Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 202203

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

.SYNOPSIS...Monsoon moisture will persist over most of Utah
through today. Drier air will gradually spread back into the area
from the northwest Friday into Saturday. Moisture start to spread
back into Utah Sunday night and Monday.


.SHORT TERM (Through 12Z Sunday)...Deep moisture remains over the
forecast area this afternoon, with satellite derived PWs in the
1.0 to 1.2 inch range, near the maximum observed this time of
year. Despite ample moisture, instability is on the low side
without a well defined forcing mechanism, so afternoon convection
is primarily confined to the higher terrain so far. Still,
expecting coverage of convection to continue to increase into the
early evening, and with enhanced southerly flow, would think at
least some of the storms would come off the terrain. Given the
very moist airmass and the possibility of heavy rain with any
storms that develop, a Flash Flood Watch continues through the
evening over central and southern Utah. Under high pressure, maxes
are running near seasonal normals over northern Utah and a little
below over southern Utah.

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 today and this evening, gusty at times, but
there is a 20 percent chance of a shift to the northwest between
22Z and 00Z. 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 23-04z.


.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-




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