Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
423 PM MDT Tuesday Sep 1 2015
Synopsis...a mild southwesterly flow will prevail across the
region through the remainder of the week. A Pacific trough will
move into the Great Basin for the weekend...and will be
accompanied by a much cooler...fall like airmass.
Short term (through 06z friday)...southwesterly flow aloft will
prevail across the interior west through the short term
period...downstream from an upper trough which is forecast to
deepen along the Pacific coast. Despite gradual drying over the
past couple of days lingering moisture has proven sufficient for
isolated convection across the eastern two thirds of the forecast
area this afternoon. Have maintained isolated convection into the
evening hours across the eastern half of the forecast area.
Southwesterly flow is forecast to strengthen a bit over the next
couple days resulting in locally breezy conditions across western
Utah while maintaining a mild airmass over the forecast area. As the
airmass continues to dry the threat for convection will be further
diminished...and likely confined only to the most favorable areas
such as Boulder Mountain and the high uintas.
While the overall idea of a strengthening southwesterly flow is
agreed upon between the various models...the degree of moisture
Thursday into Thursday evening remains the source of uncertainty
with the forecast /discussed in greater detail in the long term
section below/. Given the amplitude of the upstream trough suspect
moisture in at least a limited capacity will make it into southern
and eastern portions of the County Warning Area Thursday afternoon...and have
bumped probability of precipitation 10 to 20 percent for this period.
Long term (after 06z friday)...medium range models in good
agreement with the large scale pattern into the start of the
weekend. After this time they begin to diverge and confidence in
the evolution of the pattern decreases. Period begins with a ridge
centered over the central U.S. With a deep trough along the West
Coast. This places Utah under a pretty vigorous SW flow aloft
Thursday night into Saturday. The GFS surges higher precipitable water air into
eastern Utah Thursday night and develops decent coverage of showers and
thunderstorms...particularly across the eastern portion of the County Warning Area.
The Canadian and European model (ecmwf) are reluctant to develop much precipitation at
this time...but feel that the potential is certainly there to pull
deeper moisture north ahead of the approaching front. Given this
potential will increase the threat of precipitation just a bit on Thursday night
and Friday across the eastern half of the County Warning Area.
Expect breezy to windy south winds across much of the state Friday
and Saturday ahead the cold front associated with the deep western
trough. The European model (ecmwf)...GFS and Canadian models all bring a significant
cold front through the state Saturday night and Sunday leading to a
significant cool down. Temperatures across the state will be around
15 degrees f below normal on Sunday and some areas along the Wasatch
front may struggle to reach 70f. The medium range models Don/T
develop much precipitation along the front although believe it will be
active across northern Utah closer to the large scale dynamics associated
with the trough as it moves through the northern intermountain region.
Temperatures will then begin to rebound during the early portion of
Aviation...gusty southerly winds are expected to continue at the
slc terminal through much of the taf period. However...there is a 30
percent chance that a lake breeze could push winds to the west-
northwest between 22z and 01z. There is also a 10 percent chance
that a shower could impact the terminal through 03z...but most
activity should remain well east of kslc.
Fire weather...a weak weather disturbance crossing the area
will serve as a focus for isolated showers and thunderstorms
across the eastern half of Utah this evening. A developing trough
along the West Coast will result in an increase in southwesterly
flow through Thursday...resulting in gusty southerly winds during
the afternoon...particularly across western Utah. This may result
in local critical fire weather conditions if fuels become
Wyoming...red flag warning until 9 PM Friday for wyz277.
For more information from National oceanic and atmospheric administration/S National Weather Service visit...
http://weather.Gov/saltlakecity (all lower case)
For frequently asked questions about the area forecast discussion
http://weather.Gov/saltlakecity/general/afd_faqs.Php (all lower case)