Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 160353
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
853 PM MST Wed Nov 15 2017

.SYNOPSIS...A mild southwest flow will increase over the area
through Thursday. A cold storm system will cross the region
Thursday night and Friday.

&&

.DISCUSSION...Southwesterly flow aloft prevails across the Great
Basin this evening, downstream from a longwave trough centered off
the Pacific Northwest coast. This flow will strengthen overnight,
and as the surface gradient tightens in response to upstream
pressure falls, southerly winds may become gusty overnight from
about Salt Lake County south and westward into west central Utah.
Otherwise mid and high level moisture will continue to stream
across northern and central Utah resulting in partly cloudy skies.
Little change in overall thinking with respect to the upstream
trough as it spreads inland over the next couple of days. Going
forecast looks in good shape and no updates are planned. Previous
forecast follows...

.PREV FORECAST...

.SHORT TERM (Thru 12Z Saturday)...A deep cold trof remains
centered west of the BC coast in the ern Gulf of Alaska this aftn
with a southwest flow aloft downstream from this trof across the
Great Basin. Tropical moisture is streaming into the western
states on this southwest flow. A break in the clouds has developed
over the CWA this aftn with the cirrus shield moving off to the
south and the mid deck over the north breaking up. Temperatures
are mild and south winds are locally surfacing around the CWA.

Expect clouds to increase over the north again tonight as the next
band of moisture arrives.

The ern Pac trof gradually shears and shifts inland over the next
48s hrs or so with the swly flow increasing across the CWA. Winds
could approach advisory levels in the southwest valleys Thu aftn
but should stay sub advisory for the most part.

The tropical moisture also increases thru the day Thu and into Thu
night with precip becoming increasingly possible in areas favored
in swly flow, including far southwest UT.

The airmass stays warm in this moist pre frontal environment with
snow levels up near 10000 ft Thu aftn into the eve. Snow levels
will start to lower from the north Thu night as the associated
cold front starts to work its way into the CWA.

The new 18z GFS and 12z EC remain very similar on timing of the
trof and cold front, but the GFS is still more aggressive moving
the colder air in behind the front Fri morning, and would lower
snow levels to the valley floors north of about SLC, while they
would just reach the valley floors across the far north in the
EC. The EC has been very consistent on how its been handling this
trof and the 12z run did not change from the 00z. The GFS has
been trending towards the EC last several runs, altho there was
little change between the 12z and 18z GFS. Have continued to lean
towards the EC on snow level timing.

Expect sufficient accumulation and impacts late Thu night into Fri
across the nrn mtns to warrant an advisory and have issued one. It
is possible this may need to be expanded down into the mtn valleys
if the colder GFS solution occurs.

Both models push the front into srn UT Fri aftn with a weakening
band of precip, and turn the precip showery over the north. Gusty
and locally strong post frontal winds will gradually diminish Fri
evening and showers will end. The clearing skies and dry cold
airmass will allow temps to get fairly cold by Sat morning.


.LONG TERM (After 12Z Saturday)...Drier air moves in quickly
Friday night and Saturday, which should end any threat of
lingering showers as the trough axis moves eastward and ridging
builds in over the Great Basin. This will bring a warming trend
into Sunday with highs returning near normal, though Saturday
night will again feature crisp temperatures under clear skies and
light winds, with a few basins likely dropping to the low teens or
lower.

Global models are in great agreement in sliding the ridge axis
over Utah by early Sunday, then forecast confidence quickly
diminishes as ensembles diverge significantly next week. The
overall pattern remains progressive with additional troughs moving
into the West Coast, but deterministic models are struggling to
develop a consistent picture of how these waves will progress into
the interior western US and impact our area. In general the GFS
and GFS ensemble have more active weather for our area around the
middle of next week, while the deterministic ECMWF favors a
continuing ridge. The going forecast thus remains a compromise,
meaning a chance of precip over northern Utah, mainly in the
mountains, with near normal temperatures.

&&

.AVIATION...VFR conditions will prevail at KSLC through the
overnight hours. Southerly winds will persist, and likely become
gusty overnight with a 40 percent chance of gusts in excess of 25
KTS after 07Z.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory from 11 PM Thursday to 5 PM MST Friday
     for UTZ007>009.

WY...None.
&&

$$

Seaman/Wilensky/Van Cleave

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http://weather.gov/saltlakecity

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visit...
http://weather.gov/saltlakecity/general/afd_faqs.php



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