Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
311 am MDT sun Jul 5 2015
Synopsis...high pressure will remain centered over the southern
rockies into early next week. Abundant moisture along with weak
weather disturbances moving through the region will generate
showers and thunderstorms early this week.
Short term (through 12z wednesday)... water vapor loop shows the
ridge axis weakening over The Four Corners...with a wave over
northern Nevada ahead of a closed low well off the California
coast. Amdar 400-250mb wind observations show a 50-110kt
northwesterly jet from the Pacific northwest into the plains. A
cyclonic jet was located from the eastern Pacific into
California. GOES/GPS/rap/00z slc radiosonde observation indicate the precipitable
water value ranges between 0.55" northeast mountains...to 1.00"-
1.40" across most valleys.
Aforementioned wave shears out over the Idaho border region today.
This should provide the focus for convection as drying aloft begins
to thin out more diurnal activity across southern Utah. Dewpoints
have been falling across southwest Utah...but rising across the
north and eastern valleys. The bulk of the convection may be
exiting the forecast area early this afternoon...leaving
subsidence in its wake. For this reason have higher probability of precipitation this
morning up north than this afternoon.
With heights building back in behind the disturbance tonight believe
convection will decay by Monday morning. However diurnal convection
is expected to redevelop across the region both Monday and Tuesday
afternoon and evening given the amount of instability in place
under the ridge.
Temperatures continue to run at or above normal...though much cooler
than prior week with the ridge breaking down.
Long term (after 12z wednesday)...
drier air is expected to start to move in from the southwest
during the day Wednesday as a Pacific storm system approaches the
California coast. This system is prognosticated to move onshore and
toward the forecast area...though global models disagree on the
timing. GFS moves the system onshore during the day Thursday and
then pivots it over Utah Thursday night/Friday bringing moisture
and instability back to at least the northern two-thirds of the
forecast area. Ec is noticeably slower with the system...not
bringing it onshore until Friday. After that...the system weakens
considerably as it moves into the Pacific northwest...keeping a
dry southwesterly flow over the forecast area into the upcoming
weekend. Given the significant differences in the guidance and
lack of run-to- run consistency...have chosen to basically split
the difference with regard to probability of precipitation from about Thursday on.
Aviation...VFR conditions are expected at the slc terminal through
the morning. Southeast winds are expected to prevail before shifting
to the northwest between 18z and 20z. However...showers in the
vicinity of the terminal will likely cause periods of erratic
winds...primarily before 16z.
Fire weather...isolated to scattered showers and thunderstorms will
be the rule through the middle of next week. New lightning starts
are the primary concern...with some storms providing an opportunity
for a wetting rain. The increase in convection will trend
temperatures down a notch and humidity up a bit.
An abrupt pattern shift is currently advertised during the middle of
next week. A dry and very gusty southwesterly flow develops trending
humidity lower. Convection will still be around...though wetting
rain becomes less of a possibility as activity becomes higher
Short term/fire weather...rogowski
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