Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
353 PM MDT Wednesday Sep 2 2015
Synopsis...an upper level trough over western Canada will settle
into the Pacific northwest late this week...then swing east across
the Great Basin and northern rockies this weekend. Mild
southwesterly flow will prevail ahead of this trough through the
remainder of the week...with cool fall like temperatures arriving
over the Labor Day weekend.
Short term (through 06z saturday)...southwesterly flow aloft
prevails across the Great Basin this afternoon...downstream from a
middle level trough situated along the West Coast. This flow has
gradually dried the airmass across the forecast area over the past
couple of days...resulting in very shallow cumulus development across
the higher terrain of southern Utah...and only very isolated showers
across the north. Have maintained slight chance within a
relatively narrow axis of middle level moisture across northwest Utah
The West Coast trough is forecast to slowly deepen through the
short term period...strengthening the flow downstream across the
eastern Great Basin and resulting in breezy to windy conditions
across western Utah both Thursday and Friday. At the same time this
flow will tap into deeper moisture to the south...which will be
advected northward late in the week. The models continue to
exhibit discrepancies with respect to the trajectory of this
moisture surge...with the GFS consistently being the further west
solution pulling this plume into the eastern half of Utah...while
the ec has been taking this plume further east into Colorado. Given
these variations between model solutions have maintained going
20-30 percent probability of precipitation across the south and east Thursday through
Long term (after 06z saturday)...medium range period begins with
a deep unseasonably strong trough moving through the Great Basin.
Friday night a strong baroclinic is situated just west of Utah.
Very strong middle level flow is expected ahead of this baroclinic
zone with the GFS and European model (ecmwf) each indicating 40-50 kts of SW flow
over western Utah. This combined with a tightening pressure gradient
will keep breezy to windy conditions going through the night in
many areas across western Utah. This will also keep min temperatures
elevated that night and have increased the morning lows for
Saturday a bit to reflect this.
The windy conditions will continue into the day Saturday before the
strong cold front sweeps through the area bringing a significant
temperature drop and breezy west-northwest winds. Anticipate that
there will be a band of showers and thunderstorms associated with
the frontal boundary...particularly across central and northern Utah.
With 700 hpa temperatures dropping to -1 to -2c should even see a bit of
light snow in the high uintas.
The European model (ecmwf)...GFS...and Canadian are now in agreement that the trough
is very progressive and the flow will quickly back to the south by
Sunday night. Still much cooler air will reside across central and
northern Utah and Sunday high temperatures will generally be 10f cooler
than Saturday/S and 15-20f than Friday/S...at least across northern Utah.
For the start of next week a baggy trough lingers over the western
U.S. Leading to a slow rebound to near normal temperatures.
Aviation...gusty southerly winds are expected to continue at the
slc terminal through much of the taf period...with only a 10
percent chance that a lake breeze could push winds to the west-
northwest between 23z and 01z. There is also a 10 percent chance
that a shower could impact the terminal through 03z...with brief
gusty erratic winds as the most likely impact.
Fire weather...a developing trough along the West Coast will
result in an increase in southwesterly flow through the remainder
of the week. This will result in breezy to windy conditions across
the western half of the area Thursday into Friday. In addition to
the gusty winds...humidity values are expected to fall below 15
percent each afternoon with poor overnight recoveries. Further
east moisture is expected to increase across the eastern half of
Utah Thursday into Friday bringing a return of showers and
thunderstorms. This upstream trough is forecast to swing through
the region Saturday bringing a chance for precipitation to
northern and central Utah. Winds will also remain elevated
across most valleys through the day Saturday. A much cooler
airmass will follow this system for Sunday.
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)