Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 192220

Area Forecast Discussion
National Weather Service Salt Lake City UT
420 PM 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.


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 northeast of Las
Vegas, with a dry slot extending from central California into the
southwest Utah. 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" eastern Utah,
0.25"-0.33" along and west of I-15.

A 1000mb surface cyclone is located near Page Arizona with a cold
front crossing west central Utah to near Kanab from the southwest
thus far this afternoon. A northerly gradient of 6mb has slowed
the gusty south pre-frontal winds thus far.

24 hour trends include 5-10F of warming across northern and
eastern Utah, and 5F cooler southwest Utah. Dewpoints depression
has increased 10-20F across northern and eastern Utah, falling
5-10F across west central and southwest Utah.

This morning, MDCARS wind observations revealed the strongest
winds upstream of the closed low, so anticipating that it deepens
during the next 6-18 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 late 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. Have extended this threat
into the early evening hours.

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.

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,
while northerly winds likely to enhance across the West Desert
with deepening surface cyclone supporting a strengthening
northerly pressure gradient.

RAP not as convinced as GFS for the development of strong
easterly winds, comparing a Time Height Series over Centerville.
Also, easterly pressure gradient suppressed east of the Wasatch
Front, with little cold advection at 700mb, in fact it may
actually be slight warm advection. The potential of
downslope/canyon winds will likely be local and short lasting. No
advisory issued but something for the next shift to monitor. As
for the West Desert, northerly winds likely in the Advisory
category, though agree with UDOT that threat for I-80 restricted
to 5E Wendover and west, and SR-30 is not well traveled for high
profile vehicles.

Snow is a concern about 6500-7500ft tonight for mountains from the
Uintas to central and southern Utah mountains (wrap-around).
Though at these elevations threat is restricted to heavy intensity
snowfall to overcome the warm surfaces. So decided not to issue
an advisory at this time.

Stratiform precipitation becomes showery tomorrow afternoon with
re-development of diurnal instability. Have thunder in the
forecast for the afternoon.

Strong height falls and warming aloft Friday night end the threat
of precipitation. Ridging brings in warming temperatures and
benign weather through Saturday.

Next system approaches the north Sunday. Have enough instability
to put a 20 PoP in the mountains for the afternoon, considering
modest height falls.

A weak trough is developing late in the weekend over the Cascade
Mountain range and sliding east into southern Idaho. This feature
looks to stay mostly north but will brush the northern portions of
Utah and sweep through southwest Wyoming before sliding further
south and east over the Rocky Mountains of Colorado. Brief cold
advection associated with a front and weak jet support should
allow for a small area of showers Monday across the north before
exiting to the east by Tuesday. Increased cloud cover and moisture
should keep temperatures in a steady hold from Monday to Tuesday,
with minimal cooling in place.

Heights build once again Tuesday into Wednesday under ridging as
models depict, with warming and drying trends taking place. This is
about where similarities stop among the models. In the far extended,
another off-shore low pressure system approaches the West Coast,
however timing and track are greatly out of sync, so have low
confidence in the extended forecast. Made minor changes to going
precipitation forecast, utilizing some climatology and low end
chances for the higher terrain as a start and temperatures gradually
warm or hold steady through the extended, with the far southwest
starting to really warm up.


The SLC terminal will see VFR conditions throughout the
evening with increasing clouds. Cigs should stay above 6000 feet AGL
during the evening hours, but will drop to or below that value at
times overnight with showers in the vicinity of the terminal.
Northwest winds are expected through the overnight hours.


UT...Wind Advisory until 9 PM MDT this evening for UTZ013-020-021.




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