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FXUS65 KSLC 261534
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
934 AM MDT Sat May 26 2018

.SYNOPSIS...A low pressure system over central California will
move east into the Great basin Saturday. This low pressure will
generate widespread precipitation and cooler temperatures across
mainly northern and central Utah through the holiday weekend.
High pressure is expected to return for the middle of next week.

&&

.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, an amplified pattern over the Pacific
became more zonal, amplifying once again beginning today.

Water Vapor Satellite shows a ridge east of the Rockies with a
deep closed low centered over southern California. 400-250 MDCARS
wind observations show a southwesterly 100-125kt jet from the
Pacific over southern California into Las Vegas.

Local Observations and Trends...
12Z KSLC RAOB shows steep lapse rates and a strong wind field for
this time of the year, of uniform southerly flow. Precipitable
Water values range from 0.10"-0.15" mountains to 0.25"-0.35" most
valleys. 24hr Precipitable Water trends are generally lower by
0.10" across Utah, but 0.15"-0.25" higher across portions of
central and eastern Nevada.

A 1006mb surface cyclone is located in west central Wyoming, with
the approaching cold front over eastern Nevada.

24 hour trends: Generally 10-15F cooler except across northern
valleys. Dewpoint Depression has increased (dried) 10-30F across
the north, and decreased (moistened) 5-10F along the Nevada
border and across southern Utah and some mountain sites.

Forecast...
With 25-35kt of southerly 700mb flow, 5-8mb/3hr of falling
pressure and a 10mb southwesterly pressure gradient across
southern Utah this afternoon, synoptic gusts will be elevated.
Would not be surprised to see gusts of 35-40 mph across much of
southern Utah, especially east of I-15 including Lake Powell.

SREF indicates weak to moderate instability develops this afternoon,
over the mountains and along the Nevada border. Liked the idea of
isolated convection at these locations after inspecting high
resolution simulated reflectivity and QPF fields. 700-500mb lapse
rates will be on either side of 9C/km supporting high-based
convection over the mountains, particularly north of Boulder
Summit to the western Uintas and northern Wasatch. With a forecast
LCL near 550mb, any shower may produce locally enhanced wind
gusts.

&&

.PREVIOUS DISCUSSION...
An upper low evident in overnight satellite imagery moving into
the western Great Basin will slowly progress east over the next 24
hours, before stalling over the forecast area Sunday, bringing
cooler and unsettled weather through the Memorial Day weekend.

Ahead of this low southerly flow prevails across the forecast area
early this morning, with breezy conditions possible across the
eastern valleys this afternoon under a belt of stronger low-mid
level flow. Temperatures aloft are forecast to cool at 700mb
today over what was observed Friday, which will result in a
downward trend of several degrees in daytime max temps, although
most locations will still run a few degrees above normal. As the
primary low center translates through the southern Great Basin
this afternoon, increasing diffluence aloft will likely allow for
isolated high based convection primarily across the higher terrain
of northern and central Utah, with some potential for microburst
winds given a very deep and dry boundary layer.

This low center is expected to lift from south to north across
Utah tonight, becoming centered and stalling across northern Utah
Sunday morning. This will bring the best chance for precipitation
to northern and portions of central Utah, and have increased PoPs
20-30 percent accordingly, as well as cooled daytime temps a bit.
Precip coverage across the south is expected to remain sparse and
largely focused on the terrain.

The upper low is expected to slowly fill as it spins across
northern Utah through Memorial Day, and with increased boundary
layer moisture and cool temps aloft, anticipate at least scattered
convective coverage across northern and central Utah through
Monday evening.

The closed low from Monday will continue to fill and weaken as it
moves northeast but the GFS shows a secondary weak trough at its
heels late Monday night into Tuesday, so have held onto some PoPs
across northern Utah and southwest Wyoming. By late
afternoon/early evening Tuesday the shortwave ridge should be
capping the convection across the region so the PoPs that are in
the 18Z to 06Z time frame are mostly there due to an early
afternoon threat.

