Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 180356

Area Forecast Discussion
National Weather Service Salt Lake City UT
956 PM MDT Mon Jul 17 2017

.SYNOPSIS...Drier air moving into the area will lead to mostly hot
and dry conditions Tuesday. Monsoon moisture will spread into the
area Wednesday and Thursday and may bring the potential for heavy
rain to central and southern Utah.


.DISCUSSION...It was another active afternoon/evening across the
forecast area today. For northern Utah and southwest Wyoming, a
line of thunderstorms brought severe wind gusts (gusts > 57 MPH)
to the West Deserts, the Great Salt Lake, and the Salt Lake
Valley. In southern Utah, a training thunderstorm near Zion
prompted a flash flood warning which is still in effect as of this
writing. Area radars show that convective activity is winding
down, and high-res models suggest only a few isolated showers

Satellite water vapor imagery shows that our area remains on the
western periphery of an anticyclonic circulation centered near
eastern Colorado. However, a more subsident and drier air mass is
on our doorstep, associated with southwesterly flow aloft
stretching from California to Montana. This drier and more stable
air mass is expected to bring a relative lull in convection
tomorrow for much of our area, with any development limited to the
Uintas in northern Utah and the higher terrain in southern Utah.

Looking ahead, 00Z model runs this evening still show a more
notable monsoonal moisture push by midweek, bringing an elevated
flash flood threat to southern Utah and a better chance of
thunderstorms to northern Utah and southwest Wyoming

No forecast updates needed tonight.

Previous discussion:

.SHORT TERM (Through 12Z Thursday)...Upper level ridge centered
over the southern plains will shift slowly east through mid week.
Residual moisture in place across much of Utah this afternoon
resulting in scattered showers and storms across much of the
state. SBCAPE and precipitable water values are highest across
southern Utah although limited shear there lends storms more to
producing heavy rain than severe weather. Further north, near the
northern extent of the convection deep layer shear values of 30-40
knots indicate that a little more storm organization is possible.
Anticipate that convection will diminish after sunset in most
areas, although isolated showers may longer through the night
across southern Utah.

A drying trend will occur across northern Utah on Tuesday as the
upper high elongates bringing drier air into Utah. Anticipate that
storms will largely be confined to the southern half of Utah and
the Uinta mountains on Tuesday. On Wednesday, the models are in
good agreement that a strong monsoon surge brings deep moisture
into central and southern Utah and then into northern Utah
Wednesday night. Precipitable water values in excess are expected
for much of the state with values in excess of 1.25" across
southern Utah. GFS also indicates a a mid level shortwave moving
north out of Arizona on wednesday which would provide support for
more widespread convection. Certainly see the potential for flash
flooding across central and southern Utah with very deep moisture,
deep warm cloud layers, and the possibility of larger scale
forcing. As is always the case with such events timing of any
shortwave forcing is critical.

Temperatures will remain above normal across the area with
overnight lows remaining well above normal. Temperatures will drop
back a bit on Wednesday, especially across southern Utah, with
the increased coverage of showers and storms.

.LONG TERM...(After 12Z Thursday)...Deep moisture will continue to
stream into Utah Thursday, with PW values of 1.2-1.4" across
southern Utah and 1.0-1.2" across northern Utah. Thursday will
likely be another active convective day across Utah, though
extensive cloud cover could limit activity across central and
southern Utah. A shortwave trough moving slowly across central
Utah into SW Wyoming and upper level diffluence associated with
the right entrance region of a jet streak positioned across ID,
should allow for an increase in convective coverage across
northern Utah Thursday afternoon. A few organized storms could be
possible across far northern Utah near the UT/ID/WY border as 0-6
km bulk wind shear increases to around 40 knots.

A mid-level trough pushing across the Montana/Canadian border
will act to flattened the ridge and bring in drier air into
northern Utah Friday. A slow drying trend will take place through
the weekend across central and southern Utah, with isolated
convection expected across the higher terrain of southern Utah
Sunday and Monday, while northern and central Utah remain mainly

Forecast highs Thursday, Friday, and Saturday will remain near
average across Utah, before warming a few degrees above normal
Sunday into early next week as an upper level ridge begins to
amplify across the Western U.S.


.AVIATION...VFR conditions in store for the SLC terminal tonight
and tomorrow. Light southeast winds are expected to turn to
northwesterly around 19-21Z tomorrow.


.FIRE WEATHER...Upper level ridge remains in place over the southern
plains today with southwest flow aloft over Utah. Scattered
showers and thunderstorms this afternoon will decrease in coverage
by late evening. Drier air overspreads much of northern Utah on
Tuesday limiting the coverage of shower and storms to central and
southern Utah and in the Uinta Mountains. As the upper level ridge
shifts east by mid week, much deeper moisture will lift north
into Utah greatly expanding the coverage of showers and storms on
Wednesday and Thursday and also bring the potential for flash
flooding to southern Utah. It is not out the question that there
may be some dry thunderstorms on the leading edge of the moisture
across northwest Utah on Wednesday.

Relative humidity values will be lower on Tuesday, especially
across northern and central Utah where drier overspreads the
region. Much higher afternoon relative humidity readings can be
expected on Wednesday and Thursday for much of the state as
monsoon moisture spreads through the region.




UPDATE...Van Cleave
LONG TERM...Lukinbeal
AVIATION...Van Cleave

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