Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 152204

Area Forecast Discussion
National Weather Service Salt Lake City UT
404 PM MDT Tue Aug 15 2017

.SYNOPSIS... A weak weather disturbance will enter western Utah
late Wednesday afternoon, then impact mainly southern Utah
through Thursday. Unseasonably cool temperatures today will begin
to climb back to seasonal norms during the balance of the week.


.SHORT TERM (Until 00z Friday)...The weak upper trough passing
through Utah this afternoon will continue east and exit the state
this evening. The best convection associated with this feature
exist across central through northeast Utah along what is left of
the near 700mb baroclinic zone from earlier today. Also seeing a
few storms popping up behind the trough axis where daytime heating
and weak instability combined to form a few storms. Suspect all of
this activity will wind down early this evening as the trough axis
exiting the area and daytime heating wanes.

The remnant of the cold front that stalled across southern Utah
earlier today still serving as a focus for light showers near the
Arizona border. This weak convection is not expected to
strengthen, and should dissipate or move east of the area by early
this evening.

A second upper trough will reach far western Utah by late
Wednesday afternoon or early evening. This feature will encounter
at best a marginally unstable air mass due to decent surface
heating. This feature will be poorly organized as it tracks east
across the state. The only things working towards convection will
be somewhat cooler mid-level air and the weak dynamic support.
Will bring some loosely organized convection through mainly the
southern half of the state during the day, with this activity
confined to eastern half of the state by late in the day.

.LONG TERM (After 00z Friday)...An upper level ridge centered
near 40N and 130W with an upper level trough positioned across the
high plains and Great Lakes regions will keep a dry and warm
northwest flow in place across Utah Friday and Saturday. Just enough
moisture will linger around (PW values of 0.3-0.5" over the
mountains and 0.50-0.75" across the valleys) in the mid levels to
keep minor instability in place, allowing afternoon and evening
convection over the higher terrain. Overall, expect a drying and
minor warming trend heading into the weekend as high temperatures
push near or slightly above seasonal normals.

Looking ahead to late this weekend and early next week, the likely
question on many minds is what the weather will be like for northern
UT, southwest WY, and surrounding areas near the eclipse path,
especially Monday around noon. The short answer is the current
forecast is not optimistic for completely clear skies, although
forecast confidence remains limited this far out in time.

The consensus of today`s 12Z model runs for late next Sunday through
Tuesday is to develop a closed low off the California coast.
Southwest flow ahead of this feature would bring increased mid-level
moisture to much of our area, which combined with some weak synoptic
lift aloft (a weak jet aloft approaches our area and diffluence
develops overhead) would bring a threat of showers and
thunderstorms. Looking at model ensembles in addition to
deterministic runs, there is moderate confidence in the closed low
forming around that time, but there is limited confidence in its
position and magnitude. Those attributes will ultimately determine
how far north the enhanced moisture and synoptic support reaches,
which greatly affects the forecast near the eclipse path. For
example, the deterministic ECMWF has the low much weaker and farther
south, such that northern UT and southeast ID would more than likely
remain dry and clear. Our current forecast remains largely unchanged
and trends towards climatology, which translates to a chance of
showers/thunderstorms over our higher terrain, especially the
mountains of southern Utah and in the Uintas. Stay tuned as the
eclipse forecast details should become clearer in the coming days.


.AVIATION...VFR conditions will prevail at SLC throughout the TAF
period. Northerly winds will shift to the southeast around 03-04Z
per usual.


.FIRE WEATHER...The cold front currently stalled across southern
Utah has drawn cooler air into far northern through western Utah.
A weak weather disturbance continues to generate showers/isolated
thunderstorms across central and northeast Utah this afternoon.
This convection should end quickly this evening as the weather
disturbance exits off to the east. Lingering light showers along
the old cold front will also end over far southern Utah early this

A trailing weather disturbance will enter western Utah late
wednesday, then track slowly east across the state through
Thursday. Showers and thunderstorms that develop in association
with this feature will be of limited areal coverage are not
expected to generate wetting rains.

Temperatures will begin to climb Wednesday, then reach seasonal
norms by the end of the week. Humidity values will trend downward
through the balance of the week.





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