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FXUS65 KSLC 271602
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
1002 AM MDT Thu Apr 27 2017

.SYNOPSIS...The cool and unstable northwest flow aloft will
bring showers to Utah today. A colder and more northerly flow
aloft will develop tomorrow as an intensifying weather
disturbance moves south through the eastern Great Basin, bringing
unseasonably cold temperatures and a few snow showers. High
pressure aloft and quieter weather will return to the western
states for late in the weekend.

&&

.UPDATE...
Satellite imagery this morning shows broad northwesterly flow
across the western states. Subsidence can be seen over our area
in WV imagery on the backside of a shortwave that moved through
overnight. A cold unstable air mass lies in its wake, as seen on
the 12Z SLC sounding this morning which shows impressively steep
lapse rates up to near 500mb and visible satellite imagery / local
radars which show numerous orographic showers. Lake enhanced
showers off the Great Salt Lake have also picked up within the
last few hours. These showers are well handled by high-res
models, which have a consistent theme of lake enhancement merging
with orographic forcing along the Wasatch Front producing showers
in the afternoon hours. Elsewhere, expect showers to pick up this
afternoon as daytime heating helps to realize some of the
instability in the lower atmospheric profile across much of Utah.
Graupel showers with a few claps of thunder aren't outside the
question given the soundings today.

Showers should pick up again late tonight over northern Utah and
southwest Wyoming as the cold pool aloft rotates into our area
from the north. 700mb temps are progged to drop around -11C, which
is a 1-in-10 year type of cold according to the GEFS reanalysis,
and would easily support snow levels dropping to valley floors.
Current thinking is showers will not be widespread enough to
warrant headlines however as impacts should be highly localized.
That said, a few inches of snow this time of year could be enough
to snap tree branches given the amount of leafing that has
occurred at lower elevations, so will keep an eye on guidance
coming in today to see if anything changes. The wildcard as usual
is how active the lake enhancement becomes. There is some
suggestion of it developing in local lake effect probability tools
and high- res models, but will need to dig into the details
deeper to diagnose.,

Current forecast has a good handle on the near-term weather, so no
updates planned.

.PREVIOUS DISCUSSION...(Issued 442 AM MDT)...

The cold deep layer northwest flow aloft across the Great Basin
will persist through this evening. Shortwaves embedded in this
mean flow will serve to maintain showery precip, and bring
increasingly colder air aloft into the region. For this morning,
showers look to be fewer and less organized behind the shortwave
that moved through the region overnight. Anticipating an increase
in showers again this afternoon as the air mass becomes a bit
colder and more unstable with subsequent shortwaves supplying
dynamic lift. Can not rule out a thunderstorm or two with the
upper jet extending through west-central Utah southeast toward the
four corners region.

Amplification of the upper ridge over the eastern Pacific
beginning later today will result in an increase in the cyclonic
west-northwest flow aloft over the Basin tonight/early Friday.
Further development in the form of a closed circulation center
across the eastern Great Basin during the day Friday will result
in increased synoptic-scale lift and more widespread precip during
the afternoon through early evening hours. This precip will
gradually focus along the wrap around in the north and west flanks
of the upper low, with the best chances for precip extending from
the the northeast mountains south into south-central and southwest
Utah into early Saturday.

The upper low will gradually work southeast into the southern
Rockies Saturday night/Sunday. The deep layer northerly flow in
its wake will bring drier and somewhat warmer/stable air across
the area for late in the weekend.

.LONG TERM (After 00Z Monday)...Northwest flow remains over Utah and
southwest Wyoming late Sunday, with a couple of weak disturbances
moving through the area Sunday night and Monday. This could bring
some light showers to the northern third of the CWA, but the best
cold air and instability looks to remain well northeast of the area.

The parent trough begins to move southward on Tuesday, though models
have generally trended weaker and further east with this system
(i.e.- the GFS and Canadian have trended toward the ECMWF solution
from the last couple days), which means the bulk of the forcing goes
through Colorado and points eastward. There is still a somewhat
increased chance of precipitation across the northern half of the
forecast area Tuesday in the moist, cold advection northwest flow,
but instability is fairly meager. This weaker solution also means
that 700mb temperatures only get down to -2C, a far cry from last
night's GFS solution of -8 to -10C.

The trough exits the region Tuesday night and Wednesday, and models
are still promoting the idea of a high amplitude ridge moving over
the Intermountain West late Wednesday and Thursday. Have increased
afternoon temperatures for Wednesday and Thursday, and they may need
to be increased even more on future shifts. Current runs don't have
this ridge hanging around very long, though, with the next trough
pushing into the Great Basin next Friday or Saturday.


&&

.AVIATION...Ceilings at the SLC terminal are expected to lift above
7000 feet between 17Z and 20Z today. However, isolated to widely
scattered showers are still expected across northern Utah through
the afternoon, so briefly lower ceilings should be expected under
these showers.

&&

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

$$

UPDATE...Van Cleave
SHORT TERM...Conger
LONG TERM...Schoening
AVIATION...Kruse

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