Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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410
FXUS65 KSLC 191614
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
1014 AM MDT Thu Apr 19 2018

.SYNOPSIS...Southerly flow will strengthen today ahead of an
upper low which will cross the region through tomorrow, bringing
rain and snow to portions of the forecast area. High pressure and
resultant warm/dry weather will follow 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 northwest of Las
Vegas, with a dry slot extending from central California into the
Las Vegas area. 400-250 MDCARS wind observations show a anticyclonic
110-150kt jet over the eastern Pacific, becomes cyclonic just of
off the central California coast.

Local Observations and Trends...
12Z KSLC RAOB shows steep lapse rates up to near 570mb above the
nocturnal inversion, with uniform southwest winds of 25-45kts in
this layer. Very dry below 500mb, and moist above this level.
Precipitable Water values range from 0.10"-0.15" most locations,
to to 0.25"-0.33" near the Nevada border.

A 1008mb surface cyclone is located near Las Vegas, supporting a
4mb east-northeast gradient across the state. Winds continue to
increase from the south ahead of the storm system, especially
ridgetops.

24 hour trends include 5-10F of warming across Utah, as dewpoints
depression has increased 10-20F except for some of the highest
elevations which are 5-10F higher locally.

Forecast...
MDCARS wind observations revealed the strongest winds upstream of
the closed low, so anticipating that it deepens during the next
12-24 hours. The system is augmenting the mean synoptic pattern to
the affect of showing strong height falls centered over Las Vegas
midday, shifting east over southern Utah through tonight. This is
a dynamic situation, with 5-7mb pressure falls over eastern Utah
this afternoon.

Two concerns in the immediate short term...
1) While the strong cold front crosses Dixie mid afternoon, will
take a few hours of moistening to elevate instability. By 00Z,
the instability gradient has arrived from the west. While
measurable precipitation is unlikely given the initial dry low
level profiles late day, the threat of a late day thunder is too
high to not have some reflection in the forecast. May extend into
evening with later forecasts.

2)Ahead of the strong cold front, 50kt 700mb winds develop over
southeast Utah by evening. Model forecast soundings indicate
mixing will be near 500mb, similar to the KSLC RAOB. Winds atop
the mix layer are 50-70kts. So while most gusts will likely be
just to the cusp of advisory conditions, the threat of isolated
gusts closer to 50-60mb is high enough to warrant a wind
advisory, especially considering the threat for Lake Powell and
Glen Canyon National Recreation Area.

Ahead of the cold front, went on high side of temperature guidance
despite cloud cover due to the dynamic nature of the system and
strong winds. Went on the low side for RH for the same reason.

As the system wraps up overhead tonight, wrap around upslope could
enhance precipitation from the Uintas into West Central and
Southwest Utah. Easterly winds will likely dampen threat of much
of the precipitation due to downslope winds north of Salt Lake.
RAP not as convinced as GFS for the development of strong easterly
winds, comparing a Time Height Series over Centerville. Will take
a closer look after all the 12Z guidance arrives.

&&

.PREVIOUS DISCUSSION...
Infrared satellite imagery early this morning shows several
features set to affect our weather over the next couple days: a
deformation zone moving over Utah in advance of a circulation
center over the southern Great Basin, and another circulation
center moving down the SoCal coast. These two lows are progged to
merge into a single closed low and track through southern Utah
later today and Friday.

Water vapor imagery shows a notable dry slot currently sliding
into southern Utah. Models suggest this dry slot will keep the
eastern third of our forecast area dry through tonight, while
areas west of around I-15 and south of I-80 should see showers
fill in by late today. Temperatures meanwhile will be fairly warm
for areas east of I-15 as 700MB temps reach or exceed 0C. Gusty
south winds are expected for the higher terrain of southern Utah
today. Winds at lower elevations should pick up as well by late
in the day as the surface pressure gradient tightens, especially
for areas near the Arizona border such as Lake Powell.

As the upper-level closed low pushes eastward tomorrow, models show
a notable TROWAL signature from around the Uintas north into
southwest Wyoming, suggesting that this area could see more
significant and widespread precipitation (in comparison to the more
showery and generally lighter precip elsewhere in our forecast
area). One area that could struggle to see any precip is the Wasatch
Front, especially SLC northward, which will be downsloped by the
easterly mid-level flow. Snow levels will be higher in comparison to
recent storms, around 6,000 to 7,000 feet in the most active portion
of the event.

Ridging and associated subsidence will move into our area by early
Saturday, bringing clear skies and a warming trend.

The ridge axis over Utah heading into Sunday slowly shifts
eastward through the day, with a warm day expected in lightly
increasing southwesterly flow. This is ahead of a trough over the
Pacific Northwest that is expected to brush by northern Utah
Sunday night into Monday. This could produce some precipitation
across the far north, but 80% of the area will likely just see an
increase in cloud cover.

The trough continues eastward out of the region Monday night, with
heights and temperatures rebounding for Tuesday. Beyond Tuesday,
global models begin to get wildly out of phase, as they struggle
with how to handle a closed low off the west coast. As such, details
beyond Tuesday are very low confidence.

&&

.AVIATION...
The SLC terminal will see VFR conditions throughout the
day with increasing clouds. Southeast winds are expected to shift to
the northwest between 20Z and 22Z, but there is a 30 percent chance
that this shift occurs later than expected.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory from 2 PM this afternoon to 9 PM MDT this evening
     for UTZ013-020-021.

WY...None.
&&

$$

PUBLIC...10/Van Cleave/Schoening
AVIATION...Traphagan

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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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