Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 181040
Area Forecast Discussion
National Weather Service Salt Lake City UT
340 AM MST Wed Jan 18 2017
.SYNOPSIS...High pressure aloft will remain over the Great Basin
today. This high pressure will give way to a series of weather
disturbances beginning tomorrow and lasting into next week.
.SHORT TERM (THROUGH 00Z SATURDAY)...
Water Vapor satellite shows a ridge axis overhead, with the next
storm system approaching the west coast along 40N. 400-200mb
MDCARS wind observations indicate split flow over the western
CONUS. The westerly 90-125kt northern stream jet is over the
Canadian border, while the nose of a strong Pacific jet, sampled
at 120-135kt, is still well off the California coast. GOES/SLC 00Z
RAOB/HRRR indicate that precipitable water values vary from
0.05"-0.10" mountains to 0.15"-0.30" valleys. Blended precipitable
water product shows an atmospheric river associated with the
approaching storm system with origins from north of Hawaii
affecting the Pacific northwest.
00Z KSLC RAOB placed the base of the 5.2C inversion near 850mb.
This inversion is the focus of the forecast through this evening.
Will continue to advertise urban haze, especially now that the
work week is underway. Patchy radiational fog is possible in
valleys adjacent to the Idaho border. Stratus continues to affect
portions of the West Desert and the Western Uinta Basin. Expecting
gradual improvement in effects of this inversion during the next
18-24 hours as southerly winds increase with the next storm
Strong Pacific jet will continue to encourage forward propagation of
the aforementioned wave, even forcing it to split as it runs into
the ridge overhead today and tonight. Upper level diffluence
increases ahead of the approaching trough tomorrow morning. A
cold front crosses the state through tomorrow morning from the
west-southwest with continued cold advection through Friday. Later
tonight and tomorrow shortwave ridging builds in, decreasing
coverage of rain/snow.
Despite the splitting nature of this system, the atmospheric river
tap along with decent cold advection should provide rather
widespread precipitation to the region. Mountains should average
5-10" with light accumulations for most valleys as well. Valleys
north of Salt Lake and also colder valleys of southern Utah should
see the most valley accumulations, ranging 1-4" most of these
locations. Southwest Utah above 5000 feet will likely see some of
the highest snow amounts due to proximity to the passing
shortwave. This event does not appear to meet warning thresholds
at this time, so have held off on any Watches. Next shift will
likely be issuing advisories as the event begins late tonight and
Most all model forecast soundings for the West Desert indicate
any warm nose quickly eroding at onset of precipitation, rapidly
cooling well below 0C due to evaporational cooling. Dont think the
freezing rain risk is high enough to carry in the forecast at this
The aforementioned Pacific jet will nose into northern Mexico
Friday. Increasing diffluence aloft and PVA from energy
associated with this jet streak should spread precipitation back
into southwest Utah by Friday afternoon.
.LONG TERM (AFTER 00Z SATURDAY)...
Global models continue to depict a very active pattern for the
long term portion of the forecast period. Strong jet maximum will
be taking aim at southern California and Arizona. Upper level
diffluence combined with somewhat upslope flow should provide
sufficient lift for widespread precipitation across at least
southwestern Utah Friday evening. While coverage will be less
across the north, favorable jet dynamics along with cooling 500mb
temperatures will be more than adequate for precipitation Friday
evening into early Saturday.
A fairly strong cold front is expected to cross the state Saturday.
Behind this front, deep, moist, unstable northwesterly flow will
help precipitation to continue across the mountains into early
A brief shortwave ridge will build in later Sunday morning into
early Sunday afternoon. As a 165kt+ jet maximum takes aim at Utah
once again, another round of precipitation can be expected Sunday
afternoon into Monday morning. Another stronger, cold front is
expected to cross the state Monday morning into Monday afternoon.
This will bring another round of deep, moist, unstable northwesterly
flow later Monday into Tuesday.
A few of the global models try to build a ridge into the West late
next week but given the active pattern, will wait a bit before
buying off on this pattern change.
Biggest takeaway remains that a very active pattern will set up
across the CWA from Friday evening through at least midweek next
week. These systems are cold enough to bring snow to the mountains
and most valley locations, though it is too early to talk specific
With inversion conditions in place, expect visibilities to
fluctuate between 5-6 miles. There is a 20 percent chance dense
fog will develop between 09Z-14Z.
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