Weather Service Report


244 
FXUS65 KREV 200658
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
1058 PM PST Sun Feb 19 2017

.UPDATE...

The strong atmospheric river is beginning to push into the
California coast, with the forecast looking on track. Quick update
this evening to expire the Lake Wind Advisories for both Lake
Tahoe and Pyramid Lake. These will be replaced by a Winter Storm
Warning for Tahoe and a Wind Advisory for the West Central Nevada
Basin and Range, which include Pyramid Lake. 

Please see the previous discussion below for additional details on
the incoming storm which will bring heavy rain, snow, flooding,
strong winds. -Dawn

&&

.PREVIOUS DISCUSSION... /Issued 324 PM PST Sun Feb 19 2017/ 

SYNOPSIS...

A powerful atmospheric river storm will bring strong winds and 
very heavy precipitation with higher snow levels tonight through 
Tuesday. Keep flood mitigation in place or readily available. 
Weaker but colder storm systems may bring snow to lower elevations
at times from Tuesday through next weekend. 

SHORT TERM...

Light precipitation is already starting to move into the Sierra
this afternoon ahead of the incoming strong atmospheric river 
(AR) storm. This powerful Pacific storm will bring a complex 
combination of heavy rainfall, heavy mountain snow, and strong 
gusty winds to the region. This storm will be similar to some of 
the strongest AR storms that we've already seen this season. 
Anyone planning travel in the Sierra is strongly urged to arrive 
at their destinations before tonight, as main highways are likely 
to become impassible due to very heavy snow in higher elevations 
and heavy rain capable of producing large mud and rock slides in 
lower elevations. 

HEAVY RAIN...As we go into tonight and during the day Monday, 
snow levels are expected to rise to near 6500-7500 feet in the 
Sierra. Precipitation rates will be very intense, with high 
flooding potential for elevations below 7000 feet. Models 
continue to show very impressive amounts of precipitation with 
this AR storm, especially in the Sierra. Very strong 700mb flow 
near near 80kts+ will bring very efficient spillover into the 
lower valleys of western Nevada and Northeast CA as well. Flood 
Watches remain in effect for all those areas. Rainfall amounts of
up to 3-7 inches are likely in the Sierra through Tuesday 
morning, with 1-2 inches in the valleys of Northeast CA and 
western NV and 2-3 inches in the foothills west of Highway 395. 
Heavy rain is expected to bring widespread flooding concerns to
the region--including small stream flooding, urban flooding, mud 
and rock slides, as well as increases to mainstem rivers. See our 
hydrology section below for more details on flooding.

SNOW...Snow levels will rise tonight to near 6500 for Lassen/Plumas
Counties, to near 7000-7500 around the Tahoe Basin, and around
7500-8000 feet in Mono County and remain at those levels through
Monday evening. Snow levels could get pushed by 500-1000 feet
lower during period of heavy precipitation. Elevations above 7000
feet will get very heavy snowfall, around 2-5 feet with the 
highest amounts along the Sierra crest west of Tahoe. An avalanche
watch is in effect from the Sierra Avalanche Center. Finding a
place to put all the excessive snow will be a major challenge for
Sierra communities above 7000 feet, including around Tahoe and
Mammoth Lakes. Snow amounts between 6500-7500 will be more
variable with roughly 1-3 feet likely at that elevation. Below
6500 feet, much more limited snow amounts are likely, but not
until snow levels begin to drop Monday night into Tuesday morning.
Even in western Nevada, elevations above 6000 feet, including
Virginia City could en up with 6-12 inches of snow through
Tuesday.

STRONG WINDS...Impressive 700mb flow near 80kts+ will move into 
the area Monday and Monday night, bringing strong winds across the
region. Widespread gust 45-55 mph are expected for valley 
locations, although these will actually be damped by the heavier
precipitation during the day on Monday. We have issued wind 
advisories for the valleys of western Nevada, except for Mineral 
and Southern Lyon Counties where we have a High Wind Warning as 
even stronger winds up to 70 mph are expected. In the Sierra, the 
passes and ridges will see extreme winds Monday through Tuesday 
with gusts over 150 mph possible. High winds and heavy snowfall in
the high elevations will create whiteout conditions. Hoon

LONG TERM...Tuesday Night through Sunday...

Primary change was to lower QPF north of Interstate 80 Tuesday 
evening. Models have consolidated to a more southern moisture push
as colder air begins to move into the region. Otherwise, the 
forecast was largely left intact. 

Conditions transition colder Tuesday night as troughing over the 
west coast deepens and heights fall. Temperatures at 700mb fall to 
below -8C over Tuesday night which is generally indicative of snow 
levels down to nearly all valley floors. Temperatures aloft will 
continue to fall Wednesday and Thursday; -12 on Wednesday and -16 on 
Thursday. These cold temperatures aloft will support relatively cold 
highs in the low/mid 30s for western Nevada and mid/upper 20s for 
Sierra valleys. Conditions become even colder by Thursday night 
with overnight lows dipping into the single digits/low teens for 
Sierra valleys and low 20s for western Nevada. 

With colder temperatures moving into the region, expect precipitation
to transition to snow by early Wednesday morning for western 
Nevada valleys and continuing through Thursday morning. Expect 
another 6 inches or so for Sierra valley locations with about 
another 12 inches above 7000 feet; some localized higher totals 
will occur for typical locations like Mount Rose and the Sierra 
Crest itself. Western Nevada valleys will generally accumulate 
less than 1 inch, but locations west of Highway 395 could see up 
to 4 inches or so. 

