Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
437 am MST Monday Jan 26 2015
Synopsis...strong high pressure aloft will reside across Utah
today. This high will shift east tonight...allowing a storm
system off of Baja California California to move northward and affect the
region region beginning late tonight and continuing through
Short term (through 12z thursday)...a large scale Rex
block...characterized by an upper ridge centered across Utah
coupled with an upper low spinning off the Baja California coast...continues
to dominate the weather patter across the western Continental U.S. Early this
morning. Large scale subsidence associated with this ridge axis is
resulting in generally clear skies across the forecast area...with
only patchy fog noted around The Bear lake area.
An anomalously warm airmass aloft associated with the upper ridge
will shift across northern Utah this afternoon allowing 700mb temperatures
to soar near 7c by this afternoon. This warming will be most
notable at higher elevations...while anticipate temperatures across most
valleys to run close to sundays values.
The upper ridge axis will shift east this afternoon through this
evening allowing the Baja California low to lift northward through the lower
Colorado River valley and Great Basin tonight through Tuesday. Although
this low has tapped into deep moisture...this tap will quickly
shut off tonight as the weakening low reaches the Desert
Southwest. With the weakening nature of the wave and eroding
moisture fields it appears precipitation amounts will be limited with
this wave through Tuesday...as a weakening arc of moisture
advection lifts northward initially into southwest Utah overnight
into Tuesday morning...then quickly lifting across western Utah
during the day Tuesday. Have slowed precipitation timing a bit based on
consensus of 00z guidance...as well as lowered probability of precipitation across the
eastern valleys which will be a bit too far removed from the
better upper forcing.
Whats left of the upper wave is forecast to turn east across
northern Utah tonight through Wednesday bringing a better chance of
precipitation to the northern third of the state during this timeframe.
With 700mb temperatures remaining in the -4 to -5c range snow levels
should remain above the valley floors...with the greatest chance
for precipitation coming north of I-80.
Long term (after 12z thu)...00z GFS/European model (ecmwf) continue to agree in the
large scale pattern across the western Continental U.S. Late week...but details
diverge in evolution/placement of the return of a Rex block across
the west. Where...and to some extent when...yet another upper level
low closes off across the Desert Southwest/Baja California regions remains the
primary factor regarding areal extent and significance of precipitation
across the local area Thursday through Friday.
Models agree that a strong middle level moisture tap will once again
occur west/yet another origin from the inter-tropical convergence zone. Moisture is not a problem
west/this system rather the southern/western extent of the upper lows
circulation center will be a dictating factor regarding best forcing
of a potentially very moist airmass. European model (ecmwf) depicts this low center
closer to the socal/western Arizona border as upstream ridging becomes
more amplified from the pacnw/britcol region than that depicted in
the GFS. This allows a more focused deformation zone to develop
across western Utah that will slowly pull south as the low center
transitions into northern Mexico. GFS is a bit further east
west/resultant quantitative precipitation forecast more prolific from SW Colorado on into far eastern
Arizona and points east coincident west/best large scale lift focused there.
Run to run consistency in detail has not been overly consistent in
either model so needless to say confidence is lacking in either
solution. This said...potential exists for a healthy precipitation producer
with focus across the west and south as the European model (ecmwf) continues to
suggest. Made minimal changes to probability of precipitation outside of slight trends
towards a refined blend between these two. Likely will not see
improved consistency until the early week wave in the short
term passes downstream.
Models remain consistent that ridging will build in from the west as
the Rex transitions overhead Saturday...this bringing an end to the
aforementioned precipitation threat.
Aviation...operational weather concerns are expected to remain
minimal at the kslc terminal today. A strengthening of high pressure
aloft will be enhancing low level haze with a 30 percent chance of
high end MVFR visible at times of low sun angle and or during the
nighttime period...but VFR conditions are expected to prevail
through 03z. MVFR visible in haze is expected to be more likely after
03z this evening/overnight.
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)