Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 161143
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
443 AM MST Fri Feb 16 2018

.SYNOPSIS...A weak system will impact far northern Utah and
southwest Wyoming today and tomorrow. A colder and wetter storm
system is expected to significantly impact the entire region this
weekend into early next week.

&&

.SHORT TERM (THROUGH 12Z MONDAY)...
Antecedent conditions:
Severe Drought continues along and east of I-15 south of Salt Lake
City, with most other areas in Moderate Drought. While abnormally
dry, the Idaho border area is doing much better in the
precipitation department.

Big Picture:
During the past 5 days, 500mb height and anomaly shows an
unorganized ridge off the West Coast allowing several weak
disturbances and troughs to impact the area. However now with
significant lowering of heights over the western Pacific, seeing
amplification which is reinforcing the ridge over the eastern
Pacific.

Water Vapor loop shows all systems steering around the
aforementioned ridge, with flow turning northwest over the
Western Conus. Significant subsidence and drying noted behind the
previous trough.

400-200mb MDCARS wind observations place a strong 175-210kt
westerly jet nosing into the western Pacific, with a 160-190kt
southwesterly jet from the Southwest Conus through the Great Lakes
region. Over the eastern Pacific, a 125-170kt anticyclonic jet is
rounding the ridge.

The large scale flow is supporting an Atmospheric River around the
ridge into the Pacific Northwest. However Integrated Water Vapor
Transport is currently maximized off the coast.

Local Trends and Observations:
24 hour surface observation trends show temperatures are 10F to
20F colder and dewpoints are 20F to 30F lower.

Strong high pressure has built into the region. Gusty winds seen
in the mountains and southern Utah have abated. Precipitable
water values range from 0.05"-0.10" mountains to 0.15"-0.20"
northwest valleys.

Forecast:
The next wave and aforementioned upper level jet continue to round
the ridge and will propagate southeast from British Columbia
toward our region. Strong height falls are shown to our northeast,
which should lead to warm advection. 285-290K isentropic surfaces
indicate lift is sufficient to bring Condensation Pressure
Deficit to near zero near the Idaho border, bringing the potential
for light precipitation this afternoon and evening.
leys look to near Winter Storm
  Warning criteria in respect to forecast snow on sufficiently
  cold surfaces. Lake enhancement may prolong the event in areas.
The next colder and moist storm system that propagates toward the
region from British Columbia aids in the development of a cold
trough just to our west Sunday into Monday. This will lead to
strong height falls beginning Saturday night, maximizing by
Sunday night. This evolving storm will force a warm front through
the region from southwest to northeast tomorrow, so have continued
low PoPs northeast of Salt Lake City where isentropic surfaces
support best lift and saturation.

Global models in good agreement for most large scale parameters
with the trough. Still pretty far out to get too detailed, but at
this time believe its worthy of a Winter Storm Watch in short
order.

We appear to be in a very favorable position east of the slowly
developing trough, with the left exit region of the upper level
jet overhead, along with prolonged period of Positive Vorticity
Advection, diffluence aloft and cold advection, as well as
Integrated Water Vapor transport Sunday. Northwesterly orographic
flow will be in play, along with the possibility of lake
enhancement during the second half of the storm. This is the type
of storm that over-produces expectations (so long as the models
maintain current track).

Cold front makes its way from far northwest Utah Sunday morning to
south of I-80 by Sunday evening, before clearing the remainder of
the forecast area by Monday morning. Banding and higher
precipitation rates associated with the cold front looks likely
given Frontogenesis/Fn convergence within a saturated environment
supportive of negative Equivalent Potential Vorticity.

GFS time series at SLC shows strong and deep lift late Saturday
night and Sunday. Dendritic crosshair of omega/RH and -12F to
-18F temperatures is seen during this period, though the best lift
 is a little too low at SLC. Suspect benches into the mountains
 will see full effect of Dendritic Snow Growth.

Potential Impacts:
- Ridgetop riming and strong winds due to arrival of warm
  advection and jet late today and tonight
- Warm tomorrow ahead of the storm, about 10F above normal but
  records look safe
- Pre-frontal winds look quite gusty Saturday night, and Sunday
  south of the front. Wind advisories/warnings may be needed for
  some valleys. Ridgetop winds near 70kt possible especially
  central and southern areas.
- Most significant snow seen for valleys in quite a while late
  Sunday into early next week. Road impact timing depends how much
  these cold roads warm Saturday before the system arrives,
  though high precipitation rates possible to overcome warm roads.
  Many northern and central valleys look to near Winter Storm
  Warning criteria in respect to forecast snow on sufficiently
  cold surfaces. Lake enhancement may prolong the event in areas.

.LONG TERM (AFTER 12Z MONDAY)...
As the upper level trough digs deeper into the Great Basin
Monday, with very cold air intruding into the forecast area (700mb
temps quickly dropping to -18C by Monday night), a very unstable
air mass continues to impact the area. The EC continues to be
consistent in run to run comparisons, while the GFS had trended
closer to the EC and had shown some consistency, however with the
most recent (06z GFS vs the 00z EC) available data, the GFS once
again differs. Overall, the big picture remains the same: very
cold air with an impactful winter weather event underway through
early next week.

With this forecast package, increased PoPs on Monday evening and
Tuesday morning as the trough swings through. The 700mb baroclinic
zone finally pushes through far southeast Utah Monday night and with
the cold core aloft, kept higher PoPs for the central and southern
mountains. Cold advection remains in place through late Tuesday
across the entire state of Utah. With anticipated snowfall state-
wide, nudged temperatures down (both Max and Min) since the upper
level airmass remains quite cold as well.

Some warming and mild weather returns Wednesday through Thursday
with southwest flow in place, but just ahead of the next Pacific
Northwest trough that extended models are latching on to, forecast
to dig south through the Great Basin by Friday. Forecast models
continue to show glaring differences in details, however I must say
I`m impressed with the overall consistency, especially model run to
run.

&&

.AVIATION...
VFR conditions are expected to prevail at the KSLC terminal
through the TAF period. Southeast winds will shift to the
northwest between 19z and 21z.

&&

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

$$

SHORT TERM...10
LONG TERM/AVIATION...Dewey

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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