The global models come into agreement for a short time from about
12Z Wednesday to 12Z Thursday with a trough moving into the western
Great Basin. Southwest winds are expected to increase both Wednesday
and Thursday afternoons, especially over the western valleys. After
this point the GFS and Canadian are bullish on strengthening the
trough as it comes into the eastern Great Basin. However, the FV3
and the EC show the trough weakening after about 12Z Thursday as
another upstream trough over the Gulf of Alaska rapidly shortens the
wavelength between it and the trough over the Great Basin, thus not
allowing it to deepen but rather weakening and lifting northeast.

Confidence is low at this time as to which solution will pan out.
Have more or less split the difference for now. The 700mb temp by
00Z Sunday at SLC from the EC is 13C which corresponds to about 90
degrees at the surface while the GFS has 2C which corresponds to 68
degrees. Left the current forecast of 77 for now and will wait
for the next model runs to start trending in one direction.


&&

.AVIATION...
Southerly winds at the SLC terminal will shift to the northwest
by 22Z-23Z. There is a 10 percent chance high based convection
could impact the terminal with gusty/erratic winds between 22-02Z.

&&

.FIRE WEATHER...
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.

A spring storm system over Nevada will kick up strong gusty
southerly winds today with minimum RH in valleys ranging from
10-15% western Utah to single digits eastern Utah. Fuels are not
reported as critical so no headlines are out.

The cold front makes its way to the Nevada border by early
evening, crossing all but eastern Utah by Sunday morning. The
boundary stalls there through Monday afternoon, then crosses the
eastern valleys.

Very warm and dry today, cooling and moistening significantly
Sunday and Monday. Warmer and much drier mid week.

Isolated high-based showers and thunderstorms are possible today
over the mountains, and may bring locally gusty winds along with
the threat of lightning. This may spark a new wildfire. Showers
become widespread Monday with scattered thunderstorms, gradually
thinning each day through mid week. The chance of wetting rain
rises from near zero today to 60-80% northern Utah thinning to
5-35% southern Utah Sunday.

&&

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

$$

PUBLIC...10/Seaman/Struthwolf
AVIATION...Kruse
FIRE WEATHER...10

For more information from NOAA's National Weather Service visit...
http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php

FXUS65 KSLC 261534
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
934 AM MDT Sat May 26 2018

.SYNOPSIS...A low pressure system over central California will
move east into the Great basin Saturday. This low pressure will
generate widespread precipitation and cooler temperatures across
mainly northern and central Utah through the holiday weekend.
High pressure is expected to return for the middle of next week.

&&

.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, an amplified pattern over the Pacific
became more zonal, amplifying once again beginning today.

Water Vapor Satellite shows a ridge east of the Rockies with a
deep closed low centered over southern California. 400-250 MDCARS
wind observations show a southwesterly 100-125kt jet from the
Pacific over southern California into Las Vegas.

Local Observations and Trends...
12Z KSLC RAOB shows steep lapse rates and a strong wind field for
this time of the year, of uniform southerly flow. Precipitable
Water values range from 0.10"-0.15" mountains to 0.25"-0.35" most
valleys. 24hr Precipitable Water trends are generally lower by
0.10" across Utah, but 0.15"-0.25" higher across portions of
central and eastern Nevada.

A 1006mb surface cyclone is located in west central Wyoming, with
the approaching cold front over eastern Nevada.

24 hour trends: Generally 10-15F cooler except across northern
valleys. Dewpoint Depression has increased (dried) 10-30F across
the north, and decreased (moistened) 5-10F along the Nevada
border and across southern Utah and some mountain sites.

Forecast...
With 25-35kt of southerly 700mb flow, 5-8mb/3hr of falling
pressure and a 10mb southwesterly pressure gradient across
southern Utah this afternoon, synoptic gusts will be elevated.
Would not be surprised to see gusts of 35-40 mph across much of
southern Utah, especially east of I-15 including Lake Powell.

SREF indicates weak to moderate instability develops this afternoon,
over the mountains and along the Nevada border. Liked the idea of
isolated convection at these locations after inspecting high
resolution simulated reflectivity and QPF fields. 700-500mb lapse
rates will be on either side of 9C/km supporting high-based
convection over the mountains, particularly north of Boulder
Summit to the western Uintas and northern Wasatch. With a forecast
LCL near 550mb, any shower may produce locally enhanced wind
gusts.

