Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 042228
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
228 PM PST Sun Dec 4 2016

.SYNOPSIS...

A couple of weak systems will bring much colder temperatures
Monday through Wednesday along with low end chances for showers
Monday and Tuesday. A warmer and wetter atmospheric river
pattern sets up the end of this week with a wintry mix of
precipitation expected Thursday into Friday. Additional storms are
possible the early part of next week.

&&

.SHORT TERM...

Due to the strong morning inversions, winds have been slow to
increase this afternoon. As of 2 pm, the lowest valleys have
begun to pick up with breezy conditions now surfacing. This has
allowed for better mixing with inversions mixing out and
temperatures warming to well above normal for early December.

These winds are increasing ahead of a cold front, so this warmth
will be short-lived. This initial surface front is shallow for the
Sierra and western Nevada with the deeper cold air remaining
north and east of the region. Therefore, this first cooldown will
only take temperatures near normal. The moisture with this system
is limited to right along the frontal boundary itself, with
significant drying immediately behind it. Therefore, have reduced
precipitation chances, limiting shower potential to areas mainly
north of a Susanville-Lovelock line. Even for these areas, any
snow accumulation will be less than an inch.

The stronger secondary front and associated cold upper trough will
push into the region from the north Tuesday into Wednesday. This
will bring a major cooldown to the region with high temperatures
on Wednesday likely to only reach the 30s even in the warmest
valley locations. Showers are expected across the region on
Thursday, with the best chances north of Highway 50. This system
is once again moisture starved, so snow totals will only be a
dusting up to an inch with locally a couple inches possible if
more organized bands form.

Warm air advection and moisture begin to spread into the region
late Wednesday night into Thursday with light snow showers
possible late Wednesday night. This is the start of a complicated
forecast, please see the long term for additional details. -Dawn


.LONG TERM...Thursday through Sunday...

Forecaster confidence in mid range models is increasing since run-
to-run consistency remains high while inter-model discrepancies are
shrinking. In short, the EC, GFS, and ensembles are coming in line
with one another. It is evident now that a moderate Atmospheric
River will impact the Western U.S. late this week. The current
consensus shows that the moisture plume will be directed towards
Northern California and the Sierra. While there is room for this
track to shift over the next few days, the plume is wide enough that
there will likely be an extended period precipitation for most of
the Sierra into Western Nevada Thursday and Friday.

Integrated vapor transport (IVT) values, what we use to outline
AR events, have been steadily increasing with this system over the
past few days. The large swath of 250kg/m/s IVT extends from the
Southern Sierra into northern Oregon while the core of 500kg/m/s IVT
covers latitudes from the Central Sierra to the Oregon border.
These numbers are sufficient to show that this AR event is in the
moderate range, and will likely bring significant precipitation area-
wide. Interestingly, the area of 250kg/m/s IVT for inland stations
is fairly dense as well indicating the chance for significant
precipitation extending into the Nevada interior. Conceptually, this
makes sense as well considering that there will be quite a bit of
warm air advection accompanying this event negating some of the
blocking effects of the Sierra. As a result, precipitation chances
were increased from the Sierra into central Nevada Thursday and
Friday.

The following numbers are very preliminary and will likely change
over the next few days. Initial QPF totals for liquid equivalent
range 1-3 inches in the Sierra, 0.5-1 inch along the Sierra Front
into eastern Lassen County, and Up to 0.5 inches elsewhere. Flow is
much less impeded north of Interstate 80 with more consistent totals
ranging from 0.25-0.75 inches. Confidence in these ranges are near
moderate at this time.

However, confidence in precipitation type is very low. Snow levels
will start low with many valleys in Western Nevada trapping cold
air. As moisture moves into the region these levels rise, but it is
unclear if there will be enough wind to scour lower valleys of the
colder air. This could result in a chaotic mixture of precipitation
forming from one valley to the next all dependent on depth of
each cold pool.

These overrunning type scenarios are not too common for this
region, but there are some analogs to past events. Lower valleys,
especially east of Alternate Highway 95, tend to retain their cold
pools while Sierra valleys tend to mix out. Current forecast
atmospheric profiles show the potential for freezing rain from the
Highway 95 corridor into Central Nevada which depends greatly on
how much cold air gets trapped Wednesday. In Sierra valleys, snow
will likely transition to rain. For the Sierra Front, there could
be a mix of snow, rain, and freezing rain. These details will be
fleshed out over the next couple of days. High elevation
precipitation has a little more confidence; snow totals for
elevations above 9000 feet will be decent with current trends
suggesting a couple of feet possible. The bottom line is that
travel will be difficult in and around the Sierra as well as
Western Nevada.

Conditions improve Saturday after moisture exits the region. Then
another round or two of decent moisture moves into the Sierra
Sunday. Have begun to trend precipitation chances upward early next
week in anticipation that the storm track will remain directed
toward the Sierra. Boyd


&&

.AVIATION...

VFR conditions into this evening with west winds gusting up to 25
kts at terminals and 50 kts over the ridges. Mtn wave turbulence
will occur, but LLWS looks to be less than 20% and would most likely
affect KTVL/KTRK. Then areas of MVFR CIGS/VIS north of KSVE-KLOL
tonight in -SHSN. A break for Monday before more -SHSN possible
Monday night into Tuesday.

A larger storm is possible Thursday into Friday with CIG/VIS impacts
as well as some wind.

&&

.REV Watches/Warnings/Advisories...
NV...None.
CA...None.
&&

$$

For more information from the National Weather Service visit...
http://weather.gov/reno


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