Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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305
FXUS65 KSLC 191157
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
557 AM MDT Wed Jul 19 2017

.SYNOPSIS...Monsoon moisture spreading into the forecast area
today and tomorrow will increase shower and thunderstorm activity
and bring a potential for heavy rain. Drier air will spread back
into the area from the northwest Friday into Saturday.

&&

.SHORT TERM (Through 12z Saturday)...The ridge axis remains east of
Utah this morning. A shortwave disturbance embedded in the
easterlies across northern Arizona and far southern Utah produced a
small thunderstorm complex which was associated with heavy rain and
numerous lightning as it crossed far southern Utah overnight. This
complex now over southwest Utah is becoming more stratiform as the
environment has modified with rain-cooled air. The shortwave
disturbance is expected to lift north across southwest Utah today
and into northeast Utah tonight. This will spread deep moisture
through much of southern and central Utah, with PWATs forecast to be
in the 1.1-1.4 inch range. There is a potential for heavy rain and
associated flash flooding in prone drainages. However, cloud cover
and the aforementioned cooler surface environment may serve to limit
instability necessary for deep convection in some areas.

The flow will maintain a bit of a westerly component across far
northwest Utah today, so there is expected to be a significant
moisture gradient across that area. Showers and thunderstorms
developing on this northern fringe of the moisture surge may remain
on the drier side and and produce strong gusty winds. Moisture may
increase briefly across northwest Utah overnight.

The moisture will remain in place across much of the area tomorrow,
although the northwest will start to dry again tomorrow afternoon.
Much of southern and eastern Utah will again see showers and
thunderstorms with a potential for heavy rain.

Drier air will spread through much of northern Utah beginning on
Friday and continuing into Saturday as a shortwave trough crossing
the northern Rockies flattens the ridge. The moisture will thus
become increasingly pinned across southern through east-central Utah
where the bulk of convection can be expected.

.LONG TERM (After 12Z Saturday)...A broad ridge remains over the
Intermountain West on Saturday, with deep moisture confined to
southern and eastern Utah, while the north generally stays drier and
more stable. This should generally make temperatures and
distribution of precipitation on Saturday similar to Friday.

A shortwave trough moves into northern California from the Pacific
on Saturday, but after that models are struggling on how this
feature advances across the west. It currently looks to cross Utah
sometime late Sunday through late Monday, and when it does it will
probably draw some moisture back into northern Utah and bring
increased instability state-wide. Have increased POPs and cloud
cover a bit Sunday afternoon through Monday evening to account for
the possibility of this wave, and higher POPs may be justified in
the future if the models can lock in the details on timing/placement
of this disturbance.

Details heading into next week are fairly low confidence, though
slightly above normal temperatures will likely continue into the
last week of July. Have maintained POPs near climatology through
Tuesday and Wednesday for now, and a possibility of increasing
southerly winds with mean trough over the Pacific Northwest and a
mean ridge centered somewhere southeast of Utah.

&&

.AVIATION...There is a 20 percent chance of showers/thunderstorms
impacting the SLC terminal today, with gusty erratic winds as the
most likely impact. Away from storms, winds are expected to switch
to the northwest between 19Z and 21Z, though there is a chance that
southerlies persist past 21Z.

&&

.FIRE WEATHER...Deeper moisture will spread north through southern
and central Utah today, leading to more widespread showers and wet
thunderstorms for these areas. Northwestern and far northern Utah
will be on the northern fringe of this moisture surge and may see
drier storms producing gusty outflow winds. The moisture will remain
in place across much of the area Thursday except for a bit of drying
across the far northwest during the afternoon. The moisture will
lead to cooler temperatures and much higher humidities for much of
the fire district.

More drying from the northwest will take place on Friday and
Saturday, limiting the bulk of showers and thunderstorms to southern
through east-central Utah. Latest models indicate some moisture will
return to northern Utah for the early part of next week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flash Flood Watch from 10 AM MDT this morning through this
     evening for UTZ010-012>016-019>021-517-518.

     Red Flag Warning from 11 AM this morning to 10 PM MDT this
     evening for UTZ478.

WY...None.

&&

$$

Cheng/Schoening

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