Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 291008
Area Forecast Discussion
National Weather Service Salt Lake City UT
408 AM MDT FRI JUL 29 2016
.SYNOPSIS...A ridge of high pressure from the southern Rockies to
southern California will remain in place over the next few days.
Moisture rotating around this ridge will slowly increase across
the region today and Saturday, then remain in place next week,
with the exception of over northern Utah and southwest Wyoming,
where drier air may spread in.
.SHORT TERM (THROUGH 06Z SUNDAY)...Hot and dry high pressure
remains parked over the forecast area early this morning, but a
bit of monsoonal moisture has started to make its way into far
southwest Utah. Satellite derived PWs indicate values around
0.9-1.0 in that part of the state, compared to 0.5-0.7 inch for
most other locations. This moisture will continue to slowly move
into southern Utah today, allowing for a bit of high-based
convection to develop this afternoon. Little precipitation is
expected with anything that materializes, but should produce some
lightning and gusty outflow winds, enough to result in areas of
critical fire weather conditions. See the fire weather section of
the AFD for more details. Despite the increase in moisture over
southwest Utah, temperatures will remain quite hot in the lower
elevations of Utah`s Dixie, including the city of St. George, and
an Excessive Heat Warning remains in effect.
The ridge is progged to flatten Saturday as a Pacific Northwest
trough moves into southern Canada. This is expected to flip the flow
aloft to a more west/southwest direction, allowing the moisture to
spread across much of the rest of the forecast area, allowing for
better coverage of showers and thunderstorms Saturday afternoon.
Shortwave energy rotating into Utah will also act to increase
convective coverage. Lower levels will still be on the dry side, but
have kept POPs in the scattered range despite this. The increased
cloud cover and weakening of the ridge will also allow for slight
cooling over the forecast area, primarily over southwest Utah.
Saturday afternoon maxes should be around 5F above climo over
southern Utah, but still more like 10F above seasonal norms for
northern Utah and southwest Wyoming.
.LONG TERM (AFTER 06Z SUNDAY)...The ridge of high pressure from
the southern Rockies to southern California will shift to the
south and west a bit on Sunday, facilitating a somewhat drier
west to northwest flow for the northwest corner of Utah. Over the
remainder of the forecast area, a west to southwest flow will keep
moisture in place through Monday...with precipitable water values
of .80 to 1.00". With associated atmospheric instability, going
isolated to scattered wording for showers and thunderstorms looks
best, with best coverage over southern and eastern Utah where
deepest moisture in place. Vertical sounding profiles indicate air
mass will gradually moisten from the top down. This said,
thunderstorms could still produce gusty outflow winds, as some
drier air remains in lower levels.
GFS and ECMWF in general agreement with sliding an upper level
trough of low pressure from the Pacific Northwest through the
northern Rockies Tuesday through Thursday, producing a somewhat
drier west to northwest flow across at least northern Utah and
extreme southwest Wyoming. Some moisture will remain in place
across the southeast two-thirds of Utah with precipitable water
values still in the .75 to 1.00 inch range. Thus, will keep
isolated to scattered wording for showers and thunderstorms across
southern and eastern sections. Aside from across the higher
terrain, generally dry conditions should prevail across the
northwest third of forecast area.
.AVIATION...The SLC terminal will see VFR conditions throughout the
day with a few clouds. Light southeast winds are expected to shift
to the northwest between 17Z and 19Z.
.FIRE WEATHER...High pressure over the fire district will weaken
through this weekend. This will allow some monsoonal moisture to be
drawn northward into southern Utah today, causing the development of
some high-based showers and thunderstorms by this afternoon.
Lightning and gusty outflow winds are possible with any convection
that develops, and this will combine with the critically dry fuels
to result in severe fire weather conditions. The moisture spreads
northward into most of the rest of the fire district tomorrow,
resulting in increased coverage of showers and thunderstorms and a
larger area for potentially hazardous fire weather conditions.
Because of this threat, Red Flag Warnings and Fire Weather Watches
are in effect for much of the district. Conditions will remain quite
warm across the district over the next few days, but temperatures
will decrease somewhat with the weakening of the ridge and
increasing coverage of convection.
UT...Red Flag Warning from noon today to 10 PM MDT Saturday for
Fire Weather Watch from Saturday morning through Saturday
evening for UTZ478>484-488-489.
Excessive Heat Warning until 9 PM MDT this evening for UTZ019.
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