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fxus65 kslc 151526 
afdslc

Area forecast discussion
National Weather Service Salt Lake City Utah
926 am MDT sun Sep 15 2019

Synopsis...
warm temperatures and increasingly strong southerly winds will
exist across the region ahead of an approaching storm system
through Monday. A strong cold front will sweep through the state
Monday night through Tuesday bringing below normal temperatures
through mid to late week.

&&

Discussion...
water vapor imagery shows a closed low across east central
Arizona. This feature is expected to cross The Four Corners this
evening into tonight. This will draw moisture northward into Utah,
a process that is already underway per experimental CIRA
advective precipitable water product (especially in the 850-500mb
layer).

The increase of moisture will support weak destabilization per the
href. Isolated showers and thunderstorms remain in the forecast
this afternoon and evening from Kanab northward along The Spine of
the mountains.

Today will be quite hot, near 90f at Salt Lake City. Relative humidity values
will be quite low across central and northern valleys. Southerly
winds will become gusty with a building southeasterly pressure
gradient along with 25kts of momentum at 700mb.

&&

Previous discussion...
satellite imagery early this morning shows a subtropical wave
drifting northward through eastern Arizona, with high cloud cover
spreading across southeast Utah. Meanwhile, the next storm system
is digging along the West Coast offshore from British Columbia. As
this storm system continues to dig down the West Coast today,
flow aloft will turn more southwesterly and continue to increase.
This will help direct moisture from the subtropical wave northward
along and east of The Spine of the Utah mountains today and into
Monday. Moisture will remain fairly limited today, but just enough
will sneak into southern Utah for a few isolated showers or
thunderstorms mainly over the higher terrain. Otherwise, today
will remain sunny, very warm, and dry with across the rest of Utah
with an increasing southerly breeze. Temperatures today will top
out about 10-15f above average.

Impacts across the forecast area from the robust storm system
digging along the West Coast will be felt in earnest from Monday
through early Tuesday. Ahead of the storm system, southwesterly
winds will ramp up markedly from a combination of strong pressure
falls to our north ahead of the storm, and a belt of strong 700 mb
pre-frontal winds, which, with steep surface-700mb lapse rates,
will be able to be tapped into for gusty conditions Monday
afternoon. On the whole, pre-frontal wind gusts of 40-50 mph are
expected, mainly across the western half of Utah, for which wind
advisories may be needed. Even stronger Post- frontal winds are
possible across northwest Utah by Monday night into early Tuesday
morning, for which a high wind watch has been issued. Forecast
models continue to come into better agreement on the timing and
magnitude of strong pressure rises and cold advection behind the
front, with the potential vorticity Max tracking across northwest
Utah. This ideal overlap of parameters should Bode well for
producing strong to severe Post-frontal wind gusts of 60-70 mph in
the favored locations across the Great Salt Lake dessert, and
have travel impacts for high-profile vehicles along I-80 and
sr-30.

Temperatures tonight will remain quite mild given the increasing
southerly flow that will keep most locations from decoupling
overnight. The Salt Lake Valley in particular could see
temperatures run about 20f above average overnight, staying in the
low to mid 70s. On Monday it will remain warm, about 5-10f above
average, but will be limited some by increasing cloud cover across
the region. Through the day Monday, convection will be limited to
locations east of I-15 where the best moisture from the leading
subtropical wave will track. However, by Monday evening and night,
scattered showers and thunderstorms are expected to develop in the
strong synoptic forcing ahead of the cold front and pv Max. Any
showers or thunderstorms that develop may be capable of damaging
wind gusts given the very fast flow and dry sub-cloud layer. Storm Prediction Center
maintains a marginal risk for this threat in the day-2 outlook
for Monday and Monday night.

By Tuesday morning, colder air will be rapidly infiltrating across
northern Utah as the cold front continues to push southward into
central Utah. A rebound of showers is possible across northern
Utah Tuesday morning as the cold pool aloft transits the area.
Furthermore, on the Great Salt Lake, surface temperature to 700mb
differences will be near 20c, with equilibrium heights around
20kft, with moisture still in place under the upper-Low. Lake
enhanced showers or thunderstorms are possible Tuesday morning
from the lake extending into the Salt Lake Valley with the
northwesterly flow behind the front. The above mentioned
parameters also are supportive of waterspout development per the
szilagyi waterspout index, with even weak showers capable of
producing waterspouts.

Clearing and drying will develop by Tuesday afternoon in the wake
of the trough passage with ridging starting to build in from the
west. Temperatures will run about 15 degrees below average across
northern Utah, and about 5 degrees below average across southern
Utah.

By Wednesday flow aloft will already start to back around to the
southwest ahead of the next approaching storm system. This will
bring a warming trend with temperatures returning very near
seasonal average, and continued dry weather.

The next trough to impact the area is expected to be moving
onshore along the Pacific northwest coast to begin the long term
period. Guidance has come into better agreement with regard to the
timing of this system, bringing it through Utah and southwest
Wyoming Thursday afternoon into Friday.

As the trough and associated front move through, this will bring a
period of rain with snow at the highest elevations to primarily
northern portions of the forecast area. Have upped pops a bit with
increasing confidence. Maxes behind the system could be 15f or more
below climo on Friday for northern Utah and more like 5-10f below
climo for southern Utah.

Ec/GFS now indicating a trailing wave splitting off from the main
system will track across northern Utah during the day Saturday,
bringing some light additional precip before a drier and warmer
zonal flow returns for day seven.

&&

Aviation...
gusty southerly winds will develop at the slc terminal by 17z.
Winds may shift toward the southwest between 21-01z but are not
expected to shift to the northwest today.

&&

Fire weather...
a warming trend continues today. Locally critical fire weather
conditions are possible due to increasingly breezy conditions
along with low relative humidity. Single digit relative humidity values expected across some
northern and central valleys. Southern Utah will see higher relative humidity
today due to a moisture influx from Arizona. This will support
isolated showers and thunderstorms from Kanab northward. The
potential for cloud to ground lightning today is fairly remote.

Winds become increasingly gusty tonight, especially western
valleys, with poor relative humidity recovery. Widespread strong southerly wind
gusts are expected tomorrow ahead of an approaching strong cold
front. Widespread critical fire weather conditions due to strong
winds and low relative humidity are forecast Monday.

Mid-level moisture peaks across eastern Utah Monday. This will
support scattered showers and thunderstorms. Convection should be
fairly high-based, supporting the threat for dry lightning across
some eastern Utah valleys where the chance for wetting rains will
be lower than the surrounding mountains.

The strong cold front will cross the northwest two thirds of Utah
Monday night bringing a sharp wind shift to the northwest.
Very strong northwest downslope winds are forecast especially
across the west desert. Relative humidity values will begin to increase with the
frontal passage.

Cooler and higher relative humidity with scattered showers and thunderstorms
Monday and especially Tuesday. Critical fire weather conditions
may continue across portions of southeast and eastern Utah Tuesday
near the frontal boundary due to gusty winds and low humidity.

Drying and warming mid-week, before the next moisture surge and
approach of the next trough toward the end of the upcoming work
week.

&&

Slc watches/warnings/advisories...
Utah...Fire Weather Watch from Monday morning through late Monday night
for utz494-497-498.

Fire Weather Watch from Monday morning through Monday evening
for utz482-489.

Fire Weather Watch from 8 PM MDT this evening through late
Monday night for utz478-492-495.

High wind watch from Monday evening through late Monday night
for utz005.

Wyoming...red flag warning from through Sunday evening for wyz277.

&&
$$

Public...10/church/traphagan
aviation...Kruse
fire weather...10

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