Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Grand Junction Colorado
1108 PM MDT Fri Aug 18 2017
Short term...(tonight through Saturday night)
issued at 253 PM MDT Fri Aug 18 2017
Much drier day today as evidenced by GOES 16 low and mid level
water vapor channels. Visible satellite and observations indicate
the development of cumulus over the higher terrain mainly south
of I-70 due to steeper lapse rates, but no precipitation detected
on radar at all this afternoon. Makes sense given the almost non-
existent cape or convective instability today. The cumulus clouds
have a relatively flat appearance with high bases so not expecting
much shower activity for the remainder of the day. High pressure
is sitting to our south with the ridge building over the area,
keeping the best moisture south of the U.S. Border with some
isolated shower activity forming over southern Arizona and New
Mexico in the moisture gradient, but overall pretty dry. The San
Juans would stand the best chance of seeing some isolated showers
popping late this afternoon but overall, expect a pretty pleasant
afternoon and evening.
Saturday looks to be much of the same with another fairly dry day
expected. Some isolated shower and thunderstorm activity is
possible over the higher terrain by afternoon with cumulus
development once again. Not much precipitation expected however.
Temperatures will remain near normal through Saturday.
Long term...(sunday through friday)
issued at 253 PM MDT Fri Aug 18 2017
Sunday through next week will see a gradual change in conditions
as moisture slowly increases from the south. A trough of low
pressure will move into the Pacific northwest by Sunday, allowing
the high pressure ridge to shift eastward. This eastward shift of
the area of high pressure coupled with the cyclonic circulation
around a closed low developing off the Southern California coast,
will allow the deeper moisture to begin moving northward out of
Mexico and into southern Arizona and New Mexico. Moisture will
slowly increase across our area as precipitable water (pw) values
increase above 0.75 in southeast Utah and SW Colorado. This should result
in an increase in thunderstorm activity mainly over the higher
terrain, with some spread into southern valleys by Sunday evening.
Models hint at a shortwave moving through the flow across the
south Sunday evening which could lead to some overnight activity.
However, mid and high level moisture will continue to increase
towards Monday morning and afternoon, resulting in an increase in
cloud cover. This may cast a shadow on eclipse viewing as models
have been consistent in showing this moisture increase with
scattered to broken skies in the forecast. Details should come
into better focus this weekend, so stay tuned.
A shortwave trough will skirt across the northern rockies on
Monday with another shortwave rotating around the closed socal
low, giving US a better chance of storms Monday afternoon and
evening. Storm chances look even better as we head towards the
middle to latter part of next week when a stronger trough moves
onto the West Coast and forces this stationary socal low to
progress eastward through the Great Basin and across the western
slope within the Thursday to Friday timeframe. This should allow
the deeper moisture to traverse across our area as precipitable water values
climb above an inch in some areas. Model consistency has not been
great run to run with the timing of this trough passage.
Regardless, moisture still looks to increase with storm chances
increasing within the Wednesday to Friday timeframe, so elected to
raise precip chances and expand coverage during this time to show
this increasing trend. Timing these embedded disturbances becomes
problematic this far out so kept the increasing trend through
next week with pretty broad-brush coverage until details are
worked out with future runs. Temperatures will be near normal
until the passage of this trough towards the latter half of the
week, where slightly cooler temps will result.
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1056 PM MDT Fri Aug 18 2017
VFR conditions will prevail for the next 24 hours across the area,
with light diurnal winds.