Area Forecast Discussion
Issued by NWS Salt Lake City, UT

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
000
FXUS65 KSLC 161241
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
541 AM MST Thu Nov 16 2017

.SYNOPSIS...A mild southwest flow will continue to increase over
the area today. A cold storm system will cross the region
tonight through Friday.

&&

.SHORT TERM (Through 00z Monday)...Morning water vapor and H5
analysis continue to place the upstream cold core trough off the
PacNW coast, with an ejecting short wave rapidly translating NE
into the northern Rockies region early this morn. Downstream of
this, confluent flow over the SE Pacific has set up a subtrop
moisture tap currently streaming ENE into much of northern/central
Cali attm. For the time being said moisture tap will be largely
blocked by the central Sierra mtns locally, but trends in PWAT/IVT
will continue to be on the rise today.

The aforementioned ejecting wave will aid to drag a shallow
boundary into far northern UT this morning, with weak cold
advection below roughly H7, while isentropic glide aloft within
WAA aloft will begin to increase lift across the north. Typical SW
flow scenario setting up for today regarding precip production,
largely orographic and tied to the UT/ID border region. Have
removed PoPs from the SLC valley due to very dry low levels aided
by mixing, and downwind shadowing of local terrain (though models
continue to advertise increasing precip potential). Backing flow
this afternoon in wake of the wave`s departure should only aid
this.

Expecting rapid pressure falls today, with a surface trough
forming and deepening over west-central Utah. This coupled with
increasing southwesterly flow aloft (H7 winds pushing 40-50 knots)
will allow for gusty southerly winds across many valleys, most
notable across central and southwest Utah. Opted to roll out a
wind advisory for west-central and southwest Utah as gusts in the
45-50mph range look likely, sub advisory elsewhere. Feel the
rapid pressure falls and intensifying gradient will overwhelm
much of the negative influence from the thickening cloud cover.

Things begin to get more interesting overnight tonight and
especially Friday. The trough remains poised to translate ESE inland
at that time, coupled with a more favorable moisture advection
trajectory as the atmos river sags south towards the southern
Sierra region. Less lee-side blocking and deeper inland moisture
penetration is expected to eventually ensue late tonight/Friday
(still not ideal though).

Models largely agree now regarding timing of cold frontal passage
in the north Friday morning, with the front becoming increasingly
frontogenic midday Friday into the afternoon as the upper support
catches up the surface boundary in place across central/southwest
Utah. Expecting a period of widespread and locally significant
precip to develop along this boundary from the central Wasatch
Front south during that time (though areas across the east
downstream of the terrain should remain largely shadowed), with
embedded convection becoming likely due to rapid destabilization
below H5. Have upped water totals across many areas for these
areas this package due to the better forcing potential coincident
with the deep layer moisture in place. Snow levels will remain
quite high initially, then drop rapidly as the front arrives
(8kft+ on average through tonight, then to valley levels). This
said, not expecting much in the way of valley impacts as much of
the deep layer moisture will continue to reside on the warm side
of the trough.

Mtn advisories still look good (maybe a little overdone in the
far northern mtns), and expect focused accums to be in the
Central/Southern Wasatch and portions of the western Uintas.
Central/southern Wasatch valleys should see graupel showers in the
increasingly convective environment Friday afternoon, with the
central I-15 corridor likely seeing a brief but intense snap over
to snow during the afternoon. Will continue to have to monitor
potential of expanding winter advisories into the Wasatch Mtn
Valleys and the central I-15 corridor for Friday, but will leave
that to day shift after coordinating with partners as accums
should remain quite low due to a limited window of potential.
Intensities would be the primary factor.

A rapid building of heights remains forecast by Friday evening as
the trough axis slides east of the area. Expecting any and all
showery post frontal precip to end by midnight, with the last
holdovers tapering across the central/southern mtns. Much colder
temps are expected areawide for the weekend under clearing skies,
but the upper elevations will gradually warm Sunday within a
backing flow environment.

.LONG TERM (After 00Z Monday)...Global models are at odds with each
other through the extended period. At 12Z Monday the GFS has a
modest strength ridge axis over the western Great Basin, the EC has
zonal flow while the Canadian has a weak trough over the western
Great Basin. Consequently, have low confidence in the forecast
across northern portions of the CWA but across southern portions
pretty confident that dry and near seasonal or above temperatures
will occur.

By Tuesday night, the models come back into somewhat of an agreement
with the longwave ridge axis over the Great Basin. However, they do
differ in the amount of 700mb moisture that they spread into
northern utah. Have leaned toward the drier solution as this ridge
appears to be sharp enough to dissipate the threat of most
precipitation from reaching northern Utah. Therefore, backed off on
PoPs Tuesday night into Wednesday.

The global models move the ridge axis to the east of the CWA by
Thursday and Friday and this will allow some moisture potential to
reach the northern portion of the CWA. The EC is faster then the GFS
in breaking down the ridge but not totally sold, so hedged in that
direction with a reduced Superblend PoP solution.

&&

.Aviation...Breezy south winds with gusts to 35 mph will prevail at
the SLC terminal through today and into tonight. There is a 20
percent chance that winds could shift to NW this evening if a band
of rain sets up to the north of the terminal. Cigs will remain above
7K ft AGL though 03Z tonight after which there is a 40 percent
chance of lowering below in rain.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory from noon today to 10 PM MST this evening for
     UTZ015-016.

     Winter Weather Advisory from 11 PM this evening to 5 PM MST
     Friday for UTZ007>009.

WY...None.
&&

$$

Merrill/Struthwolf

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



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.