Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 181141

Area Forecast Discussion
National Weather Service Salt Lake City UT
541 AM MDT Mon Jul 18 2017

.SYNOPSIS...Drier will move into northern Utah today. Monsoon
moisture will spread into the area Wednesday and Thursday and may
bring the potential for heavy rain to central and southern Utah.


.SHORT TERM (Through 12Z Friday)...The center of the high pressure
has shifted to near the Four Corners region this morning. However,
the ridge is still relatively flat. As a result, there is still
enough of a westerly component across much of the area to prevent
deeper moisture from Arizona from spreading into Utah today. In
fact, northern Utah will actually dry a bit today. This will
maintain warm temperatures and keep convection more isolated across
the north and limited to the Uintas and Book Cliffs. Southern Utah
will continue to be a bit more active with the stronger storms
capable of producing locally heavy rain.

The ridge will amplify tonight into tomorrow, allowing moisture to
quickly spread into the area as the flow turns more southerly.
Expect this moisture surge to keep a few nocturnal showers and
thunderstorms in place tonight, then thunderstorm coverage will
increase more significantly tomorrow. Thunderstorm activity will be
aided by a shortwave disturbance currently over New Mexico which
will rotate into southwest Utah tomorrow afternoon.

Although an expected increase in cloud cover may limit deep
convection, given the forcing associated with the shortwave as well
as favorable instability parameters per the SREF (median MLCAPE near
750 J/kg or median SBCAPE near 1000 J/kg across portions of southern
Utah) and PWATs upwards of 1.25 inches, an increased potential for
flash flooding will exist with tomorrow`s thunderstorm activity.
There may be a need for a Flash Flood Watch for southern Utah for
tomorrow, but there is not quite enough confidence at this time.

The aforementioned shortwave energy is progged to rotate into east-
central Utah tomorrow night and into Thursday. Nocturnal
thunderstorms are expected to follow this shortwave. A brief lull
may exist behind it across southwest Utah, but the ridge will bulge
back up behind it by peak heating Thursday. With ample moisture still
in place, southern and eastern portions of the forecast area are
expected to remain wet. Although PWATS are progged to increase
briefly into far northwest Utah tomorrow night, the bulk of the
moisture looks to stay out of that area, a common theme among most
models as a shortwave trough crossing the Pacific Northwest and into
Idaho brings more westerly flow into the area Thursday. As a result,
that area may miss out on most of the thunderstorm activity, but if
any storms do occur there, gusty outflow winds will be likely.
Otherwise, clouds and precipitation will result in cooler
temperatures areawide tomorrow and Thursday.

.LONG TERM (After 12z Friday)...A broad trough continues to move
north of the area on Friday, and the westerly flow that develops
across the Great Basin should help push drier air into northern
Utah. Much of southern and eastern Utah should remain moist on
Friday, with precipitable water values hovering around 1 inch,
allowing for a continued threat of showers and thunderstorms.

As the trough heads further east into the Great Plains, a flat ridge
builds over much of the western CONUS, with a general zonal flow
becoming established over Utah. Most model solutions have a weak
disturbance moving across northern California and Nevada late Friday
into Saturday, but subtle differences in the placement of this
shortwave will impact flow direction over Utah, and subsequently the
placement of moisture across the state. The 00Z GFS came up with a
new solution that returns moisture to northern Utah on Saturday,
while the Canadian and ECMWF still have the idea of the north
remaining dry through the weekend. Have kept the forecast leaned
toward the drier EC/Canadian solutions, primarily because this is
more consistent with past runs, but confidence in the weekend
forecast has obviously taken a bit of a hit.

The general idea heading into the beginning of next week seems to be
an amplifying ridge centered over the Four Corners. Have kept
temperatures near or slightly above climatology through this period.


.AVIATION...Winds at the SLC terminal are expected to switch to the
northwest around 19-21Z today. VFR conditions should prevail through
the day under mostly clear skies.


.FIRE WEATHER...High pressure has shifted towards the Four Corners
region this morning. The airmass will trend drier across northwest
Utah today while some moisture will linger across the south.
Across northern Utah, this will keep any thunderstorms fairly
isolated and limited to the vicinity of the Uintas and Book Cliffs.
However, southern Utah will continue to see isolated to scattered
mostly wet storms today.

Moisture will increase from the south overnight and into Wednesday.
This should keep some showers and thunderstorms active across mainly
southern Utah through the night, then expect scattered to numerous
wet storms across much of southern and central Utah and isolated
storms for northern Utah tomorrow. Thursday will remain wet
especially across southern and eastern portions of the fire
district. After that, a drying trend is expected for the end of the
week and into the weekend.





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