Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 151148
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
427 AM MDT Sun Sep 15 2019

.SYNOPSIS...
Warm temperatures and increasingly strong southerly winds will
exist across the region ahead of an approaching storm system
through Monday. A strong cold front will sweep through the state
Monday night through Tuesday bringing below normal temperatures
through mid to late week.

&&

.SHORT TERM (Through 00Z Thursday)...
Satellite imagery early this morning shows a subtropical wave
drifting northward through eastern Arizona, with high cloud cover
spreading across southeast Utah. Meanwhile, the next storm system
is digging along the west coast offshore from British Columbia. As
this storm system continues to dig down the west coast today,
flow aloft will turn more southwesterly and continue to increase.
This will help direct moisture from the subtropical wave northward
along and east of the spine of the Utah mountains today and into
Monday. Moisture will remain fairly limited today, but just enough
will sneak into southern Utah for a few isolated showers or
thunderstorms mainly over the higher terrain. Otherwise, today
will remain sunny, very warm, and dry with across the rest of Utah
with an increasing southerly breeze. Temperatures today will top
out about 10-15F above average.

Impacts across the forecast area from the robust storm system
digging along the west coast will be felt in earnest from Monday
through early Tuesday. Ahead of the storm system, southwesterly
winds will ramp up markedly from a combination of strong pressure
falls to our north ahead of the storm, and a belt of strong 700 mb
pre-frontal winds, which, with steep surface-700mb lapse rates,
will be able to be tapped into for gusty conditions Monday
afternoon. On the whole, pre-frontal wind gusts of 40-50 mph are
expected, mainly across the western half of Utah, for which wind
advisories may be needed. Even stronger post- frontal winds are
possible across northwest Utah by Monday night into early Tuesday
morning, for which a High Wind Watch has been issued. Forecast
models continue to come into better agreement on the timing and
magnitude of strong pressure rises and cold advection behind the
front, with the potential vorticity max tracking across northwest
Utah. This ideal overlap of parameters should bode well for
producing strong to severe post-frontal wind gusts of 60-70 mph in
the favored locations across the Great Salt Lake Dessert, and
have travel impacts for high-profile vehicles along I-80 and
SR-30.

Temperatures tonight will remain quite mild given the increasing
southerly flow that will keep most locations from decoupling
overnight. The Salt Lake Valley in particular could see
temperatures run about 20F above average overnight, staying in the
low to mid 70s. On Monday it will remain warm, about 5-10F above
average, but will be limited some by increasing cloud cover across
the region. Through the day Monday, convection will be limited to
locations east of I-15 where the best moisture from the leading
subtropical wave will track. However, by Monday evening and night,
scattered showers and thunderstorms are expected to develop in the
strong synoptic forcing ahead of the cold front and PV max. Any
showers or thunderstorms that develop may be capable of damaging
wind gusts given the very fast flow and dry sub-cloud layer. SPC
maintains a Marginal Risk for this threat in the Day-2 outlook
for Monday and Monday night.

By Tuesday morning, colder air will be rapidly infiltrating across
northern Utah as the cold front continues to push southward into
central Utah. A rebound of showers is possible across northern
Utah Tuesday morning as the cold pool aloft transits the area.
Furthermore, on the Great Salt Lake, surface temperature to 700mb
differences will be near 20C, with equilibrium heights around
20kft, with moisture still in place under the upper-low. Lake
enhanced showers or thunderstorms are possible Tuesday morning
from the lake extending into the Salt Lake Valley with the
northwesterly flow behind the front. The above mentioned
parameters also are supportive of waterspout development per the
Szilagyi waterspout index, with even weak showers capable of
producing waterspouts.

Clearing and drying will develop by Tuesday afternoon in the wake
of the trough passage with ridging starting to build in from the
west. Temperatures will run about 15 degrees below average across
northern Utah, and about 5 degrees below average across southern
Utah.

By Wednesday flow aloft will already start to back around to the
southwest ahead of the next approaching storm system. This will
bring a warming trend with temperatures returning very near
seasonal average, and continued dry weather.

.LONG TERM (After 00Z Thursday)...
The next trough to impact the area is expected to be moving
onshore along the Pacific Northwest coast to begin the long term
period. Guidance has come into better agreement with regard to the
timing of this system, bringing it through Utah and southwest
Wyoming Thursday afternoon into Friday.

As the trough and associated front move through, this will bring a
period of rain with snow at the highest elevations to primarily
northern portions of the forecast area. Have upped POPs a bit with
increasing confidence. Maxes behind the system could be 15F or more
below climo on Friday for northern Utah and more like 5-10F below
climo for southern Utah.

EC/GFS now indicating a trailing wave splitting off from the main
system will track across northern Utah during the day Saturday,
bringing some light additional precip before a drier and warmer
zonal flow returns for day seven.

&&

.AVIATION...
The SLC terminal will see VFR conditions throughout the day with
mostly clear skies. Southerly winds will increase during the
morning, becoming gusty by midday.

&&

.FIRE WEATHER...
Warming and drying continues today with a slight increase in
southwesterly winds ahead of the next approaching storm system.
This will result in some locally critical fire weather conditions,
although fuels remain fairly moist since the last widespread
precipitation event. A few isolated thunderstorms are possible
over southern Utah this afternoon, mainly over the higher terrain
east of I-15, with coverage increasing across all of eastern Utah
on Monday. Southwest winds will become quite strong ahead of an
approaching cold front on Monday across much of Utah. A cold front
passage Monday night, particularly across northwest Utah, will
result in a sharp wind shift with very gusty post-frontal winds
into early Tuesday morning. Temperatures drop and humidity rises
considerably behind the cold front passage for Tuesday. Scattered
showers and thunderstorms are possible across northern Utah from
late Monday through early Tuesday as the storm system moves
through. Temperatures will moderate into mid-week, but remain
below average.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Fire Weather Watch from Monday morning through late Monday night
     for UTZ494-497-498.

     Fire Weather Watch from Monday morning through Monday evening
     for UTZ482-489.

     Fire Weather Watch from this evening through late Monday night
     for UTZ478-492-495.

     High Wind Watch from Monday evening through late Monday night
     for UTZ005.

WY...Red Flag Warning through Sunday evening for WYZ277.

&&

$$

SHORT TERM...Church
LONG TERM...Traphagan
AVIATION...Traphagan
FIRE WEATHER...Church

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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