Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
346 PM MDT Wednesday Aug 31 2016
Synopsis...high pressure will shift east of the area through
tonight. A series of Pacific troughs will move across the forecast
area tomorrow through early next week.
Short term (through 06z sunday)...a Pacific trough continues to
encroach on the area this afternoon with heights beginning to fall
along the Utah/Nevada border. The attendant jet along the southern/eastern
periphery of the long wave trough is slowly shifting across Nevada,
with future eastward translation overhead the primer needed for
increased convective threats over the next 48 hours.
High based and shallow convection has started to form over NE Nevada
this afternoon and is expected to gradually expand in areal coverage
into northwestern Utah through the evening hours as the
aforementioned jet noses in. Instability parameters do not support
much in the way of storm threats, but gusty outflow winds may work
across the northwest deserts towards the middle of the night due to fast
moving training virga showers.
Available moisture will remain modest at best and at the mid levels
over the next 36 hours with very dry sub-cloud layer conditions.
Lapse rates/instability will also remain less than ideal with any
cape remaining very thin. Thus, thinking that any thunderstorm
formation will need the dynamic trigger of the jet to form deep
enough updrafts for charge separation. This should arrive across the
southwest late tonight as the jet shifts in with a speed maxima
rounding the southern periphery by dawn, with this forcing expanding
across all areas during the afternoon when the jet shifts overhead
driving scattered convection from SW-NE. Expecting widespread
outflow winds with this development, and the threat of microburst
winds focused tomorrow afternoon through the evening aided by
modestly fast storm motions.
Synoptically driven southerly winds will also be trending up through
Friday as the mid level cold front approaches from the west.
Convective threats will be trending down from west to east Friday as
the moisture tap shifts east over the eastern half of the area, but
similar impacts from any storms will remain present where formed due
continued maintenance of a very dry sub-cloud layer. Organization
may be easier to come by due to adequate shear and passage of
another short wave across northern Utah, but coverage trends will be
This latter wave will be the trigger to shift the slow moving cold
front east across northwestern Utah Friday night into Saturday
driving modest cold air advection (h7 temps falling to between 5-8 c during the
day). With this, temps along the Wasatch front are expected to fall
down around 80 f and below climo for the first time in a while, but
south and east of the front warm and breezy conditions will continue
with another round of diurnal convection likely across the east.
Long term (after 06z sunday)...the large Pacific trough is
expected to cross the Great Basin during the first part of the
upcoming week. Winds are progged to remain elevated over central
and southern Utah on Sunday ahead of the weakening frontal
boundary. Some lingering moisture may allow for a few showers over
the higher terrain of northern Utah as the southern half of the
forecast area remains dry.
The approaching trough is expected to bring a second somewhat
stronger cold front through on Monday. Ec indicates a bit more
moisture with this feature than the GFS, but both keep things
relatively dry. By Monday afternoon, maxes could run up to 10f
below seasonal normals, very fall-like. Global models then
indicate the back side of the trough moving across Utah and
southwest Wyoming through midweek. A rather dry airmass should
inhibit any convective development but the trough will limit
warming of temperatures through day 7, with maxes progged to
remain at least 5f below seasonal norms.
Aviation...the slc terminal will see VFR conditions through the
night with any ceilings well above 7kft. There is a 10 percent chance of
virga/showers developing at or near the terminal after 06-08z which
may produce gusty outflow winds. Otherwise, northwest winds have
become established and southerly winds are not expected to return
Fire weather...a red flag warning remains in effect Thursday
and Friday across northern and west central Utah due to high
confidence of critical fire weather conditions due to combination of
gusty winds, dry microbursts, and low relative humidity across the western valleys,
and lightning after a prolonged dry period for the northern
An approaching Pacific storm system has begun to tap deeper mid
level moisture from the south today...and as the upper jetstream
moves into the area late tonight high based showers and isolated
largely dry thunderstorms will develop across the southwest then
lift and expand across most all areas tomorrow. With very dry low
levels in place and the elevated nature of the moisture...do expect
any thunderstorms will have minimal rainfall.
Additionally...fast NE storm motions will likely keep any rainfall
that does occur brief in nature...and enhance gusty outflow and
microburst wind threats.
Mid level moisture will begin to shift over the eastern half of the
district Friday, but have maintained a widely scattered thunderstorm
threat similar to Thursday east of the I-15 corridor.
Winds on Friday will also be peaking across the area maintaining the
potential for critical fire weather conditions across the northern
and west central portions of the district.
The cold front will nose into northern Utah Saturday then gradually
sag south into central Utah late weekend. Passage of this front
across the northwest Sat/sun will maintain gusty southerly winds
over the south and east possibly through the weekend.
Utah...red flag warning from 6 am Thursday to 10 PM MDT Friday for
Wyoming...Fire Weather Watch from 2 PM Thursday to 8 PM Thursday for
Short term/fire weather...Merrill
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