Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 210214

Area Forecast Discussion
National Weather Service Salt Lake City UT
814 PM MDT Sat Oct 20 2018

.SYNOPSIS...A slow moving low pressure system will cross the
region through early next week. High pressure will return for the
middle of the week.


Water Vapor loop shows a closed low off the central California
coast, with a strong wave pivoting from southern California toward
southern Nevada. This pattern is advecting in moisture from the
south-southeast, as evident from the experimental CIRA Advective
Precipitable Water product in both the 700-500mb and 850-700mb
layers. As moisture continues to deepen, SREF indicates
instability increases across southwest Utah overnight, with a
slight chance or chance of seeing thunder. The threat of showers
will increase overnight as diffluence aloft increases, and as lift
associated with the aforementioned wave increases as well.

The trend in models during the past 24 hours has been to slow
down the increase in moisture. So adjusted PoPs to adhere to this
trend, but still expect a likely probability of seeing showers
across southwest Utah by morning.

Also updated the T/RH hourly curve using the latest observational
and model data.


Afternoon satellite imagery and objective analysis continue to
show a mid level ridge stretching from the Pacific Northwest
eastward through the Snake River Plain and extending into the
central Rockies. Meanwhile a broad upper low continues to slowly
spin along the California coast with several shortwave features
rotating around the mean circulation center. Southerly flow
downstream from this low is helping to advect moisture northward
into the Desert Southwest, with the northern periphery of this
moisture axis spreading into southern Utah this afternoon.

The upstream low is forecast to drift northward overnight before
beginning a slow trek inland Sunday. As this occurs moisture will
continue to increase across primarily southwestern Utah overnight
into Sunday morning, before spreading across the remainder of the
area later Sunday into Monday. This will bring an increase in
showers along with a few thunderstorms beginning late this evening
across the southwest, then slowly spreading into central Utah
overnight into Sunday morning. This activity will likely not reach
the Wasatch Front until late Sunday into Monday, and have left the
forecast for the northern Utah valleys generally dry through
Sunday afternoon while increasing the PoPs across the southwest by
10-20%. However given the expected showery nature of the
precipitation have left PoPs in the chance category. Snow levels
should remain at or above 10 kft.

The upper low is expected to evolve into an open wave as it slowly
crosses the forecast area Tuesday. Have maintained 20-30% PoPs for
through the day Tuesday, although the focus for precipitation will
likely be shifting east with time.

The apex of a shortwave trough will be over eastern Utah Tuesday
night allowing for weak zonal flow to prevail for a couple of
days. This will bring in a slightly drier airmass, however some
isolated showers will still be possible over higher elevation
areas of northern Utah Tuesday night into Wednesday.

Continuing into late next week...a series of short wave ridges will
propagate through the forecast area associated with a building long
wave ridge to our west as indicated by NAEFS/EPS guidance. This will
allow for drier air and warmer than average temps to prevail through
the end of the week and early next weekend with much of the forecast
area 5-10 degrees above normal Friday and Saturday.

We are confident in ridging moving into the forecast area late next
week and next weekend, however uncertainty exists as to how strong
the ridge will be. The deterministic EC and current EPS guidance is
currently favoring a stronger ridge with a closed mid level
anticyclone developing over southern Utah, while the GFS and NAEFS
guidance keeps it as an open ridge. If this stronger ridge develops,
it will promote further drying and even warmer temps for southern
Utah next weekend.


VFR conditions will prevail at KSLC through the TAF period.
Winds become southeast around 03Z. Mid and high level clouds will
build northward toward the terminal late tonight and tomorrow.





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