Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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732
FXUS65 KSLC 220409
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
1009 PM MDT Wed Sep 21 2016

.SYNOPSIS...A strong cold and moist storm system will impact most
of the region through the end of the week.

&&

.DISCUSSION...Evening water vapor imagery and objective analysis
show an upper trough digging along the Pacific Coast inducing a
southwesterly flow aloft downstream across the forecast area.
A surface front along the leading edge of this currently extends
across northern Utah, and served as a focus for strong
thunderstorms this evening as a subtle wave lifted north along
this boundary. Convection along this boundary has largely shifted
north as the aforementioned wave continues to lift northeast,
however there is enough of a push from the remnant cold pool to
shove this boundary through SLC over the next hour. However this
boundary has largely outrun its upper support, and this coupled
with a wedge of drier air aloft will likely keep any precip from
SLC/Provo southwest into west central UT in the chance/scattered
category through the overnight hours, while the better organized
precipitation remains west near the NV border.

Meanwhile moisture in place across southern Utah which resulted in
fairly widespread rainfall through the day is beginning to creep
northward, and will continue to advect into central and
eventually northeast Utah overnight into the morning hours
Thursday. This may leave much of the Wasatch Front wedged between
this moisture axis, and the better forcing to the west for much of
the day Thursday, until the upper low finally rotates inland and
into the Great Basin later in the day.

Updated to reduce overnight POPs along the central/southern
Wasatch Front back into west central Utah. Previous discussion
follows...

.SHORT TERM (Through 00z Saturday)...Weather will become
increasingly active across the area over the next 48 plus hours.
Set up moving forward combines anomalously high low/mid level
moisture from the subtropics that is poised to merge with the
advancement and passage of a strong early winter type storm system
overhead. Impacts from Flash Flooding across the south are
possible, widespread heavy rain across the north is expected, as
are much colder temps and accumulating snow above 7000ft during
this period.

Afternoon water vapor and H5 analysis indicate continued east-
southeast advancement of this positively tilted trough stretching
from southern Britcol to NorCal. Downstream a strengthening
southwesterly flow aloft continues to tap this low/mid level
moisture remnant from what was Tropical storm Paine, with said
moisture advecting north from the desert southwest into southern
Utah. Widespread light to moderate rain has been experienced
across the south already today, as this moisture has interacted
with shearing vorticity stretching across the south remnant from
an old closed Low that was over Baja. Convection has, for the time
being, remained largely non existent due to a lack of instability,
but that will change over the next 12 hours as well.

The aforementioned trough will continue to translate east-
southeast tonight with the center of the cold core low poised to
move into Nevada. As that occurs increasing diffluence aloft will
begin to drive large scale forcing across the southwest by
daybreak, and all areas by the afternoon. Slight cooling at the
midlevels as this occurs will aid steeper lapse rates, and
formation of modest instability across the southwest by morning
and all areas by the afternoon. As such, areal coverage and
significance of precip is expected to increase and become more
convective, rapidly expanding south to north during the day. Due
to the anomalous nature of the moisture coupled with the strength
of the upper level trough, opted to issue a Flash Flood Watch for
much of southern/eastern Utah through early Friday morning when
convective potential wanes, as locally heavy rain aided by a deep
warm cloud layer is expected.

Strong deep layer shear coupled with this instability and moisture
(and some mid level drying ahead of the approaching cold front)
will allow for the potential of convective organization on
Thursday. Severe thunderstorms are possible, with hail and
damaging winds possible impacts from any organized cores. SPC
currently has the forecast area under a marginal risk for this
potential.

The attendant cold front of the upper trough will largely stall to
our west through tomorrow, then push east across the area
tomorrow evening into the overnight hours bringing much colder
temperatures by Friday morning. Steadier precip will begin to
focus across the northern half of the area as the cold core
settles in, with a trowal forming overhead. Combining the duration
of precip potential and presence of the deep layer moisture,
anticipating in excess of an inch of rainfall along the Wasatch
Front, with mountain areas potentially seeing 2 to 3 inches of
liquid by Friday night. H7 temps forecast to bottom out in the -3
to -4 C range will allow for snow levels to lower to around 7000ft
Friday, with accumulating snow expected elevations above, and
focused across the northern mountains. Anticipating need for a
Winter Weather Advisory for these higher mountain areas Thursday
night/Friday, with impacts largely relegated over 8000ft.

&&

.LONG TERM (after 00Z Saturday)...The cold upper low is forecast
to be lifting into nrn WY at the start of the long term period
with wrap around precip in nw flow remaining in place across nrn
UT orographically enhanced into the Wasatch mtns.

This should taper off overnight with a few lingering showers over
the nrn mtns on Sat morning pretty much ending in the afternoon.

The upstream ridge remains centered off the coast thru Sun with a
cool dry northerly flow over the CWA. Clearing skies Sat night
should allow temps to dip to near or below freezing in the
normally colder valleys.

Dry conditions persist Sun thru Wed as the upstream ridge slowly
progresses into the Great Basin and brings a warming trend to the
CWA. Expect temps to return to near seasonal normals by Tue and
warm a little more Wed.

&&

.AVIATION...An outflow boundary is expected to reach the SLC
terminal at about 0415Z with winds shifting from south to northwest
at 20 with gusts to 35 mph possible for about an hour or less. There
is a 30 percent of lightning with and behind this boundary. Winds
are expected to shift back to southerly by 06z but may take until 07
or 08Z. After 06z cigs should remain above 7k ft AGL through 00Z.

&&

.FIRE WEATHER...Deep moisture moving north from a remnant tropical
system will combine with a strong early winter type storm system
late tonight through Friday before gradually tapering over
northern portions of the district Saturday. Currently this
moisture is producing widespread wetting rains across the
southwest and surface RH has reached levels not seen for some
time. As the storm system carves into eastern Nevada tonight this
moisture and associated rainfall/thunderstorm potential is
expected to spread north encompassing much of the area tomorrow. A
strong cold front will work west to east across the district
Thursday evening into the overnight hours maintaining widespread
precip, and likely making it more steady across the north through
Friday before tapering Saturday. Snow levels by Friday morning
will lower to near 7000ft, with accumulating snow expected to
focus over the higher mountain areas in northern Utah.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flash Flood Watch through late Thursday night for UTZ012-013-
     019>021-518.

WY...None.
&&

$$

Seaman/Struthwolf/Merrill/Wilensky

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visit...
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