Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 301122
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
522 AM MDT Thu Mar 30 2017

.SYNOPSIS...An upper low will develop across the Great Basin
today, then slowly drift east along the Utah Arizona border
tonight through Friday. High pressure will return Saturday,
followed by another cold front early next week.

&&

.SHORT TERM (Through 12Z Sunday)...A rather vigorous shortwave
trough currently digging through the northern Sierra is forecast
to continue to dig southeast into the Great Basin today, before
evolving into a closed low and slowly drift eastward along the
UT/AZ border tonight through the day Friday. Downstream from this
wave strengthening warm air/moisture advection is resulting in a
couple bands of precipitation early this morning...one drifting
across southern ID and far northern UT, with a second upstream
across northeast NV. Through the morning hours the greatest chance
for precipitation across the forecast area will generally remain
north of I-80. Increased PoPs across this area while trimming back
morning PoPs across the central and southern Wasatch Front.

The upstream trough is forecast to close off as it crosses
central/southern NV this afternoon. Some uncertainty remains
regarding how early and how far west this low will form, which
would have a significant impact on precipitation across northern
Utah. At this point the consensus in the models is to push a cold
front through northern Utah during the afternoon hours, reaching
the Wasatch Front late in the afternoon. Considerable upper
diffluence overtop of this surface boundary would support fairly
widespread precipitation along the boundary this afternoon and
evening across the north, and most of the 00Z hi-res models
support this idea. May see a few thunderstorms along the boundary
as well given the strength of the large scale ascent and modest
instability. However, if the low closes off earlier than forecast
it may cause the frontal boundary to hang up west of the Wasatch
Front, which is not all that uncommon, and would result in a
substantially reduced probability of precipitation this afternoon
and evening across the Wasatch Front and points east.

Further south, as the upper low traverses central NV this
afternoon, a strong cold front will push through west central and
southwest UT late this afternoon and this evening, likely bringing
a band of heavy precipitation along with a few thunderstorms. As
the upper low slows its forward progress and gradually traverses
the UT/AZ border this evening through through tonight and into the
day Friday, wrap around moisture will likely bring a fairly
prolonged period of precipitation to west central and southwest
Utah including the I-15 corridor. With the 700mb cold pool
forecast to dig through this area, snow levels should fall to near
4000 feet this evening, and remaining at most valley floors into
Friday morning, before possibly rising a bit Friday afternoon as
warmer air advects into the area. Confidence in precipitation
across central and southern Utah is much higher given the
proximity to the upper low and the strength of the cold front,
and going winter weather highlights across central and southern
Utah seem to handle the situation well.

Focusing back on the north, as this low tracks well south of the
area, and the 700mb cold pool digs well west of the area, snow
levels across the north are expected to remain above most valley
floors with the exception of far western areas near the NV border.
As the flow develops an easterly component overnight, will likely
see precipitation quickly diminish along the Wasatch Front.
Trended PoPs lower by 20-30% overnight into Friday, and the day
shift may be able to further reduce these. With the cold pool
dropping west, and then passing south of the area, a wedge of
relatively warm air will reside across northeastern portions of
the area including the Uintas and Uinta County Wyoming. This will
likely keep snow levels above 8000 feet through tonight, and
possibly never falling below 7000 feet through the event.
Maintained the going winter highlights for these areas, although
made some adjustments for Uinta county to delay the onset and
raise the snow level.

Winds will also be a concern with this system. With a deepening
surface low crossing southern Utah through the day today. With
strong pressure falls and a tightening gradient during the
afternoon, coupled with a strong cold front late this afternoon
and early evening, went ahead and issued a Wind Advisory for the
southern tier of zones for this afternoon and evening hours.
Additionally, a stout gradient on the back side of the cold front
across northern Utah this afternoon and evening should promote
gusty winds across the northwest deserts. Went with a Wind
Advisory for this area, as the upper support with the upper low
remaining west of the area doesn`t seem to support High Wind
Warning criteria.

Of additional concern will be a window for easterly winds along
the northern Wasatch Front tonight into Friday morning. This will
need to be monitored, although the lack of 700mb cold advection
within the easterly flow would seem to preclude a higher end wind
event.

As the upper low slowly moves into the southern Rockies Friday
night into Saturday, lingering precipitation should wind down
across central and southern Utah by Saturday morning. In the wake
of this system, low amplitude mid level ridging is forecast to
build into the Great Basin Saturday afternoon through Saturday
night.

.Long Term (After 12Z Sunday)...Low amplitude mid level ridging
will begin to shift downstream Sunday in advance of the next
trough depicted in globals to arrive Monday. The globals are
trending closer regarding detail of this trough, depicting a
strong cold frontal passage Monday morning/midday, spin up of a
mid level low over southern Idaho, and net sharpening as it tracks
overhead later Monday into Tuesday. If details remain in place
regarding the strength of the cold front, we can expect a period
of intense precip with snow levels lowering towards and likely
down to the lower valley floors by midday. Will have to continue
to monitor accumulating snow potential over the next several days.

The mid level low will continue to deepen with downstream
progression, likely closing off over Colorado Monday night into
Tuesday. With this, a period of neutral to weak cold air advection
within a NW-N flow should be sufficient for maintaining showery
precip along the terrain into Tuesday morning before tapering, this
as heights begin to rebound and build in from the west.

The remainder of the long term period will be dominated by mid level
ridging over the area, with stable and warming conditions becoming
the norm into the late week period.

&&

.AVIATION...Cigs will continue to lower at the KSLC terminal this
morning, with an increasing chance of light showers intermittently
tracking near or over the terminal come midday. A cold front is
expected to move across the terminal between 21-23z switching winds
from southerly to a moderately gusty northwesterly with passage.
Steadier MVFR rainfall is expected with this frontal passage, likely
lingering into or through evening hours.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Wind Advisory from 2 PM this afternoon to 9 PM MDT this evening
     for UTZ019>021.

     Winter Storm Warning from 6 PM this evening to 6 AM MDT Saturday
     for UTZ009-015-016-517-518.

     Winter Weather Advisory from 6 PM this evening to 6 AM MDT
     Saturday for UTZ007-008-010-014.

     Wind Advisory from 2 PM this afternoon to 4 AM MDT Friday for
     UTZ005.

WY...Winter Weather Advisory from 3 AM Friday to 6 AM MDT Saturday
     for WYZ021.

&&

$$

Seaman/Merrill

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