Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
926 PM MST sun Jan 25 2015
Synopsis...strong high pressure aloft will center over Utah
Monday. This high pressure will shift east of the state on
Tuesday...allowing a storm system off of Baja California California to
move north and impact much of the state late Monday night
through early Wednesday.
Short term (until 00z thursday)...the center of the strong upper
ridge encompassing the western Continental U.S. Will center across Utah on
Monday. This upper ridge will continue east...with the axis of the
ridge moving into The Rockies late Monday night or early Tuesday.
The well-defined upper low currently off the West Coast of Baja California
California will lift north behind the exiting ridge on Monday.
This low will gradually open up as it moves into the western Great
Basin late Monday night/early Tuesday. The tropical moisture
tapped by this low will stream north into Utah ahead of the
advancing low. Light showers could reach into southwest Utah as
early as Monday evening...with better organized and widespread
valley Rain/Mountain snow late Monday night as the now open wave
trough reaches southern Nevada/southwest Utah.
The best chance for precipitation will remain with the remnant of the
upper low across central/northern Nevada through western Utah
during the day Tuesday. Precipitation is expected to turn heavier and
more widespread across northern Utah Tuesday night as the flow
aloft turns more to the west and bring the last of the trough
east across northern Utah.
Weak dynamic lift supported by modest cooling aloft and a moisture
source originating in the tropics all suggest a widespread precipitation
event with amounts approaching .40 to .60 inches across the far
north Tuesday night through early Wednesday. Prefer the European model (ecmwf)
solution on qpfs...though suspect most of the available guidance
will be a little low based on the air mass in play. Lingering
light showers will remain across the north behind the exiting
weak trough Wednesday afternoon.
Previous long term (after 00z thursday)...it appears precipitation
shuts down fairly quickly Wednesday evening in the wake of the
departing short wave trough and following weak short wave
ridge moving in overhead.
The next weak low ejecting out of the eastern Pacific brings another round
of tropical moisture into the western states and differences in the
models on the placement of the low create disagreement on the
northward extent of the precipitation forecast to accompany this feature.
The 12z ec moved farther north with the circulation center by 12z
Friday and spreads precipitation across much of the County Warning Area that retreats slowly
south through Sat night as the low sinks S. The GFS remains farther
south initially and keeps precipitation out of the far north...then
shifts it southward faster.
Both models do end up with the low far enough south for the threat
of precipitation to end across the County Warning Area by 00z Monday with dry and warmer
conditions forecast for the first part of next week.
Expect these subtle differences will not be resolved until we get a
few days closer to this time frame so have kept probability of precipitation on the low side
and hedged the difference between the two solutions through sun. If the
ec is right there might be fairly significant water amounts across
the County Warning Area from this event.
The other element in the forecast would be the potential for a
locally strong downslope east wind event along the Wasatch front Sat
morning. Magnitude depending on how far south the closed low is by
12z Sat with the farther north ec depicting the stronger event.
Aviation...light and variable winds will turn to the southeast
between 05z and 06z. Urban haze could briefly lower visibilities
to between 5 and 6 miles until the southeast winds become
established at the terminal. Skies will remain mostly clear
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http://weather.Gov/saltlakecity/general/afd_faqs.Php (all lower case)