Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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839
FXUS65 KSLC 252118
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
318 PM MDT WED MAY 25 2016

.SYNOPSIS...The elongated upper level trough over the Great Basin
will shift slowly across the state through Thursday. Unsettled
conditions associated with this trough will persist through the
upcoming weekend.

&&

.SHORT TERM (Through 12z Saturday)...A Deformation axis
stretching from northern Utah into central Nevada is quite
apparent in afternoon water vapor, while subtle ridging exists
across the far south and southeast. A weak upper level circulation
exists along the OR/ID border sagging south, while a closed low
in SoCal continues to slowly churn east. The latter will begin to
impinge on northern Arizona/southern Utah overnight, while the
former will do the same on northwestern/west-central Utah and NE
Nevada.

Area of best instability and forcing remains within the aforementioned
deformation axis this afternoon. Numerous showers and a few storms
exist along the UT/ID border, with coverage become more scattered
then isolated in nature working south to Utah County and west to
Great Basin NP. These coverage trends should peak over the next
few hours then gradually wane this evening with loss of solar and
diurnal instability.

Interaction of these waves will, however, continue to enhance the
deformation overnight into tomorrow, with a net south-southeast
translation of its axis coincident with the closed low`s eastward
progression. Still expecting namely isolated nocturnal showers to
develop over the southern mountains late this evening and
overnight tied closely to the northern periphery of the closed low,
and isolated coverage associated with the deformation axis and
upstream wave.

Expecting areal coverage of showers and storms to expand to most
all areas tomorrow, influenced by both the remnant deformation
(then bisecting the area) and upstream wave working south along
the UT/ID border. Opted to increase PoPs across the south and west
where these features will have the most impact. Convection should
remain quite benign with any deeper updrafts short lived, but
capable rainers.

The axis of the mean long wave trough will finally shift east
tomorrow night through Friday allowing a northwesterly flow aloft
to spread into the area. Suitable moisture/instability will exist
for a continuation of diurnal convection, though coverage trends
will be waning and trending more tied to the terrain come Friday.

.LONG TERM (After 12Z Saturday)...A broad upper trof remains to the
north of the CWA at the start of the long term period with latest
GFS showing a short wave lifting out of the nrn Rockies and
subsidence spreading into the northern CWA in its wake. The EC
roughly agrees so have lowered pops for the north Sat into Sat eve
and raised them over the south.

A weaker trof splits south into CA on Sat as well and this feature
is forecast to bring a return of convection to much of the CWA
starting later Sat night and continuing thru Sun night. The EC
phases a nrn branch trof with this weak wave and shifts the main
trof axis into the eastern CWA Mon aftn with drying for the west.
Meanwhile the GFS follows a similar solution but does not shift the
main trof axis east of the CWA until almost midday Tue. This
solution keeps convection fairly active on Mon and maintains a
threat of rain for Memorial Day activities.

Have leaned towards the GFS solution here and kept a mention of rain
thru Mon as just a slight slowing of the trof in the EC would bring
it in line with the GFS.

Both models bring a ridge into the Great Basin by midweek for a
break in the cooler wetter pattern.

&&

.AVIATION...Expecting winds at the SLC terminal to be light
southwesterly through the afternoon due to nearby scattered showers
and thunderstorms. Winds may become erratic near these passing
storms, with only a 20 percent chance of seeing northwest winds
prevail this afternoon. Expecting a more prevailing southeast switch
after 03z. Ceilings should remain above 7000 feet through the TAF
period.

&&

.SLC Watches/Warnings/Advisories...
UT...None.
WY...None.
&&

$$

Merrill/Wilensky/Dewey

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