Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 161133
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
333 AM PST Thu Feb 16 2017

.SYNOPSIS...

Windy conditions are expected today with a short period of
mountain snow and valley rain mainly before noon. Another storm
system Friday into Saturday will bring less wind but additional
rain and snow, with best snow potential for Mono County. A much
stronger storm is still on track to bring wind and more
significant precipitation early next week. Keep flood mitigation
in place or readily available through at least the next week.

&&

.SHORT TERM...

The first of several storm systems to affect the region in the
upcoming week is spreading precip into northeast CA, with wind
gusts increasing. The latest guidance indicates that the bulk of
the rain and snow will occur during the morning hours from Alpine
County northward, with about a 2-4 hour period of heavy rain and
snow, which will likely lead to some quick rises for rivers in
northeast CA. Current projections keep the main rivers within
their banks, except possible minor flooding for poor drainage
areas. Snow levels initially in the 7500-8500 foot range early
this morning may quickly drag down to around lake level during
this heavier burst although snow may not accumulate on paved
surfaces at this elevation. Current snowfall projections in the
advisory areas look on track for this quick hitting storm, with
the most significant travel impacts likely for the main passes
during the morning with rapidly worsening conditions as snowfall
rates could surpass 2"/hour. Gusty winds are also expected in the
advisory areas, especially on the Sierra ridges where peak gusts
have already surpassed 110 mph.

Farther south into Mono County, the snow should begin by late
morning, but the storm loses much of its punch by this afternoon.
This will limit snow amounts, with up to 6" possible near the
crest but less than 3" for the rest of the county. Gusty winds
will likely become the prevailing impact in Mono County, with the
current wind advisory remaining intact.

For western NV, the main impact will be gusty winds ahead of the
main precip band with slight downslope enhancement possible this
morning, then a decent round of gradient winds this afternoon
after the precip band weakens. We will also add northwest NV and
the Surprise Valley to the wind advisory--while the downslope
enhancement may be less likely, the afternoon winds appear a bit
stronger, as secondary shortwave brushing across Oregon will keep
700 mb winds near 50 kt over this area into the mid-afternoon. A
short period of rain will spill into the Reno-Carson area by late
morning, with precip amounts generally near 0.10 inch for the
urban areas, with up to 0.25 inch for western foothills.

For tonight, very little precip is expected to linger as this
storm continues to weaken as it departs to the east. The only
possible exception is for far southern Mono County, where the
leading edge of the next storm may produce light snow shortly
before daybreak.

For Friday and Friday night, moisture will increase as next low
moves toward the southern CA coast and splits off from the main
upper level flow. Moisture band associated with this low will
mainly affect southern CA, but broad area of upper level
divergence with southeast flow aloft will direct this moisture into
the Sierra, especially south of the Tahoe basin. Light precip is
also likely to spread across much of western NV, with a short
period of moderate precip possible south of US-50 during the day
Friday. Areas of light rain may continue in lower elevations,
with better chances shifting to north of I-80 by Friday night.
QPF values and rainfall rates are not very significant, which is
a bit of good news as hydrologic impacts should not be very
significant for this event.

Snow levels look to be somewhat lower for much of this storm, as
model soundings show a deep isothermal layer extending below 700
mb to near the surface. Light snow could even mix with rain in
foothill locations around Reno-Carson City, but little or no snow
accumulation is expected. For the Sierra, the best snow totals
look to be on track for Mono County with a foot or more possible
near the crest and at least 6" possible along much of US-395.
Farther north to the Tahoe basin, lesser snow amounts are likely,
but several inches could accumulate even down to lake level
(although daytime accumulation on pavement may be limited to the
main passes due to lighter snowfall rates).

By Saturday, as the low moves east and weakens, light rain and
snow showers remain possible for both the Sierra and in lower
elevations as some wrap around moisture bands linger over the
region. By this time, the isothermal layer becomes less pronounced,
leading to snow levels likely rising to at least 6000-6500 feet
across most areas. MJD

.LONG TERM...Saturday night through Wednesday...

