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Area forecast discussion 
National Weather Service Grand Junction Colorado
421 am MDT Friday Sep 4 2015

Short term...(today through saturday)
issued at 419 am MDT Friday Sep 4 2015

A small vorticity maximum will carry an area of showers and a few
thunderstorms through the forecast area this morning. It will
travel roughly along a line from Blanding to Rifle...and
dissipate between 9 am and noon.

Precipitable water values over The Four Corners are near a maximum
for the week this morning due to a southwesterly fetch of middle
level moisture which has moved into southwest Colorado. A
slightly unorganized vorticity maxima will move northeast to
central Colorado by noon today. All of the forecast models have
been and continue to bring precipitation over much of the region
by late this afternoon...however am having a hard time finding a
good dynamical lifting mechanism other than a 70 knots jet streak
aloft and daytime heating. Prognosticated convective indices are weak
today which is surprising with relatively clear skies in place.
Looking at 00z initialization of the 300mb jet steak and the
upstream 00z Flagstaff sounding...the NAM seems to be initializing
a bit better than the GFS with the upper level jet. This all being
said...with good shear and moisture today do expect this will
trump the other factors and produce numerous showers and
thunderstorms during the afternoon and into the evening before
tapering off after sunset. Most activity will be across western
Colorado and extreme eastern Utah.

Convective indices are much more favorable for thunderstorms
developing on Saturday afternoon with the NAM indicating cape
values upwards of 1000 j/kg across eastern Utah and extreme
western Colorado. The 700 mb trough barreling through the Pacific
northwest looks to push energy into NE Utah and northwest Colorado on Saturday
afternoon...so am expecting showers and thunderstorms to develop
ahead of this disturbance mainly over NE Utah and northwest Colorado.
Showers and thunderstorms on Saturday will likely be higher-based
with some drying occurring across the region...so will impact
higher terrain the most with valleys more likely to avoid rain as
the day progresses.

Daytime temperatures will remain near normal or a couple of
degrees below normal over the next couple of days...and nighttime
temperatures a bit above normal with increased dewpoints and cloud
cover.

Long term...(saturday night through thursday)
issued at 419 am MDT Friday Sep 4 2015

Midrange models are in decent agreement at the start of the
period. The strongest vorticity maximum will round the bottom of the trough
over the northern rockies Sat evening. This should tighten the
gradient and increase surface winds in the evening. Then drier air is
projected to move in as the winds veer from south-southwest to west-southwest through
Sunday. The European model (ecmwf) keeps the area dry through Monday while the GFS
draws a little bit of moisture into the area from the south late
Sunday night and Monday. Will lean toward the slightly wetter GFS
which has done a better job verifying precipitation from residual
moisture this Summer.

Tuesday and Wednesday in both models have weak west-northwest flow aloft and
drier air over the area with a low-areal coverage...diurnal
pattern to shower development. Thursday looks like the driest day
with a strong ridge established over the Great Basin.

&&

Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 419 am MDT Friday Sep 4 2015

A small disturbance will bring an area of isolated to scattered showers
and a few thunderstorms cutting through the forecast area and
traveling along a line from Blanding to Rifle. Showers should be
in the vicinity of kmtj...kgjt and kril between 12z and 15z but
flight conditions should not be degraded.

Scattered to numerous showers and thunderstorms will develop
after 18z and continue through about 04z this evening...caused by
moisture flowing into the area from the south and a trough
approaching from the northwest. All taf sites will be affected by the
storms though the southern half of the area will have the higher
areal coverage. Mountains will be frequently obscured. Ktex and
kdro have about a 30 percent chance of MVFR visibilities in stronger
showers. Kase...kege and kril have about a 20 percent chance.

After 04z showers and thunderstorms will decrease...then storms
over the southern half of the area will increase again after bout
09z as another disturbance moves in from Arizona and nm.

&&

Gjt watches/warnings/advisories...
Colorado...none.
Utah...none.
&&

$$

Short term...jam
long term...cirrocumulus
aviation...cirrocumulus

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