Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 201116

Area Forecast Discussion
National Weather Service Salt Lake City UT
516 AM MDT Wed Sep 20 2017

.SYNOPSIS...A mild southwest flow will return to the area today. A
cold upper trough will move into Utah starting tonight. This
colder storm system will linger over the area through the end of
the week and into the upcoming weekend.


.SHORT TERM (Thru 00Z Saturday)...A deep cold upper low is
centered over southern Alberta early this morning and is elongated
e-w just north of the US border. The ern lobe lifting thru the
nrn Plains is the trof that crossed nrn UT yesterday. The western
lobe dropping into the PacNW is the next upper trof forecast to
drop into the Great Basin and eventually cross our CWA.

This deep elongated low rotates cyclonically over the next 24
hours with the western lobe deepening thru the PacNW and backing
the flow to swly over the CWA. The old stalled front from
yesterday lifts north as a warm front today but doesn`t make it
much past SLC as colder air with the digging trof starts to spread
into nwrn UT during the aftn. South winds increase south of this
front today and temps rebound across all but the far north. Some
precip could develop over the far northwest this aftn as the
thermal gradient tightens and upper support starts to increase.

The wrn end of the large upper trof continues to dig into the
Great Basin tonight thru Thu pushing the cold front and the
precip slowly se. A closed low gradually develops out of the the
digging trof and this becomes the dominant low Thu night into Fri
as the Canadian portion of the low minors out and lifts off to the

Models indicate a broad swath of precip along and behind the
front but it advances at the speed of the upper trof and takes
most of the day Thu before it reaches SLC. South winds look to
become locally strong ahead of the front Thu with another day of
mild temperatures in the pre frontal airmass.

Guidance begins to diverge on the evolution and location of the
closed low Thu night and how fast the precip tapers off on Fri
with the EC slower. Leaned towards the EC and kept at least chance
pops over most the CWA for Fri.

.LONG TERM (After 00Z Saturday)...The EC and GFS are close in the
general pattern of the major trough over the western CONUS through
Monday but then differ on how quickly the trough departs from the
region Tuesday and Wednesday. The differences between the EC and
the GFS on Saturday is the strength of the short wave that rotates
around the base of the main trough over the Great Basin. The GFS
is broader with the main trough and does not show much energy with
the short wave while the EC is sharper with the short wave and
thus has better potential for creating lift across the CWA. Have
leaned toward the EC which keeps better consistency with previous
forecast. The GFS dries it out completely across the CWA save the
Uintas and SW Wyoming. Just seems odd that the GFS removes all
threat of precip under such a cold core at 500mb for this time of
year. Will keep scattered showers in the forecast over most of the
CWA through Sunday before finally shrinking the coverage to only
SW Wyoming and the Uintas by Monday morning.

Temperatures will remain well below normal through the weekend and
early next week despite a gradual warming trend after Sunday. The
min temperatures are expected to be very close to freezing or sub-
freezing in some western valleys Saturday, Sunday and Monday
mornings. Cloud cover will be the determinant factor as to how
cold the temperatures will get. Bottom line is that preparation
should be made now to protect gardens and orchards should sub-
freezing temperatures occur any one of those nights.

.AVIATION...Southeast winds are expected to remain at the SLC
terminal through about 20-21Z before a switch to northwest occurs.
There is a 40 percent chance that winds will not shift to
northwest until 01-02Z. If this delay in the wind shift occurs
then south winds could gusts over 20 mph through the afternoon.
Any showers should hold off until after 12Z Thursday.


.FIRE WEATHER...It will be warm, dry and locally windy today, with
southwesterly winds increasing ahead of the next storm. This next
cold front will move slowly through the state late today through
late Thursday, with locally strong southwest winds ahead of the
front. Temperatures will be much cooler behind the front with
widespread wetting rain expected, along with accumulating snow in
the northern and central mountains. Cool, showery conditions could
continue into Friday and Saturday, with drier conditions heading
into next week.





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