Scientific Forecaster Discussion
fxus65 kslc 241627
Area forecast discussion
National Weather Service Salt Lake City Utah
1027 am MDT Tue Apr 24 2018
Synopsis...a weak upper level trough across Wyoming will continue
to move southeast across northern and eastern Utah today. High
pressure aloft will settle across the interior west midweek and
produce very warm temperatures for the latter half of the week.
Discussion...morning water vapor and 500 mb analysis continue to
depict downstream progression of an elongated short wave trough
currently tracking through Wyoming. Upstream mid level ridging
continues to dominate the western Great Basin, while a cut off low
remains nearly stationary off the Pacific coast. Passage of the
aforementioned wave to the north has enhanced a mean north-S oriented
mslp gradient across the area, while a building surface high over
central Wyoming has aided development of a NE oriented gradient
across the central/northern Wasatch front east into western WY,
with modest cold air advection up through roughly 700 mb banking up across the
Continue to note elevated canyon wind gusts in the prone areas
in the north such as Farmington and parleys canyons, with the
former peaking at 45 mph last hour. Expecting the shallow cold air advection and
attendant NE gradient to relax through mid day (with further
downstream translation of the short wave) allowing these gusts to
subside through that time. Should see a diurnal redevelopment
tonight however as a secondary progressive short wave tracks
through the northern rockies southeast into the plains states through mid
morning tomorrow. Nothing excessive, just notable easterly gusts
at the canyon mouths.
Have made no changes to going forecast this morning. Temps near
seasonal norms, tranquil weather, and the aforementioned winds
were well captured for today. Previous discussion below.
Previous discussion...a high amplitude ridge will build over the
far western Continental U.S. Tomorrow, with Utah expected to remain in the
downstream northerly flow as another trough diving southeast along
The Rockies will keep the ridge axis west of the forecast area.
This trough will just graze southwest Wyoming but is not expected
to bring any precipitation to the area. Although the airmass will
trend warmer tomorrow, the northerly flow and proximity to this
trough will limit the extent of the warming trend through
Thursday. Still, temperatures will run above normal both days. The
ridge will finally shift overhead late Thursday.
Warm high pressure is expected to remain in place through Friday,
resulting in maxes 15-20f above seasonal normals with dry
conditions and light winds. The ridge is progged to shift east
beginning Saturday as a broad Pacific low moves onshore along the
California coast, but the global models continue to disagree with
regard to what will happen to the low after that.
The latest GFS indicates the low splitting as it moves over
California, with the front piece sliding across the northwestern two-
thirds of Utah during the day Saturday, bringing a cold front
through. As this part of the trough pivots northeastward on Sunday,
the front retreats back into far northwest Utah. The front
eventually moves back through Monday into Tuesday as the second
piece of the trough phases with a second Pacific system that moves
across Arizona for day seven and beyond.
In the ec, the Pacific low moves slowly across Utah and southwest
Wyoming Saturday night into early Tuesday, waiting to split until it
moves into Utah and not doing so nearly as sharply as the GFS
indicates. Another Pacific system is then expected to dive into
California for day seven before phasing with another system and
moving into Arizona after day seven.
These differences in guidance impact the temperature forecast
noticeably over the weekend, with the ec being warmer for Saturday
and the GFS being warmer on Sunday due to their differences with the
front. With lack of agreement and run-to-run consistency, have gone
with a blend for temperatures over the weekend. However, the system
is not particularly cold regardless so temperatures should stay at
or above seasonal normals. The various solutions with regard to the
front also impact the expected precipitation over the weekend, so
have broadbrushed things, keeping pops in the slight chance to
chance range. Despite the disagreement continuing for Monday and
Tuesday, both versions of things would result in unsettled weather
for at least northern and central Utah, so have maintained pops
through day seven.
Aviation...northwest winds will continue at the slc terminal
through the day. Expect a shift to the south between 03-05z.
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