Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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156
FXUS65 KSLC 250948
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
348 AM MDT Sun Sep 25 2016

.SYNOPSIS...Building high pressure will bring dry conditions with
a warming trend through the first part of the week. A storm
system moving up from the south is expected to bring moisture back
to the area by midweek.

&&

.SHORT TERM (through 12z Wednesday)...Mid level ridging centered
just off the west coast is inducing a dry northerly flow
downstream across the Great Basin this morning. At the surface,
high pressure is centered across western Wyoming resulting in
light winds across northern Utah, which combined with clear skies
is allowing for strong radiational cooling. Locations across the
Wasatch Back have dropped into the mid to upper 30s early this
morning, and coupled with lingering moisture from recent rainfall
could in patchy fog early this morning. The potential for patchy
fog also extends into the Cache Valley and other sheltered
outlying locations. Additionally frost is expected across these
areas, as well as locations within the Sanpete and Sevier Valleys.
Further south a northerly pressure gradient is inducing locally
windy conditions through the gaps/canyons of Washington county.
These winds are expected to remain below advisory criteria, and
should begin to diminish during the mid to late morning hours.

The upstream mid level ridge is forecast to build inland, and
eventually become centered over the forecast area by Monday
afternoon. This will allow for a warming trend areawide through
the short term period. An upper low which is forecast to develop
over northwest Mexico today is forecast to drift northward early
in the week, with associated moisture potentially reaching far
southern Utah Tuesday afternoon. As such have maintained low pops
across portions of this area Tuesday afternoon through Tuesday
night.

.LONG TERM (after 12z Wednesday)...Elongated closed low over
western Arizona prior to 12Z Wednesday will open up and move north
across the CWA during the day Wednesday with characteristics of an
easterly wave. Therefore introduce pops to CWA with and after the
passage of easterly wave axis. Have not gone too bold with pops at
this time as most of moisture remaining above 700mb at Cedar City
and SLC initially. However, PWs do climb to between 0.50 and 0.75
of an inch across southern half of CWA at start of this extended
period and across the entire CWA by 00Z Thursday.

Have brought in some drying across the west central and southwest
valleys late Wednesday night into Thursday in the wake of the
easterly wave type trough as the flow becomes weakly anti-cyclonic
at 500mb. Start increasing pops again over the south late Thursday
night and early Friday as another disturbance moves northeast into
the the CWA. By Friday afternoon, the jet associated with this
leading disturbance is aimed towards the four corners while the
main trough approaches with its jet angling into Idaho.
Consequently, the northern CWA ends up under diffluence Friday
afternoon through Saturday morning. Have increased pops there
accordingly.

The surface low that forms over northwest Utah under this
diffluence aloft will track east northeast into southwest Wyoming
by Saturday night dragging an accompanying cold front across
northern Utah. Showers are expected to accompany this front but
then cooler and drier conditions will follow for Sunday.

There are 40-50 meter 500mb height discrepancies between the GFS
ensemble members associated with the southwestern portion of this
leading edge of the trough which will probably impact the timing
some but also its strength. Discrepancies between strength and
timing are even greater on the backside of the trough where 500 mb
height differences are as much as 60-75 meters. These variations
lend to a lesser degree of confidence as we get farther into this
extended.

&&

.AVIATION...No operational weather concerns for SLC terminal through
the TAF period. Clear skies and light winds will prevail.
Southeast winds will shift to northwest between 17 and 18z with a
30 percent chance of switching at 16Z.

&&

.FIRE WEATHER...High pressure aloft will build across the region
today through Monday, then remain in place for Tuesday. This will
result in dry conditions along with a warming trend. Winds will
remain northerly today, however as the high becomes centered
overhead winds will become light and driven by diurnal slope
flows. This high will shift east midweek, and be replaced by
southerly flow which will allow moisture to return northward
across the area.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

Seaman/Struthwolf

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For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php



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