Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 201547
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
947 AM MDT Fri Apr 20 2018

.SYNOPSIS...An upper level low pressure system will slowly make
its way across the southern and central portion of the region
today. High pressure will bring a warming and drying trend for the
upcoming weekend.

&&

.DISCUSSION...
Antecedent Conditions...
Extreme Drought is occurring from Provo south to Cedar City and
to the southeast. Severe Drought is occurring surrounding the
Extreme Drought, in areas south of Ogden and Duchesne to the
Arizona border. Drought conditions scale down to Abnormally Dry
along the Idaho and Wyoming border.

Big Picture...
During the past 5 days, the amplified pattern over the Pacific has
shifted toward being more zonal as the magnitude of the jet has
significantly decreased. This is leading to a higher potential of
cutoff systems as opposed to consolidated troughs.

Water Vapor Satellite shows a deep closed low over southeast Utah.
400-250 MDCARS wind observations show a cyclonic 100-135kt jet
over from eastern Nevada, across Arizona into the southern
Rockies.

Local Observations and Trends...
12Z KSLC RAOB shows steep lapse rates up to near 300mb above the
nocturnal inversion, with uniform east-northeast winds of
25-45kts in this layer. Very dry below 600mb, and moist above
this level. Precipitable Water values range from 0.10"-0.15"
mountains, to to 0.40"-0.65" most valleys.

A 1008mb surface cyclone is located near the Great Salt Lake,
supporting a 6mb northwesterly gradient to the far northwest
corner of Utah. This is support gusty winds across the west.

24 hour trends include 10-20F cooler south and southeast of Salt
Lake City, 5F warmer to the northeast. Dewpoints depression has
decreased 20-40F except 5F higher across the northwest.

Forecast...
Wrap around precipitation expected to become showery as the storm
system pulls away this afternoon. Strong height rises and
midlevel warming will lead to showers dissipating late day through
the evening. Expect a slight chance for thunder based on SREF
guidance from SPC this afternoon. Snow level 6-7kft rising to
8kft late day.

Gusty northwest winds in the wake of the surface low will affect
western Utah, gradually diminishing through the afternoon.

&&

.PREVIOUS DISCUSSION...
A wind advisory remains in effect for Great Salt Lake Desert
until 9am this morning due to a strong surface gradient and
favorable northerly winds. These winds will weaken some this
afternoon as the gradient relaxes. Most mesonet sites are
reporting wind in the 30-40 mph currently but should lower by this
afternoon. Local strong downslope winds are occurring along the
Northern Wasatch Front where gusts are over 30 mph with isolated
gusts in excess of 50 mph. These winds should decrease rapidly by
mid morning if not earlier.

Areas of precipitation are becoming more widespread in the past
hour or two across Utah and Juab counties as the warm advection
becomes more favorable there. The GFS 500-700mb winds are normal
to the equivalent potential temperature contours at this time
across Utah County. However, the latest 06Z GFS guidance shows
that this area vanishes by about 12Z with a new area developing
across southwest Wyoming west southwestward into Salt Lake and
Tooele counties then southward into central Millard County and
Iron County by mid to late morning. Have based PoPs on this idea
which resulted in boosting the PoPs at Tooele, Salt Lake City,
Delta and Cedar City for the morning time frame.

The 700mb temperatures are just cold enough that when the
precipitation intensity increases the rain changes over to snow.
UDOT cameras showing that snow is occurring along the I-15
corridor between Beaver and the I-70 interchange. This is expected
to continue into the morning hours before the 700mb temps warm
enough to have a change over back to rain. Another area where snow
is possible this morning is in the Price area where dew points
are so low that any precipitation that falls will cool the air
mass and have snow occur. By afternoon the 700mb temps warm enough
that a slight chance of rain showers will occur.

The 700mb temperatures lower to -2 to -4 C Degrees over southwest
Wyoming midday which will also be cold enough for snow there. As
the the flow becomes more Nine good upsloping precip is expected
on the north slopes and ridge tops of the Uintas today into
tonight.

Instability is best across the southeast corner of the CWA this
afternoon with LI`s forecast to be below 0 there. However, already
having a thunderstorm popup in northern Carbon County moving NW.
Will update forecast to include isolated thunderstorms for the
north central mountains for today.

Lowered temps under the swath of most likely precip today by a
couple to 3 degrees which seems to better fit the 850 temp
forecast and the 700-850mb thickness.

This closed low moves east of the CWA by midnight so have trimmed
PoPs for this evening and removed all but a band over the high
Uintas. Ridging builds in Saturday through Sunday with a warming
trend.

The ridge shifts further east of the area Sunday night, as a weak
shortwave trough moves across the Pacific Northwest. This
continues to look like a brush by storm for Utah, with the shallow
associated cold front pushing into northern Utah and southwest
Wyoming early Monday morning. Have maintained some 20-40 percent
POPs across the northern third of the CWA Sunday night through
Monday, but in general the best moisture and forcing will likely
remain north of the Utah/Idaho border. Cooling across the north is
expected to be modest as well, with little if any impact for
central and southern Utah.

Any showers should mostly dissipate by Monday evening, as the trough
slides into the Great Plains. Heading into Tuesday through Thursday,
models have finally come into better agreement, with 00Z runs
building a high amplitude ridge over much of the western CONUS for
the middle of the week. This means above average temperatures for
the area and generally dry conditions. Have kept some low end POPs
for some mountain locations through this period, in part because the
GFS keeps a little lingering moisture over the Rockies, and in part
because this is the first time models have really come into
agreement for this time frame. If this ridge remains consistent,
even these low POPs will probably be able to be removed.

Looking beyond Thursday, models move a closed low from the Pacific
into the west coast late Thursday into Friday, though details
differ. Will keep an eye on this feature as it moves closer in time
from its current day 7-10 position, but current model solutions do
not have this low greatly impacting Utah, with the idea that it may
push north of the area as it moves further inland.

&&

.AVIATION...
Winds at the SLC terminal may continue to be erratic
today maintaining a north-northeast direction but speeds varying
between 10-25 KTS. Ceilings are expected to remain below 6 kft
through the afternoon, but only a 20 percent chance of MVFR
conditions due to passing rain showers. Overnight, the winds are
expected to shift to be from the SE between 07-09Z.

&&

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

&&

$$

PUBLIC...10/Struthwolf/Schoening
AVIATION...Bonnette/Dewey

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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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