Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
253 PM MST Wednesday Dec 17 2014
Synopsis...a series of weak weather systems will cross the
region through tomorrow evening...with a cool front expected
across the north Friday night into Saturday. Otherwise weak
inverted conditions expected across the north for the rest of this
Short term (through 00z sunday)...
water vapor loop shows a shortwave ridge overhead...with a weak
disturbance crossing Arizona. The next splitting trough has made
its way inland with a ridge over the eastern Pacific. Amdar
400-250mb wind observations indicate a 75-110kt cyclonic jet from
northern California into northern Mexico. GOES/GPS/rap/12z radiosonde observation
indicate the precipitable water value ranges between 0.08"
northeast mountains...to 0.45" southern valleys. Blended
precipitable water product shows an increase in moisture from the
west within the trough axis...with over 150% of normal across much
of the west.
Pattern is characterized by a moist airmass and periodic weak ascent
due to weak disturbances progressing across the region. High
resolution short range guidance suite indicates highest
probabilities for light snow to advance into the central mountains
from the southern mountains overnight.
The pattern shifts tomorrow evening as the last splitting trough
crosses overhead...followed by an amplified ridge early Friday.
Pattern is progressive with a weak cool front across the north
Friday night...with lingering effects into Saturday.
Otherwise northern valleys will be under the effects of an
inversion. However effects will partially be mitigated by lack of
snow cover...relatively warm ground and high dewpoints...and mostly
cloudy conditions. Pattern shift aloft Friday and Saturday appear
modestly better for mixing...though not enough to totally eradicate
inverted conditions. At present expect majority of fog and lower
temperatures to be restricted to the West Shore of the Great Salt
Lake into the west desert.
Long term (after 00z sunday)...
focus in long term shifts to a Stout Pacific moisture tap
advecting into the pacnw states...and its evolution as it
translates east into the intermountain region in the Sunday night
through Monday night timeframe. By 00z Sunday the core of this
hose will be punching into central Idaho and then points southeast
thereafter as it rolls over an amplifying ridge along the Pacific
coast. Of note...precipitable water anomalies are prognosticated to run some 2 to 3
Standard deviations above normal across the far north. Models
agree a period of modest warm air advection within northwest flow will be
occurring across the northern third of the state coincident with
this...with a broad area of upglide from the northern third of
Utah north into western Wyoming. Model quantitative precipitation forecast across the northern
mountains is quite generous through Monday night with values in
excess of 1.5" as far south as the southern Wasatch. GFS/European model (ecmwf)
does have the core of the upper jet to our northeast however so
southern extent of precipitation and amounts remain suspect at this
time...especially south of Interstate 80. The gefs ensemble mean
hints at inconsistency in detail as well with 500 mb heights higher
across northern third than those in both the 12z operational GFS
and European model (ecmwf)...this potentially indicating focused precipitation more to our
northeast. Confident in running with likely probability of precipitation for all northern
mountain areas still but precipitation amounts remain suspect at this
A progressive short wave descending southeast through the northern
plains Monday night into Tuesday is expected to cut the Pacific tap
significantly...limiting areal extent and significance of precipitation
across the north in the Tuesday/Wednesday timeframe. This said...mountain snow
showers may persist through this time due to persistent northwest flow
aloft and somewhat modest moisture in place. Probability of precipitation reflect this.
Another note...better model agreement regarding a colder and more
put together trough days 9-10...at least in the 12z global runs. One
to watch potentially but run to run variations have been numerous.
Aviation...light northwest surface winds will remain in place at
the kslc terminal through 03-04z before returning to the south. A 20
percent chance exists that the current ceilings near 7000ft above ground level will
lower to 6000ft above ground level at times through 06z...with this
potential increasing to a 30 percent chance after 06z tonight.
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)