Scientific Forecaster Discussion
fxus65 kslc 100241
Area forecast discussion
National Weather Service Salt Lake City Utah
741 PM MST Fri Dec 9 2016
Synopsis...a persistent and quite moist westerly flow will allow
for another round of widespread precipitation across the north
Saturday through Saturday night. Southern Utah will remain south
of the storm track until Saturday night. A relative break from
active weather will occur early next week.
water vapor satellite shows a closed low nearing British Columbia
with zonal flow into the interior west from the eastern Pacific.
400-200mb mdcars wind observations place a 110-140kt westerly jet
over the Pacific northwest and northern rockies. GOES/slc 00z
radiosonde observation/hrrr indicate that precipitable water values vary from
0.15"-0.25" southern mountains to 0.45"-0.55" northern valleys.
Blended precipitable water product shows an atmospheric river from
near Hawaii nosing into the Great Basin.
Based on radar trends and latest guidance, removed most pops south
of the northern Wasatch mountains through the rest of the
Rap indicates an embedded wave approaches from northern Nevada
overnight. As it does so, precipitation chances increase across
the north after midnight. Local/NCEP WRF models bring
precipitation as far south as slc, while the hrrr is much drier
south of the Idaho border. Only made minor tweaks to pops as felt
like they were consistent with the latest model runs.
Valleys north to Ogden should remain rain, while snow levels may
allow for a mix of rain and snow Wasatch back and Cache Valley.
Limited to no impacts in the valleys overnight. No changes to the
winter headlines for the northern mountains.
Updated the T/TD/rh curve based on observations and lamp data.
Also bumped up winds in the higher terrain to be more consistent with
observations and model trends.
afternoon water vapor and 500 mb analysis depict a near zonal flow
across the Continental U.S. Lying south of an impressive long wave trough
over Canada, anchored by a deepening low dropping south from the
Arctic. Deep moisture continues to be tapped from the subtropics
over the eastern Pacific with a significant plume penetrating into
the western Continental U.S. Near the norcal coast and points interior. A
strong and near zonal jet remains aloft of this plume but has
shifted north over the northern rockies states today.
Our forecast area is now well south of the focused jet energy, and
although deep moisture remains plentiful, lack of forcing has and
will continue to limit precip potential tonight across the north.
This said, the Utah/Idaho border will continue to be most favorable for
continued precip due to closer proximity to the aforementioned
jet and associated short wave energy embedded in it. Have
continued to reduce pops across northern Utah and quantitative precipitation forecast/snow over
the next 12-18 hours this forecast, but maintained high pops along
the border and northern mtns for a high pop low qpf period into
A more favorable jet orientation and another round of weak warm air advection at
the mid/lower levels will aid to increase precip across the north
once again Saturday morning however, this driven by subtle pattern
amplification upstream as a progressive short wave tracks east-southeast out
of the pacnw. Good agreement in models regarding this. Snow levels
will remain high, near 7000ft across the north most areas, but
have retained a rain/snow mix in the trapped valleys such as the
Cache through the day.
As the aforementioned short wave gradually encroaches the area and
passes Saturday night, guidance remains in near lockstep of
tightening baroclinicity over northern Utah, and a shift to cold
advection with passage of a mid level cold front through the
middle of the night. Expecting areal coverage of precip to expand
south to roughly Cedar City at that time, with focused precip
within the tightest thermal gradient over northern/central Utah,
with lowering snow levels to valley levels early Sunday morning.
Do not foresee much snow at all in the northern valleys early
Sunday morning though the cold air advection will support it, as the deepest
moisture will be at that time shifting downstream ahead of the 700 mb
-8c layer. Did however extend the Winter Storm Warning for the
northern mtns into Sunday morning as significant snow is expected.
See west-southwest for details. Additional headlines may be needed for the
Wasatch mtn valleys such as Park City, but will let later shifts
decide on this. By mid morning Sunday, precip largely will come to
an end as drier air advects in from the northwest behind the front.
Weak warm air advection will then again set up across the far north Sunday night
into Monday bringing a chance for light snowfall, but not
expecting anything of significance. Have generally chance/slight
chance pops for generally the Logan area mtns.
Global models continue to be at odds with each other and diverge
more as we head into the extended. The ec has been the most
consistent from run-to-run and actually handled the most recent
series of storms better than the the GFS building my confidence in
leaning heavily towards the ec solution. When comparing the last 4
runs of the GFS there has been a lot of variation in the handling
of the trough off the Pacific northwest coast. Here is where the ec takes
the trough and stretches it westward allowing for more of ridge to
develop across the West Coast Tuesday and eastward to over the
Great Basin by Thursday. This will keep the main moisture plume
confined to the northern portions of the County Warning Area. Meanwhile, the GFS
brings this trough southeast across Oregon and strings it out as
its connection with the longwave trough over the central Continental U.S. Is
severed. The latest 18z GFS run is heading towards the ec solution
with a much flattened wave crossing the northern rockies.
Based on the aforementioned reasoning have lowered pops
significantly across southern Utah Tuesday through Thursday per the
ec guidance. By Friday, the ec brings the trough that it had
stretched westward on Tuesday and consolidates it over the eastern
Pacific and then brings it east southeast Ward across Oregon into
the Great Basin at the same time a new long wave trough develops
over western Canada. The combination of these features could bring
widespread precipitation to the entire County Warning Area Friday with colder temps
VFR conditions are expected at the slc terminal overnight. Winds
are currently shifting to southeasterly and will persist out of
the south through much of Saturday. Winds could become gusty after
about 14z. Cigs will also fall below bkn070 about 14z with a 30
percent chance of MVFR conditions in rain possible after about
Utah...Winter Storm Warning until 9 am MST Sunday for utz007>009.
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