Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 200314
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
814 PM MST Sun Feb 19 2017

.SYNOPSIS...An elongated upper level trough will continue to
exit the eastern Great Basin tonight. High pressure aloft will
develop over Utah Monday, with a mild southwest flow aloft Tuesday.
A potentially strong winter storm could impact the region beginning
midweek.

&&

.DISCUSSION...
Water Vapor Satellite shows the trough axis has moved through Utah.
400-200mb MDCARS wind observations place a 130-165kt anticyclonic
from Oregon into Arizona. GOES/SLC 00Z RAOB/HRRR indicate that
precipitable water values vary from 0.10"-0.20" mountains to
0.35"-0.55" valleys.

00Z KSLC RAOB indicated significant instability early this
evening. This was much more than advertised by the SREF. The
passage of the trough axis and drier air moving in aloft supported
the development of isolated thunderstorms, with graupel and small
hail, but no measured wind gusts greater than 40 mph. This
threat will wane this evening with loss of daytime heating, so
didnt mention in this update.

Currently seeing peak of the meager cold advection, which will
turn to warm advection overnight. This may support some rain and
high elevation snow especially north of Interstate 80. Going
forecast had this well depicted so only minor changes.

Tweaked cloud coverage based on recent satellite trends and latest
model data. Also refreshed the temperature/RH curve.

&&

.PREVIOUS DISCUSSION...
An elongated upper trough with an axis extending from the Pacific
Northwest states through the Four Corners region and into western
New Mexico continues to slowly shift east this evening. Utah is
already mostly behind the trough axis, but is still seeing a few
instability showers. These showers should diminish after sunset
with partial clearing of skies through this evening. Overnight
mins should generally run a few degrees cooler tonight as a
result.

A ridge will build into the area later tonight and clouds will
quickly increase again as 700mb warm advection develops.
Precipitation is expected to fill in again across the north, with
light precipitation expected with snow levels near 5500-6000ft. A
moist southwesterly flow will continue into tomorrow allowing
precipitation to continue at times through tomorrow night. The
northern Wasatch mountains and adjacent valleys will be favored in
this flow regime, but rising snow levels will keep mountain
accumulations limited. However, a tightening MSLP gradient and 700mb
winds 45-55kt developing ahead of a deepening upstream system will
bring breezy to windy conditions to the western valleys and a
potential for strong gusts along exposed ridgelines late tomorrow
into Tuesday. A fairly well-mixed airmass on Tuesday is expected to
bring a return of much warmer temperatures.

The storm will eject through the West Coast and end up grazing
northern Utah Tuesday afternoon. However, this will push a cold
front into northern Utah late Tuesday afternoon through Tuesday
night with enough cold air to potentially bring snow to the valley
floors. With the responsible trough exiting the region Wednesday
morning and another trough deepening over the West Coast,
convergence into the baroclinic zone stalled over central Utah is
expected to strengthen. This will focus precipitation from around
west-central through northeast Utah before the front finally pushes
through the state Thursday. A cold, moist and somewhat unstable
northwesterly flow will then follow for Thursday.

This pattern will bring a return of more winter-like conditions to
the area with a potential for accumulating snow to the valley
floors. Some uncertainty still exists with regard to the
positioning of the baroclinic zone and temperatures relative to the
boundary, which would affect timing and location of heaviest snow.
For now, have kept snow amounts relatively light in the valleys but
some mountains, particularly the southern Wasatch and Uintas could
end up seeing significant snow amounts.

Once the trough slides to the east Thursday evening, weak
shortwave ridging is possible heading into Friday. Any ridging
will be very low amplitude, so warming will not be significant;
right now, models indicate 700mb temps near the UT/ID border
increasing from -16C to -13C. Enough moisture lingers that some
light precip may be possible in this weak warm advection,
particularly across far northern Utah.

The next shortwave trough looks to slide across the Great Basin late
Friday through Saturday. Details with this trough begin to differ
significantly in the global models, but there is currently agreement
that the best forcing remains south and west of the forecast area,
eventually sliding through Arizona on Sunday. For now, have
maintained some low level POPs through next weekend, especially in
the southern half of the CWA.

With weak synoptic-scale forcing through most of the extended, the
biggest takeaway for the end of the week through the weekend is
probably that the airmass does not warm up significantly. This means
that temperatures should remain below seasonal normals through this
time, which may be a minor shock after the warm February we`ve been
having so far.

&&

.AVIATION...
The threat for showers and isolated thunderstorms near the SLC
terminal has largely ended for now. Erratic winds will be
possible through about 04Z and then should become southerly for
the remainder of the night. Expect a period of lower ceilings and
a few showers between about 09Z and 15Z as the next wave moves
through, with occasional reductions below 7000 feet AGL.

&&

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

$$

PUBLIC...Rogowski/Cheng/Schoening
AVIATION...Kruse

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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