Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
444 am MDT Wednesday Sep 2 2015
Synopsis...an upper level trough over western Canada will settle
into the Pacific northwest late this week...then swing east across
the Great Basin and northern rockies this weekend. Cool fall like
temperatures are expected over the Labor Day weekend.
Short term (until 06z sunday)...the upper low currently along the
britcol coast will continue on a slow southward drift into the
Pacific northwest late in the week. The resultant unseasonably
deep trough across the West Coast states will leave the Great
Basin under an increasingly strong southwest flow through the
first half of the upcoming weekend.
The increasing southwest flow aloft combined with the surface
trough along the Utah/Nevada border will result in strong
southerly winds across the western and northern valleys into the
first half of the weekend. Not anticipating any wind highlights
today...but could see some issues develop by late Thursday as the
advancing upper trough nudges a fairly strong cold front closer to
the state. Friday and Saturday will see the strongest winds as the
cold front positions near the Utah/Nevada border. An high wind
event could form in this pattern as near 700 mb winds reach 40
plus knots and the thermal gradient tightens in association with
the surface front.
GOES imagery still shows higher precipitable water values across Arizona and
extreme southern and eastern Utah. Looking at a few thunderstorms
forming along the boundary between the moisture plume and the
drier air to the west across south-central through northeast Utah
this afternoon/evening. Over northern Utah and area of .60-.70
inch precipitable water values may help fuel convection later this afternoon.
The past couple of model runs of the NAM/GFS have indicated a band
of precipitation stretched southwest to northeast across northwest Utah
late this afternoon and evening. This band of precipitation appears to be
concentrated near the surface trough and underneath an area of
upper divergence associated with the entrance region of a modest
upper jet. Suspect the quantitative precipitation forecast fields are a bit overdone in the two
models...but do see the potential for at least isolated convection
forming across the northwest per the model solution.
Question for upcoming weekend will be how much cooling to expect
and whether any significant precipitation develops with the upper trough
and associated surface front. At this point it appears that cooler
fall-like temperatures are a given behind the cold front across west-
central through northern Utah this weekend. As far as precipitation would
expected convection to form along the frontal boundary and then
extend back into the colder air to the northwest. Convergence into
the surface boundary supported by dynamic lift from shortwaves
ejecting out ahead of the main trough will supply the lift
necessary for convective precipitation across northwest Utah. Moisture
will be somewhat limited...so not anticipating any heavy precipitation or
strong thunderstorms at this time through Saturday evening.
Long term (after 06z sunday)...global models have started to come
into better agreement with a fairly strong...cold Pacific trough
expected to cross the northern intermountain region/northern rockies
Sunday. In a flip from earlier runs...the 00z ec is actually a bit
quicker with this system. The 00z Canadian continued to be the
slowest model...with the system crossing the area later Sunday. This
system is also quite a bit deeper as represented in the Canadian.
In general...a fairly potent Pacific trough is expected to cross the
northern intermountain/northern Great Basin area during the day
Sunday. The associated cold front is expected to move across
northern Utah early Sunday...reaching at least central Utah by
Sunday afternoon before weakening as the trough accelerates away
from the region. Unlike the GFS runs of the past several days...the
00z run now depicts 700mb temperatures falling to near -1c along the
Utah/Idaho border by 12z Sunday. This matches fairly well with the
The global models diverge quite a bit after Sunday. The GFS takes
the shortwave dropping down the Pacific coast in the wake of the
previous system and develops somewhat of a cut-off low off the
California coast Monday. With the low in this position...moisture
surges into portions of Utah. Much of the remainder of the global
model guidance varies significantly from this solution. Forecast
package already included low end probability of precipitation across the southern mountains.
Left these in the current package given the significant variation
between the global models. Even these low end probability of precipitation are too high for
most of the global guidance.
The biggest message for the long term portion of the forecast is a
fairly significant cool down can be expected Sunday across much of
the state behind the first real cold front of the approaching fall
season...with temperatures falling 10-15 degrees from saturdays
highs. Portions of the Wasatch front may struggle to even reach 70f
Fire weather...a fairly strong trough will slowly cross the
northwestern portion of the country over the next several days.
Ahead of this system...expect gusty southerly winds across much of
the district through at least Saturday. The best chance for
humidities to fall below critical levels will be across the west
deserts through this period. Localized critical fire weather
conditions will be possible through Saturday in locations with
critical fuels...especially across the western deserts. At this
time...these locations do not have critical fuels...so held off on
any fire weather watches or red flag warnings.
A cold front will cross the district Sunday...bringing much cooler
temperatures and a reduction in winds through at least early next
Aviation...gusty southerly winds will continue at the slc terminal
through the day.
Wyoming...red flag warning until 9 PM Friday for wyz277
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