Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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770
FXUS65 KSLC 291527
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
927 AM MDT Thu Sep 29 2016

.SYNOPSIS...A weakening weather disturbance will track north
across Utah this afternoon through this evening. A cold Pacific
storm system will enter the Pacific Northwest early Saturday, then
strengthen as it settles into the Great Basin by early Monday.
This storm system will impact much of Utah during the early
portion of the upcoming week.

&&

.DISCUSSION...
Water Vapor loop shows ridging over the Rockies, while a weakening
closed low lifting into central Nevada. MDCARS winds observations
show a 90KT-115KT southwesterly jet across western Canada.
GOES/HRRR/12Z KSLC ROAB indicate precipitable water values range
from 0.40"-0.55" mountains to 0.80"-1.20" most valleys.

Dewpoints have risen 10-15F during the past 24 hours across
southern and eastern Utah, with values rising through the 50s
currently. This along with precipitable water values indicate the
uptick in moisture across this part of the forecast area. Weak
waves will continue to shear northeast across southeast Utah this
afternoon providing ascent as will the coupled upper level jet
near the four corners late day. SREF instability parameters
indicate plenty of instability develops this afternoon. Given all
this and a warm cloud layer of around 5kft, have decided to issue
a Flash Flood Watch.

Most other parameters were tweaked but not significantly so.

.PREVIOUS DISCUSSION...
The remnant of the decaying upper low currently over southern
Nevada will track north through western Utah today, then into
western Wyoming this evening. This feature will be moving through
a decent moisture field (PWAT values around .75 to 1.00 inches).
This combination of weak dynamic lift and abundant moisture will
lead to increasing convection this afternoon. South-central and
southeast Utah could see some deep convection where the upper
level divergence is strongest. Looking at the potential for
locally heavy rainfall in those areas, with some flash flooding
possible in areas prone to such events.

Moisture will be slow to clear out of Utah Friday. Will likely
see an increasing trend in mainly terrain-based convection during
the day, though not anticipating strong/organized convection at
this time due to the lack of significant dynamic/thermally driven
lift.

A next upper level trough will reach the Pacific Northwest early
in the upcoming weekend. This feature looks to be quite potent,
with a well-defined closed low settling into the western Great
Basin late in the weekend. Ahead of this feature an increasing
southwest flow will serve to maintain breezy conditions and
temperatures above early October norms. Somewhat drier air will
move into the area, though still could see some terrain-based
convection firing up Saturday afternoon. Potential shortwave
energy ejecting out of the main upper level trough could bring
some organized convection to mainly northwest Utah Sunday
afternoon and evening.

Global model consistency regarding the amplification and net
evolution of the next pacific trough seems to be coming more in
line. In the big picture, the idea of a somewhat less progressive
and more amplified system tracking across the eastern Great Basin
early next week is winning out, solutions similar to those
represented by the EC ensemble mean and some members of the GEFS
from days prior. Though details will continue to unfold in time,
do expect a relatively cold and wet system to impact the north
early next week, with primary impact across the north.

A strong cold front is expected to pass across the forecast area
Monday, this as yet another closed upper low carves into Nevada then
points east across the state Monday into Tuesday. With more
consistency in guidance run to run and model to model, have noted an
uptick in QPF as a combination of large scale forcing and modest
moisture combine within the period of cold frontal passage on
through mid level axis passage. With higher confidence coming to
form, opted to raise PoPs fairly significantly across namely
northern and central portions of the area. H7 temps as cold as -6C
should support snow accumulations across the northern mountains once
again, with snow levels potentially lowering to or just below 7000ft
across the north.

Still not overly confident on track and evolution of the trough
Tuesday on, though global guidance does suggest a somewhat
progressive easterly translation through midweek. Have trended grids
along these lines, but wouldn`t be overly surprised to see things
slow down a touch in later runs. Time will tell.

&&

.AVIATION...
Southerly surface winds will persist at the KSLC terminal through
late this afternoon, with a 60 percent chance of a brief switch to
the northwest in the 22-02z window. Any cigs developing after 19z
are expected to remain well above 6k agl,with a 30 percent chance
of vicinity showers between 20-02z. Expecting VFR conditions to
prevail throughout the TAF period.

&&

.FIRE WEATHER...
Yesterday`s ERC values were below the 50th percentile across the
state.

Convection will affect mainly areas east of Interstate 15 today.
With moistening profiles, the threat for heavy rainfall will exist
through this evening. Tomorrow there is the possibility of another
round of convection for the same area, though it looks like it
will be lower in coverage.

Significant storm system develops along the west coast this
weekend, expecting to see an increase in southwest winds and
lowering RH values.

The storm system crosses Utah early next week, bringing a cooling
and moistening trend with a good shot of wetting rain/high
elevation snow across northern and portions of central Utah.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flash Flood Watch from noon MDT today through this evening for
     UTZ012-013-020-021.

WY...None.
&&

$$

PUBLIC...Rogowski/Conger/Merrill
AVIATION...Verzella
FIRE WEATHER...Rogowski

For more information from NOAA`s National Weather Service visit...
http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php



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