Area Forecast Discussion
Issued by NWS Reno, NV

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
783
FXUS65 KREV 160344
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
744 PM PST Wed Nov 15 2017

.UPDATE...

The previous trends from an earlier update continue to hold. Snow
levels are lower where the pcpn intensity is higher...not so much
where pcpn is lighter. This is a common occurrence. But there is
one anomaly right now. Light snow continues falling in far
western Lassen County near Bogard. That area is seeing less pcpn
intensity now...but it is likely that warm air advection has not
returned to the area and driven snow levels up yet. There is
another area of heavier pcpn well to the west that should impact
western Lassen later tonight. Ahead of this...warmer air may
briefly push snow levels above 7000 feet...but then they should
fall again.

Farther south into the Tahoe Basin...the northern part of the
basin is seeing heavier pcpn at this point. Snow levels have
dipped below 7000 feet as evidenced by snow over Donner Summit and
a little lower on the western slope where lift and dynamic cooling
are able to overcome warm air advection. We still feel that
decreasing pcpn intensity will briefly allow snow levels to rise
after about 9 pm for a short period of time before heavier pcpn
moves back into the region after midnight. That should drop snow
levels again or...at most...hold them steady. This is a struggle
between advection of warm air and lift causing cooling. At times
we see this manifest as an isothermal airmass that teeters
between rain and snow. It all depends on the pcpn intensity. Model
soundings still look too warm based on current trends so we have
had to go cooler tonight and early Thursday.

Winds remain an issue as well. The strongest wind gusts have
decreased this evening...but that has more to do with moisture and
spill over pcpn moving into western Nevada. This increased
moisture and some instability help to dampen the winds. Winds are
still rather strong into the southern portions of the basin and
range country where moisture is not increasing as yet. That will
happen later tomorrow. So we will continue to see periods of
decreased winds overnight when pcpn does occur...but when the
intensity or spill over decreases the winds will increase at the
surface again.

Bottom line: Snow level variations could result in higher totals
in Lassen/Plumas counties down to 5500 feet and the Tahoe basin
down to 6500 feet by the time the snow ends Friday morning.
Periodic strong winds east of the Sierra support maintaining the
warnings and advisories into Thursday. Updates out soon.

&&

.PREVIOUS DISCUSSION... /Issued 514 PM PST Wed Nov 15 2017/

UPDATE...

Recent web cam images show snow falling and accumulating at
Bogard Rest Area in far western Lassen County. Snow is also
falling at Donner Summit in the Tahoe Basin and near Chester in
Plumas County. This is a little lower than we had originally
forecast. But...this may be just a temporary condition owing more
to precipitation intensity than an overall lowering of snow levels
due to encroaching colder air. In fact...forecast soundings would
have one believe that snow levels should be steady or rising from
late afternoon into the early evening. Now...the models often
struggle with snow levels...so this is not a unique case. Radar
imagery indicates a lessening of precipitation intensity this
evening. This is likely to allow snow levels to rise again.

The latest HRRR shows an increase in precipitation again later
tonight. We will show a drop in snow levels early this evening
followed be rises after about 7-8 pm. Then we will indicate falls
again later tonight. This will change snow accumulations a little
across parts of Lassen and Plumas counties...but not enough to
change the winter storm warning as yet. Some of the lowest areas
with snow (below 6000 feet) may see little accumulation.

PREVIOUS DISCUSSION... /Issued 109 PM PST Wed Nov 15 2017/

SYNOPSIS...

A strong atmospheric river storm will impact the region through
Thursday. Strong and damaging winds are expected this afternoon
across western Nevada along with high wildfire danger before
precipitation arrives. Precipitation will continue to intensify
this afternoon through Thursday. A cold front will drop snow
levels Thursday and will provide heavy spillover rainfall for
western Nevada.

SHORT TERM...

The first significant atmospheric river event of the season is
just starting to move into the west. Light rain and high elevation
snow is falling throughout northeast California, the Sierra and
western Nevada. Precipitation will only intensify over the next 36
hours with snow levels a bit slower to fall. Winds are slowly
starting to pick up, with the strongest winds anticipated this
afternoon through Thursday afternoon.

Here is what has changed:

* Increased QPF (liquid precipitation) totals along the Sierra
  crest. NCAR ensemble guidance is showing a 70% chance for
  localized areas of 10" of liquid over a 48 hour period along the
  central Sierra crest. In addition, model guidance continued to
  trend upward in liquid totals.

* An areal flood watch was issued for portions of northeast
  California, the Sierra, and the western Nevada Sierra Front.
  While mainstem river flooding is not expected, significant rises
  are likely. Small creeks and streams along with poor drainage
  areas in urban areas and recent burn scars may see minor
  flooding. While not in the watch, Hardscrabble Creek will also
  bear monitoring.

