Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 02/14/19

Area Forecast Discussion

National Weather Service Reno NV

 731 PM PST Wed Feb 13 2019

.UPDATE...

We are currently in a lull between the moderate to heavy rain and
snow of earlier today and the next part of the system that will
bring more rain and snow to the region starting late tonight into
Thursday. With this we expect minor flooding to develop again in
some low water crossings in Douglas County and more nuisance
flooding around the Lake Tahoe area. There is still likely to be
ponding of water on the west shore of Lake Tahoe tonight where
snow banks block drains. Anyone up there should be alert to this
and try to avoid driving through standing water. You may not know
the depth of the water.

The winds across the region have decreased a considerable amount.
But as the precipitation has largely ended and the upper level jet
associated with this system begins to lift north...we should an
increase in winds by the early morning hours. We currently have
High Wind warnings for areas east of the Sierra...and those will
stay in place. The Wind Advisory for the basin and range country
this evening will be dropped. While winds there may increase
through the evening...gusts should remain below 40 mph.

&&

.PREVIOUS DISCUSSION... /Issued 321 PM PST Wed Feb 13 2019/

SYNOPSIS...

Significant storm continues to impact the region with heavy rain,
high elevation snow, and strong, potentially damaging, winds.
Localized ponding of water may cause minor flooding concerns,
especially in areas where plentiful low level snow is blocking
drainage. A colder storm will impact the region late Thursday
night through Saturday bringing another round of snow and travel
impacts to all elevations.

SHORT TERM...

A robust atmospheric river is bringing copious amounts of
moisture to the region along with warm air. Based on temperatures,
forecast soundings, webcams, and ESRL profiler data, snow levels
are hovering between 7000-8500 feet. They have at least stabilized
somewhat though compared to the wide variability we saw this
morning. Heavy Sierra cement is falling up high while periods of
heavy rain occur down low.

Travel will continue to be messy due to snow, slush, heavy rain,
and water ponding across the region. Periodic road closures have
occurred already earlier today, so if you have travel plans,
especially in the Sierra, it is best to check with CalTrans and/or
NDOT for the latest.

The rain combined with the plentiful low level snowpack is
leading to localized minor flooding around the Tahoe Basin and
into the Carson Valley. This is mainly due to snow piles blocking
drainage channels and/or funneling water in unintended
directions. In addition, there is still the possibility that minor
flooding could occur in smaller creeks and streams coming out of
the Carson Range. A Flood Advisory is in effect through this
evening when rainfall rates will lessen for the night. Keep in
mind another round of heavier rainfall is coming in on Thursday.
Please see the hydrology section below for additional details.

Winds are strong and turbulent aloft, but haven`t surfaced much
today due to the precipitation acting as a stabilizing feature not
allowing the stronger winds to mix down. Stronger winds are
anticipated for Thursday as a nearly 200 mph jet impinges on the
region. Winds may be lighter than forecast in heavier
precipitation bands, but strong, potentially damaging, gusts are
possible in precipitation lulls. These gusts are likely to be in
the 50 to 70 mph range with locally high speeds possible in wind
prone locations. Across Sierra ridgelines, wind gusts could
potentially reach 150 mph on Thursday. Wind speeds will remain
elevated Thursday night ahead of a cold front, then will be breezy
Friday post-frontal.

There are two frontal features coming through, the first late
Thursday afternoon and the second before daybreak Friday. Both
will bring colder air back into the region with snow levels
crashing back to all valley floors. Significant snow accumulation
is likely in the Sierra with accumulating snow also likely in
western Nevada with the Friday morning commute impacted. QPF and
snow totals were raised again with the character of the snow
transitioning from heavy and wet back to more dry and powdery by
Friday morning. Several feet of snow is likely in the Sierra with
1 to 2 feet possible down to Lake Tahoe Level. Farther south into
the eastern Sierra of Mono County, 1 to 3 feet is likely west of
Highway 395 with 4-12 inches east of there. In western Nevada,
several inches of snow is possible down to valley floors with 4-10
inches going into the VC Highlands. The combination of powdery
snow along with strong winds may bring whiteout conditions late
Thursday into Friday morning, especially in the Sierra. While
winter warnings will be needed for this next portion of the storm,
we will opt not to issue anything at this time given the current
suite of warnings out at this time. -Dawn

LONG TERM...Saturday through Wednesday...

This weekend through the middle of next week, northeast CA and
western NV will remain under a large-scale upper trough. This brings
high confidence that afternoon temperatures will remain below
average through the period, with 20s and 30s for most areas Sunday
through Tuesday. At the higher elevations (above 8000 feet),
widespread single digits and teens can be expected with deep cold
air entrenched over the region.

As far as precipitation, confidence is high that any showers will be
in the form of snow. There will be three waves moving into the upper
trough: one Saturday, one Sunday, with a third possible around next
Wednesday. As far as snowfall amounts, these are of low confidence
as it will depend on the exact track of the waves. Also, with snow
being of a convective nature, localized areas could have burst of
heavier snow showers that are unpredictable given their small-scale
nature. Keep in mind that while we have light snowfall accumulations
forecast for the valleys over the weekend, this is for non-paved
areas. If snow is not persistently moderate to heavy, or falling
between the evening and mid-morning hours, it will have a hard time
sticking to roads in areas where the air temperatures are in the
~mid 20s or higher (roads can heat 15-30 degrees above air
temperatures in the afternoon this time of year).