There is a chance for valley rain showers and mountain snow showers
Saturday night and Sunday. Any snow accumulation is expected to
be light, probably less than an inch or two with some local
accumulations of a few inches possible near the Sierra crest.

**Sunday/Monday Precipitation and Hydrologic Outlook**

We did not change the ongoing the Sunday/Monday forecast this
morning. Confidence continues to increase that the Sunday/Monday
storm is going to be the biggest precipitation producer of the
week. However, there continues to be significant uncertainty on
how fast snow levels will fall. Latest model simulations still
vary on snow level depending on how shortwaves develop and track
as the offshore trough approaches the west coast.

Our current snow level forecast is in the 6000-7000 foot range
during the heaviest precipitation late Sunday night into Monday
and dropping below 6000 feet (in the Sierra) late Monday night
near the end of the main precipitation event. Our message has not
yet changed: the snow level forecast remains highly uncertain at
this point and could change by 1000 feet or more.

The current hydrologic situation is already primed for flooding and
moderate amounts of precipitation have shown quick responses on area
rivers, and especially smaller creeks and streams. Consider that
creeks, streams, and rivers are already running with high base flows
and there are 2 light to moderate storms anticipated before the
Sunday/Monday event. In addition, the very deep snowpack in the
Truckee/Tahoe and Mammoth Lakes areas have shown how quickly water
drainage areas can dam up causing significant urban flooding. Not to
mention all the rock and mudslides that have continued to impact
area roads, even the last few days when it wasn`t raining. All the
areas that have seen flooding during the past few weeks: including
the Susan, Pit, and West Fork of the Feather Rivers could very well
see flooding again if snow levels don`t fall quickly with this
storm.

**Monday Winds**

Winds have continued to come in stronger with this system as well.
700 mb winds are projected to reach 60-70 kts by Monday morning with
ridge winds again gusting in the 120-140 mph range Sunday evening
into Monday. Currently there is moderate confidence that these winds
could mix down into western Nevada on Monday which would create
another hazardous travel day for high profile vehicles and air
travel.

**The rest of next week**

Latest model simulations have trended cooler bringing a cold front
from the north into the Great Basin Tuesday night. This pattern
cuts off the Pacific Ocean moisture tap so snow amounts would be
lighter in the mountains but snow levels would be down to all
valley floors by Wednesday. -Zach/Jon

&&

.AVIATION...
A series of storms will impact the Sierra through the middle of
next week. The first storm is currently creating ridge wind gusts
to 100+ mph and ridge gusts will increase today to 120+ mph.
Surface winds are expected to peak at KRNO/KCXP/KTVL/KTRK between
12-18Z this morning with gusts to 30-45 knots and still staying
windy this afternoon with gusts 20-35 kts. At KMMH winds will pick
up a few hours later and should gust strongly most of the day
with peak gusts 40-55 knots. There will be low level wind shear
and rotors across the region through this morning before the wind
direction and speed better lines up between the surface and
midlevels.

Precipitation is expected to be mainly between 14Z-23Z today with
a few lingering showers this evening and then the next storm
arriving late tonight. Snow levels will start above Sierra
terminals today with a few inches of snow possible on runways
near the end of the event. KMMH may see a rain snow mix this
afternoon and Western NV terminals should see only -SHRA today.
JCM

&&

.REV Watches/Warnings/Advisories...
NV...Wind Advisory until 4 PM PST this afternoon NVZ003-005.

     Winter Weather Advisory until 4 PM PST this afternoon NVZ002.

     Lake Wind Advisory from 10 AM this morning to 6 PM PST this
     evening for Pyramid Lake in NVZ004.

CA...Winter Weather Advisory from 7 AM this morning to 4 PM PST this
     afternoon above 5500 feet in CAZ071.

     Wind Advisory until 4 PM PST this afternoon CAZ070.

     Wind Advisory from 7 AM this morning to 4 PM PST this afternoon
     CAZ073.

     Winter Weather Advisory until 4 PM PST this afternoon CAZ072.

&&

$$

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



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