Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 302144
Area Forecast Discussion
National Weather Service Salt Lake City UT
344 PM MDT TUE AUG 30 2016
.SYNOPSIS...A ridge of high pressure currently over the Great
Basin will shift east tomorrow. An upper level trough of low
pressure will move across the region this weekend.
.SHORT TERM (Through 06Z Friday)...Afternoon water vapor and H5
analysis depict a progressive short wave lifting NE into eastern
Oregon, this ejecting from the parent trough gradually nosing into
the PacNW region and riding its southern periphery jet. High
pressure currently over the area will gradually shift east in
response to these features over the next 24 hours allowing a
transition to increasing southwesterly flow aloft. Very warm and dry
conditions will hold through Wednesday as this southwesterly flow
aloft becomes established, with temps holding near 10 degrees above
climo for many areas through that time.
The aforementioned southern periphery jet and tightening baroclinic
zone across northern Nevada will gradually nose into western Utah
late in the day Wednesday. Increased dynamic lift associated with
this combined with mid level moisture pooling within the zone does
look sufficient to support isolated high based convection along the
UT/NV border late day. Vertical profiles in BUFRs remain less than
ideal for deeper convection, but do support high based shower/virga
development beginning during the evening hours and continuing
overnight. With further eastward translation of the trough overnight
and through Thursday, and a continuation of increasing moisture
advection from the south, areal coverage of showers and an increased
threat of some storms will spread southwest to northeast Thursday
morning on through the day. With a very dry subcloud layer in place
at that time (especially in the north), anticipate minimal rainfall
potential, but elevated outflow threats will be present and aided
by fast NE storm motions.
Synoptically driven southerly winds will also be trending up late
tomorrow through Thursday, then likely peaking across much of the
area 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.
.LONG TERM (AFTER 06Z FRIDAY)...The broad upper level trough works
its way through the PacNW on Friday, ramping up SW winds across
the region, particularly across western Utah. Afternoon
convection may pop over the higher terrain, particularly in the
southeast third of the forecast area where PWATS exceeding 0.75
inches combine with decent instability and CAPE values. The
trough then pushes into the northern Rockies Friday into
Saturday, keeping most of the moisture well to the north but
shoving a cold front into northwest Utah by late Friday evening.
Global models diverge on initial 700mb temperature drop, with
the GFS quite colder than the ECMWF. Went with Saturday highs
about 5 degrees F below seasonal norms, with a further descent
through the weekend as the cooler air sinks south. While winds
decrease immediately behind the front, elevated SW winds will
continue south of this surface boundary into the weekend.
This swath of dry, stable air will likely preclude afternoon
convection for much of the weekend. As the main trough glides
across Utah Monday into Tuesday, the GFS intimates another cold
front nudging into the forecast area on Monday, with the EC
suggesting this push a full day later. Either solution keeps the
region cooler and arid well into the early work week.
.AVIATION...Winds at the SLC terminal are expected to remain
out of the northwest through this evening, although they may become
variable at times. The shift back to south will likely occur between
02-03z, with a 20 percent chance of occurring by 01z. VFR conditions
will prevail through the TAF period.
.FIRE WEATHER...Very warm and dry conditions will hold through
Wednesday as high pressure gradually shifts east and southwesterly
flow aloft increases ahead of an approaching storm system. A lobe
ejecting from this trough will graze northwestern Utah Wednesday
evening bringing gusty southerly winds and high based showers or
isolated storms capable of strong but brief outflow winds but
minimal rainfall to northwest zone 478. Thursday looks to be the
most active day regarding both increasingly gusty southerly winds
across the western valleys, and high based storms with minimal
rainfall potential becoming possible most all areas during the
afternoon and evening hours. Have opted to issue a Fire Weather
Watch for combined effects of gusty winds...low humidity...nearly
dry thunderstorms with fast storm motions and gusty outflow winds.
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
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.
UT...Fire Weather Watch from Thursday morning through Friday evening
SHORT TERM/FIRE WEATHER...Merrill
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