Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 262147
Area Forecast Discussion
National Weather Service Salt Lake City UT
347 PM MDT Sun Mar 26 2017
.SYNOPSIS...High pressure will shift east tonight ahead of the
next Pacific storm system for Monday and Tuesday.
.SHORT TERM (THROUGH 00Z THU)...A short wave ridge is shifting
east of the area this afternoon downstream from the next Pac trof
moving into the Great Basin. High clouds along and west of the
ridge axis have been increasing across the CWA today and will
overspread the entire area this evening.
An initial portion of the trof will bring warm advection lift to
the nrn CWA tonight with a threat of spotty light precip
developing towards sunrise. The precip fills in across the wrn
2/3 CWA thru Mon morning as the main portion of the trof arrives
and a cold front enters the state. It persists thru the aftn and
spreads into the east with the front.
The airmass gets cold enough to lower snow levels to around 6000
ft by late Mon altho they could get locally lower in heavier
precip. Once again, it appears the impact of snow on travel during
the daytime over mtn passes will be limited to the highest
routes over 9000 ft by the fact that the airmass is not all that
cold and a high enough sun angle allowing solar heating to filter
thru the clouds keeping road surfaces warmer than the air. This
is expected to change after sunset as the airmass continues to
cool overnight and snow levels lower. Expect more widespread
impacts over the major passes with some impacts possible as low as
the passes along I-15.
Have opted not to issue any winter weather highlights at this time
due to the expected delayed impacts, but we may need one for the
Monday night period, depending on just how long the snow lasts.
Precip should taper off Tue aftn thru the eve as the system moves
off to the east and short wave ridging builds in from the west.
Expect drier and warmer weather for Wed under the short wave
ridge with high clouds increasing in the afternoon.
.LONG TERM (After 00Z Thursday)...A ridge will be over the CWA to
start the extended period but it will be short lived as a storm
system moves in from the Pacific Northwest during Thursday.
Unfortunately, the EC and GFS are at odds in how quickly this system
moves into Utah. The GFS is now faster and colder than the EC and
and tracks into northwest Utah while the EC takes aim farther south.
Since the GFS has been somewhat more consistent have leaned in that
direction but because EC is supported in some regards by the
Canadian didn`t stray to far from a blend.
The GFS has the 700mb baroclinic zone blasting through the CWA to a
Duchesne-Price-Escalante line by 00Z Friday while the EC has the 700
mb baroclinic zone from NE NV southward ELY then southwest to
Tonopah. obviously this makes a big difference in timing of
precipitation and type of precipitation as the GFS has the 700 mb
temp down to -10 degrees C across the northern Wasatch while the EC
has -1 degree C by 00Z Friday. Due to the track of the EC taking the
brunt of the storm farther south, the coldest the 700mb temp gets at
SLC through Friday is -3 degrees C. This would mean that all
precipitation in the valleys and below 7000 ft would be in the form
of rain. Meanwhile the -10 degrees C by the GFS would easily cause
precipitation to be in the form of snow down to the valley floor.
Have gone with a hybrid of the two and with a rain and snow mixture
for Thursday night from northern Utah down to the southwest valleys,
excluding Dixie. The EC does support cold enough temperatures at
700mb to produce snow over southwest valleys so confidence higher
there that snow will be encountered in the valleys.
By late Friday, the EC and GFS have similar solutions with the 500
mb low close to the four corners region which aids in higher
confidence. They both show a rather strong northerly flow at 700mb
of 40-50 kts across western valleys from Friday morning through
Friday night. In combination with a strong surface gradient winds
could approach and possibly exceed high wind criteria, especially in
the north south oriented canyons of Washington County.
The storm pulls away Saturday but some mountain showers may linger
over the south and eastern locations into the afternoon. The ridge
builds in Saturday night but the EC operational model has another
disturbance already moving in across northern Utah Sunday afternoon.
However, sided with the GFS as the EC ensemble does not support this
solution of the EC operational run. Wanted to go fully dry for
Sunday but kept some sub climo PoPs in for now until further model
runs show consistency in keeping the ridge in place.
.AVIATION...The SLC terminal will see VFR conditions throughout the
evening push. Winds are light and variable at this time and will
likely remain this way until a more persistent southeast flow
develops after about 02-03z this evening. Cigs will lower after
midnight with a chance of showers in the vicinity as early as 10Z,
but will likely hold off until 13-14Z.
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