Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 211035

Area Forecast Discussion
National Weather Service Salt Lake City UT
435 AM MDT Wed Sep 21 2016

.SYNOPSIS...A strong cold and wet storm system will impact most
of the region through the end of the week.


Water vapor loop shows a cold long wave trough entering the
Pacific Northwest. The former closed low off northern Baja and
Tropical Storm Paine are now shearing northeast over Southern Utah
and Arizona. MDCARS wind observations show a 100kt-125kt southwesterly
jet along the Canadian border. GOES/HRRR/00Z KSLC RAOB indicate
the precipitable water value ranges from 0.30"-0.45" mountains to
1.25"-1.65" far southern valleys.

Gusty winds across northern Utah which originated across southern
Utah where rain was causing strong evaporative cooling will
subside through the morning hours.

The sheared feature, formerly closed low off Baja, is providing
good area of rain across the south this morning. The feature will
continue to weaken then lift north across the state as the
upstream storm system begins to adjust the wind field.

As the day progresses, instability will develop and deepen thanks
to diurnal warming and increasing low level moisture, while
diffluence aloft and PVA serve as forcing mechanisms. SPC SREF
indicates at least 30kts of effective shear. WRF simulated
reflectivity indicate convective cores as opposed to stratiform
precipitation. There is a threat of organizing strong storms due
to the shear, forcing, and initially dry lower levels prime for
strong dry microburst winds.

In addition, even though storm motion will be fairly quick,
training is possible. Across Southern Utah there is a potential
for locally heavy rainfall as the warm cloud depth increases to
about 5kft during the afternoon. With precipitable water values
this high and dewpoints around or above 60F, certainly enough
moisture in place. Have added mention of heavy rainfall. No watch
as of yet due to cloud cover and steady rainfall this morning and
questions regarding afternoon storm coverage.

With sunset, expect majority of showers and storms to coalesce in
the vicinity of the initially shallow cold front entering
Northwest Utah. The boundary will sharpen and deepen as the trough
approaches from the west overnight. Coverage of precipitation will
blossom as the cyclonic jet arrives, especially in vicinity of the
shear axis. A corridor of heavy rainfall will develop where this
feature sets up (currently near the I-15 corridor), which
progresses eastward tomorrow, translating with the upper level jet
shear axis.

Fairly strong cold advection will occur tomorrow night which will
keep the precipitation going as snow levels start to near 8500 ft.
Snow levels may be even lower than that later tonight in areas of
higher precipitation rates.

All the ingredients are coming together for a cold and wet day
over northern utah Friday. This potent and cold storm will be
centered over northern Utah Friday with good 700mb cyclonic
curvature and convergence early on as the axis moves through in
the morning then favorable upslope flow into the Wasatch Mountains
and north central mountains in the afternoon. At 500mb there is
wrap around warm thermal advection. In light of these forecast
conditions have gone with categorical stratiform precipitation
along the I-15 corridor from about Delta northward to Idaho as
well as the adjacent mountains and over the Great Salt Lake
Desert. Have boosted precip amounts for Friday and Friday night
with over 0.50 of an inch in the valleys and over 1 inch in most
of the northern Utah mountains except for the Uintas which will be
too far east to benefit from the good dynamics aloft during this
period. The Uintas will get the brunt of their precip prior to 12Z
Friday. Snow levels over northern Utah will have reached about
7000 ft by Friday morning and should stay at about this level
through Saturday morning before rising again. Locally a foot or
more of snow is expected in the Wasatch, Oquirrh and Stansbury
Mountains at 8500 ft and above.

Have brought down temperatures into the 40s in the northwest valleys
due to the likelihood of continuous rain all day. The 700-850
thickness under these conditions actually supports only a max of 42
degrees at SLC but played it cautiously for now. Mid to upper 40s
should get peoples attention that this will be a cold day ahead.

Farther south, over the western valleys and spine of the central
mountains, the best chance for precip in the long range will be
Friday afternoon behind the 700mb trough axis as the moist northwest
flow impacts this area. Temperatures will be cool in the 50s and 60s
in the western valleys and 70s in the eastern valleys. Snow levels
in the southern mountains will get down to near 7500-8000 ft.

Snow showers likely to continue into late Friday night in the
Wasatch while just a chance elsewhere in the northern Utah. This is
due to the favorable northwest flow likely creating lake-effect off
of the GSL. The flow aloft becomes northerly Saturday afternoon and
this will cut off any additional instability off the GSL into the
Wasatch. Perhaps the Oquirrhs could see snow showers through the
day. Cold aloft and some lingering moisture for chance of snow and
rain showers Uintas and southwest Wyoming as well.

Across southwest Utah there is a chance of getting strong canyon
winds Saturday afternoon and night as the surface gradient tightens
up with the strong high over northern Utah and northeast winds at
700 mb increasing to 25-30 kts. Breezy conditions will occur
elsewhere across southern Utah Saturday night and into Sunday.

By Monday we have a strong ridge building back in over the CWA with
dry conditions and a warming trend.


South winds will prevail at the SLC terminal throughout the TAF
period. Expect increase in winds mid morning with gusts over 30
mph possible. Cigs are expected to come down to about BKN070 late
this afternoon or evening with rain showers. There is a 30 percent
chance of cigs lowering to or below BKN060 between 01 and 06Z in
rain showers. Erratic wind gusts are possible with these showers.


Yesterday`s ERC values were below the 50th percentile near
Arizona, while many stations across central and northeast Utah
were in the 80th to 89th percentile.

Pool of deep moisture will continue to be pulled north across the
area ahead of a cold storm system along the Pacific Northwest
Coast yielding copious showers and thunderstorms. This storm will
make use of the deep moisture and provide plenty of rainfall late
Wednesday into Saturday. Snow Levels are likely to fall to near
7kft by Friday.





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