Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 211542
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
942 AM MDT Fri Jul 21 2017

.SYNOPSIS...Drier air will gradually spread back into the area
from the northwest today and Saturday. Moisture will start to
spread back into Utah Sunday night and Monday.

&&

.DISCUSSION...Mid level moisture trends continue to dry from the
northwest this morning as flow aloft trends more west/northwest in
response to the northern branch becoming more zonal in nature.
Surface dewpoints remain elevated across the area however (largely
remaining in the 50s), this indicating plentiful low level
moisture in place. 12z KSLC RAOB supports this with the bulk of
PWAT (1.18") holding below H5, with a significant dry slot in
place between H5-H3. The deepest moisture continues to hold pat
across namely central/southern Utah, with peak values over the
southeast.

Updated earlier to reduce morning PoPs across the west, while
shifting a slight chance for convection north into the central
Wasatch Front for this afternoon. Shear continues to be minimal
for most areas, though SPC mesoanalysis and RAP do indicate
potential for 30 kts across the far north. This coupled with
plentiful low level moisture and dry slotting aloft, expect to see
a few deeper updrafts forming as far north as KSLC this pm.

Primary focus later this shift will be to monitor any heavy
rainers that develop across the central/southern/eastern portions
this afternoon. Aforementioned steering flow now supports
potential for down/drainage propagation across the
south/southeast, but lack of any notable features/shear across the
south should keep flash flood threats more isolated to the most
prone basins, thus no flash flood watch attm. Something to watch
close though. Previous discussion below...

PREVIOUS DISCUSSION...The drying trend continues through Saturday
across the north, as the northern trough exits the region and
ridging eventually amplifies a bit over the Great Basin. This will
lead to a warming trend in the afternoons, from the near-normal
temperatures today to a few degrees above normal temperatures on
Saturday.

The ridge of high pressure will be centered just west of the
forecast area at the start of the period, with a drier air mass
still still in place over much of the northern half of Utah.
Moisture will remain in place over southern Utah, with a moisture
axis extending west and northwest into Nevada. Thus, there will be
a potential for showers and thunderstorms during the afternoon
and evening over west central and southern portions of Utah.
Showers and thunderstorms will be possible from the central Utah
mountains to the Uintas as well.

Models suggest a weak upper level disturbance will drift into
northern Utah Sunday night, supporting slight chance wording for
showers and thunderstorms over northwest Utah, with a slight chance
for convective activity to continue over west central and southern
Utah.

Deeper moisture will spread across forecast area late Monday
afternoon and night, promoting a chance for showers and
thunderstorms.

Models begin to diverge for Tuesday through Thursday period, with
GFS indicating a somewhat vigorous upper level disturbance and
associated cold front sliding across the Great Basin Tuesday and
Wednesday.  Given ample moisture with precipitable water values in
excess of an inch, mid level disturbance, and diffluent flow aloft,
GFS suggests a rather active period of showers and thunderstorms,
with some potential for severe weather given increasing shear. Drier
air then progged to advect into region Thursday. ECMWF on the other
hand, keeps base of upper level trough along Pacific Coast, and
ejects a series of weaker upper level disturbances across Great
Basin from Tuesday through Thursday. Majority of GFS ensemble
members support GFS deterministic run.  All considered, will keep
mention of showers and thunderstorms for Thursday, though will scale
back chances a bit.


&&

.AVIATION...Southeast winds currently in place at the KSLC
terminal are expected to switch to the northwest between 18-19z
per norm. A 20 percent chance exists that buildups and isolated
storms will be in the vicinity after 21z, most likely forming off
the Oquirrhs and tracking east across the southern approach.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

DISCUSSION/AVIATION...Merrill
PREVIOUS DISCUSSION...Schoening/Barjenbruch

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