Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Salt Lake City Utah
649 am MDT Tuesday Oct 21 2014
Synopsis...a dry and mild southwesterly flow will give way to a
Pacific cold front today. Dry and mild southwest flow will return
for Thursday and Friday.
Update...quick update to add thunder Ogden northward this morning
Short term (through 12z friday)...
water vapor loop shows an ejecting wave approaching from northern
California. Amdar 400-250mb wind observations reveal a cyclonic
90- 115kt jet off the Pacific northwest coast. GOES/GPS/rap/00z
radiosonde observation indicate the precipitable water value ranges between 0.25"
northeast mountains...to 0.75" most valleys.
Think probability of precipitation may be on the high side for the wastach front this
morning but didnt feel confident enough to lower from likely to
Expect the cold front to pass around 15z at Salt Lake City. Rap 850-
700mb thickness and 850mb saturated equivalent potential
temperature suggests that the thermal ridge is beginning to shift
east at this time. Expect the boundary to slow and begin to stall
across central or southern Utah this afternoon with loss of upper
Precipitable water values are greater than +2 Standard deviations
for this time of year. This should help aid in shower development
despite much of the colder air aloft remaining across Idaho and
eventually the northern rockies.
Snow levels look to lower to about 9500ft today. This could have
some impact on the Mirror Lake Highway in the Uinta
Mountains...where snowfall could amount to 2-4" on a snowboard.
Roads are warmer and with much of the event during the
daytime...solar insolation should also help to counteract impacts
except perhaps during peak intensity.
As for winds...continue to advertise best gusts across northwest
Utah and southwest Wyoming Post frontal today. Warm sector doesnt
look too breezy as we lose the pressure gradient and 700mb flow
is around 20kts.
End precipitation late this evening...with shortwave ridging
building in for Wednesday. A moist westerly Pacific flow looks to
supply middle and high level cloudiness across the far west and north
during the second half of the work week as temperatures moderate.
Long term (after 12z friday)...
high pressure continues to build over the area Friday before
shifting east Saturday...continue the warming trend. By Friday
maxes are expected to be up to 10f above seasonal norms.
Models continue to indicate that a Pacific northwest storm system
will impact the region for the latter half of the weekend and into
early next week. However...models have diverged a bit on the details
compared to yesterday. Both bring the system and initial front in
Saturday night/Sunday morning...dropping 700 mb temperatures into the -3c
to -4c range over northern Utah. However...the system is more split
in the ec compared to the GFS...with the southern wave moving into
southern Utah during the day Sunday before the northern piece digs
into northern Utah Sunday night/Monday. The GFS solution brings the
entire system through on Sunday...impacting primarily northern and
central Utah. Regardless...all guidance is showing unsettled weather
during this period so have kept probability of precipitation at least scattered during this
period. Behind the exiting trough...strong high pressure is expected
to rebuild by day 7.
Aviation...VFR conditions are expected at the slc terminal through
the morning. Southeast winds will persist before the passage of a
cold front between 14z and 16z...shifting winds to the northwest.
Showers in the vicinity of the terminal will become more numerous
with the front this morning...reducing ceilings to or below 6000 feet above ground level
at times through about midday. Clouds are expected to decrease by
Fire weather...a cold front crosses northern and central Utah
today. Good chance for precipitation especially across northern
Utah. Snow level is around 9500ft. The front will also bring a
notable increase in humidity across much of the state into middle
Temperatures rebound gradually late week with decreasing humidity.
Another wet cold front expected next weekend. Southerly winds may
become gusty ahead of the front.
Short term/fire weather...rogowski
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