Scientific Forecaster Discussion
fxus65 kslc 191126
Area forecast discussion
National Weather Service Salt Lake City Utah
526 am MDT sun Aug 19 2018
Synopsis...building high pressure will keep a dry and stable
airmass over the area today. A more moist airmass will make its way
back into Utah and southwest Wyoming for Monday through Wednesday.
Short term (through 12z wednesday)...the upper low that grazed
northern Utah 24 hours ago is now moving into the Great Plains while
a ridge continues to build into the western Great Basin. This
pattern will keep a dry and stable airmass in place over Utah with
clear skies and warmer temperatures expected today. However,
visibilities will be reduced in some areas due to smoke from the
West Coast that was advected into Utah yesterday.
The ridge axis will shift overhead by early tomorrow morning, then
continue eastward during the day tomorrow as an upstream trough
settles into the Pacific northwest. This will bring a strengthening
southwesterly flow which will start to draw moisture back into the
area. Models continue to indicate precipitation developing tomorrow
afternoon across northern Utah just ahead of a deepening surface
trough over northwest Utah, although the GFS thinks this will happen
earlier in the day. It remains to be seen how much measurable
precipitation will actually fall during the afternoon, as the
subcloud layer may remain relatively dry. As such, showers and
thunderstorms may produce more winds than rain initially. However,
model guidance has dewpoints increasing during the evening and
overnight hours, with slc for example reaching the upper 40s. This
coincides with increased forcing for ascent across northern Utah as
the trough ejects a weak ripple through Idaho which in turn pushes a
cold front into far northern Utah. As such, think showers and
thunderstorms will start to trend more wet during the evening hours
and continue through the night.
The shortwave trough will remain near the Pacific northwest on
Tuesday, albeit a bit farther south, with southwest flow continuing
over Utah. Moisture will increase more on Tuesday, with pwats
nearing or exceeding 1 inch for most areas except near the Idaho
border by late afternoon. More widespread precipitation can be
expected across the forecast area as a result, with storms
continuing to trend towards the wetter side.
Long term (after 12z wednesday)...an upper level trough over
central Idaho midday Wednesday is shown to be a little more
progressive and slightly deeper by the ec than the GFS as it moves
east into Wyoming by Thursday morning. The Canadian supports the ec
as well. Have increased pops over the central and northern Wasatch
front and mountains and SW Wyoming for Thursday morning as 500mb
vorticity looks significant to set off showers.
In the wake of this shortwave passage, drier air and much lower
precipitable waters move in across northwest and western Utah but held onto (although cut
back) some minimal pops over the uintas and most mountains south of
I-80 due to the propensity of models tending to dry the air mass out
too fast. Removed all pops from Friday afternoon through Sunday
midday as precipitable waters drop below 0.50 of an inch. A little moisture moves
back into the picture Sunday afternoon across the far south as
another trough moves into Pacific northwest and the flow aloft backs
to more southerly.
Temperatures during this extend period are expected to remain at or
slightly above normal levels.
Aviation...no weather concerns at the slc terminal today. Winds
will likely switch from southeast to northwest between 16 and 17z
this morning, with a 20 percent chance of holding off until after
Fire weather...high pressure over the western Great Basin this
morning will shift over Utah by late tonight. This pattern will
maintain a dry and stable airmass over the area. As the high shifts
east tomorrow, southerly winds will strengthen, allowing moisture
start spreading back into the area.
The winds are expected to become strong and gusty over the western
Utah valleys tomorrow afternoon and evening. Although relative
humidities will also begin to rise, they are expected to remain low
enough over west-central Utah to combine with the winds and
critically dry fuels to result in hazardous fire weather conditions.
Isolated to scattered showers and thunderstorms are expected to
develop across northern Utah tomorrow afternoon and evening. These
may initially begin dry and produce gusty winds, but models suggest
that storms will trend more wet by the evening. With moisture
continuing to increase through midweek, coverage of convection will
increase across the area for Tuesday and Wednesday with increasing
potential for wetting rain. A drier airmass is then expected to
spread back into the area for the latter part of the week.
Utah...Fire Weather Watch from Monday afternoon through Monday evening
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