Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
332 PM MDT sun Aug 2 2015
Synopsis...a weak weather system will slowly cross the region
from south to north through tonight. A drier southwesterly flow
will follow through midweek.
Short term (through 12z wednesday)...weak trough currently making
its way northward approximately along the Utah/Nevada border. This has
shifted the flow aloft to a more southerly direction...drawing the
monsoonal moisture over southern Utah northward into the rest of
the forecast area. Satellite derived precipitable waters indicate values of up to
1.0 approaching Interstate 80...and the moisture will continue to
move to the Utah/Idaho border through this evening. Best coverage of
showers and thunderstorms has shifted to central Utah this
afternoon as more subsident air moves into far southern Utah.
Focus of the precipitation will continue to track northward with
the continued movement of the wave.
Overnight...the disturbance is expected to take a more easterly
direction...moving across northern Utah on Monday. Scattered
showers and thunderstorms will thus continue as a drier and more
stable airmass moves into southern Utah. This drier air will
continue northward through Tuesday as weak ridging develops behind
the exiting wave. However...lingering moisture and instability
will keep a few showers and thunderstorms over primarily northern
Utah Tuesday afternoon and evening.
Long term (after 12z wednesday)...the subtropical ridge axis
centered over the Arizona/nm border subtly shifts eastward on Wednesday
as a low amplitude trough sneaks toward northern Utah...promoting a
relatively dry southwesterly flow aloft. A weak shortwave embedded
within the flow accompanied by increasing precipitable water will provide ample
dynamics to encourage afternoon and evening convection in northern
Utah. This feature also cools down 7h temperatures a few degrees...
delivering temperatures about 5 degrees below seasonal norms for the
northern half of the state. Have lowered appropriate Thursday maximum
temperatures a bit to reflect this. Lingering instability over the uintas
may be the only chance for a dollop of convective activity on
Friday temperatures warm rapidly ahead of an encroaching and deepening low
pressure system swinging into California. Models diverge at this
point on timing and impact of this low as it progresses toward Utah.
The GFS has remained staunch in delivering precipitation into the County Warning Area by
Friday...while the European model (ecmwf) remains slower...less moist and less
extensive in coverage. In either case...expecting midlevel southwest
winds to ramp up by Friday evening across much of the state. Have
introduced probability of precipitation in southern Utah by Friday 18z and extended them a
bit further north on Saturday to gently nudge toward GFS solution.
Drier air rapidly replaces the exiting system Saturday into
Sunday..bringing a zonal upper level flow and subsidently quiet end
to the weekend.
Aviation...the west-northwest winds at the slc terminal are not
expected to persist very long as southerly winds should arrive
between 22-00z and continue through the night...with occasional
gusts 20kts or greater. There is a 20 percent chance of
thunderstorms at the slc terminal between 22-02z. Occasional rain
showers will then continue through the overnight hours and into
early Monday afternoon. Isolated embedded thunderstorms will occur
with these showers...along with a 40 percent chance of occasional
ceilings 6kft or lower in the heavier showers.
Fire weather...a weather disturbance is tracking along the
Utah/Nevada border this afternoon...allowing deep moisture to
spread into the southern two-thirds of Utah. Scattered to numerous
showers and thunderstorms have developed in this moist and
unstable airmass...and the focus of convection will shift
northward this evening as the disturbance continues
northward...then cuts over northern Utah tonight and tomorrow.
There is a chance of wetting rain with any of these showers and
thunderstorms. With the northward movement of the
disturbance...drier air will move back into the district from the
south tonight through Tuesday. Under the influence of the
wave...scattered showers and thunderstorms will continue over
northern Utah tomorrow before coverage decrease Tuesday under weak
high pressure. Southerly winds will begin to increase Tuesday and
Wednesday as another trough begins to approach the area.
Short term/fire weather...traphagan
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