Winter Weather Advisory 21, NWS Reno, NV

Area Forecast Discussion
National Weather Service Reno NV
237 AM PDT Wed May 15 2019

.SYNOPSIS...

A significant change to a much cooler and wetter pattern will
begin today. A series of storm systems will bring periods of rain
and mountain snow with the best chances today into Thursday
night, and Saturday night into Sunday. Breezy conditions are
expected for the upcoming week with stronger winds possible
Thursday and during the weekend. Cooler than normal temperatures
will last well into next week.

&&

.SHORT TERM...

Our weather begins the change to a much more active pattern that
will continue into this weekend and through much of next week.

Although not unusual to see snow in Sierra in May, this storm is
unusual in the matter of how much moisture and cold air aloft it
possesses by May standards. It is not a strong storm when compared
to a typical winter Sierra storm, but it has a lot of moisture
with it and will still likely produce travel impacts on the passes
mainly Thursday night into Friday morning.

Rain/Snow:

* The initial band of rain will spread from the Tahoe Basin
  northward into Oregon later this morning. Snow levels will
  remain high (>8,000') and will remain there much of the day.

* The heaviest band of moisture will move through the Sierra
  Wednesday evening into Thursday morning after which it will
  quickly taper to showers. This is when snow levels will drop
  below 7,000' and mountains will see a transition from rain to
  snow by early Thursday morning, but roads may still be warm and
  may initially have a difficult time accumulating snow on roads.

* Lighter showers continue through the day Thursday with a colder
  wave moving through Thursday night into Friday morning. Although
  less moisture will be associated with this wave, it will be the
  best chance of seeing travel impacts on the passes with a few
  inches of snow accumulation possible and also for seeing snow
  showers down at lake level.

* Sierra snow amounts through Thursday morning range generally
  from 2-7" above 8000 feet, and up to 3" between 7000-8000 feet
  across the Tahoe Basin while the Sierra in Mono County could see
  8-15" above 8,500'. The colder port of the storm Thursday
  night/Friday AM could add an additional 2-5" at higher elevations.

* Western Nevada valleys will see all rain with amounts generally
  around 0.25-0.33" through the Highway 395 corridor. Heavier
  totals are more likely east of roughly Lovelock-Fallon where 1/3
  to 3/4" of rainfall is possible. Spot flooding in poor drainage
  areas is possible, but no river flooding is expected.

Winds/Cold:

* Breezy winds continue each day through Friday, with gusts of
  25-35 mph. Thursday is most likely to have a longer period of
  stronger winds with occasional gusts to 45 mph. Sierra ridge
  gusts are expected to reach near 75 mph at times. Choppy and
  hazardous conditions are expected across area lakes and boating
  conditions won't be favorable on any lake for the next few days.

* Both Thursday and Friday will be quite cold with highs about 15
  to 25 degrees below normal. Highs in the mid-50s expected for
  western Nevada with low to mid 40s for Sierra valleys. The
  breeze will make it feel even colder.

Travel:

* Anyone with travel plans over the Sierra should prepare for
  occasional winter driving conditions including chain controls
  and plan for extra travel time. Open Sierra pass roads could
  have snow accumulation issues especially Thursday night into
  Friday morning. Check with CalTrans for the latest road
  conditions and chain controls if you are planning on travel.

* Persistent windy conditions, rough air should be expected
  through the weekend with increased turbulence and impacts to
  high profile vehicles on wind prone roads. Fuentes

.LONG TERM...Saturday onward...

The anomalously cool and wet pattern has a high likelihood of
continuing through the weekend into next week based on ensemble
guidance. The day 6-10 forecast extreme percentile based on the GEFS
reforecast analogs continues to indicate a large area through much
of CA/NV, stretching into the Intermountain West with percentiles
greater than 0.9-0.95. Translation: it's going to continue to be
unusually wet for May.

A brief break still looks likely for Saturday morning before another
system with a deep Pacific moisture tap moves into the west coast
midday Saturday into Sunday. Have continued to increase pops/qpf
(liquid totals). Snow levels start lower with this system than with
the initial system for Wednesday/Thursday as cold air is already in
place. Generally anticipate snow levels in the 6000-6500 foot range,
though daytime roadway accumulation will be hard to come by outside
of the highest Sierra passes (~8500' and higher). Even there, snow
is unlikely to stick for long during the day given how warm the
roads get this time of the year. Overnight is another story, with
accumulation possible for all Sierra passes, and possibly even light
snow down to 6000 feet or so. Precipitation becomes more showery by
midday Sunday with another brief break possible Monday.

Where it continues to look interesting is Tuesday as a 120+ kt jet
could slam into the Sierra and usher in a slug of atmospheric
river (AR) moisture. There are a few ensemble solutions that take
the jet farther south and AR detection probs are showing the best
inland trajectory being between about 31-37N, so it it possible
the moisture could be focused south of the region. Either way,
some simulations show a large area of low pressure carving out
over the west with potential deformation bands bringing additional
chances for rain/snow through the end of the week. We'll see, but
this time frame bears watching for sure.

While breezy conditions are likely through the weekend into early
next week, the two peak periods to watch for now would be Sunday and
again Tuesday. Winds should begin to taper by the middle into the
end of next week, though the potential is there for strong NE winds
across the Sierra.

High temperatures will remain well below normal through at least the
middle if not end of next week -- no signs of any heat waves in
sight. Widespread freezing temperatures in valley locations is
unlikely due to abundant clouds and wind, but a few outlying valleys
could be flirting with freezing temps at times (Monday looking
most likely at this time) which could affect sensitive vegetation.
-Dawn

&&

.AVIATION...

Light rain moves into the region today, but will be spotty in
nature. The best chances will be through the northern Sierra into NE
CA and extreme NW Nevada, including KTRK and KTVL. Showers are
possible near/at KRNO, KCXP, and KMMH today, but it will be more hit
or miss in nature. Expect mid-level overcast skies throughout the
region with potential mountain obscuration. MVFR cigs/vis are
possible in any rain showers.

Widespread rain and higher elevation snow are expected tonight into
Thursday morning, bringing further reductions to cigs/vis along with
widespread mountain obscuration. The wet pattern will continue into
the weekend with the most likely dry window from late Friday through
midday Saturday. Sierra terminals could see very light snow
accumulation overnight with these systems, but any snow will quickly
melt during the day.

Winds will be gusty each day now through potentially early next week
with gusts likely exceeding 20-25 kts at area terminals, highest in
the afternoon. Periods of wind shear are possible, especially during
the overnight and morning hours as winds are stronger aloft and
lighter at the surface due to decoupling. Turbulence and mountain
waves are likely as well, especially along and downwind of the
Sierra. -Dawn

&&

.REV Watches/Warnings/Advisories...
NV...Lake Wind Advisory from 1 PM this afternoon to 11 PM PDT
     Thursday for Lake Tahoe in NVZ002.

CA...Lake Wind Advisory from 1 PM this afternoon to 11 PM PDT
     Thursday for Lake Tahoe in CAZ072.

&&

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