FXUS65 KSLC 182116
Area Forecast Discussion
National Weather Service Salt Lake City UT
316 PM MDT Mon Mar 18 2019
High pressure aloft will shift north into western Canada and the
northern Rockies early this week. The next storm system will
arrive midweek, with impacts primarily across the southern and
central portions of the forecast area. Another storm system will
follow by late this weekend.
.SHORT TERM (Through 12z Friday)...
GOES West Water Vapor imagery shows high pressure continues to
build across the Pacific Northwest and British Columbia, while an
upper level shear zone axis is in place on its southern periphery
across southern Idaho and northern Utah. A shearing trough passage
to our west this morning has brought some high thin cirrus across
Utah. Expect that boundary to continue translate southeastward
this evening, allowing skies to become mainly clear tonight. A
second shortwave will dive south out of Saskatchewan and pass
mainly to our east on Tuesday, while a trailing piece of the wave
shears out over northern Utah. A subtle airmass exchange with this
wave passage across northern Utah will result in temperatures
just a couple degrees cooler than today, but with a slightly
increased northerly breeze.
Our focus then turns to the upper-level low seen on water vapor
imagery near 35N/140W. This low will translate eastward undercutting
the high pressure to the north, to track across the southern Great
Basin and southern Utah from late Wednesday through Friday. Expect
increasing cloud cover across the state on Wednesday, with
precipitation arriving across southern Utah late Wednesday.
Increased easterly flow Wednesday across northern and central Utah
ahead of the low will increase downslope warming and drying, which
will help push temperatures into the upper 50s to low 60s, despite
the increasing cloud cover.
The slowing and negatively tilting trough will result in a period of
enhanced upper-level diffluence over southern Utah from late
Wednesday through Thursday that will coincide with increased
southeasterly 700 mb flow and moisture advection into the southern
and central Utah terrain. This system will focus precipitation on
the southeast aspects from Bryce to Escalante to San Rafael Swell
and up into Price. Accumulating snow is expected mainly above 6000
feet, with significant accumulations of 7 to 17 inches possible
above 7000 feet. Minor accumulations are possible across Castle
County as low as 5500 feet where trapped colder air and dynamical
cooling may help to lower snow levels. From a hydrology perspective,
the precipitation maximum with this event will more focused over
south central Utah (1-1.5 inches) instead of southwest Utah like the
last few events. While the lower elevation rain and snowmelt will
likely result in some streams and rivers running high, fast and
cold, the overall flooding threat appears low at this time due given
the cooler temperatures and lower snow levels. The only wrinkle
could be that precipitation will become more convective Thursday
afternoon as the trough axis cross southern Utah, increase lapse
rates. Thus locally heavier rain is possible and could cause more
localized flood issues later Thursday, but this would be more a
short fuse event and lower confidence from this lead time. Showers
will continue into Thursday night but will start to diminish as the
trough axis shifts to our east.
Temperatures will trend cooler from Wednesday into Thursday as the
cold pool associated with the low encompasses the state. However,
this will mean near normal values for northern Utah and only
slightly below normal values for southern Utah. Gusty canyon winds
are likely to develop Wednesday and Thursday mornings north of Salt
Lake as the low tracks by to our south and increases the west-east
pressure gradient across the northern Wasatch. Expect these winds to
remain sub advisory at this time.
.LONG TERM (After 12z Friday)...
The filling upper level low will begin to lift out of the area to
start the long term forecast period. Expect precipitation to be
ongoing, through predominately valley rain and higher elevation
A quick period of shortwave ridging will cross the area
Saturday...followed by the next trough later Saturday into Sunday.
Ensemble guidance is in rough agreement on this system...with the
best forcing across the northern half of the state. This system is a
bit colder than the previous and snow levels should be
lower...though at this time appear to remain above valley floors.
Kept high pops across the area as previous packages have been
carrying. Impact-wise, this seems to be a fairly average system for
march with the potential for accumulating snow in the mountains.
A longer period of ridging and resulting warmer/drier conditions can
be expected early to the middle of next week. This is supported by
nearly all the ensemble systems.
Operational weather concerns will remain minimal at the KSLC
terminal through the valid TAF period. Light northwesterly
surface winds currently in place are expected to phase to a
southeasterly surface wind between 03-04z per norm. VFR
conditions will be maintained under largely clear skies.
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