Winter Storm Watch 5, NWS Reno, NV

Area Forecast Discussion
National Weather Service Reno NV
252 AM PDT Tue Apr 4 2017

.SYNOPSIS...

Dry and warmer conditions with widespread high clouds are
expected through midweek. Winds will begin to increase Wednesday
afternoon, followed by a sharp change to a cooler and wetter
pattern with periods of stronger winds Thursday through the
weekend. The best chances for heavy rain and mountain snow are
expected from late Thursday night through Saturday morning.

&&

.SHORT TERM...

The quiet portion of the upcoming week's weather will be today and
Wednesday as a low amplitude high pressure ridge moves eastward from
California to the Great Basin. Widespread cirrus coverage will be
present both days as abundant high level Pacific moisture spreads
across the region. This cloud cover may limit daytime heating by
a few degrees, but highs should still make their way into the
lower-mid 60s for lower elevations and lower-mid 50s for the
Sierra today. Further warming of about 5-10 degrees is likely
Wednesday as increasing southwest flow will bring some improved
mixing, even with the presence of this cirrus. Milder overnight
temperatures are also expected due to this widespread cloud
coverage.

By Thursday, southwest flow aloft will continue to increase as a
strong low pressure approaches the west coast, leading to a
further increase in wind speeds. Gusts could surpass 40 mph
by Thursday afternoon especially from US-50 northward, while
Sierra ridge gusts reach 75-85 mph. A weak leading shortwave ahead
of the main storm will bring increasing mid level clouds, with
light rain showers possible mainly north of I-80 and near the
Sierra crest during the day. This thicker cloud cover along with
cooling 700 mb temperatures will trend max temps downward, with
highs returning to the 60s in most lower elevations west of US-95,
near 70 degrees from US-95 eastward, and lower 50s near the
Sierra. MJD

.Long Term...Thursday Night and beyond...

In what has seemed to be "the winter of atmospheric rivers", we have
yet another storm headed our way this weekend. Integrated Water
Vapor Transport detection ensemble tools would categorize this as a
solid moderate atmospheric river event, but factoring in the time of
year, this is a strong storm for April. For example, the last time
Tahoe City saw a 2-day total of 2"+ liquid precipitation in April
was back in 2003! Confidence is high for heavy rain and snow late
Thursday night through Saturday, but lower for snow levels and
subsequently snow totals and flood potential.

Here are the highlights with regards to next weekend:

PRECIPITATION: The initial push of moisture will move into the west
late Thursday night going into early Friday morning with the
heaviest precipitation rates likely to be Friday for much of
northeast California, the Sierra, and western Nevada. There will be
a secondary maximum in rates late Friday night into Saturday
morning, but the focus for this piece looks a bit farther south,
from the central Sierra southward. Areas of steady rain and snow
will taper to showers Saturday afternoon into evening, with the
most recent forecast simulations showing little additional
precipitation for Sunday. Liquid precipitation totals indicate
chances for 2-5 inches along the Sierra crest with 1-3 inches in
the Tahoe Basin, eastern Sierra foothills, and northeast
California. 0.5-1 inch is possible along the Sierra Front with
lesser amounts into west central Nevada.

SNOW LEVELS: After several days of seeing snow levels trending
downward, a shortwave has put a literal kink in the forecast as it
digs south and the flow turns from westerly to southwesterly,
allowing warmer air to once again filter into the region Friday
afternoon and evening. Snow levels are likely to lower quickly
Friday morning in the heavy precipitation, around 6000 feet near the
Oregon border down to 7500 feet in the southern Sierra. Snow levels
may jump 500-1000 feet during the brief period of warmer air working
into the region, then quickly fall by Saturday morning as a cold
front moves through the area. Snow levels will be low enough
Saturday afternoon and evening for a mix of rain and snow down to
all valley floors in Western Nevada. The biggest potential
variation in snow totals will be in the 6000-7500 foot range where
there is less certainty in the heavier rounds of precipitation
falling as rain or snow.

HYDROLOGIC CONCERNS: Despite the brief increase in snow levels,
overall snow levels for the event will not be especially high,
especially given the time of year. While rises are likely on all
mainstem rivers, the current snow level forecast is unlikely to
produce flooding in most basins. The Middle Fork of the Feather
River basin is of greatest concern at this time due to the catchment
being at a lower elevation. Keep in mind, that all rivers are
already running high and small changes in how quickly snow levels
fall may result in big differences in the river forecasts. What is
likely the greatest concern at this time are lower elevation creeks
and streams, especially those coming out of the Carson Range and the
Virginia Mountains. The ground is already saturated from this record
winter and there will be significant runoff, increasing flooding
concerns. Also be prepared for rises on all terminal lakes,
including but not limited to: Washoe, Silver, Swan, and White Lakes.

WINDS: The jet stream moves across northern CA/NV on Friday with
strong winds looking likely as 700 mb winds increase to 50-70 kts
and gusts mix down to the surface. There is the potential for
downslope enhancement, however, with the plentiful moisture it is
likely that the wave activity will be dampened out along the Sierra
Front with stronger winds possible downstream into the West Central
Nevada Basin and Range. The region may experience periods of
erratic stronger gusts between waves of heavier precipitation,
instead of continuous strong gusty winds. Post-frontal breezy
conditions should continue into Saturday.

While this storm is nothing like the huge AR events we saw in
January and February, it has the potential to be a significant snow
event for the high Sierra, with several feet of snow likely. Major
travel impacts are likely across all passes beginning late Thursday
night lasting through Saturday night. If you have travel plans next
weekend, it may be best to either get out ahead of the storm, or
wait until it is over.

There is a chance for a final wave to push through the region
Monday, which could yield snow down to western Nevada valley floors,
but confidence is lower due to wavering forecast model solutions.
Beyond this, the flow becomes amplified across the Pacific, with
lesser chances for direct hits along the west coast from any deep
Pacific moisture plumes. However, there isn't a signal for a strong
ridge building over the west either, with continued chances for
additional systems moving through the region. -Dawn

&&

.AVIATION...

Ridging rebuilds over the west today into Wednesday with afternoon
breezes and broken to overcast high level clouds. Southwest winds
will begin to increase Thursday leading to areas of turbulence,
especially downwind of the Sierra.

A significant storm system is looking likely late Thursday night
into the weekend, with strong winds and periods of heavy
rain/snow. Please see the long term section above for additional
details. -Dawn

&&

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

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