Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
501 am MST Tuesday Jan 27 2015
Synopsis...a storm system will slowly cross the region through
early Wednesday. Another system will develop during the latter
portion of the week across the Desert Southwest...and my impact
portions of central and southern Utah.
Short term (through 12z thurs)...an upper low currently centered
along the Southern California coastline will continue to fill and weaken
today as it lifts northeast through the Great Basin. The
associated middle level trough axis is currently sweeping northward
across Arizona/southeast California...and helping to force an arc of
precipitation across southern Nevada/southwest Utah. This precipitation
will continue to spread northward while likely diminishing with
time as it spreads across central and eventually northern Utah later
this morning through this afternoon. With 700mb temperatures
only falling into the -2 to -3c range precipitation type will
remain liquid across all valleys through this afternoon...with any
higher elevation snowfall remaining minor.
The remnants of the aforementioned upper low and associated deeper
moisture are forecast to generally lift up and over the top of the
forecast area tonight into Wednesday morning with the greatest
chance for precipitation remaining north of the forecast area.
That said...sufficient lift and moisture will remain present
across northern Utah to bring another period of light higher
elevation snowfall overnight into Wednesday morning...with
accumulations again remaining minor. The best chance for valley
precipitation looks to remain north of I-80 and in the form of rain. Middle
level ridging will briefly follow this wave bringing a downward
trend in precipitation Wednesday afternoon into Wednesday evening.
Valley inversions in place across northern Utah will likely weaken
significantly today through tonight as 700mb temperatures continue to
cool into the -5c range by Wednesday morning. May not completely
mix out all valleys...but do expect an improvement in air quality
during this timeframe.
Long term (after 12z thu)...06z GFS just viewed has trended towards
previous runs of the European model (ecmwf) regarding positioning/evolution of the
upper low late in the week. The next splitting feature poised to
take aim towards the weakness across the Desert Southwest /left over
from the short term trough/ looks to close off either over the
western Utah border or just west by Thursday night. Significance of
this relates most closely to location of the middle level deformation
zone extending from its north and east leading up to that time on
Continued to trend probability of precipitation up across central/southern Utah along and
beneath this anticipated deformation...but if future runs maintain
consistency probability of precipitation remain a fair bit low across those regions as both
forcing and significant moisture will be present. As the low
continues to spin up and gradually carve south/east towards and
along the US/Mexico border Friday into Saturday the deformation will
follow suite...this focusing precipitation more and more across the
southern portions of the area. Assuming this holds true...will
finally see a much needed moderate precipitation producer across namely
southern Utah during that time.
Global models suggesting a pattern shift to ensue thereafter as a
more defined belt of westerlies punches into the western Continental U.S. Late
next weekend. Short wave energy riding this belt is apparent in
guidance...and although confidence in any detail remains minimal at
best that far out...opted to trend probability of precipitation to climatology.
Aviation...valley haze will at times restrict surface visibility at
the kslc terminal to MVFR levels through much of this morning.
Additionally...a period of ceilings at or below 6000ft above ground level is expected
to occur at times in the 17-21z as the leading edge of a weak but
moist system moves through. The best chance for light rain showers
is in this window as well as after midnight tonight as the wave
skirts northern Utah.
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)