Area Forecast Discussion
Issued by NWS Reno, NV

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000
FXUS65 KREV 200342
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
842 PM PDT Thu Oct 19 2017

.UPDATE...

Looking at observations around the region, some valleys are
holding onto the strong and gusty winds, while wind speeds in
other valleys have weakened as is typical in the evening. Expect
that wind speeds will increase over the next several hours as the
cold front nears and the strongest winds aloft move overhead.
Mountain top observations are reflecting this with speeds steadily
increasing as of 8:30 this evening. Forecast sounding and cross
sections would indicate a brief period of downslope wind
enhancement along the lee-side Sierra Front from approximately 10
pm through 2 am. As precipitation begins, expect that wave
activity will be dampened out and winds will weaken. The bulk of
the precipitation will come in a quick burst along the cold front
overnight with activity largely over by sunrise Friday morning.
It`s still a question how quickly snow levels will drop, but would
not be surprised to see light snow accumulations between
6000-7000 feet in the Sierra. Allow extra time for your Friday
morning commute in the event of slick roads.

Updated the forecast this evening to expire the remaining Red Flag
Warnings due to humidity values increasing across the region. Also
expired the Lake Wind Advisory for Pyramid Lake due to the lake
being closed to the public overnight. Wind speeds, however, will
likely increase overnight on Pyramid Lake. All other wind
headlines remain intact this evening. -Dawn

&&

.PREVIOUS DISCUSSION... /Issued 608 PM PDT Thu Oct 19 2017/

UPDATE...

Humidity has increased throughout the northern Sierra, northeast
California, and the western Nevada Sierra Front. Conditions remain
dry, however, through the central Nevada Basin and Range in
addition to the eastern Sierra and Mono County east of Highway
395. With this in mind, have allowed the Red Flag Warnings for the
Tahoe Basin and western Nevada Sierra Front to expire, while
continuing the warnings elsewhere through 8 pm this evening. Winds
will remain strong and gusty and likely increase this evening as
the cold front nears, but with humidity also rising and
precipitation expected overnight it will limit the fire danger.
-Dawn

PREVIOUS DISCUSSION... /Issued 303 PM PDT Thu Oct 19 2017/

SYNOPSIS...

A fast moving storm will bring strong winds through tonight, with
colder temperatures and a short period of valley rain and mountain
snow late tonight into Friday with possible travel impacts over
most passes for the early Friday morning commute. Light snow is
possible Saturday morning for the Surprise Valley, otherwise dry
and warmer conditions are expected this weekend and next week.

SHORT TERM...

The forecast including the update from earlier today is on track
for strong winds through tonight ahead of the approaching cold
front. There will also be a short period of higher precipitation
rates late tonight/early Friday morning with the frontal passage.
The highest precip amounts are expected northeast CA and near the
Sierra, but a quick burst of rain is also likely to reach the
Reno-Carson vicinity and western NV mainly from I-80 northward.

For the Wind Advisory areas, gusts up to 55 mph are expected with
gusts up to 70 mph at wind prone locations. (Wind prone locations
include Highway 395 between Susanville and the NV/CA border,
along I-580 in the Washoe Valley, and also for foothill locations
around Reno, Carson City and Minden). There is a small (about 25%)
probability for a brief period of high winds (60+ mph in valleys,
75+ mph in wind prone areas) for the Reno-Carson vicinity late
tonight between 11 pm and 2 am just prior to the onset of
precipitation. Drivers on Highway 395/I-580 should be prepared for
strong cross winds and travel restrictions. Lake wind advisories
also remain in effect for Lake Tahoe and Pyramid Lake for wave
heights up to 4 feet.

Latest computer simulations continue to show a few hours of
heavier precipitation tonight between 10 PM and 5 AM as the cold
front quickly moves through. For locations that transition to
snow, snowfall rates 1-2 inches/hour are possible for 1-3 hours
during the frontal passage. Snow levels are forecast to sharply
fall overnight to 5000-5500 feet in northeast CA, to 6000-6500 in
the Tahoe Basin. Depending on how quickly rain turns to snow,
there is potential for up to a couple of inches of snowfall west
of Susanville in northeast CA (near and above 5500 feet), and in
the Tahoe Basin, with 2 to 6 inches above 7500 feet. Less moisture
is transported into Mono County, but at the higher passes (Sonora
and Tioga) and for higher elevations on Mammoth Mountain, a few
inches of snow are probable early Friday morning, with precip
winding down before 8 AM.

The bottom line is to be prepared for some slick or slushy roads
across most Sierra passes tonight above 7000-7500 feet, and also
on the higher elevation routes west of Susanville such as Highways
36 and 44. The pre-dawn portion of the Friday morning commute
will see the greatest travel impacts.

For total liquid precipitation amounts, expect around 0.50 inch
with locally up to 0.75 inch along the Sierra crest west of Tahoe
and into northeast CA west of Susanville, and 0.25-0.50 inch for
the rest of the Tahoe basin. For western NV, expect up to 0.10
inch across far western NV including Reno-Carson City-Virginia
City, with higher amounts up to 0.25 inch possible north of I-80
and foothills west of the main urban areas.

