Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
1018 am MDT Sat Sep 5 2015
Synopsis...an upper trough will cross the northern Great Basin
today. Dry westerly flow will develop over northern Utah Sunday
and continue through middle week. Limited moisture will linger over
the south Sunday and Monday.
Discussion...water vapor loop shows a deep trough over the
Pacific northwest. Amdar 400-250mb wind observations show a
75-105kt cyclonic jet along the West Coast over to southern Nevada
and northeast into the northern rockies. GOES/GPS/rap/12z slc radiosonde observation
indicate the precipitable water value ranges between 0.25"
mountains...to 0.70"- 0.85" across the southern valleys.
Msas analysis shows a surface trough stretching from Bear Lake
southwest through Delta. Tslc tdwr radar along with mesowest data
suggest the cold front is through Ogden and approaching the Salt
Lake Valley from the northwest. As heating occurs ahead of the
boundary...it may slow before continuing through central Utah later
this afternoon when the rap indicates deeper cold air arrives.
Ahead of the boundary...expect southwest gusts up to 50 miles per hour. Later
on this afternoon...expect greater gusts to develop just behind the
boundary as the baroclinicity is enhanced due to heating.
Downslope/canyon locations such as Point of the Mountain will also
likely see enhanced Post frontal gusts.
Isolated to scattered convection is expected east of Interstate 15
and north of Interstate 80 this afternoon. Better coverage is
anticipated across the northern mountains closer to the dynamics of
the storm system to our north.
Previous discussion...the deep cool upper trough is
beginning to shift into the northern rockies early this morning. The
associated cold front lays from near Logan southwestward to southwestern Tooele Colorado.
Scattered elevated convection has persisted overnight along and
just east of this boundary north of about slc and expect this to
continue through the morning.
The upper trough is forecast to lift northeastward through the northern rockies and
this will send the cold front southward through the northern and central County Warning Area
today. Isolated to scattered surface based convection should
develop in a marginally moist airmass across southern and eastern Utah today
and a few cells could persist tonight across the south as the
front stalls out across southern Utah.
South winds ahead of this front should reach advisory levels
across southwestern Utah but the front has advanced too far for winds to get
strong across the northern Wasatch front and northwest deserts...so
have cancelled the advisory for these areas. Dont expect the Post
frontal winds to become too strong as the thermal gradient behind
the front is not all that tight and there is not very much forcing
due to the ejecting nature of the parent trough. Frontal timing may
also make the advisory marginal for the Salt Lake and Utah valleys
but have left it in place in these areas for now.
The stalled front could provide a focus for convection across the
south Sunday and Monday while the north will be cool and dry. Expect
drying to spread south Tuesday and temperatures will modify across the
High pressure is prognosticated to rebuild in earnest over the forecast
area beginning midweek. This will result in generally dry and
stable conditions with a gradual warming trend in temperatures.
GFS indicates a weak grazing system will curb the warming trend
Friday night/Saturday morning while the latest ec maintains strong
ridging. Regardless...temperatures are expected to remain at least
slightly above seasonal norms through day 7.
Aviation...concerns focused around wind shift with frontal passage.
Expect primarily northwest flow through valid taf period...though
there is a 20 percent chance for a period of southwest flow until
around 18z. Also will need to monitor for possible extension of low
level wind shear beyond 19z. While some blowing dust possible into
early afternoon...expect VFR ceilings and visibilities.
Fire weather...red flag warning continues through this evening for
the western valleys. Relative humidity values between 10 and 15 percent...along
with very strong winds will contribute to critical fire weather
conditions. A significant wind switch to the northwest is occurring
with the frontal boundary...which is south of Ogden and nearing Salt
Lake City as of 1000 am. Even behind the boundary...winds remain
gusty for several hours...especially in prone downslope/canyon
The cold front is forecast to make its way into central Utah by this
evening...with winds diminishing tonight. Cooler conditions can be
Convection appears isolated most locations...but lightning and
enhanced/erratic winds will be a threat concurrent with the larger
scale wind system for the remainder of today.
A warming/drying trend is forecast for the first half of next
Utah...red flag warning until 10 PM MDT this evening for utz478-492-495-
Wind Advisory until 10 PM MDT this evening for utz003-004-015-
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