Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Grand Junction Colorado
1044 am MDT Tuesday Sep 1 2015
Short term...(today through wednesday)
issued at 445 am MDT Tuesday Sep 1 2015
Models show a little anticyclonic flow this afternoon behind
departing short wave from Monday afternoon. Air mass remains
relatively moist so afternoon thunderstorms will develop...but
will lack structure as upper air boost and shear component is
missing or lacking. Storms will favor the high terrain...although
one/two storms may drift over the valleys around early evening.
Moisture content is a notch lower for Wednesday in a weakly
sheared environment. The number of afternoon storms will be
less...and mostly confined to the southwestern and west-central mountains
of western Colorado. The storms that do form will be of short
duration. More sunshine both days will result in slightly higher
temperatures for Tuesday and Wednesday.
Long term...(wednesday night through monday)
issued at 445 am MDT Tuesday Sep 1 2015
Due to limited moisture Wednesday...as mentioned in the previous
section...there is little chance that showers and thunderstorms
will continue past sunset Wednesday night. The previous discussion
mentioned that the European model (ecmwf) and GFS began to diverge late in the week
and this incongruity holds true with the latest model runs.
However...early on models were similar...both featuring a trough
of low pressure over the Pacific northwest and high press over
On Thursday...southwest flow continues and GFS draws a generous
slug of moisture from old Mexico across Arizona and New Mexico
some of which appears to be drawn from tropical depression 14e. In
contrast...the European model (ecmwf) was considerably slower holding the deep
tropical moisture from 14e over the northern plateau of Mexico at
days end. These disparities Don/T appear to have much affect on
Thursday/S weather which will feature scattered diurnally driven
showers and thunderstorms over the higher terrain with the
southern and central Colorado mountains favored.
However...this changes Thursday night into Friday as the GFS digs
the trough over the Pacific northwest farther south relative to
the European model (ecmwf) while directing the subtropical jet directly overhead.
As a result...the GFS generated considerably more quantitative precipitation forecast over the
forecast area than the European model (ecmwf) which held moisture and the jet south
of the area for a drier output. The ec remained drier Friday as
well as the deeper tropical moisture just reaches the forecast
area/S border with New Mexico by 00z/Saturday. In comparison...the
GFS delivers tropical moisture well ahead of the ec with
precipitable water values ranging from 0.9 to 1.0 inch by days
end. Consequently...the GFS points to a much more active day
across the region Friday and Friday evening.
On Saturday...the moisture finally arrives according to the European model (ecmwf)
while a dry slot develops in the GFS solution which has begun to
swing the Pacific northwest trough inland ahead of the ec. So
models understandably differ with respect to coverage of moist
convection...but opposite to Friday.
By Sunday both models were in better agreement...though not
completely in sync. A shortwave ejecting from the Pacific
northwest trough lifts across the northern rockies driving the
moisture plume southeast of the forecast area. As a result...
considerably less thunderstorm activity is expected Sunday. In
addition...a cool front will move over the northernmost reaches
of the forecast area which should bring a bit of cooling to areas
north of the I-70 corridor. According to the European model (ecmwf)...this pattern
holds into Monday. However...the GFS quickly deepens the Pacific
trough causing moisture to surge back into the area.
So...in short...hard to add a lot of detail to the extended period
as models struggle to resolve details in the current pattern.
Temperatures will largely depend upon the arrival and persistence
of moisture later this week and into the weekend. In general...
expect cooler than normal temperatures by a few degrees in
response to clouds and showers and Sunday/S cold front.
Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1044 am MDT Tuesday Sep 1 2015
Moisture and instability remain in place today across western
Colorado and eastern Utah. This will allow the usual afternoon
showers and thunderstorms to form on the ridges and spread across
the high country. The ktex...kase...kege and possibly kril appear
to have the better chances of being impacted by the passing
storms. Gusty and erratic outflow winds will be the main threat to
aviation interests from 01/19z to 02/03z. Confidence too low to
place restrictions to flight categories in prevailing forecast
at this time. Afternoon upvalley winds near 310/10 may affect Landing
traffic at kase during the middle afternoon hours until disrupted by
passing storms. Overall VFR expected to prevail over the next 24
long term...no layers