Scientific Forecaster Discussion
fxus65 kslc 181606
Area forecast discussion
National Weather Service Salt Lake City Utah
906 am MST Mon Nov 18 2019
high pressure aloft will remain the dominant feature
across the region through Tuesday. A deepening low pressure
system will settle into California Tuesday night, then turn east
and work slowly across southern and central Utah midweek.
Widespread rain and snow will impact mainly southern and central
Utah late Tuesday night through Thursday.
quiet weather today with ridging surface and aloft. Strongly
inverted valleys will see reduced warming today. 12z kslc radiosonde observation
shows several subsidence inversions. Air quality is expected to
continue to deteriorate through tonight.
looking at a couple more days of warm and dry conditions
underneath the high pressure aloft over the Great Basin.
Changes will finally come to the region towards the middle of the
week. The agent of this change will be the shortwave moving through
the western Aleutian chain early this morning. This feature will
eventually catch up with the slower moving vorticity Max currently
near 140w. Strong ridging behind the trailing shortwave will
allow it dig south and elongate a trough along the West Coast late
Monday night/early Tuesday. This trough will evolve quickly into
a closed low as it settles over central/Southern California
Increasing deep layer southerly flow ahead of the developing upper
low will allow for a tap of the exceptionally moist tropical air
mass over baja/western Mexico. This moisture will reach into
southern Utah Tuesday night, with a good portion of this moisture
expanding north and east across the state Wednesday.
The initial round of precip should reach southern Utah along the
leading edge of the moisture plume Tuesday evening. Showers will
likely focus along the leading edge with some enhancement possible
in the terrain upglide regions.
Rapidly increasing synoptic-scale lift will commence Wednesday as
the now well developed upper low begins to slowly advance towards
the southern Great Basin. Dynamic support from vorticity lobe(s)
ejecting out ahead of the main low along with good upper
divergence, terrain upglide and the now well primed air mass
should bring widespread moderate/heavy precip into southwest Utah
by Wednesday morning. Initial snow levels will be on the high
side, though with strong lift generating heavier precip, these
snow levels should start to come down later Wednesday
morning/afternoon. The heaviest snows should focus on the areas
of best moisture and synoptic lift across the southern mountains.
Accumulations should easily exceed a foot, with favored areas
running close to two feet.
Increasing dynamic/thermal support for lift as the upper low
advances closer to southwest Utah will maintain peak precip
intensity rates through Wednesday evening. Portions of northern
Utah will begin to see an increase in precip intensity/areal
coverage as the thermal gradient tightens near the mid-level
deformation along the northern flank of the upper low late
Wednesday night/early Thursday. This favorable synoptic situation
will have a relatively short life across the north as the arrival
of the upper low center over southern/central Utah will create a
deep layer and somewhat drier easterly flow across the north
during the late morning and afternoon.
Precipitation will begin to transition to a more convective
nature as the cold core of the low settles into central/southern
Utah. Peak convective precip intensities/areal coverage will
likely continue into mid-afternoon Thursday, then begin to taper
off as drier air begins to work into the northern/western zones
and the cold air aloft works east out of southern/eastern Utah
late in the day.
By Thursday night, the low will be pulling east of the forecast
area, allowing for precipitation to taper off. There still remain
some minor difference between guidance members but current
thinking is that associated precip is not expected to linger much
past Friday morning. A north to northwest flow over the weekend
should keep conditions dry, but maxes are generally expected to
remain at least 5f below seasonal normals.
Global models and most ensemble members show a trough moving over
the top of the California Ridge that could potentially bring
additional precip and cold air to Utah and southwest Wyoming early
next week. However, significant spread exists with regard to timing
and track. For now, have introduced some pops for day seven to
northern Utah, per the model blend.
operational weather concerns will remain minimal at the kslc
terminal through the taf period. Southeasterly drainage winds will
shift to the northwest between 20-21z with VFR conditions
persisting through at least 18z tomorrow.
Utah...Winter Storm Watch from Tuesday afternoon through Thursday
afternoon for utz518.
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