Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
505 am MDT Thursday Sep 18 2014
Synopsis...an upper trough will slowly advance into the western
Great Basin today through tonight then form a closed low off the
Southern California coast by late Friday. Moisture will increase
across Utah today ahead of this trough and remain into Friday.
Short term (thru 00z sun)...Utah is under a S to southwesterly flow this
morning with a high centered just to our east over western Colorado and an
elongated trough along the West Coast. The southerly flow will persist
today but not increase all that much as the upper trough starts to
split. The trough splits completely tonight with the northern branch
sliding past to our north and the southern branch forming a closed low
off the Southern California coast.
Moisture does spread north today in the S-southwesterly flow but lift is
weak across the south and any precipitation there will be spotty from
convection driven pretty much solely by daytime heating. Dynamics
are a bit better across the northwestern County Warning Area and convection should become
somewhat more widespread. The threat of strong convection appears
low as the best support does not reach northwestern Utah until tonight past
peak daytime heating.
Still expect fairly widespread rain across the northern County Warning Area overnight
lingering into Friday morning and scattered showers south tonight and
east Friday. There is still some potential for local heavy rain as
the airmass will become fairly moist and the convection should be
Drying spreads from the northwest across the County Warning Area Friday afternoon as the northern
branch trough departs and the closed low consolidates off to our
southwest. This low is forecast to start lifting northeastward on Sat and
will tap the moisture in place to our south once again with precipitation
spreading back into southwestern Utah Sat afternoon.
Long term (after 00z sunday)...models continue to remain in
phase regarding the track and evolution of the closed low that
will settle into socal early weekend. Minimal movement of this
will allow a Stout subtrop moisture tap from the southeast
beginning late Saturday before an upstream long wave trough kicks
the low northeast through the area Sunday into Monday. Should see
fairly widespread coverage of showers/storms peaking Sunday before
track of low allows a net drying trend southwest to northeast late
Sunday night through Monday.
Models remain consistent thereafter through midweek advertising a
net warming/drying trend. This as middle level ridge amplification
occurs in advance of the aforementioned long wave trough approaching
the Pacific coast.
Confidence in detail regarding the evolution and significant of this
long wave wanes rapidly later next week. Important to note that both
the European model (ecmwf) and GFS do transition this east and over the eastern Great
Basin in some manner for the late week period. Only trended probability of precipitation to
climatology for Thursday as timing/detail vary in guidance...but could
very well be addressing a fall-like storm come day 7.
Aviation...southerly surface winds are expected to become
moderately gusty this morning at the kslc terminal as a tight
pressure gradient has developed over northwestern Utah. This
gradient will relax some this afternoon...and with high temperatures once
again forecast to be very warm...a 60 percent chance exists for a
switch to northwest winds between 21-23z. Dry conditions will
persist today but an approaching storm system will spread showers
and storms into the area after 02/03z tonight.
Fire weather...low pressure currently off the northern California
coast will continue to track inland today. Southerly flow aloft in
advance of this has allowed moisture to increase across the area and
the combination of that moisture and the influence of the
approaching low will yield showers and storms across much of the
district for a 24 hour period beginning this afternoon. This low
will split and stall over socal Friday night/Saturday allowing some
drying to begin the weekend...but will then lift northeast through
the district increasing wetting rain potential again for another 24
hour period beginning early Sunday morning. High pressure will
return early next week allowing a net warming/drying trend through
long term/fire weather/aviation...Merrill
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