* Extended the strong winds through Thursday night. Winds will be
  periodically be dampened out by the heavier precipitation, but are
  expected to continue to be gusty. The High Wind Warnings will be
  extended by 12 hours through Thursday afternoon.

What we have confidence in:

* Strong, potentially damaging winds for the Sierra Front and along
  the I-590/Hwy 395 corridor. The strongest winds are anticipated
  this afternoon through Thursday afternoon, though gusty winds
  will continue through Thursday night.

* Period of moderate to heavy rain with significant snowfall for
  areas above 8000 feet. The peak intensity will be tonight through
  Thursday evening.

* Liquid totals of 7-8" look likely along the Sierra crest with 2-4"
  through the Tahoe Basin. 1-3" is possible for the western Nevada
  Sierra Front and the Highway 395 corridor in Mono County. 0.25"-
  0.50" is possible across the Basin and Range including areas such
  as Lovelock and Fallon.

* Snow totals above 8000 feet could easily reach 2-4 feet with local
  amounts over 5 feet possible. The character of the snow will be
  heavy and wet.

Possible forecast problems:

* How quickly snow levels will fall. Guidance has been slower in
  bringing the cold front through, however the heavier precipitation
  rates may drag snow levels down faster. Snow totals below 8000
  feet have a big boom or bust potential. Some ensemble guidance in
  the 6000-7000 foot elevation range shows anywhere from a few
  inches upwards of 3 feet of snow being possible depending on snow
  level timing.

* While no mainstem river flooding is expected at this time,
  significant rises are likely. This will be the first true test of
  the hydrologic system this season. If snow levels stay up longer
  and liquid totals are higher than forecast it is possible there
  may be additional hydrologic concerns. -Dawn

LONG TERM...Saturday through next week

A quiet weekend will be followed by another storm early next week.
There is still quite a bit of variability with this storm for next
week, but there is potential for another round of valley rain and
mountain snows. Lower forecast confidence for how travel will be pre
and post Thanksgiving for next week as well.

The Monday-Tuesday storm continues to have variable snow level
scenarios, ranging from near 6000 feet to above 9000 feet. We are
still leaning toward the warmer side of the guidance range (snow
levels likely above 8000 feet) as the downstream longwave pattern
continues to favor a deeper trough over the eastern US, which
typically keeps the west milder. The strength of this system and
precipitation amounts also vary with the guidance sources, as the
warmer scenarios may tap into a deeper source of subtropical
moisture and produce a period of enhanced rainfall, especially north
of I-80.

After this storm pushes through, the overall trend favors a few dry
days leading into Thanksgiving as high pressure ridge builds from
the southwest US into the Great Basin. While there are still some
uncertainties with next week`s weather pattern, at this time there
do not appear to be significant weather impacts for the early part
of the Thanksgiving holiday peak travel period. ELW/MJD

AVIATION...

Winds will continue to increase this afternoon, with winds lasting
through Thursday night. LLWS will be likely as southerly winds
develop across valleys and southwest ridge level winds increase to
55-70 kts with gusts approaching 90kt. The critical period for winds
will be through 00z Friday (late Thursday afternoon), especially for
the eastern Sierra along the Highway 395 corridor extending out to
Hwy 95. Sustained winds 25-35 kts with gusts to 50 kts will be
widespread and there could even be a period of stronger winds from
Reno south toward KMMH this afternoon and evening.

Rain and high elevation snow will continue to intensify this evening
through Thursday. As this rain band shifts southward overnight,
surface winds will ease although they will remain fairly steady.
Also ridge level winds are going to remain strong and turbulence
will persist. Moderate to occasionally heavy rain in the Tahoe Basin
will push CIGS/VSBY down to MVFR levels tonight into Thursday
morning. This band of rain will shift southward into the eastern
Sierra of Mono County for Thursday with lowering CIGS/VSBY at KMMH.
As far as snowfall, accumulation is going to be limited to later
Thursday and Thursday night as snow levels will be a bit slower to
fall tonight and early Thursday than previously thought. A couple
inches are possible at KTRK/KTVL Thursday night with lesser
confidence in an inch or two at KMMH. -Dawn/Hohmann

&&

.REV Watches/Warnings/Advisories...
NV...Flood Watch through Thursday evening NVZ002-003.

     High Wind Warning until 4 PM PST Thursday NVZ003-005.

     Winter Storm Warning until 4 AM PST Friday above 7000 feet in
     NVZ002.

     Wind Advisory until 4 PM PST Thursday NVZ001-004.

CA...Flood Watch through Thursday evening CAZ071>073.

     Winter Storm Warning until 4 AM PST Friday above 6000 feet in
     CAZ071.

     High Wind Warning until 4 PM PST Thursday CAZ070.

     Winter Storm Warning until 4 AM PST Friday above 8000 feet in
     CAZ073.

     Winter Storm Warning until 4 AM PST Friday above 7000 feet in
     CAZ072.

&&

$$

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



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.