Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Grand Junction Colorado
1129 am MDT sun Mar 29 2015
..updated aviation discussion...
issued at 933 am MDT sun Mar 29 2015
As mentioned earlier...a weak frontal zone and narrow band of
moderate middle to upper level moisture will be over southeast Utah and southern Colorado
this afternoon. A weather spotter in Cortez reported altocumulus
castellanus this morning...an indication of middle level instability. Forcing
is weak...but latest model soundings favor weak cape and strong
middle to upper level lapse rates which should be enough to support
isolated to scattered convection over the abajos and San Juan
Mountains...with isolated thunder also possible this afternoon and
early evening. Have updated probability of precipitation and weather forecast for
southern portions to increase chance of rain slightly and include
possibility for thunderstorms. A genearlly westerly steering flow
should keep any showers that do develop over the mountains.
Otherwise...another sunny and near record warm day is expected
across the region.
Short term...(today through monday)
issued at 335 am MDT sun Mar 29 2015
The weak baroclinic band...best seen in 500-300 deformation...was
stretched between the I-70 and Highway 50 corridors this early
morning. The best forcing this early morning was in the Elk Mountains
with light showers from Vail Pass south to Monarch Pass. Showers
briefly drove snow levels to around 10kft feet but cameras showed
only wet roads over Vail Pass.
Today and tonight...light mountain showers are possible this
early morning under this deformation/frontal band. It will weaken
as it sags into the San Juans today but provides a focus for
afternoon showers. Moisture remains marginal across the south
with precipitation water values at 0.4 inch and 700mb specific humidity
at 4 g/kg. Afternoon showers will be all rain at all elevations
with 700mb temperatures at 8c. Instability looks insufficient to produce
lightning though an isolated strike in the eastern San Juans is
possible. Southern buildups should collapse quickly with sunset.
Meanwhile drier air north of I-70 will bring mostly clear skies
today and tonight.
Monday...moisture begins to return to the greater region from the
west for isolated afternoon buildups mainly over the mountains.
These will produce more virga and erratic outflow winds than
Temperatures remain very warm for late March. Near record highs
are possible across the south this afternoon while the rest of
the forecast area will be around 15 degrees above normal.
Long term...(monday night through saturday)
issued at 335 am MDT sun Mar 29 2015
Moisture and clouds will continue to feed into the area from the
southwest...being fed north over Arizona and nm as air rotates around
an old upper low over the Baja California. The main longwave anchors over
North America will remain the hudson's Bay low...the ridge over
the central and northern rockies...and the large trough off the
pacnw coast and in the Gulf of Alaska. In the GFS...moderately
strong shortwave troughs will round the bottom of the two longwave
troughs Monday night and Tuesday...which will cause the overall
pattern to slide east. Thus the western ridge becomes flattened
and slides east as a cold front moves into Great Basin Tuesday...and
through eastern Utah and western Colorado Tuesday night and Wednesday. This sends
maximum and min temperatures down from 5 to 10 degrees Wednesday and Thursday. The European model (ecmwf)
has less amplitude with this trough and is about 24 hours faster
with it...with the cold front pushing through by late Monday night
The ec rebounds the ridge Friday and Saturday and shows a
shortwave running along the US/can border. The GFS brings a
reinforcing trough and cold front into the area Thursday which
drops temperatures even more. If the GFS comes true fruit trees and
tender vegetation may be at risk of a freeze early Thursday...Fri...and
Sat mornings...with the Thursday night/Friday morning low temperatures being the
Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 1129 am MDT sun Mar 29 2015
VFR conditions expected through the period. Typical diurnal wind
trends should take hold by middle afternoon after a few places like kgjt
and kase flush out the unusual northeast wind currently observed.
Isolated -shra/thunderstorms and rain possible over the Southern Mountain areas 19-03z...but
expect it to stay generally east of ktex.