Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
418 PM MST Thursday Dec 18 2014
Synopsis...a weak weather disturbance will exit the region this
evening...yielding to a weak ridge early Friday. An increasingly
moist northwest flow aimed at northern Utah is expected for the
weekend into early next week.
Short term (through 00z monday)...a weak split system continues to
cross Utah this afternoon...generating enough instability to allow
spotty showers to continue with snow levels near 4500ft. Expect
showers to quickly wane this evening due to both the loss of daytime
heating and the departure of the trough. Brief clearing of skies
tonight behind the trough will allow temperatures to fall enough to
bring a threat of patchy fog to some valley locales.
Shortwave ridge tomorrow will bring a more stable airmass to the
area...but this will be short-lived as a series of weak disturbances
will arrive tomorrow evening. Expect another round of spotty showers
mainly across the north with snow levels again near or just above
the valley floors.
Warm air advection at 700mb will strengthen Saturday night...about
the same time deep moisture starts to arrive over northern Utah.
This will be the beginning of a prolonged wet period for northern
Utah. However...warming airmass will lead to higher snow levels with
most valleys seeing rain and mountains receiving relatively high-
Long term (after 00z monday)...impressive atmospheric river /AR/
event punching into the Oregon coast will translate southeast and
peak over the northern third of the forecast area Sunday into
Monday. Orientation and location of the Arkansas such that vapor transport
will remain quite solidified into the interior mountain west
..naefs mean integrated water vapor transport values currently
falling within a 15 to 30 year return interval locally for this time
of year. Noteworthy. The strong upper jet axis aiding influx remains
northeast across western Wyoming in guidance however...but still a
solid 135 knots aloft across northern Utah. Warm air advection peaking coincident
with peak precipitable water...0.7 inches Sun night...a key to the prolonged
period of ascent upglide and precipitation forecast across the north
through that time. Vertical profiles in bufr soundings remain quite
stable through this event...but anomalous moisture combined with at
least some dynamic lift within northwesterly flow aloft is owing to
increased confidence of the bullish storm total quantitative precipitation forecast reflected in
guidance...in excess of 2 inches some northern mountain areas.
Confidence of realizing or exceeding this remains highest for the
mountains north of I-80 though some areas favored by northwest flow in
the central Wasatch could see similar values. Warm air advection advection peaks 700 mb
temperatures to near -2 c...thus rain/snow line throughout the bulk of the
event should hover around 7500 feet. Continued to bump probability of precipitation in the
northern mountains and upped quantitative precipitation forecast in regards to this thinking.
Further amplification of the West Coast ridge Monday night into
Tuesday will break the Arkansas tap and allow heights aloft to
build...this forcing a rapid decreasing trend of precipitation Monday
night. Models now in agreement of a dry or mostly dry forecast
Tuesday/Wednesday areawide and trended probability of precipitation in that direction.
Run to run and model to model agreement continues to run amok
regarding details of a stronger trough Christmas evening night through
the Holiday...though 12z runs have come to better agreement for the
time being. Growing confidence in a significant cool down and some
snow to Low Valley locales along the I-15 corridor exists...but
amounts remain highly suspect due to the potential of a fairly
progressive nature of the short wave upon passage.
Aviation...occasional ceilings lowering to sub 6000ft above ground level levels will
continue at the kslc terminal through 00-01z. Loss of sun angle will
promote further scattering of these low clouds and the lingering
snow showers over the surrounding terrain. Light northwest surface
winds are expected to prevail through 03z...though may be light and
variable at times through 23z.
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