Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
817 PM MDT Monday Mar 30 2015
Synopsis...a warm southwest flow aloft will develop across
Utah Tuesday...followed by a couple of cold Pacific weather
disturbances for the middle portion of the week. High pressure
aloft will return for late Friday and into the first half of the
Discussion...water vapor loop shows a ridge over the
intermountain west...with a weak disturbance over southwest Utah.
Amdar 400-250mb wind observations show a 75-125kt anticyclonic jet
over western Canada. GOES/GPS/rap/00z slc radiosonde observation indicate the
precipitable water value ranges between 0.15"-0.25" northern and
western Utah...to 0.33"- 0.45" across the southeast valleys.
Blended total precipitable water product indicates a Pacific tap
into the Pacific northwest and northern rockies.
High based convection across south central Utah is being enhanced by
the approaching disturbance. Have expanded mention across eastern
valleys for the remainder of the night...but drop mention across
central/southern mountains along with south central Utah after
midnight due to the progression of the wave. With in cloud lightning
being observed kept mention of thunder this evening. Have also had
several reports of dry microbursts in the 45-60 miles per hour range. Will
continue to message short term trends through tweets.
Rap 850-700mb thickness/saturated equivalent potential
temperature/streamlines indicate region will be in the warm sector
overnight ahead of an approaching cold front. Have increased
overnight lows closer to lamp guidance...and updated the diurnal
Sky grids updated to reflect diminishing convective debris across
the southeast zones overnight as middle and high clouds begin to
increase across the northwest zones by morning.
Next shift will be challenged by pre and Post frontal wind forecast
and assessing need for any highlights. Tomorrow may be the warmest
March day ever recorded in Salt Lake City.
Previous discussion follows...
Convection tied to the higher southern terrain persists late this
afternoon. This convection does appear to have some dynamic support
from the vorticity maximum currently off the southwest corner of
Utah. This dynamic lift will sustain convection into early this
evening. The weakening of the vorticity maximum and the loss of daytime
heating should bring showers/isolated thunderstorms to an end by
The low-amplitude ridge to the west of Utah at this time will slide
east across the state late tonight/Tuesday morning. Trailing this
ridge will be an increasingly strong and warm southwest flow ahead
of what will be a couple of cold Pacific troughs midweek. The first
trough will move east across the northern rockies and the northern
Great Basin Tuesday night/Wednesday. The associated surface cold
front will enter northwest Utah around midday Tuesday...then stall
waiting for the arrival of the colder air aloft later in the
afternoon. The tightening surface gradient during the afternoon
along with good mixing will allow temperatures to climb to record level and
produce strong southerly winds over the western valleys. The front
will accelerate east during the late afternoon and evening...with
strong northwest Post-frontal winds and a dramatic drop in temperatures
during the evening. Wind gusts could become quite strong over the
Salt Flats...and may require highlights in subsequent forecasts.
The trailing second trough has for several models been shown to
deepen over the Great Basin in response to upstream ridging off the
West Coast. The latest versions of the GFS/European model (ecmwf) have closed the
timing gap difference...though the European model (ecmwf) is still a bit faster
pushing the trough east through Utah on Thursday. Precipitation will likely
concentrate along and north of the associated baroclinic positioned
from southwest through east-central Utah. Low-level convergence
along this boundary along with vorticity lobe rotating through the
mean trough should have little trouble generating lift and solid
precipitation near and north of the boundary. Further to the north
scattered cold air instability showers are likely...with this
activity confine to the higher terrain and adjacent valley
locations. Precipitation will show a decreasing trend Thursday night as the
main trough moves through the state an into western Colorado by late
Thursday night. Clearing skies behind the trough will bring cold
temperatures to northern and western Utah...with the potential for
an hard freeze in the active agricultural area.
Medium range models are in general agreement building shortwave
ridging across the forecast area Friday...followed by a period of
mild and dry southwesterly flow aloft Saturday into early Sunday as
an upper trough develops along the West Coast. As such have gone
with a dry forecast for this period along with a warming trend area-
wide. The models diverge in the evolution of this trough toward the
latter portion of the weekend into early next week...which leads to
a low confidence forecast for the days 6-7 period. The previous
forecast is fairly in line with the GFS solution...which slowly
brings this trough inland and pushes a frontal band into northern Utah
Aviation...VFR conditions will prevail at kslc through the
tonight...with southeast drainage winds redeveloping 03z-05z.
Southeast winds increase late tonight...with threat of Airport
weather warning criteria gusts by the middle of Tuesday morning
ahead of an approaching strong cold front.
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