Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 210407

Area Forecast Discussion
National Weather Service Salt Lake City UT
1007 PM MDT Thu Jul 20 2017

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


Discussion...Infrared satellite imagery this evening shows an area
of convective activity stretching from northern Mexico, through
the Four Corners states including Utah, and northeastward into
the northern Plains. Water vapor shows a drier and more subsident
air mass moving eastward across the Great Basin and approaching
the Nevada/Utah border. PWAT analysis from GOES-16 and the Blended
TPW product shows a notable gradient across the Great Basin from
0.2" in western NV to > 1" in southwest Utah. Forecast models
bring this drier air into the northern portion of our forecast
area tomorrow.

Closer to the surface, a few areas of convective activity continue
on area radars, with the main areas of activity being the west
deserts of Utah and along the far NE corner of the state near Bear
Lake. Recent SPC mesoanalysis shows features which help explain
current convection, specifically an axis of elevated instability
roughly across central Utah and better shear farther north along
the UT/ID border. The last few runs of the HRRR have a good handle
on these features, and continue the idea of isolated to scattered
showers/thunderstorms past midnight, with the best chance being
across central Utah. Have tweaked the forecast slightly to
indicate at least some chance of nocturnal thunderstorms for most
of the state.

Looking ahead, drier weather is in store for western and northern
Utah valleys into this weekend while afternoon storms continue
for most of the mountainous areas. Next pulse of monsoon moisture
looks to occur by late Sunday into early next week. See
discussion below for more details.

.SHORT TERM (Through 12Z Sunday)...

The ridge over the area is expected to flatten by tomorrow as a
trough moves over the top of it, shifting the flow aloft to a more
westerly direction. This will allow a drying trend over the forecast
area, primarily over northern Utah and southwest Wyoming. GFS
indicates PWs over northern and central Utah dropping to the 0.5 to
0.7 inch range by Saturday afternoon as PWs in excess of 1.0 inch
persist over southern Utah. As a result, coverage of convection will
decline significantly over primarily the northern half of the
forecast area while the threat of thunderstorms with heavy rain
continues for southern Utah.

.LONG TERM (After 12Z Sunday)...The ridge over the region remains
centered to the west of UT at the start of the long term period
with the drier air that had spread into the CWA still in place
across the north and east. Deep moisture will remain pooled along
the AZ border extending northwest thru NV. Expect most convection
Sun will remain limited to the south but isolated storms will be
possible up the mountain spine into the Uintas, and northward
along the NV border.

The high is forecast to shift east thru Mon allowing the moisture
to our south and west to spread back into the CWA with a
corresponding increase in convection. The deepest surge looks to
arrive Tue altho guidance begins to diverge at this point. The GFS
has a short wave that moves into the northwest states and nrn
Rockies on Tue, then spreads drying over the north for Wed while
the EC keeps the deep moisture in place thru Wed and lifts the
wave weakly past on Wed night with only a little drying Thu.
Raised pops to scattered for the entire CWA for Tue and kept a
mention area wide thru Thu, but confidence in the forecast drops
off quickly after Tue.


.AVIATION...Southerly winds will prevail at the SLC terminal
through the rest of this evening and overnight, though decreasing
in general as the night goes on. There is a 30 percent chance
that showers and thunderstorms in the vicinity of the terminal
will result in periods of erratic winds between about 05-08z.


.FIRE WEATHER...Monsoonal moisture remains over the fire district
this afternoon on the back side of an upper ridge. Showers and wet
thunderstorms have developed, primarily over the higher terrain,
and will continue into the evening. Heavy rain will be the primary
threat with any thunderstorms. Given the deep moisture, relative
humidities will remain high and southerly flow will remain breezy.

Drier air will start to move into the district from the northwest
beginning tonight, and this drying trend will continue into the
weekend. The most noticeable drying will occur over northern Utah,
with deeper moisture remaining pooled over southern Utah through
the weekend. Moisture is expected to return to the district early
next week.

UT...Flash Flood Watch until midnight MDT tonight for UTZ010-012>016-



UPDATE...Van Cleave

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