&&

.PREVIOUS DISCUSSION...
An upper low evident in overnight satellite imagery moving into
the western Great Basin will slowly progress east over the next 24
hours, before stalling over the forecast area Sunday, bringing
cooler and unsettled weather through the Memorial Day weekend.

Ahead of this low southerly flow prevails across the forecast area
early this morning, with breezy conditions possible across the
eastern valleys this afternoon under a belt of stronger low-mid
level flow. Temperatures aloft are forecast to cool at 700mb
today over what was observed Friday, which will result in a
downward trend of several degrees in daytime max temps, although
most locations will still run a few degrees above normal. As the
primary low center translates through the southern Great Basin
this afternoon, increasing diffluence aloft will likely allow for
isolated high based convection primarily across the higher terrain
of northern and central Utah, with some potential for microburst
winds given a very deep and dry boundary layer.

This low center is expected to lift from south to north across
Utah tonight, becoming centered and stalling across northern Utah
Sunday morning. This will bring the best chance for precipitation
to northern and portions of central Utah, and have increased PoPs
20-30 percent accordingly, as well as cooled daytime temps a bit.
Precip coverage across the south is expected to remain sparse and
largely focused on the terrain.

The upper low is expected to slowly fill as it spins across
northern Utah through Memorial Day, and with increased boundary
layer moisture and cool temps aloft, anticipate at least scattered
convective coverage across northern and central Utah through
Monday evening.

The closed low from Monday will continue to fill and weaken as it
moves northeast but the GFS shows a secondary weak trough at its
heels late Monday night into Tuesday, so have held onto some PoPs
across northern Utah and southwest Wyoming. By late
afternoon/early evening Tuesday the shortwave ridge should be
capping the convection across the region so the PoPs that are in
the 18Z to 06Z time frame are mostly there due to an early
afternoon threat.

The global models come into agreement for a short time from about
12Z Wednesday to 12Z Thursday with a trough moving into the western
Great Basin. Southwest winds are expected to increase both Wednesday
and Thursday afternoons, especially over the western valleys. After
this point the GFS and Canadian are bullish on strengthening the
trough as it comes into the eastern Great Basin. However, the FV3
and the EC show the trough weakening after about 12Z Thursday as
another upstream trough over the Gulf of Alaska rapidly shortens the
wavelength between it and the trough over the Great Basin, thus not
allowing it to deepen but rather weakening and lifting northeast.

Confidence is low at this time as to which solution will pan out.
Have more or less split the difference for now. The 700mb temp by
00Z Sunday at SLC from the EC is 13C which corresponds to about 90
degrees at the surface while the GFS has 2C which corresponds to 68
degrees. Left the current forecast of 77 for now and will wait
for the next model runs to start trending in one direction.


&&

.AVIATION...
Southerly winds at the SLC terminal will shift to the northwest
by 22Z-23Z. There is a 10 percent chance high based convection
could impact the terminal with gusty/erratic winds between 22-02Z.

&&

.FIRE WEATHER...
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.

A spring storm system over Nevada will kick up strong gusty
southerly winds today with minimum RH in valleys ranging from
10-15% western Utah to single digits eastern Utah. Fuels are not
reported as critical so no headlines are out.

The cold front makes its way to the Nevada border by early
evening, crossing all but eastern Utah by Sunday morning. The
boundary stalls there through Monday afternoon, then crosses the
eastern valleys.

Very warm and dry today, cooling and moistening significantly
Sunday and Monday. Warmer and much drier mid week.

Isolated high-based showers and thunderstorms are possible today
over the mountains, and may bring locally gusty winds along with
the threat of lightning. This may spark a new wildfire. Showers
become widespread Monday with scattered thunderstorms, gradually
thinning each day through mid week. The chance of wetting rain
rises from near zero today to 60-80% northern Utah thinning to
5-35% southern Utah Sunday.

&&

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

$$

PUBLIC...10/Seaman/Struthwolf
AVIATION...Kruse
FIRE WEATHER...10

For more information from NOAA's National Weather Service visit...
http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php