Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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306
FXUS65 KSLC 271542
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
842 AM MST Mon Feb 27 2017

.SYNOPSIS...The next cold Pacific storm system will impact much of
the area through at least Tuesday evening. An increasingly dry and
mild westerly flow aloft will set up for the latter half of the
week.

&&

.DISCUSSION...
Water Vapor Satellite shows an upstream trough over Idaho and
Nevada. The next storm is a closed low along the Washington/Oregon
coast. 400-200mb MDCARS wind observations place a 120-140kt
southwesterly jet from southern California across southern Utah.
GOES/SLC 12Z RAOB/HRRR indicate that precipitable water values
vary from 0.10"-0.20" mountains to 0.30"-0.40" most valleys.

Going forecast in good shape. Bumped up PoPs where it is currently
snowing per observations and webcams. Also tweaked QPF based on
latest model data but it did not result in many zone based
changes. Going headlines remain the same.

Well, its happening. Visibility sensors, traffic flow and traffic
incidents all indicate significant challenges to the morning
commute especially Salt Lake Valley northward. Utah county is just
getting into the action now.

The cold front stalled for a time overnight allowing the 700mb
reflection to catch up, tightening up the baroclinic zone which
is currently from north of Delta across central Utah county. This
boundary will continue to sag into central Utah this afternoon.
Meanwhile after a break in precipitation north of the frontal
band, snow is expected to once again redevelop as 700mb cold
advection becomes maximized across the north, and the 500mb cold
pool grazes the northern part of the state, increasing
instability.

Meanwhile a strong jet will continue to bring in higher moisture
content across southern Utah well ahead of the cold front. Gusty
pre-frontal winds may near 45 mph at times especially northeast of
Kanab to Hanksville including Lake Powell.

&&

.PREVIOUS DISCUSSION...
The axis of the approaching upper trough currently extends across
the northern Sierra Nevada through eastern Oregon. The 700mb
baroclinic zone is draped over northwest Utah per the latest RAP
analysis, near where a band of snow currently exists. The boundary
will continue to strengthen through this afternoon as it slowly
moves south into central Utah, with snow filling in along and
behind it while gusty southerly winds will be common across
southern Utah ahead of it. With frontogenesis being strongest
across central Utah, the heaviest snow is expected across Utah
County through west-central Utah and the adjacent mountains later
this morning through this afternoon.

Southern Utah may see some light precipitation in the prefrontal
environment through around midafternoon, but the bulk of
precipitation there will occur late this afternoon through the
overnight hours as a plume of moisture arrives from the southwest,
combining with the cold front which will be crossing the area during
that time. Overall, much of the forecast area will see accumulating
snow, with the greatest amounts across central Utah as previously
mentioned. Going winter weather highlights will continue, although
the southern Utah zones probably could have been fine with a later
start time given the greatest impacts will be during the latter part
of the day.

There may be a brief lull in the weather late tonight into early
tomorrow morning as the initial shortwave trough exits the area and
a second disturbance, which is currently located off the British
Columbia coast, approaches Utah. This trough will cross the area
Tuesday afternoon which will bring snow showers back to the portions
of northern and central Utah. A cold and moist northwesterly flow
will keep lingering showers in place through Tuesday night. Warm
advection at 700mb will then develop for Wednesday, keeping some
clouds in place across northern Utah with a few snow showers
possible across the far north.

High pressure over the forecast area is expected to weaken some
Thursday night into early Friday as a weak wave grazes northern
Utah and southwest Wyoming. Moisture looks limited so associated
precip should be as well, but it looks to sag a weak frontal
boundary into northern Utah.

Behind this wave, EC/GFS shows a more zonal flow developing over the
state for the first part of the upcoming weekend with moisture
increasing over northern Utah in weak warm advection. This will
allow for a chance of precip, with a couple of weak shortwaves
enhancing instability.

Global models both show another grazing system for the latter half
of the weekend into day seven, but the latest GFS is 12-18 hours
slower bringing the system in compared to the EC. All guidance shows
a good amount of cold air moving into northern Utah with the
boundary with some decent dynamics but very limited moisture, so
have kept POPs low at this time.

&&

.AVIATION...
Occasional LIFR with prevailing IFR snowfall will continue to
impact the KSLC terminal into the early afternoon hours before
improving to intermittent snow showers and largely MVFR conditions
after 22z. The best potential for this LIFR snowfall is through
17/18z. Additional accumulations of 1 to 3 inches are expected on
untreated surfaces, with most of this accumulation occurring
through 21z this afternoon. North to northwest winds will remain
in place through today.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory until 4 AM MST Tuesday for UTZ001>003-
     006-007-011-012-016-020-518.

     Winter Storm Warning until 4 AM MST Tuesday for UTZ004-008>010-
     014-015-517.

WY...Winter Weather Advisory until 4 AM MST Tuesday for WYZ021.

&&

$$

PUBLIC...Rogowski/Cheng/Traphagan
AVIATION...Merrill

For more information from NOAA`s National Weather Service visit...
http://weather.gov/saltlakecity

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



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