Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 181604

Area Forecast Discussion
National Weather Service Salt Lake City UT
1004 AM MDT Fri Aug 18 2017

.SYNOPSIS...A warming and drying trend will continue across the
area today. Moisture will slowly increase again from the south
this weekend and continue into early next week.


.Discussion...Water vapor satellite imagery this morning shows an
west-to-oriented deformation zone over southern Utah, with a few
altocumulus in the area. Elsewhere across Utah, generally
westerly flow aloft is in place. Area radars remain quiet at this

A quiet weather day is in store for the majority of the forecast
area. Highs are expected to be run a degree or two warmer than
yesterday, but still within a few degrees of normal for this time
of year. Northern Utah and southwest Wyoming will see mostly clear
skies, while southern Utah will see a few showers and
thunderstorms. Instability looks to be limited, so any
thunderstorms that form should be on the weaker side. On the other
hand, storm motions will be quite slow to almost stationary, so
there is a small flash flooding threat if storms develop over the
most sensitive drainages in southern Utah.

The big forecast question continues to be cloud cover for the
eclipse Monday. 12Z runs that arrived this AM continue to suggest
thicker cloud cover over southern Utah, with thinner and higher-
based clouds over central/northern Utah and southwest Wyoming.
Shower and thunderstorm activity looks more significant over
southern Utah, with perhaps high-based showers through the
afternoon elsewhere. To our north into Idaho, an upper-level
trough axis looks to push through around the time of the eclipse,
bringing drier air and clearing most locations except some high
clouds in far SE Idaho. The farther west you go into Idaho, the
better the chances of clear sky conditions. To our northeast in
Wyoming, high clouds look to remain over much of the state except
perhaps the SE corner of the state.

No changes need to the previous forecast, so no updates this am.
See previous discussion for further forecast details.

Previous discussion...issued 429 AM MDT...

.SHORT TERM (Through 12Z Sunday)...Dry and stable northwest flow
aloft prevails across the forecast area early this morning, where
GOES derived PW values are generally around 0.5-0.6 inches in the
lower elevations, and surface dew points are in the mid 30s to mid
40s. This should result in a dry day across most of the forecast
area, with a small threat for high based convection across the
higher terrain of southern Utah.

The flow aloft is forecast to back heading into the weekend,
allowing an upper low currently along the southern California
coast to begin pulling moisture northward into the forecast area.
Initially Saturday into Saturday night this moisture will be high
based, as it spreads into central and portions of northern Utah.
For now have maintained low PoPs across areas from roughly the
western Uintas/southern Wasatch Front southward into central Utah.

.LONG TERM (After 12Z Sunday)...Global Models suggest some
increase in moisture Sunday and Sunday night and even into Monday,
as upper level ridge axis shifts east of Great Basin,
facilitating a southerly, though still weak, flow. With moisture
advection, precipitable water values will increase to three-
quarters of an inch to an inch. Models also point to a weak mid
level disturbance crossing region, though GFS still a bit less
progressive. All considered, existing slight chance to chance
wording for primarily southern and eastern portions of forecast
area justified, though some potential exists for northwest and
north central Utah as well.

With additional mid level disturbances rotating through the
region, the overall pattern through through Thursday looks a bit
unsettled, with a potential for convection each day.

A trough of low pressure will glide across the northern Rockies
late Thursday through Friday. This will facilitate a somewhat
stronger and drier flow.

High temperatures during the period will remain mostly unchanged,
generally near seasonal normals.


.AVIATION...With a very dry and stable air mass over the region
today, VFR conditions will prevail at KSLC through the TAF period.
A light southeast drainage wind should turn to the northwest
around 18Z, though the switch could be delayed an hour or so.


.FIRE WEATHER...A stable west-northwest flow prevailing across the
region will maintain warm and dry conditions today, with a small
threat for high based showers/thunderstorms across the south. A low
pressure developing over southern California will help flip the
larger scale flow around to southerly this weekend into early next
week. This will allow for a gradual influx of moisture into the
region. Initially this moisture will be high based this weekend,
but gradually increase by early next week allowing for a better
chance for showers and thunderstorms.




UPDATE...Van Cleave

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