Scientific Forecaster Discussion
fxus65 kslc 250403
Area forecast discussion
National Weather Service Salt Lake City Utah
1003 PM MDT Mon Apr 24 2017
Synopsis...a series of progressively colder and energetic storm
systems will cross the region through the first half of the
Short term (through 00z friday)...the shortwave that moved east
across northern Utah during the late afternoon/early evening hours
has moved off into Wyoming at mid-evening. Shower activity,
though somewhat reduced behind exiting shortwave, continues across
the north as a moist and fairly deep layer cold advection pattern
persists at this time.
Convective precip will increase in areal coverage and intensity
later tonight/Tuesday morning as vorticity Max currently over the
Pacific northwest digs southeast in response to upstream ridging
over the eastern Pacific. Continued deep layer moisture and cold
advection associated with this feature will bring increasing
synoptic-scale lift to much of northern/western Utah. Through at
least early Tuesday morning. Convective precip will continue to
move southeast with the vorticity Max, resulting in scattered
convection across the south by late morning or early afternoon.
Could see a few strong storms in the core of the cold pool aloft
as it moves across the south late morning through mid-afternoon.
Looking at a short break in organized precip as low-amplitude
shortwave ridge to the west creates an anti-cyclonic west-
northwest flow across much of the Great Basin. Strong
amplification of the mean ridge position over the eastern Pacific
near 140w Thursday will return the Great Basin to an active
cyclonic northwest flow aloft by the end of the short term period.
Long term (after 00z friday)...long-range models are in fairly
good agreement in the long term through the weekend. A moist,
cold, and unstable northwesterly flow aloft Thursday evening will
keep precipitation in place over mainly the northern mountains and
adjacent upslope regions before the next shortwave disturbance
carves into Utah during the day Friday. This trough is forecast to
pinch off into a closed low over eastern Utah and western
Colorado by Friday evening.
Precipitation will become more widespread on Friday with the arrival
of this next storm system. However, with more of an easterly flow
developing across northeast Utah and southwest Wyoming along the
northern periphery of the closed low Friday night into Saturday,
precipitation will become more favored across southwest Wyoming and
the north slopes of the uintas as well as portions of western Utah,
rather than along the Wasatch front. That being said, with 700mb
temperatures generally in the -8 to -10c range across much of
northern and central Utah, snow levels should be able to reach the
valley floors at times Friday through Saturday.
The airmass will start to trend more stable Saturday night into
Sunday as high pressure builds into the western Continental U.S.. however, the
mean ridge position will remain to the west of the forecast area,
keeping Utah under a northwesterly flow aloft. Models suggest that
weak ripples embedded within this northwesterly flow will continue
to graze northern Utah through the early part of next week.
Aviation...the slc terminal is expected to see VFR conditions
through late tonight. However, intermittent showers may briefly
result in MVFR conditions in heavier rain. Rain showers are expected
to increase after 08-09z with a corresponding lowering of ceilings and
visby with occasional MVFR conditions possible. Winds may become
erratic due to showers, otherwise they should remain out of the
north. However, there is a 40 percent chance a southeast wind
develops after 05z.
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