Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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006
FXUS65 KSLC 260353
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
953 PM MDT Tue Jul 25 2017

.SYNOPSIS...A couple of weak weather disturbances will cross
northern Utah tonight and again Wednesday. A gradual drying trend
is expected for the latter portion of the week as high pressure
aloft strengthens across the region.

&&

.SHORT TERM (Through 00Z Saturday)...A shortwave currently over
northeast Nevada is supporting a fairly well defined line of
convection along the Utah/Nevada border at mid-evening. This band
will likely hold together the next couple of hours, then shrink a
bit and remain closely tied to the best dynamic lift and elevated
instability across far northern Utah late tonight.

Down south dynamic subsidence behind an exiting shortwave over
northwest Colorado has brought a quick end to convection this
evening. Can`t rule out a few convective elements working north
out of northeast Arizona later tonight, but doubt that any
organized convection will reform overnight through early
Wednesday.

PWat values looks to stay elevated through Wednesday. Light flow
aloft, abundant moisture and good differential heating should fire
off terrain-based convection across southern/central Utah
beginning late morning. Northeast Utah may get a little slower
start as some lingering cloud cover and fairly stable rain-cooled
air inhibit convection through a good portion of the morning.

Far northwest Utah may a have a little more success generating
convection a series of vorticity lobes eject out ahead of the
weakening trough moving through the western Great Basin.
This convection will likely not stray too far from the best
dynamic support, which will leave the best threat near the Idaho
border and far northern mountains.

Convection should clear out after sunset Wednesday evening, then
fire up once again over the higher terrain Thursday. Somewhat
drier air will work into the state beginning Thursday as the flow
aloft turns a bit more westerly . The gradual drying trend will
continue into Friday, with the best available moisture once again
pinned down near the Arizona border region.

.PREVIOUS LONG TERM (After 00Z Saturday)...A mid-level ridge will
be centered just SE of the forecast area near the four corners at
the start of the long term period. Additionally a shortwave
trough will be centered near the California coast. This setup will
place the fcst.area under southwesterly flow at mid-levels with
weaker more terrain- driven/erratic flow at lower levels. Enough
residual moisture remains over at least the southern/eastern parts
of the fcst.area to spark at least isolated convection Friday and
Saturday afternoons. Weak waves breaking off from the larger
Pacific trough will potentially provide enough dynamic lift to
generate some cumulus or weak t-storms over Northern Utah Saturday
afternoon, but think this area will be too moisture-starved to
include higher PoPs at this time.

Heading into Sunday guidance diverges considerably with the
EC/Canadian building a mid-level ridge over Nevada while the
deterministic GFS slowly propagates the four-corners ridge
northward ultimately centering it over the eastern portion of
Utah. The GFS solution would allow for fairly good moisture
transport on the western periphery of the ridge and the GFS is
quite bullish on PoPs/QPF for Sunday and Monday and is also cooler
with regards to max-temps due to precip/cloud cover The EC/CMC
solution would keep the area on the eastern periphery of the
amplifying Nevada ridge which would result in a much drier
northerly flow likely squashing precip chances over most of the
region. Leaned towards the drier/warmer EC/CMC solutions this
package because there is also support for this solution from a
number of members of the GFS ensemble. It seems probable that a
large source of the current model divergence is due to the
possible (difficult-to-model) interaction between EPAC tropical
cyclones (Hilary and Irwin) the mechanics of which may have
significant impacts on the evolution of the western US synoptic
pattern.

&&

.AVIATION...The shortwave moving through northeast Nevada this
evening has generated a line of showers/thunderstorms currently
near the Utah/Nevada. This line will become less organized as it
advances east, with mostly isolated over/near the terminal after
around 08z. The lingering showers and storms should push north of
the terminal around 13z-14z. VFR conditions will prevail, though
ceilings could briefly drop to around 6 kft in heavier precip.

Prevailing winds ahead of the convection will be out of the south,
then turn to the west-northwest as this activity nears the
terminal around 08z. Light northerly winds will exists late
tonight through much of the day Wednesday.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

Conger/Carr

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