Area Forecast Discussion
Issued by NWS Reno, NV

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017
FXUS65 KREV 101208
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
408 AM PST Sat Dec 10 2016

.SYNOPSIS...

Periods of heavy rain, high elevation snow, and strong gusty
winds will impact the Sierra tonight through Saturday with periods
of light to moderate precipitation in western Nevada. Sunday and
Monday will see a break in significant precipitation before
another round of storms moves into the region the midweek.

&&

.SHORT TERM...

Widespread rainfall is pushing across the northern Sierra this
morning with lighter precipitation spilling over to western Nevada
foothills and valleys. This copious moisture is associated with a
warm Atmospheric River (AR) storm with a supply of subtropical
moisture with origins near Hawaii. This is producing a warm storm
with very high snow levels around 8,500-9,500 feet.

The peak of the heaviest rainfall will occur over the next 12-15
hours across the Sierra. This combined with saturated soils will
result in plenty of water runoff resulting in rises in rivers,
creeks, and streams today. The copious amounts of rainfall falling
throughout the Tahoe Basin will result in drastic rises on the
Truckee especially near Truckee where the river is forecast to reach
moderate flood stage this afternoon and a River Flood Warning is in
effect for this portion of the river. There will be rises on the
Truckee River downstream as well through the Reno/Sparks area, but
no flooding is anticipated. Rivers will quickly fall again by
Saturday evening into Sunday.

As of 3am, observed rainfall amounts are around 0.50-0.75 inches
right along the Sierra crest with about 0.25-0.50" around the Tahoe
Basin. As the main push moisture begins to arrive we will see rain
rates increase around 0.25-0.50"/hr. The overall forecast amounts
remain unchanged as latest suite of guidance showed little
significant change in QPF amounts. As precipitation winds down late
afternoon and evening, we should see widespread rainfall amounts in
the 2-3" range with 0.50-1.00" across far western Nevada.

Heavy snowfall is expected for very high elevations in the Sierra
(>9,000`) where anywhere from 1 to 3 feet of heavy wet snow is
expected. The best chance to see the higher end of that range looks
to be across the Sierra crest through Alpine and northern Mono
counties. Most Sierra passes should see mainly rainfall but higher
passes such as Mount Rose Highway and Carson Pass could see
significant snowfall. Snow levels will fall near 7,000 feet between
about 5-10pm tonight when moisture is starting to taper off. It is
during this period where a few inches of snow will be possible for
lower passes including Donner and Echo.

The other component to this storm has been isolated areas of strong
and gusty winds. Most of the gusty conditions have been confined
mainly to the Hwy 395 corridor from Reno through to Gardnerville.
Most gusts overnight have recorded in the 40-50 mph range with wind
prone locations such as the Galena Creek Bridge reaching 82mph. Have
extended the Wind Advisory for the Reno, Washoe Valley, Carson City,
Carson Valley area until noon today. Gusts will be sporadic with
frequent peaks and lulls as strong winds mix down to the surface
during breaks in the rain. Gusty winds will also work their way east
across the basin and range this afternoon ahead of the rainfall. We
get a break in the weather Sunday and Monday after which our next
system will arrive late Monday into Tuesday. Fuentes


.LONG TERM...Tuesday through Friday...

For Tuesday...Model simulations are in good agreement that the next
moderate AR will reach the east slopes of the Sierra before Tuesday
morning. Last few model runs have been consistent in showing decent
spillover north of I-80 and POPS were increased accordingly. The
operational models are also showing somewhat better snow level
agreement for Tuesday morning, especially north of I-80. Snow levels
Tuesday morning could be as low as 3,500 feet MSL near the OR border
sloping up to around 4,000 to 5,000 feet near the I-80 corridor.
Snow levels should rise slowly from west to east during the day
Tuesday as a warmer air mass crosses the Sierra. The upshot is there
is potential for a period of snow in western NV down to valley
floors Tuesday, especially in the morning and from the I-80 corridor
northward.