Weak ridging provides a relative pause in activity Thursday as 
another cold system drops down the Pacific coast. Also, the moisture 
tap switches from deep subtropical to more seasonable eastern mid 
latitudinal Pacific. This will result in lower, but not 
insignificant, QPF than we have recently seen. Models are less 
certain with more inter-model inconsistencies to note on timing and 
intensity of this wave. GFS has been more robust while the EC splits 
energy over the eastern Pacific. Either way, expect at least some 
more mountain snow for the Sierra; how much will be the question to 
answer over the next few days. Boyd

AVIATION...

Winds will continue to increase tonight as this storm system 
intensifies. This will lead to turbulence aloft and near the surface 
along with Low Level Wind Shear - LLWS will be very strong by late 
Monday morning for Sierra locations. 

Showers with intermittent MVFR conditions are moving over Sierra TAF 
sites this afternoon. By this evening conditions continue to 
deteriorate as moisture and IFR/MVFR cigs/vsbys spread through the 
region from the west. Snow levels rise through the night into 
Monday. The heaviest precipitation is likely during the day Monday. 
All rain at the terminals by then. Gusty winds remain a possibility 
east of the Sierra late Monday afternoon into Monday evening when 
precipitation starts to decrease.

Lower cigs and vsbys return Monday night and snow levels start to 
lower. This will produce IFR conditions and accumulating snows for 
the Sierra terminals. By late Tuesday snow levels are likely to the 
lower valley floors with minor accumulations east of the Sierra. But 
the amount of precipitation will be much lower by then. Conditions 
become colder Wednesday and Thursday with the potential for light 
accumulating snow at western Nevada TAF sites. Boyd

HYDROLOGY...

* Major flooding now expected for the Middle Fork of the Feather
  River near Portola.
* Truckee River at Reno and Vista along with the Carson River near
  Carson City forecast to reach monitor stage. 
* Flooding likely for creeks, streams, urban areas and drainage
  basins in the Tahoe Basin, eastern Sierra, northeast California
  and western Nevada early Monday through early Tuesday.

A strong atmospheric river will push into the region late Sunday 
night through Tuesday morning bringing heavy rain and rising snow 
levels on Monday. Currently, snow levels are expected to be around
6500 feet in northeast California and around 7500 feet for much 
of the Sierra and western Nevada. Precipitation totals in the 
Sierra could reach 3 to 6 inches, with 2 to 3 inches in the 
Carson Range and much of the Susan, Pit, and Middle Fork of the 
Feather river basins. 1 to 2 inches of rain is likely through much
of the Greater Reno/Sparks/Carson City metro areas and along the 
Highway 395 corridor into Mono County.

Unprecedented amounts of rain and snow have already fallen this 
winter and the ground is at record saturation levels. It will not 
take much to bring renewed flood concerns throughout the Sierra, 
northeast California, and in western Nevada. Around the Tahoe Basin, 
the eastern Sierra, and western Nevada, main threat for flooding 
will be along creeks, streams, and drainage basins/farmland where 
water collects from creeks running out of the mountains. Flooding is 
also possible through both the Carson Valley and Washoe Valley along 
with Hardscrabble Creek in the Virginia Range. Urban flooding is 
probable in the Tahoe Basin as snow berms won't allow for water to 
properly drain from the region. The other concern will be the 
increased potential for rock and mudslides in areas of steep 
terrain, with road closures possible.

The river basins to watch at this time will be the Middle Fork of 
the Feather, the Susan, and the Pit in northeast California. Snow 
levels may be just low enough for keep the Susan and Pit rivers from 
flooding, but with copious amounts of additional rainfall expected, 
these will need to be closely monitored. The Middle Fork of the 
Feather is now forecast to reach major flood stage. 

Snow levels below 8000 feet usually prevent flooding along the 
Carson, Truckee and Walker Rivers. However with significant runoff 
anticipated, the drainages which flow into the mainstem rivers may 
overcome the slightly lower snow levels. This includes, but is not 
limited to, Steamboat Creek and the North Truckee Drainage. The 
current forecast takes the Carson River near Carson City and the 
Truckee River at Reno and Vista to monitor stage. Small changes 
to precipitation totals and/or snow levels may result in minor 
flooding.

Snow levels are anticipated to fall by Tuesday morning or afternoon, 
which also coincides with weakening moisture transport. This will 
help lessen new flood risks, but it may take a day or two for 
ongoing flood waters to subside. -Dawn

&&

.REV Watches/Warnings/Advisories...
NV...Wind Advisory from 10 AM Monday to 4 PM PST Tuesday NVZ005.

     Flood Watch through Tuesday morning NVZ002.

     Winter Storm Warning until 4 PM PST Tuesday above 6500 feet in 
     NVZ002.

     High Wind Warning from 10 AM Monday to 4 PM PST Tuesday NVZ001.

     Flood Watch from 4 AM PST Monday through Tuesday morning NVZ003.

     Wind Advisory from 10 AM Monday to 4 PM PST Tuesday NVZ003-004.

CA...Flood Watch through Tuesday afternoon CAZ071.

     Winter Weather Advisory until 4 PM PST Tuesday above 5500 feet 
     in CAZ071.

     Wind Advisory from 10 AM Monday to 4 PM PST Tuesday CAZ070.

     Flood Watch through Tuesday morning CAZ073.

     Winter Storm Warning until 4 PM PST Tuesday above 7000 feet in 
     CAZ073.

     Flood Watch through Tuesday morning CAZ072.

     Winter Storm Warning until 4 PM PST Tuesday above 6500 feet in 
     CAZ072.

&&

$$

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

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Western Regional Climate Center, wrcc@dri.edu