Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 291508
Area Forecast Discussion
National Weather Service Salt Lake City UT
908 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.
Water vapor loop shows ridging over the southwest CONUS. MDCARS
wind observations show a 50-70kt northwesterly jet from the
Canadian Rockies into the plains. GOES/HRRR/12Z KSLC RAOB
indicate the precipitable water value ranges from 0.25"-0.35"
northern mountains to 0.70"-1.00" far southern valleys.
Instability axis most pronounced across southern Utah. RAP
indicates several weak waves rotating around the ridge from
southern Nevada into southwest Utah. As a result the convection
today will be focused across the south with more stable conditions
across the north. This aligns well with the HRRR and local WRF
Storms today will carry the risk of locally strong wind gusts
given the inverted-v profiles.
Also increased sky cover based on satellite trends and latest
guidance. Tweaked temperatures down a couple notches especially
south given the amount of clouds. Still close enough to carry the
Excessive Heat Warning for Dixie.
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.
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
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...Southerly winds at the SLC terminal are expected to
shift to the northwest between 18-20Z today. Otherwise, little
threat of convection continues into today.
Primary concerns revolve around new starts due to lightning during
the next several days. A Red Flag Warning has been issued for
portions of central and southern Utah Friday and Saturday, and a
Fire Weather Watch has been issued for northern Utah Saturday.
Yesterday`s ERC values were above the 90th percentile for Central
and Northern Utah, with areas greater than the 80th percentile
across Southwest Utah.
Thunderstorms today will be most prevalent across south central
and southern Utah. Tomorrow the threat becomes widespread across
the region. Many of the thunderstorms will contain little
rainfall, but gusty and erratic winds. This combination will not
only support new fires, but also lead to explosive fire growth
Otherwise hot conditions will begin to recede this weekend due to
the greater coverage of convection. The same can be said about RH
values, very dry today will trend a bit more moist each coming day
through the weekend.
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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