Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 201020
Area Forecast Discussion
National Weather Service Salt Lake City UT
420 AM MDT Thu Apr 20 2017
.SYNOPSIS...The next cold front will cross the region today with
a cold northwest flow for Friday. A ridge builds in for the
weekend supplying a warming trend.
.SHORT TERM (THROUGH 12Z SUNDAY)...
Water Vapor Satellite shows the former Eastern Pacific closed low
shearing southeast into the Great Basin. 400-250mb AMDAR wind
observations places a 100-120kt cyclonic jet from off the central
California coast across southern Utah and the Central Rockies.
00Z KSLC RAOB/GOES/HRRR indicate precipitable water values range
from 0.10"-0.20" mountains to 0.30"-0.50" most valleys.
Energy from the aforementioned closed low will slide through the
region this afternoon and tonight. Height falls are subtle today,
with better height rises Friday, though even these are
diminishing with time. Jet dynamics should foster showers as the
upper level trough progresses through. Models in good agreement on
the strength of the cold advection this afternoon and night.
Models are a bit faster with the system, bringing in the first
round of showers this morning as far east as the Wasatch Front.
Not a particularly wet system given a weaker tropical tap than the
previous storm. Light snow may accumulate on cold surfaces across
the Wasatch Back, Bear River Valley, and Southwest Wyoming. Any
road impacts expected to be minor and brief during times of
enhanced precipitation rates.
Cold front is expected to reach the Wasatch Front through Delta
by noon, continuing across the remainder of the state through the
evening hours. Went on the low side of guidance for high
temperatures across the western valleys.
Pre-frontal southwesterly winds may become gusty in the greater
Lake Powell area this afternoon with contributions from 30-35kts
700mb flow and 5mb/3hr pressure falls. Post frontal winds look more
interesting tonight given 30-35 kts 700mb flow, a 9mb northwesterly
gradient across the state, 5-8mb/3hr pressure rises and modestly
strong 700mb cold advection.
Have issued a wind advisory for Castle Country south to the
northern portion of Capitol Reef National Park this evening
through tomorrow morning. Judging from local WRF a critical layer
near 700mb is more likely to develop south of Castle Dale, where
gusts up to 55 mph may occur especially near Fremont Junction.
Ridging builds in this weekend. Could see some outlying areas in
a hard freeze. Went on the lower end of temperature guidance
across the north Saturday given the northeasterly pressure
gradient with a surface high in place.
.LONG TERM (AFTER 12Z SUNDAY)...
The global models start off the extended period with a trough
along the Pacific Northwest Coast which they handled differently
and never really come together through the remainder of the
The EC 500 mb heights are about 60 m higher by 06Z Monday than the
GFS operational run and GFS ensemble while the EC ensemble is closer
to the GFS. Therefore have leaned in favor of the GFS and bumped the
PoPs some across northern Utah, especially over the mountains as
nose of the jet crosses northern Utah during this time. Following
this system, the EC is 24 hrs slower in bringing the next short wave
across northern Utah as it has a ridge in place through Tuesday.
Meanwhile the GFS has another shortwave moving across the northern
two thirds of the CWA already Monday night. Most models seem to
support the GFS and considering the fast nearly zonal flow nature of
this pattern would not be surprised that numerous shortwaves are
riding the jet. Have gone with an increase in PoPs Monday night.
The GFS is more bullish on bringing a stronger trough in Thursday
with -8 degrees C 700 mb temps into northern Utah by 12Z Thursday.
The EC is weaker and has -4 degrees C 700 mb temps across northern
Utah at the same time. The GFS remains progressive with this trough
over Colorado by 12Z Friday while the EC builds a strong ridge off
the West Coast and digs the trough into Arizona and is at least 24
hrs behind in moving the trough eastward to over the Rockies. This
made day 6 and day 7 very challenging but ended up with a blend of
the models which supported higher than climo PoPS for the northern
two thirds of the CWA. Temperatures will likely be 3-10 degrees
below normal from Tuesday through Thursday.
South winds will prevail the SLC terminal through about midday
before a shift to northwest occurs with the onset of showers. VFR
conditions should prevail through about 22-23Z when MVFR
conditions are expected in lower cigs and visby through about 02Z.
There is a 30% chance that the MVFR conditions never materialize.
Northwest winds could gust over 20 mph between 19 and 02Z.
UT...Wind Advisory from 9 PM this evening to 11 AM MDT Friday for
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