Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 151548
Area Forecast Discussion
National Weather Service Salt Lake City UT
848 AM MST Wed Feb 15 2017
.SYNOPSIS...High pressure aloft will maintain dry and mild
conditions across the region today. Southwesterly flow will
develop Thursday, followed by an active pattern late in the week
into early next week.
Water Vapor Satellite shows a ridge from southern California
across the Great Basin. Several storm systems are approaching the
West Coast. 400-200mb MDCARS wind observations place a 100-130kt
cyclonic jet into the Pacific Northwest. GOES/SLC 12Z RAOB/HRRR
indicate that precipitable water values vary from 0.05"-0.10"
mountains to 0.20"-0.30" valleys. Blended Precipitable Water
product shows an atmospheric river into the West Coast with
origins from near Hawaii.
High pressure surface and aloft will support sunny skies and light
winds. There is a possibility of a few strands of high clouds
making it into Northwest Utah by sunset. Webcams show fog in the
Cache Valley and Bear River Valley which was in the forecast.
Updated the diurnal curve with observations and latest model data.
Otherwise very little change to the going forecast.
Mid level ridging extends across much of the Interior West this
morning, downstream from a predominantly zonal flow which extends
across the eastern Pacific. With the ridge axis shifting overhead
today will continue to see temperatures aloft climb, resulting in
a warming trend area- wide. Under sunny skies, temperatures should
run a good 5 to 10 degrees above climo. The only exceptions would
be the Cache and Bear River Valleys where shallow cold air
This ridge axis is forecast to shift east Thursday allowing a
southwesterly flow aloft to overspread the region, and with deeper
mixing max temps should continue to exhibit a warming trend.
The large scale pattern is expected to transition and become more
progressive heading into the weekend. The first in a series of
waves to cross the region is forecast to eject across the northern
Great Basin Thursday night primarily impacting areas north of
I-80 with valley rain and mountain snow. The models have been
trending weaker and further north with this wave, and as such
would suspect most valleys south of I-80 will remain dry, although
have retained slight chance PoPs for now.
In the wake of this wave, shortwave ridging is expected to
translate across the forecast area early Friday, as an upstream
wave approaches and then digs along the CA coast. As this wave
drops along the coast, backing flow will allow a plume of theta-e
advection to spread first into southwest Utah late Friday, then
northward into central and eventually northern Utah Friday night,
weakening with time. Have increased PoPs across the southwest
Friday afternoon/evening, while maintaining chance to slight
chance further north into Friday night given the weakening trend
of this feature. With the upper low expected to pass well south of
the forecast area, would only anticipate showers into the day
Saturday, and have kept PoPs in the chance category for this
timeframe. Snow levels Friday through Saturday are expected to
remain well above the valley floors given 700mb temps in the -2 to
-4C range and no surface cold air in place.
This extended period begins with fairly good agreement between
global models of an elongated/splitting trough from central Cal to
central Baja. Because of this stretched out system the best
dynamics will be either under cutting the CWA or going by to the
north. What is left of the trough axis moves through the CWA
Sunday afternoon. Although have continued with likely PoPs over
the higher terrain and northern valleys, the amount of precip will
be modest at best. Have been leaning towards the EC solution past
few runs and will continue to do so through the rest of the
extended as it has been the most consistent in its trend of
weakening the systems as they move through the Intermountain west
early next week.
In the wake of this trough axis on Sunday, a weak shortwave ridge is
expected to build over the region Monday, although some moisture
could creep into the northwest corner of Utah late in the day. Have
trimmed back PoPs during this period but may not have backed off
enough as the models show some blotchy QPF but don`t see the
mechanism creating the precip. The trend over the past several runs
of the global models has been to gradually back off and foresee this
to continue as the overall ridge position appears to remain over the
Great Basin. Have nudged temps up a couple of degrees for Monday and
Tuesday as the EC is now showing 0 to +2C degrees at 700mb across
most of the CWA. Depending on the amount of moisture/cloud cover
these temps could be underdone by several degrees.
By Wednesday, the EC shows a sharper and thus a slower trough moving
into the western Great Basin, while the GFS is more progressive and
has 700mb temps about 5-6 degrees colder than the EC. Confidence is
low at this point as there are multiple waves across the Pacific
shown by all the global models which often leads to timing and
amplitude problems. Have taken a broad brush approach for now with
above normal climo PoPs as patterns remains active.
Operational weather concerns are minimal at the SLC terminal
today. Light and variable winds this morning will likely increase
from the northwest between 18Z and 20Z.
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