Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
804 PM MST Thursday Dec 25 2014
Synopsis...the upper level low pressure system over western Utah
will settle into The Four Corners region early Friday...then exit
the region Friday night. A new round of unsettled weather will
develop late in the weekend and remain through the first half of
the new week.
Discussion...water vapor loop shows a closed low over south
central Utah. Amdar 400-250mb wind observations indicate a
100-135kt cyclonic jet from along the West Coast to the northern
Mexican border and into the Midwest. GOES/GPS/rap/00z radiosonde observation
indicate the precipitable water value ranges between 0.1"
mountains...to 0.25" most valleys.
Storm evolution on track! Two focus areas overnight. First up
north...cancelled the Winter Storm Warning for the Cache Valley as
much of the accumulating snow winds down. Untreated roads of
course may continue to be hazardous.
For the northern Wasatch front...expect much of the snow has ended
as well. However...hrrr-3km and local high resolution WRF models
continue to indicate gusty easterly downslope winds developing after
midnight...due to the upper low re-positioning across south
central Utah. Recently snowbasin top of Strawberry has shifted
from west-southwest to NE...which is usually the first sign of a developing
downslope event for Davis and Weber counties. Still...winds are
only gusting 22mph up there as of now. These winds...gusting 35-50
miles per hour...would cause a significant blowing snow issue with Bountiful
bench nearing a foot of snow. Low visibilities and deteriorating
Road surface conditions can be expected due to the blowing snow.
Have issued a blowing Snow Advisory and added blowing snow to the
weather grid overnight.
No change to the rest of the headlines further south. Not expecting
much more for the Salt Lake Valley...but showers have lined up into
the Tooele Valley to justify keeping the warning going for zone
3...and Tooele County for a continued advisory for zone 5.
Focus further south is expected to be along Interstate 15 from
Fillmore to Cedar Cedar given north-northwest upslope flow on the
back side of the storm system. Boosted probability of precipitation and snowfall especially
for these areas.
Previous discussion follows...
The upper low sliding south through western Utah this afternoon will
eventually turn east and settle into The Four Corners region by
early Friday. Widespread light to locally moderate snow persists in
the wrap around warm advection in the north and west flanks of the
low late this afternoon. Accumulating snows are expected to remain
fairly close to the low center overnight. Looking at heavier snows
to return to central and southern Utah later this evening as the low
center moves into extreme southern Utah. Northern Utah will see snow
decrease in areal coverage and intensity during the evening as a
backdoor cold front moves west out of southwest Wyoming and brings
more stable air and downslope conditions to northern Utah.
Have left all of the warnings/advisories in place with the afternoon
forecast package. Expect that the various highlights across northern
Utah will be dropped overnight as the snow and winds gradually
dissipate. Southern and central Utah will likely see accumulating
snows continue along the I-15 corridor and over the higher terrain
in close proximity to the upper low.
The anti-cyclonic northerly flow trailing the exiting fore corners
low Friday night will bring an end to any lingering snow by early
Friday evening. Very cold temperatures are expected as cloud cover
diminishes and winds decrease. Some fog could develop by Saturday
morning...though at this time the areal coverage looks to be limited.
The shortwave ridge moving southeast across the Great Basin Saturday
will maintain dry conditions with temperatures increasing a bit compared to
Friday. Any increase in temperatures looks to be minor due to extensive
snow cover and generally stable conditions.
Global models continue to remain in quite good agreement regarding
the big picture evolution of the next and very cold trough late
weekend into early next week. Rapid amplification of the West Coast
ridge will allow a short wave to carve south through the Great Basin
Saturday night setting up a broad long wave trough across much of
the western Continental U.S. Through that time. The airmass associated remains
Continental...quite dry and very cold with 700 mb temperatures dropping into
the -15 to -17 c range Monday through Tuesday.
The nose of a 100kt cyclonic jet will push into northern Utah Sunday
night...coincident with modest near 700 mb baroclinicity pushing in from
the north...this associated with the strongest period of cold
advection late Saturday night into Sunday. Continue to hold onto
Likely Mountain probability of precipitation and high chance valley probability of precipitation across the northern
half of the forecast area as said forcing should produce a broad
area of light snow and or snow showers. High snow ratios and very
cold temperatures in place could allow even very light snowfall to stick to
roadways...but those impacts will need to be looked at closer as the
12z guidance in better agreement with the upper trough evolution
Aviation...at the slc terminal...MVFR conditions in light snow are
expected to prevail during the early evening. Conditions will become
predominately VFR after between 04z and 06z as snow showers decrease
in coverage. There is a 30 percent chance of MVFR/IFR conditions in
snow for periods after 06z. Northwest winds are expected through the
Utah...Winter Storm Warning until noon MST Friday for utz003-004-
Winter Weather Advisory until noon MST Friday for utz002-005.
Wyoming...Winter Storm Warning until noon MST Friday for wyz021.
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