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Area forecast discussion
National Weather Service Grand Junction Colorado
549 PM MDT Wed Sep 20 2017

Short term...(tonight through Thursday night)
issued at 405 PM MDT Wed Sep 20 2017

The strong and broad Pacific trough continues to dominate much of
the western Continental U.S. Today with the tight wind gradient persisting
aloft under a +110kt upper level jet. This is evident on the
latest visible satellite imagery this afternoon with the presence
of some cloud streets over portions of the northern Great Basin
resulting from those turbulent winds aloft.

The forecast remains on track over the next several days as the
Pacific trough is expected to elongate and dig further south. This
will turn our upper level flow to the southwest with the gradient
tightening even more as the jet dives into the Great Basin on
Thursday. These winds will mix down to the surface under mostly
sunny skies Thursday with peak mixing occurring by late afternoon.
As a result, have decided to hoist a Wind Advisory for portions of
northwest Colorado and southeast Utah where the strongest
sustained winds and gusts are expected. Several other Lower Valley
locations were borderline on advisory criteria, so this may need
to be re-evaluated by later shifts. Additionally, given the
critical fuels status and low relative humidities under the dry
southwesterly flow, have also issued red flag warnings for those
select fire weather zones in southwest Colorado. Again, other
zones were close to reaching criteria but confidence was not high
enough to issue at this time. More details can be found in the
fire weather discussion below.

Breezy conditions will continue Thursday evening as the jet
remains overhead. Overnight lows will remain mild for this time of
year. But that will soon change as we head closer to the weekend.

Long term...(friday through wednesday)
issued at 405 PM MDT Wed Sep 20 2017

By Friday the Pacific trough will not have progressed much further
east and will still be situated over the Great Basin. We will
finally begin to see some precipitation on Friday as showers and
thunderstorms develop ahead of the cold front associated with this
storm. The Euro remains the wetter solution at this time but the
higher terrain still looks to be favored for activity. 700mb
temperatures support the presence of some light snow showers at
the higher mountain peaks throughout the day with the northern
mountains seeing the best chances given the proximity to the cold
front. As the previous forecaster mentioned, the front appears to
stall out across our region on Saturday so another round of
scattered convection will develop.

The trough looks to finally lift out of our area during the day on
Sunday as a ridge of high pressure amplifies off the West Coast.
The GFS wants to carve one final trough to our southwest before
the ridge moves further east early next week but, given the
continued model differences, confidence is low at this time.

Temperatures will be the main focus going into the extended
period with the arrival of the cold air mass beginning Friday and
continuing through the weekend. Widespread below seasonal
temperatures are expected across the region - at least by some 10
to 15 degrees. Near or even below freezing temperatures look to be
possible for the northern valleys Sunday, Monday, and maybe even
Tuesday morning so that will definitely need to be monitored over
the next couple of days for any potential freeze highlights.


Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 549 PM MDT Wed Sep 20 2017

Wind will be the main threat to aviation operations over the next
24 hours. Strong southerly flow ahead of strong storm brewing over
the Great Basin will lead to gusts of 25 to 45 mph over the
majority of the valley taf sites Thursday afternoon. Pockets of
low level wind shear early Thursday will also be a threat before heating helps to
mix these winds to the surface. VFR should dominate the forecast
over the next 24 hours. Probability of very low but some temporary
lower flight criteria could occur due to blowing dust. Ridgeline
turbulence is likely tomorrow with this strong flow in place.


Fire weather...
issued at 405 PM MDT Wed Sep 20 2017

Winds will be very gusty ahead of an approaching storm
system in the Great Basin. Southerly winds gusting 35 to 45 will be
common over much of eastern Utah and western Colorado by tomorrow
afternoon. Afternoon humidity in the lower teens will also occur
over many of the lower elevations of southwest Colorado where
fuels are critically dry. Therefore the combination of wind and
low humidity will create red flag conditions over far SW Colorado.
Winds look to remain strong through the weekend before this
system moves eastward. Southeast Utah will avoid critical
conditions due to non-critical fuel condition. Still, with such
strong winds some control problems are anticipated on any existing
or new fires.


Gjt watches/warnings/advisories...
Colorado...Wind Advisory from noon to 9 PM MDT Thursday for coz001-002-007-

Red flag warning from 1 PM to 7 PM MDT Thursday for coz207-290.

Utah...Wind Advisory from noon to 9 PM MDT Thursday for utz022-027-029.



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