Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 222211

Area Forecast Discussion
National Weather Service Salt Lake City UT
411 PM MDT Fri Sep 22 2017

.SYNOPSIS...The upper low over northern Nevada will head southeast
into southern Utah late Saturday, then turn northeast towards
Wyoming late in the weekend. Unseasonably cool temperatures will
cover much of Utah, with periodic valley rain and mountain snow
concentrating on the northern and central portions of Utah and
into southwest Wyoming through the weekend.


.SHORT TERM (Until 06z Tuesday)...The upper low currently over
northern Nevada will continue on a slow southeast drift toward
southwest Utah by late Saturday. From There the low will migrate
east across southern Utah, then takes a hard turn to the northeast
into southwest Wyoming by late in the weekend. Unseasonably cool
temperatures with an increasing threat for precip will exist over
the weekend.

Increasing convection along the east side of the upper low is
associated with vorticity lobe lifting north-northeast through
northeast Nevada. This feature may bring a short period of
increased convection to northwest Utah this evening. Dynamic lift
from the vorticity lobe should get a boost as a developing jet
streak forms across far northwest Utah increases upper divergence
and aids convection. Not anticipating any significant activity
with the areal coverage remaining scattered and ending over most
of the area by around midnight.

Another concern overnight will be the potential for below freezing
temps in areas where active gardens and agricultural activity
still exist. Over southwest Utah clearing skies this evening,
light winds and dew points only in the 30s will lead to temps
falling quickly this evening. A secondary vorticity rotating
around the upper low could bring some mid-level clouds back to
the southwest in time to avoid an hard freeze, though suspect that
isolated pockets of mid to upper 20s will form in the normally
colder outlying area. Up north the higher valleys and outlying
areas will likely see more cloud cover with some additional
precip overnight. This may be sufficient to save vegetation from
an hard freeze, though some of the normally colder areas of the
Wasatch mountain valleys and Cache valleys could hit critical
temps late tonight, especially if some clearing develops.

The areal extent of convective precip will likely be at a minimum
much of Saturday prior to the arrival of the upper low over
southwest Utah. Precip will then begin to increase in areal
coverage late Saturday/Saturday night as mid-level warm advection
wrap around develops on top of the cool/moist air mass over
western/northern Utah. This favorable wrap around pattern will
persist as the upper low lifts northeast towards Wyoming Sunday.
Somewhat concerned about southwest Wyoming and northeast through
far northern Utah receiving heavy precip late Sunday through
Sunday night. Both the GFS and ECMWF show an uptick in QPFs during
that period, though the ECMWF puts the maximum values further to
the east across the northeast Utah mountains through southwest
Wyoming. Much of this precip based on temps aloft could end as
snow, with significant accumulations possible. No highlights at
this time due to some uncertain on the focus of heavy precip, and
the expectation that amounts and impacts will be more advisory

Precipitation will wind down Monday as the upper low lifts to the
northeast and weakens during the day. Replacing the low will be a
north-northwest flow which will persist across the Great Basin
much of the week.

.LONG TERM (After 06Z Tuesday)...A mean trough remains over much of
the western CONUS heading into Monday night, with one shortwave
exiting to the east of the forecast area through Colorado, and
another digging south along the Utah/Nevada border. This second
wave will continue to move southward through the day Tuesday,
before beginning to cut off into a closed low over Arizona on
Wednesday. The main impact with this wave will be to reinforce the
cool airmass over the area for the beginning of the week.
Initially, this wave has little additional moisture associated
with it, so precipitation looks unlikely in most locations Monday
night through Tuesday night.

This lack of moisture could change starting Wednesday, depending on
where/how this closed low sets up. The GFS continues to keep
conditions fairly dry across Utah and southwest Wyoming through the
end of the week, as the low digs too far south/east to impact Utah,
before it dissipates through Friday and Saturday.

The ECMWF, on the other hand, has had a couple of runs now where the
closed upper low sets up further north and/or west, which could
bring some additional moisture into Utah from the southeast
Wednesday through Friday. This would primarily impact the southern
half of the CWA, though there remains a lot of uncertainty in all of
these solutions. For now, have introduced some slight chance POPs to
the southeastern quadrant of the CWA Wednesday through Friday, and
kept temps in that area from warming too quickly, to account for the
EC solution.

Either way, a warming trend looks likely for the northern half of
the area through at least Thursday, bringing high temperatures
for mid-to-late week close to normal values for late September.


.AVIATION...Periodic ceilings below 3000 feet will remain possible
at the SLC terminal through roughly 00Z with passing rain showers,
with any low ceilings expected to scatter out after 00Z. Northwest
winds will prevail through the evening before switching back to the
south around 05Z.







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