colorado where there is 500
j/kg of MLCAPE and sufficient shear. It will not take much to mix
winds down to the surface, which will likely be enhanced by
evaporation since dew point depressions are upwards of 40 degrees.
I would expect the chance for an isolated strong storm to
continue in that area until this evening. The best chances for
precipitation starts late this evening mainly in the southern and
central mountains of Colorado. Most places will be dry by sunrise
tomorrow with the exception of a few showers. Models agree that as
the trough lifts northeastward the forcing swings through tonight
and exits the area tomorrow afternoon. Snow levels drop to 12 kft
tonight so a dusting of snow at those elevations can not be ruled
out especially in the San Juan and Elk Mountains.

Tomorrow the midlevel jet passes directly overhead so conditions
will be just as windy as today. The synoptic dry slot also slides
across the region, therefore limiting the chances for precipitation.
The one area that has residual moisture is the upper San Juan River
Valley so showers are possible there tomorrow. The relative humidity values are
expected to be lower than today in southeast Utah and far southwest
Colorado with similar wind speeds so critical fire weather
conditions are possible. Therefore a red flag warning has been
issued for fire zones 490 and 491 in Utah. There is not enough
coverage in the Colorado zones to warrant a warning at this time,
but there is a potential for localized critical conditions. Cooler
air filters in behind the system tomorrow night. By Saturday
morning the 700 mb temperatures drop 5 to 10 degrees over the 24
hr period with the greatest change across the northern half of the
forecast area. The low temperatures in the Yampa River Valley
could reach the upper 20s and perhaps mid 20s in spots. These
forecasts are based on values that are below a majority of the
guidance. There is uncertainty on the amount of moisture linger
and result in cloud cover, which could keeps things warmer. There
is also a chance that the air mass is cold enough for a hard
freeze regardless of the clouds. A freeze watch has been issued
for portions of northern Colorado for Saturday morning.

Long term...(saturday through thursday)
issued at 324 PM MDT Thu Sep 19 2019

Winds should weaken over the weekend on the backside of the upper
level jet, but breezy conditions can't be completely ruled out for
Saturday. Strong winds aloft will blanket the region as the
trough lifts northeast. Transport winds shift from a westerly
component Saturday morning to northwest as a transitory ridge builds
to our west. A slow warming is also expected for northern valleys
under the freeze watch, and temperatures will remain below normal
for a day as colder air Ushers in behind the exiting trough. Another
chilly night is likely Saturday due to clearing skies under the
approaching ridge, with below freezing temperatures possible for
northern valleys in northwestern Colorado. Temperatures will rebound
a few degrees Sunday, as the ridge builds over the County Warning Area.

Ridging will be short lived as the next Pacific northwest system
moves southeast, and deepens over the Great Basin. Models have
handled the track of this system rather poorly, so confidence is low
on precipitation totals/timing. The Euro still has a more aggressive
southward track, forming a closed low over eastern Nevada by Monday
afternoon and keeping the system south of the County Warning Area. This scenario
keeps US trapped under dry air for majority of the region, with the
exception of The Four Corners region and far southern Colorado.
Multiple GFS ensemble models follow the Euro, however the mean 500
mb height ensemble quickly pushes the trough across the Great Basin
without forming a closed low. This northerly track allows
for a plume of moisture to extend into the region bringing the
potential for wetting rain (or even snow!) Monday night into
Tuesday. 700 mb temperatures range from 0 to +2 degrees c in
southwest Colorado, so we could see some flakes as low as 9000 ft
overnight. Based on model inconsistencies, I wouldn't hold my
breath for a blanket of snow by Tuesday morning. Stay tuned
winter weather enthusiasts!


Aviation...(for the 00z tafs through 00z Friday evening)
issued at 526 PM MDT Thu Sep 19 2019

Strong southwest flow is pushing showers and isolated
thunderstorms through the region this evening. These robust gradient
winds could be enhanced by the showers to produce very sudden wind
gusts in the 40 to 55 mph range through sunset. Showery
precipitation is likely to persist through the overnight hours
into Friday afternoon. Coverage is limited and therefor low
confidence has been relayed in this taf forecast. Expect some
areas of obscured terrain and turbulence over the ridges the next
24 hours.


Fire weather...
issued at 332 PM MDT Thu Sep 19 2019

An approaching upper level trough will continue to mix strong
southwest winds to the surface across southeast Utah this evening.
Gusty conditions will persist on Friday, as the system progresses
and strong winds mix down to the ground. Combination of gusty
winds with low relative humidity and dry fuels will result in
critical fire weather conditions across southeast Utah. Therefore,
a red flag warning was issued for this afternoon and evening
until 9 PM MDT, as well as Friday for wind, low relative humidity,
and dry fuels. Portions of far southwestern Colorado could reach
critical conditions Friday afternoon, later shifts will evaluate
the potential for greater coverage.


Gjt watches/warnings/advisories...
Colorado...freeze watch from late Friday night through Saturday morning for
coz002-005. flag warning until 9 PM MDT this evening for utz490.

Red flag warning from noon to 9 PM MDT Friday for utz490-491.



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