Scientific Forecaster Discussion
fxus65 kslc 191223
Area forecast discussion
National Weather Service Salt Lake City Utah
522 am MST Tue Feb 19 2019
Synopsis...the storm system still lingering over the area this
morning will exit to the east later today. A new and potent storm
system will impact mainly the southern two-thirds of the state
beginning late Wednesday night and continuing into Thursday night.
Short term (through 00z saturday)...an elongated upper trough
remains in place, extending from the northern rockies through
Arizona this morning. Meanwhile, an embedded shortwave disturbance
is currently noted across southeast Idaho and into northern Utah. As
this piece of energy crosses the area this morning, expect
temperatures at 500mb to begin warming aloft, leading to
stabilization of the airmass from above. However, steep lapse rates
will remain in place from the surface to above 700mb, so expect the
possibility of snow showers to continue into the afternoon hours.
Main concern for the next several hours is the lake-effect snow that
produced locally significant accumulations in the Tooele Valley
overnight. With the flow aloft backing, the band has started to
transition to the Salt Lake Valley. Although the band looks less
well-defined in the latest radar imagery, surface winds per latest
mesowest still indicate good convergence over the gsl. This,
combined with the aforementioned steep low-level lapse rates and
relatively moist airmass suggest favorable conditions for the lake
effect to continue through this morning, with impacts to the morning
commute especially in the Salt Lake Valley.
Otherwise, extensive stratus remains in place across much of western
Utah through at least this morning. Temperatures will be quite cold
today, with 700mb temperatures ranging from -16c across the south to
-19c across the north. Afternoon maxes are forecast to range from
around 15 to 25 degrees below climo today.
The flow aloft will briefly become anticyclonic late tonight into
early tomorrow morning as a low-amplitude ridge shifts across the
area. However, warm advection will quickly develop at 700mb late
tonight ahead of the next approaching trough, with snow beginning to
fill in again across the north. This warm advection is expected to
wane by late tomorrow afternoon. Snow accumulations across northern
Utah are not expected to be particularly significant except perhaps
locally near the Idaho border.
The trough will end up pinching into a closed low as it dives south
through western Nevada tomorrow night into Thursday. This will then
shift the focus of precipitation into southern Utah. Models slowly
track a 700mb circulation across southern Utah Thursday into Friday.
As moisture spreads into the area ahead of said circulation,
favorable southwest through southeasterly upslope flow will occur
for an extended period across southern through east-central Utah.
With a continued cold airmass resulting in snow levels down the all
valley floors, significant snow is expected. As a result, have
issued a Winter Storm Watch for this event.
Northern Utah will quickly shift to an easterly flow aloft Thursday
which will not Bode well for snow along the Wasatch front and
adjacent mountains. In addition, the easterly mslp gradient will
allow for gusty canyon winds once again along the Wasatch front,
developing late tomorrow night and continuing through Thursday
night. At this time, Ridgetop winds do not support a larger
The trough will fill and start shifting east at a faster pace on
Friday. North to northwesterly flow aloft will spread across the
entire forecast area as a result. Snow will linger especially along
the mountains and adjacent I-15 corridor from Salt Lake to around
Cedar City before tapering off Friday evening.
Long term (after 00z saturday)...by Friday night, the only
precipitation continuing with the ongoing storm should be isolated
to the higher terrain areas of central and eastern Utah. This should
come to an end by Saturday morning, as the parent trough and surface
feature continues well east of the forecast area...finally allowing
for a dry period to take over across southern Utah next weekend and
early next week.
Switching focus to northern Utah...the next weather system is
expected to move into the forecast area late Saturday into Sunday.
In contrast to the previous system, this one is characterized by a
low amplitude trough over the entire northwest Continental U.S.. poleward of a zonal
polar front jet (pfj) will be much cooler 500 mb/700 mb temperatures. The
pfj is currently forecast to align almost due east-west directly
over central Utah by Saturday afternoon. This will leave the precip
(all snow) confined to the northern third of Utah and southwest
Wyoming Sunday as cooler temps aloft increase the instability aloft
allowing for scattered snow showers to develop along higher terrain
areas and other weak minor short wave troughs. The lack of
significant forcing will keep any precip on light side.
Heading into early next week, model consistency becomes something to
be yearned for. The general trend is that the pfj will remain
aligned over Utah with a jet Max moving onshore off the Pacific
coast during the early part of next week. When and where this jet
Max moves onshore will determine how the trough evolves. As the jet
Max makes its way inland, it will increase the amplitude of the
trough and create a slightly more potent system. However, due to
model inconsistencies between the timing and deepness of the system
attm, the current forecast package went with a blended approach of
slight chance-chance pops across much of the area.
Aviation...the primary concern for the kslc terminal this
morning will be a lake effect snow band currently near the terminal.
This band is expected to create light snow with minor accumulation
at the terminal under northwest winds and VFR conditions. That
said...there is a 30% chance the primary band of snow moves over the
terminal before 14z creating 1-2 inches of snow accumulation on
untreated surfaces with IFR conditions. After 14-15z, the lake
effect band should dissolve leaving VFR mid-level ceilings in its
Utah...Winter Storm Watch from Wednesday evening through Friday
afternoon for utz517-518.
Winter Storm Watch from Wednesday evening through late Thursday
night for utz012-013-019>021.
Winter Weather Advisory until 1 PM MST this afternoon for utz003.
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