Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 211033

Area Forecast Discussion
National Weather Service Salt Lake City UT
433 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.


.SHORT TERM (Through 12Z Sunday)...Isolated showers and
thunderstorms have continued to form across Utah and southwest
Wyoming this morning, as monsoonal moisture remains in place in
all but the far northwestern portion of the CWA. A shortwave
trough will continue to slide along Idaho and Montana through the
day, and increased westerly flow south of this feature will allow
drier air to slowly but surely push even further into the
northern third of the CWA.

For most of the forecast area, moisture will largely remain in
place, which should result in scattered showers and thunderstorms
forming again this afternoon, with the focus in the southern and
eastern half of Utah. Heavy rainfall remains the primary threat,
with precipitable water values hovering near 1 inch. Have decided
not to issue a flash flood watch for today, as the threat of
flooding is not expected to be widespread enough to justify the
watch, especially with the best instability and forcing remaining
east of the CWA. That said, there will continue to be some areas
of concern if heavy rainers remain over one place for too long,
particularly in the southeastern portions of the CWA, where slow
down-basin storm movement through the afternoon/evening could be
a problem.

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.

.LONG TERM (After 12Z Sunday)...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

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

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...There is a 20 percent chance of showers/thunderstorms
at the SLC terminal today, primarily before 18Z, with erratic
winds and lightning as the most likely impacts. Winds will likely
switch to the northwest around 17-19Z, with a slight chance that
southerlies persist past 19Z.


.FIRE WEATHER...Isolated thunderstorms continue across Utah this
morning, as monsoonal moisture remains over much of the state.
This activity will increase again this afternoon, with a focus on
the southern and eastern half of the state, as drier air begins
working into northern Utah. In areas that do get thunderstorms,
wetting rains are likely. The drying trend continues through the
weekend, but moisture is quick to return to the entire state for
the first half of next week.





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