Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
417 am MDT Tuesday Sep 1 2015
Synopsis...an upper level trough developing over western Canada
will settle south into the Pacific northwest midweek. A mostly dry
and warm southwest flow aloft will form across the Great Basin...
followed by cooler fall like temperatures spreading across the
region over the weekend.
Short term (until 06z saturday)...the upper low currently along
the central britcol coast will continue on a southward drift
through Tuesday. This upper low will eventually settle into the
Pacific northwest midweek///then linger across the northern half
of the western region through the upcoming weekend.
Persistent overnight convection over south-central Utah has been
confined to an area of low-level convergence underneath a modest
upper level jet across northern Arizona through western Colorado.
Water vapor imagery suggest a weak vorticity maximum may exist over
northwest Arizona which is driving loosely organized convection
advancing into extreme southern Utah. Suspect this dynamic feature
will usher the convection off to the east and bring an end to
showers/storms during the middle-morning hours.
GOES precipitable water imagery shows moisture returning north across western
Utah...with deep layer moisture advancing north out of Arizona and
into extreme southern Utah. Convection will likely form up again
over the higher terrain beginning early this afternoon...then
continue across southern through east-central Utah along the axis
of best moisture into this evening. Northern and western Utah
will likely see an increase in convective cloud cover during the
day...with a minimal chance of precipitation as the air mass remains less
than supportive of deep convection.
Looking at an increasingly warm southwest flow develop across the
Great Basin in advance of the Pacific northwest trough midweek.
Breezy and warm conditions will exist across the area with
terrain-based convection across mainly southern and eastern Utah
within the deep layer moisture.
A well-defined baroclinic zone associated with the upper trough will
approach northwest Utah late in the week. This feature will likely
stall across far northwest Utah and become the focus for showers
and storms along and behind the boundary. The various model
guidance are not generating much if any precipitation with this feature
which is concerning considering the moisture just to the south
and east. Inclined to think that some precipitation will develop as
moisture converges into the boundary with dynamic lift from
ejecting shortwaves out of the main trough supporting fairly
Long term (after 06z saturday)...global model consistency has
increased a bit over 24 hours ago...with the bulk of the 00z global
model guidance starting to come more toward a common solution.
A large...relatively cold Pacific trough will be crossing the
Pacific northwest at the start of the long term forecast period.
Both the 00z GFS and 00z ec eject this trough across the northern
intermountain region and perhaps across the northern Great Basin by
Sunday. Timing issues remain between these models...with the GFS
still trending faster than the ec. The 00z Canadian however...still
favors a deeper and slower system...with the trough finally ejecting
Sunday and moving into southern Canada late Sunday/early Monday.
There are two main differences in the evolution of this trough that
directly impact Utah. The first is just how cold the system will be
as it moves through the area. By Sunday morning...the 00z ec
features 700mb temperatures around -2c near the Utah/Idaho
border...whereas the warmer GFS averages near 0c. Given that this
system has slowed down a bit from earlier runs...it appears as if
Saturday will be a bit warmer than previously expected. Increased
temperatures a bit on Saturday...and these may need to be adjusted a bit
warmer if things run even slower in later model runs. Kept the trend
of lowering sundays highs below climatology as mentioned by previous
The other main difference for Utah is just how quickly southerly
flow advects deep moisture into Utah. The 00z GFS is much more
progressive with the deep moisture surge...bringing deep moisture
into the area prior to the long term forecast period. As the trough
moves inland and the flow becomes more southwesterly across much of
Utah...the chance of precipitation will likely decrease by Saturday
afternoon...especially across the south.
After the trough clears the area...the ridge will build once again
across the region...keeping a relatively benign pattern in
place...with warming temperatures through early next week.
Fire weather...a drier and more stable airmass will build into the
area today. A large and fairly strong Pacific trough will slowly
approach the Pacific northwest over the next several days. This will
increase winds across much of the district Tuesday through Thursday.
This trough will cross into the Pacific northwest Friday...and into
the northern intermountain region Saturday...keeping winds elevated
through at least Saturday. Occasional periods of localized critical
fire weather conditions will be possible Tuesday through
Saturday...with the best chance Wednesday and Thursday.
Aviation...elevated southerly winds will continue at the slc
terminal through much of the day. There is a 10 percent chance winds
will shift to the northwest between 21-01z.
Wyoming...Fire Weather Watch for wyz277 from 11 am Wednesday through
9 PM Friday.
For more information from National oceanic and atmospheric administration/S National Weather Service visit...
http://weather.Gov/saltlakecity (all lower case)
For frequently asked questions about the area forecast discussion
http://weather.Gov/saltlakecity/general/afd_faqs.Php (all lower case)