Although the moisture feed quickly shuts down by daybreak Friday,
isolated snow showers and cold temperatures could keep patchy
slick conditions on Sierra roads through the morning commute.
Winds decrease but remain brisk in valleys Friday, and will
continue to be strong across ridges especially in Mono County.
Temperatures will be much cooler, with highs only in the 50s for
lower elevations and 40s near the Sierra, about 15-20 degrees
cooler than today.

On Saturday, a shortwave moving across the Pacific Northwest will
bring warm advection cloud cover to much of the region, with
light precipitation possible north of Susanville and Gerlach. The
Surprise Valley could see up to 1 inch of snowfall Saturday
morning as enough cold air is likely to remain in place during
this shortwave passage.

Otherwise for this weekend, drier conditions and lighter winds will
return along with a warming trend. Highs on Saturday will be held
down by the cloud cover, mainly in the 60s except 50s near the
Sierra and areas north of Susanville-Gerlach. By Sunday, most
valleys should reach the 70s with 60s near the Sierra and near the
Oregon border. MJD

LONG TERM...Monday and beyond...

A quiet, mild week is still on tap for next week. High pressure is
expected to settle over the Sierra by Monday with warm temperatures
and light winds. Areawide temperatures will likely be in the 70s,
with Sierra valleys not much cooler than the lower elevations due
to strong inversions setting up. Even ridge and mountain top
temps below 10,000 feet will get into the 60s next week. The ridge
may retreat to off the west coast later next week which could
allow for a weak back door cold front passage, but at this time
only a modest cooling is anticipated with precipitation unlikely.
With the dry air in place, overnight lows will be seasonable due
to strong radiational cooling. MJD

AVIATION...

Concern today will be the winds with gusts from the southwest to 35
kts at area terminals. Then tonight, winds will continue to be an
issue along with precip moving into the Sierra. Detailed concerns
for the terminals are grouped below:

KRNO/KCXP: The main wind impacts are likely through 09Z, with the
strongest gusts up to 45 kt expected between 05z-09z, just prior
to the precip onset with widespread turbulence and periods of
LLWS expected. Chances of seeing any MVFR CIGS from rain is about
30-40% and focused from 09-12Z. After 12Z Friday, VFR conditions
prevail with west to northwest winds gusting up to 30 kt through
the day.

KTVL/KTRK: The main wind impacts are likely through 08Z, with the
strongest gusts up to 45 kt expected between 04z-08z, just prior
to the precip onset with widespread turbulence and periods of
LLWS expected. Precip is most likely from 08Z-12Z, mainly as rain
the form of rain with MVFR CIGS, then mix with and change to snow
between 11Z-12Z before ending. IFR CIGS/VIS are possible after the
changeover, but no accumulations are expected on runways. After
12Z Friday, VFR conditions prevail with southwest winds gusting
up to 25 kt through the day.

KMMH: Winds to be gusty here, but also expecting them to be more
aligned with winds aloft. Therefore less LLWS is expected, but it is
still not out of the question. Peak gusts to 45 kts could occur
from 05-12Z, with gusts around 35 kt continuing through much of
Friday. Restrictions to CIGS/VIS are less likely as the terminal
should largely remain shadowed out with precip staying closer to
the Sierra.

For the weekend into next week, much lighter winds are expected with
VFR conditions for all terminals.

FIRE WEATHER...

Strong and gusty winds are on track for early evening and overnight
as a quick moving low pressure moves through the region. The main
period of concern for fire weather will through late afternoon
as drier air will overlap with increasing winds. Wind gusts could
reach 30 to 40 mph by late afternoon with relative humidity in
the 10-20% range. The lowest humidities are expected for Zones
453 and 273, but winds will be stronger in zones 450 and 272.

The period of strongest winds tonight will not coincide will the
driest air so we should see the critical fire weather threat diminish
as winds increase but moisture also increases. Expecting humidity
to be in the 40-50% range by sunset in the Tahoe Basin, 30-40% in
the Sierra Front, and 20-30% for the Basin and Range/Mono County.
Moisture will arrive a few hours later and we should see humidity
rise rapidly during the remainder of the evening.

Peak wind gusts are expected this evening until the cold front
moves through the region. Widespread valley gusts of 45-55 mph
will be possible during this time frame, with wind prone
locations and foothills reaching up to 70 mph and ridge tops near
100 mph. The cold front should reach the I-80 corridor between 2-4
am Friday morning. Fuentes

&&

.REV Watches/Warnings/Advisories...
NV...Wind Advisory until 5 AM PDT Friday NVZ005.

     Lake Wind Advisory until 5 AM PDT Friday for Lake Tahoe in
     NVZ002.

     Wind Advisory until 5 AM PDT Friday NVZ003.

CA...Wind Advisory until 5 AM PDT Friday CAZ070-071.

     Wind Advisory until 5 AM PDT Friday CAZ073.

     Lake Wind Advisory until 5 AM PDT Friday for Lake Tahoe in
     CAZ072.

&&

$$

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


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