Scientific Forecaster Discussion
fxus65 kslc 191109
Area forecast discussion
National Weather Service Salt Lake City Utah
509 am MDT Thu Sep 19 2019
Synopsis...a storm system will move through the area late today
through Friday. The associated cold front will stall over northwest
Utah this morning, then work east across the area through tonight.
The cooler temperatures and precipitation with this storm will be
followed by drier and warmer conditions this weekend.
Short term (through 00z monday)...latest water vapor imagery
this morning shows the low center of the upstream storm system
centered over the far northern Sierra Nevada, with southwesterly
flow aloft remaining in place over Utah. Meanwhile, the surface
boundary has been analyzed along a line from west-central Utah
through the southern end of the Salt Lake Valley. With the trough
making slow forward progress this morning and carving farther south
while the flow aloft backs to more southerly, the surface boundary
may retrograde slightly this morning.
Strong and gusty southerly winds will develop across much of
southern, central, and eastern Utah ahead of the cold front by late
morning or early afternoon. May see a few sites reach Wind Advisory
criteria, but this is expected to remain mostly isolated, so decided
against issuing any wind highlights.
The front will strengthen this afternoon before starting to move
again as the upper jet noses into northwest Utah and the trough
begins to swing through the state. Precipitation will also begin to
fill in across northern Utah late this afternoon and into tomorrow
given this jet energy and as deeper cold advection develops across
the area. A much colder airmass will spread into the area behind the
front, and snow levels will likely fall to at least 8000ft with
minor accumulations on the high peaks. Afternoon Max temperatures
will be 10-20 degrees below normal tomorrow across northern Utah,
and 5-10 degrees below normal across the south.
The main trough will exit the area tomorrow evening, but a secondary
vort lobe is expected to rotate into Utah behind it late tomorrow
night into Saturday. However, there is little moisture for this weak
system to work with, so the only real impact with this wave is that
it will keep cool temperatures in place into Saturday. A progressive
shortwave ridge will build into the area Saturday night, followed by
a warmer southwest flow again on Sunday as the next storm system
arrives over the Pacific northwest.
Long term (after 00z monday)...the operational ec and GFS are in
step up with each other through 12z Monday with the trough moving
into the western Great Basin but then a significant divergence
occurs between them as the GFS digs the trough to the Nevada-Utah
border by 00z Tuesday while the ec tracks the trough south
southwestward towards the southern sierras. As a result the timing
of and extent of precip is much different between models. Have
leaned towards the ec as the ensemble mean of the GFS and ec are
similar to the ec operational while the GFS is an outlier. However,
the confidence is not exactly high as the ec ensemble mean has been
somewhat variable past few runs with a slight hint of a more
northerly track of the low then the ec say from 2 forecast cycles
Although that said, have decreased the threat of the precipitation
across the north and increased it some across the south for Monday
and then lowered pops across the board for Tuesday and Wednesday but
did not completely remove it. A drier northwest flow should prevail
Wednesday with the closed low at its farthest southern position but
as it moves back northeast across Arizona it could bring showers
back in across The Spine of the mountains from south central Utah to
Aviation...shallow northerly flow across the slc terminal this
morning will likely get erased by southerly winds aloft mixing down
to the surface about 13-14z. There is a 20% chance that southerly
winds aloft do not surface. The winds will then shift back to the
west northwest between 19 and 20z. Winds could gust over 25 mph
between 23 and 01z. Cigs will lower after about 04-06z with showers
developing after midnight.
Fire weather...a cold front will stall over northwest Utah this
morning as the associated storm system remains to centered over
Nevada. Strong and gusty south-southwest winds can be expected
across much of central, southern and eastern Utah today ahead of
this front. These winds will combine with low humidities to create
hazardous fire weather conditions where fuels are critically dry.
The storm and associated cold front will then make more forward
progress through Utah this afternoon through tonight. Precipitation,
with snow above around 8000 feet, will gradually fill in across
mainly northern and central Utah this evening and into tomorrow.
Much cooler temperatures will follow the front.
Drier conditions will develop for the weekend, with southwest flow
increasing a bit on Sunday ahead of the next storm system.
Utah...red flag warning from noon today to 10 PM MDT this evening for
Red flag warning until 10 PM MDT this evening for utz495.
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