Scientific Forecaster Discussion
fxus65 kslc 300847
Area forecast discussion
National Weather Service Salt Lake City Utah
247 am MDT Tue may 30 2017
Synopsis...strong high pressure aloft will move slowly east
across Utah through Wednesday. A weak Pacific trough will cross
northern Utah during the latter half of the week.
Short term (through 12z thursday)...
water vapor loop shows a ridge overhead. Amdar 400mb-200mb wind
observations reveal split flow over the west with one anticyclonic
jet over western Canada with another cyclonic jet along the
California coast to the Mexican border. GOES/hrrr/00z kslc radiosonde observation
indicate precipitable water values range from 0.15"-0.25"
mountains and deserts to 0.30"-0.45" most other valleys.
Warming and drying into midweek with a ridge overhead. With the
ridge axis overhead today, 700mb flow becomes southerly,
increasing mid level moisture and thus instability. Sref indicates
modest instability building today and especially tomorrow.
Today, any high-based convection will likely remain near the
higher terrain with flow still rather weak. With the trough
approaching tomorrow, flow will increase so included isolated
convection for many valleys as well.
Little to no rainfall is expected across the valleys due to the
high-based nature of this activity. However this activity will
pose a gusty/erratic wind threat due to inverted-v BUFKIT forecast
soundings indicating a good amount of dcape. With simulated
reflectivity from convective allowing models showing weak values,
and relatively weak mid level flow, decided not to include any
enhanced wording for dry microbursts in the forecast for tomorrow.
Went on the high side of guidance for temperatures and low side
for relative humidity given recent performance. Southerly winds will pick up
ahead of the trough across the western valleys tomorrow afternoon,
with gusts expected to remain around 35 mph.
Long term (after 12z thursday)...
an upper level ridge will be present across the northern plains
Thursday afternoon with another broad area of high pressure over
the eastern Pacific. A weakening shortwave trough is expected to
cross the northern intermountain region Thursday afternoon and may
extend as far south as northern Utah. Guidance varies on the
exact trajectory of this trough and its impact on Utah. For now,
anticipate the drier ec solution is more likely. This would bring
mainly a threat of high based convection with attendant dry
microburst threat to northern Utah.
Subsidence can be expected behind this wave Friday as heights rise
across the County Warning Area. This should limit any convection to maybe the
western uintas Friday afternoon/evening. Another dry day can be
expected Saturday with the ridge overhead.
Model to model consistency regarding an upper level trough expected
to impact the Pacific northwest and possibly as far south and east as Utah
continues to be poor. The 00z ec and Canadian remain similar,
keeping the wave near the US/Canadian border while the 00z GFS
continues to insist this feature will have trouble rounding the
upper level ridge and instead retrograde west and become a closed
low over the west into midweek. Continued to lean toward the more
progressive ec and Canadian solutions.
winds at the slc terminal are expected to shift to the northwest
between 17-19. Otherwise, another day under high pressure across
clearing will be restricted to good across the north and east today,
with all areas reaching excellent tomorrow and Thursday with
increasing transport winds and mixing.
A ridge will give way to a splitting trough later this week.
Isolated to perhaps widely scattered showers and thunderstorms can
be expected, primarily in the mountains and during the afternoon
and evening hours. In addition to lightning, there is a threat of
gusty/erratic winds near any showers.
A warming/drying trend peaks Wednesday, then slightly backtracks
at the end of the work week. Poor overnight relative humidity recovery is
expected across southern Utah through Wednesday. Light winds for
the most part, though southerly winds increase Wednesday ahead of
the trough, with gusts up to 35 mph across the western valleys and
lesser gusts for Dixie.
Short term/fire weather...rogowski
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