Beyond Tuesday...there is low confidence in the amount of spillover
east of the Sierra crest , but overall there is good potential for
wet conditions through the rest of the week. The general trend is
for lower snow levels for the second half of the week, with a chance
for well below normal temperatures and periods of accumulating snow
down to all valley floors by the end of the week. JCM

&&

.AVIATION...

Wind, rain, and high elevation elevation snow will continue into
this evening across northeast California and western Nevada. The
main concern is for moderate or even severe turbulence and areas of
LLWS (within 020 AGL) in the lee of the Sierra crest this morning.
Rotors are evident at KRNO this morning such that surface winds
could be erratic varying between calm and southwest gusts up to
around 30 kts through this morning. Winds aloft diminish by
afternoon leading to some improvement in these conditions but it is
still likely to be turbulent in the lee of the Sierra.

As far as CIGS/VIS and precipitation at terminals (Sierra and
western NV) through this afternoon, rain is expected with snow
levels above 8000 ft MSL. For western NV, MVFR CIGS/VIS are possible
through this evening with -RA/RA. For Sierra (KTRK/KTVL) and
northeast CA (KSVE) terminals, CIGS and/or VIS expected to be lower
with below landing minimums possible for KTVL (less than 3 SM VIS)
and near mins for CIGS (015) at KTRK possible through this evening.
Conditions should begin improving by late this evening but mountain
obscuration and IFR/MVFR CIGS could persist in the Sierra overnight.
JCM

&&

HYDROLOGY...

Main areas of concern for flooding are the creeks along the west
shore of Lake Tahoe, Donner Creek, and the Truckee River along
Highway 89 to the Martis Valley, this includes the city of Truckee.

The warm atmospheric river storm has arrived in the northern
Sierra. The peak of the heaviest rainfall will occur over the next
12-15 hours across the Sierra. This short but potentially high
intensity rainfall event combined with saturated soils will lead
to rapid rises on small creeks, streams and the mainstem Truckee
River. As of 3am, observed rainfall amounts are around 0.50-0.75 inches
right along the Sierra crest with about 0.25-0.50" around the Tahoe
Basin. As the main push moisture begins to arrive we will see rain
rates increase around 0.25-0.50"/hr. The overall forecast amounts
remain unchanged as latest suite of guidance showed little
significant change in QPF amounts. As precipitation winds down late
afternoon and evening, we should see widespread rainfall amounts in
the 2-3" range with 0.50-1.00" across far western Nevada.

The copious amounts of rainfall falling throughout the Tahoe
Basin will result in drastic rises on the Truckee especially near
Truckee where the river is forecast to reach moderate flood stage
this afternoon and a River Flood Warning is in effect for this
portion of the river. There will be rises on the Truckee River
downstream as well through the Reno/Sparks area, but no flooding
is anticipated. Rivers will quickly fall again by Saturday evening
into Sunday.

Depending on the amount of shadowing, there could be some minor
flooding of creeks along the east shore of Lake Tahoe and creeks
leading out of the Carson Range and into the foothills west of Reno,
Carson and Minden. Rock and mud slides could also impact highways
near steep terrain, especially I-80, to go along with the usual
ponding of water on roads and poorly drained urban areas.

Rainfall intensity is forecast to decrease after midday though the
flood threat may persist into this evening in the Truckee area.
For the Truckee River downstream of the Martis Valley to Reno-
Sparks, the water levels will rise this evening/night but no
flooding is forecast.

Elsewhere rain will increase flows, but no flood issues are expected
for the mainstem Susan, Carson, and Walker rivers.  Brong/Fuentes

&&


.REV Watches/Warnings/Advisories...
NV...Wind Advisory until 4 PM PST this afternoon NVZ001-004.

     Lake Wind Advisory until 4 AM PST Sunday for Lake Tahoe in
     NVZ002.

     Flood Watch through this evening NVZ002-003.

     Wind Advisory until noon PST today NVZ003.

CA...Wind Advisory until 4 PM PST this afternoon CAZ073.

     Lake Wind Advisory until 4 AM PST Sunday for Lake Tahoe in
     CAZ072.

     Flood Watch through this evening CAZ072.

&&

$$

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



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