BOTTOM LINE FOR THIS WEEKEND: be prepared for slick, snow-covered
roads in the higher elevations (pass level) of the Sierra and
northeast CA with LENGTHY TRAVEL DELAYS QUITE POSSIBLE. For valleys,
periodic slick travel conditions are also possible, mainly between
the evening and mid-morning hours. -Snyder

AVIATION...

..STRONG-SEVERE TURBULENCE POSSIBLE OVER AND DOWNWIND OF THE SIERRA
THROUGH THURSDAY...

THROUGH THURSDAY MORNING: Snow levels are rising this afternoon with
most terminals going over to rain. The only exception is for KMMH
where a mix of snow and rain is still possible for a few more hours
before that terminal goes over to all rain. No meaningful snowfall
accumulation is expected for Sierra terminals through Thursday
morning, although a brief slushy accumulation cannot be ruled out
for KTRK/KMMH in heavier precipitation for a few more hours.

Prevailing conditions at Sierra terminals will remain generally IFR
through tonight, more due to VIS than CIGS. In western NV,
conditions to be generally VFR with occasional dips to MVFR possible
(mainly to due VIS). Higher terrain obscuration to remain extensive
in/near the Sierra, and occasional out in the Nevada basin and
range.

Terminal/low level winds are not expected to kick up as much as
previously thought this afternoon and tonight. HOWEVER, EXPECT
STRONG TURBULENCE OVER THE SIERRA with a 100-150 kt jet moving
overhead. There have already been at least a couple severe
turbulence PIREPs with possible injuries and damage to smaller
aircraft this afternoon over the Sierra.

WINDS THURSDAY: Stronger winds ensue on Thursday as a surface low
falls below 990 mb near the northern CA coast (in response to an
incoming upper low). Winds in western Nevada on Thursday could cause
extensive delays to air traffic and periods of downright dangerous
flying conditions with moderate-extreme turbulence, strong low level
winds shear, and unpredictable near surface rotors. A High Wind
Warning is in effect. Ridge top winds 100+ knots are likely with
the jet stream over the Sierra strengthening to 150-180 knots
Thursday as the polar and subtropical jets collide with an
incoming upper low. This is a particularly dangerous situation for
aviation in the lee of the Sierra the next few days.

PRECIPITATION/CIGS/VIS THURSDAY AND FRIDAY: Moderate to heavy
precipitation is expected to keep prevailing IFR conditions for the
Sierra, especially once rain goes back to snow later Thursday.
Significant snow accumulations at Sierra terminals are expected
starting later Thursday afternoon, with 1-2 feet possible through
Friday. Snow accumulations of up to a few inches will be possible at
KRNO/KCXP/KMEV Thursday night and Friday morning as a cold front
swings through the area. Areas farther east including KLOL/KNFL
could see a light accumulation (up to an inch) as well Friday
morning. -Snyder/Zach

HYDROLOGY...

Major Rivers in the area are expected to rise significantly but
remain below flood levels as a result of today and Thursday
rains. Deep dry, and cold snowpacks above ~ 6000-7000 feet will
have the capacity to absorb a significant amount of rain before
releasing any rainfall or contributing any melt. In lower
elevations and on sunny aspects, where the existing snow is
shallow, rain will initially pass through these areas after some
delay, and then start to melt and contribute to runoff. While
mainstem river flooding remains unlikely,the Middle Fork of the
Feather River could reach monitor stage.

So far during this storm it appears the snowpack is retaining most
of the rain as expected and gaged rivers and creeks have had
modest rises, roughly as expected. How well the snowpack will
continue to mitigate runoff when the second wave of heavy rain
occurs tomorrow is a point of some uncertainty, and could result
in higher than expected flows, but impacts are expected to remain
confined to areas outlined below.

The greatest concerns are lower elevation creeks and streams
along with terminal lakes in valley areas. In addition, areas
where snow is blocking drains in urban areas and/or mountain
communities could lead to ponding of water or funneling of water
into undesired locations.

The other concern is the deep snow and cold temperatures
preceding this event which have formed ice and deep snow in many
stream channels especially along the west shore of Lake Tahoe.
This ice and snow may impact creek stage and flow measurements. It
is expected that the ice and snow in the channel will be gradually
melted as the streams rise, but it is possible that the snow and ice
may restrict the channels with a rapid stream rise, and result in
flooding impacts at lower than typical levels and in unexpected
locations. Bardsley/Dawn

&&

.REV Watches/Warnings/Advisories...
NV...High Wind Warning from 4 AM Thursday to 4 AM PST Friday
     NVZ004-005.

     Winter Storm Warning until 10 PM PST Thursday above 7000 feet in
     NVZ002.

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

     High Wind Warning until 10 PM PST Thursday NVZ003.

CA...Winter Storm Warning until 10 PM PST Thursday above 5000 feet in
     CAZ071.

     High Wind Warning from 4 AM Thursday to 4 AM PST Friday CAZ070.

     Winter Storm Warning until 10 PM PST Thursday above 7000 feet in
     CAZ072-073.

&&

$$

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