Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 200944

Area Forecast Discussion
National Weather Service Salt Lake City UT
344 AM MDT Sun May 20 2018

.SYNOPSIS...A warming and drying trend will occur today and
tomorrow ahead of a storm system which arrives Tuesday. Most
areas will see isolated to scattered thunderstorms through mid
week, and an increase in gusty southerly winds through Monday. A
cooler and moister pattern sets up mid week with the arrival of
the next storm.


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...
The pattern has begun to re-amplify across the Pacific with a
closed low near 45N 150E, and another near 50N 175W. The later
system is juxtaposed with a strong jet located south of the
Aleutian Islands.

Water Vapor Satellite shows generally lower heights over the
western CONUS. A shortwave ridge is progressing east across
eastern Utah, with an approaching negatively tilted shortwave
trough from central Nevada into northern Arizona. 400-250 MDCARS
wind observations show a cyclonic 125-150kt jet located south of
the Aleutian Islands. This jet splits over the eastern Pacific
with a ridge over western Canada and lowering heights off the
Oregon coast.

Local Observations and Trends...
00Z KSLC RAOB shows weaker flow and warming temperatures, compared
to the 12Z RAOB, and a 1C subsidence inversion near 560mb.
Precipitable Water values range from 0.25"-0.33" mountains to to
0.50"-0.70" valleys. This reflects little change in Precipitable
Water compared to 24 hours ago, with slightly higher values across
the higher terrain.

A 1025mb surface anticyclone is located across the Dakotas, with
a 1008mb surface cyclone located near Las Vegas. This is creating
a 5mb easterly pressure gradient across southwest Wyoming and
northeast Utah resulting in pockets of higher than normal canyon
winds north of I-80.

Radar and IR satellite indicate isolated to scattered weakening
showers east of I-15.

24 hour trends: Generally 5-10F warmer. Dewpoint depressions are
5-10F higher (drier) across northwest Utah, and 5-10F lower
(moister) along the mountains and eastern valleys.

Flow will become increasingly southerly across the region with
lowering heights along the California coast. A weak shortwave
trough over Nevada sould open, sending several weak waves across
the region today. This could support (mainly) elevated convection
across south central Utah early this morning. Instability
parameters are depicted to be moderately stronger than yesterday,
with mid level cooling weakening the mid level subsidence
inversion. This is shown as 700-500mb lapse rates steepening to
near 9C/km. Due to the better instability and weak waves
propagating across the region, anticipate better coverage of
deeper convection this afternoon and evening with many areas
seeing a 40-50% chance for rain, tapering down to 20% along the
Arizona border. Stronger storms may bring small hail and locally
gusty winds (due to dry lower levels as depicted by likely
probabilities of greater than 1000J/kg across southern and
southeastern valleys in particular). Storm motion should be from
the SSW between 15-25kts.

Convection across northern Utah and southwest Wyoming will be slow
to dissipate tonight, but expect all convection to dissipate by
late evening across southern Utah as increasingly stronger
southerly flow draws drier low and mid level air into southern
Utah from Arizona, due to a low closing off over central
California (coincident with the arrival of the aforementioned
Pacific jet).

Convection Monday afternoon should favor northern and some central
areas, with no convection expected across the south during the
afternoon. This should change by evening, as the closed low nears
Las Vegas, with height falls supporting new convective
development across portions of southwest and south central Utah
with lingering convection further north. Storm motion should
generally be from the SSE between 15-25kts. Arrival of dynamical
forcing should also bring more expansive cloud cover to the region
Monday night, lasting through Tuesday night.

Best dynamical lift appears to be across southwest Utah Monday
evening, and eastern areas Tuesday as measured by the positioning
of the left exit region of the upper level jet and diffluence
aloft. The question for southwest Utah is whether dynamical
forcing is strong enough to overcome the dry airmass. The answer
is not yet definitive so left a mention of isolated evening and
overnight convective activity for all areas except near Lake

Instability parameters improve across central and some southern
Utah areas Tuesday thanks to cooling aloft. Expect to see
isolated convection closer to the Arizona border, increasing to
scattered coverage across northern and central areas Tuesday
afternoon and evening, which may linger overnight across the north
supported by better instability and proximity to the decaying
closed low across northeastern Nevada. Storm motion should
generally be from the SSE between 15-25kts once again.

A warming and drying trend is expected through Monday as southerly
winds become increasingly gusty ahead of the aforementioned closed
low. Cooler temperatures and moistening lower levels expected
Tuesday due to expansive cloud cover and cooling in the column
driven by the arrival of the closed low.

The closed low is expected to be centered over far northwest Utah
Wednesday morning. Current thinking is that the system will
weaken as it lifts northward during the day Wednesday. As a
result, northern portions of the forecast area will remain moist
and unsettled, with convection possible through the afternoon
hours. The threat of showers and thunderstorms becomes isolated at
best beginning Wednesday morning as the system continues away
from Utah and southwest Wyoming.

High pressure should move over the area Thursday in between the
exiting system and another low off the California coast. The ridge
is expected to remain over the area through Saturday, amplifying as
the low gradually approaches California. Threat of convection in
this drier and more stable airmass will be limited and confined to
the higher terrain of northern Utah. As the ridge amplifies, the
forecast area will see a warming trend, with maxes expected to run
up to 15F above seasonal normals by Saturday.

Latest GFS ultimately moves this low up and over the ridge on
Sunday, weakening the high slightly but ultimately keeping a dry and
generally stable airmass over the area past day seven. In the EC,
the system shifts the high east of Utah and southwest Wyoming on
Sunday bringing the low across the state early next week. Given the
recent pattern, am more apt to believe the EC solution but it is too
far out to have more than low confidence in any solution.


The SLC terminal will see VFR conditions throughout the day with
any cigs remaining above 6000 feet AGL. Southeast winds are
expected to shift to the northwest between 18Z and 20Z. There is a
30 percent chance that showers in the vicinity of the terminal
will result in periods of gusty and/or erratic winds this


ERC values at RAWS sites are trending higher areawide, and range
from the 80th to 89th percentile southeast valleys, 50th to 69th
percentile southwest Utah and eastern valleys, and below the 50th
percentile further north and west.

Better chance for deep convection most locations today as a
disturbance crosses the region during peak heating. Drying will
occur across the south Monday, limiting convection to the northern
and central areas. Thunderstorms could ignite new fires considering
a low chance of wetting rain.

A warming and drying trend is expected through Monday as southerly
winds become increasingly gusty. Single digit RH values are expected
across southern and southeast valleys this afternoon, and southeast
valleys Monday afternoon.

A broad storm system organizes across central California Monday,
impacting the region Tuesday and Wednesday bringing another round
of showers and thunderstorms, along with a slightly cooler and
moister airmass.





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