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Area forecast discussion 
National Weather Service Salt Lake City Utah
838 am MDT Friday Oct 31 2014

Synopsis...a mild and increasingly strong southerly flow will
remain over the area through early Saturday. A vigorous Pacific
storm system will reach Utah Saturday. Cool and unsettled weather
will remain over the area Saturday night through early Monday.


Discussion...water vapor loop shows the ridge shifting east of
The Rockies as a trough develops along the West Coast. A weakening
southern Pacific system is currently lifting overhead. Amdar
400-250mb wind observations reveal a cyclonic 100-135kt jet off
the Pacific northwest. GOES/GPS/rap/12z radiosonde observation indicate the
precipitable water value ranges between 0.15" 0.40"

Today will be the warm before the storm. Southerly winds increase in
response to pressure and height falls ahead of the trough. Modestly
strong southeasterly pressure gradient and 30kt southerly flow as
shown by the rap should equate to conditions just shy of Wind
Advisory across the western valleys this afternoon.

Southerly winds and deepening mixing will allow for temperatures to
rise well into the 70s with +7c 700mb temperatures.

Aforementioned weakening wave has brought scattered to at times
broken middle and high level clouds. Expecting partly to mostly sunny
skies. Increase in moisture is increasing instability across
southeast Utah late made probability of precipitation non-zero...but still no
mention of showers through sunset.

Previous discussion follows...
the cold core eastern Pacific trough approaching 130w early this
morning will bring a dramatic change in temperatures and precipitation
this weekend. The axis of the trough will reach the West Coast
this evening. A vigorous shortwave rounding the base of the trough
will lift northeast across southern Nevada early Saturday...then
lift quickly north through Utah Saturday afternoon.

The associated surface cold front will enter western Utah early
Saturday...then lift northeast with the shortwave during the day.
Increasing frontogenic forcing along this boundary late Saturday
morning/afternoon along with strong dynamic lift should easily
support deep convection along the frontal boundary. Substantial
deepening of the surface low over northeast Nevada in the area of
fairly strong upper divergence suggests that the strongest
convection will exist across northern Utah during the afternoon.

Besides the convection winds will be a major issue with this storm
tonight/Saturday. Ahead of the front we will see the typical
increase in southerly winds with the favorable surface gradient and
increasing low-level wind support. Winds could approach Wind
Advisory criteria this afternoon...but suspect that tonight may be
the more likely period for these stronger winds. Because of the
southwest to northeast track of the surface front...the still strong
low-level southwest flow will likely mix down and create additional
strong southerly winds across the western and northern valleys
Saturday late morning through mid-afternoon.

Pattern gets a bit complicated for the balance of the weekend as the
ejecting shortwave pulls away in the northern rockies... leaving the
main trough hanging back across the Great Basin. The best dynamic
forcing will shift into the southern end of the trough over northern
Arizona/extreme southern Utah. The focus of organized precipitation will
remain near the baroclinic zone stretched out along a southwest to
northeast axis through the center of the state. Increasing
instability as the cold air settles into the state combined with
dynamic lift and convergence into the baroclinic zone should result
in significant precipitation near the boundary Sunday through Sunday night.

North of the baroclinic zone a decent pool of cold/moist air
exists...but the dynamic lift may turn out to be weak between the
exiting system in the northern rockies and the system well to the
south. Looking for instability showers late Saturday night/early
Sunday...with one last weak shortwave bringing some organization to
the precipitation Sunday afternoon and evening. Precipitation type will be snow at
the higher elevations...with a mix of rain and snow throughout in
the valleys. Intensity of the precipitation will likely determine if snow
falls in the lower valleys...with minimal accumulations expected.
Some orographic enhancement to the snowfall is possible in the
mountains...though the mean layer is not all that strong and may not
produce significant enhancement to snow totals.

Building heights beginning late Monday should signal the end of
precipitation by Monday evening. The upper ridge building in from the
eastern Pacific will bring a period of dry and mild conditions back
to the region through much of the week. A series of shortwaves
moving through the northern rockies will spread some clouds into
northern Utah...with no real threat of any substantial precipitation during
the extended period.


Aviation...the slc terminal will see VFR conditions throughout the
day with increasing high and middle level clouds. Winds should be
southerly throughout the day...with gusts in excess of 30 miles per hour
expected to begin between 16z and 18z.


Fire weather...high pressure has started to shift east over the
last 12 hours allowing a strengthening southwesterly flow aloft to
develop this morning. Very warm and dry conditions will persist
today as such but changes will come Saturday into early week as a
cold Pacific storm moves through the region. Southwesterly winds
will ramp up significantly Saturday as a strong cold front pushes in
from the west. This front will bring widespread wetting rains and
high elevation snow to much of the district Saturday afternoon into
the overnight hours...and very gusty southwesterly winds...with
focus across the north. New snow accumulations will generally stay
above 8000 feet with up a to a couple of inches possible. A
secondary piece of the storm will track through The Four Corners
region Sunday into Monday bringing a focused round of precipitation to the
southeast at that time.


Slc watches/warnings/advisories...


fire weather...Merrill

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