481
FXUS65 KREV 212230
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
230 PM PST Thu Nov 21 2024

.SYNOPSIS...

* Strong and gusty winds expected Friday evening across the Sierra
  Front, impacting recreation and aviation travel.

* Periods of rain and heavy mountain snow expected Friday morning
  into Saturday, creating travel impacts in the Sierra.

* A break in the unsettled weather late Saturday into Sunday
  before more gusty winds, rain and snow showers arrive early next
  week.

&&

.DISCUSSION...

The first wave of stormy weather is winding down as the band of
showers is receding northward per latest radar observations. The
seemingly incessant rain showers in Lassen and Plumas County will
continue throughout the day, resulting in some flooding concerns
into the weekend (see Hydrologic section below more details).
Elsewhere, we`ll have a brief intermission in the rain and snow
until Friday. Friday afternoon and evening will feature strong and
gusty southwest winds across the Sierra Front where gusts may reach
up to 45-55 mph with locally stronger gusts possible for wind prone
areas such as the US-395 and I-580 corridors. While much of the
afternoon will be windy, timing of strongest winds likely arrive
after 5-6 PM PST and aren`t expected to let up until after midnight.
Be prepared for bumpy flights into and out of regional airports,
difficult travel for high-profile vehicles, and choppy lake
waters much of Friday.

Our next window for wet weather will be early Friday through
Saturday as the southwesterly fetch of rich moisture traverses
southward. Showers arrive to the Tahoe Basin between 8-10 AM PST,
providing rain for communities near lake level and snow above 7000-
7500` throughout the day Friday. Showers slowly expand southward
across the Sierra towards Mammoth Lakes during the afternoon and
evening with snow levels remaining above 7500`, allowing snow along
Sonora and Tioga passes. Snow levels fall to 5500-6000` around the
Tahoe Basin early Saturday, allowing for snow to reach Sierra passes
and locales near lake level. Snowfall totals will be heaviest above
7000` where 18-24" are most likely, but there is 15-30% chance of
the highest Sierra peaks reaching 3 feet of snow. Lower elevation
Sierra locales such as Truckee, South Lake Tahoe, and Mammoth Lakes
will most likely see 1-4" with a 10-30% chance of 6+" through
Saturday.

A few light rain showers may sneak their way into the Greater Reno-
Carson City-Minden in the late morning and afternoon, but much of
the spillover showers will stay north of the I-80 corridor. That is
until Friday night when latest CAMs advertise better potential for
steady, spillover rain into the Sierra Front. Blended guidance
indicates at a sharp gradient in rainfall totals near US-95 where
areas west of this corridor see 0.5"+ of rain while areas to the
east only get a tenth or two through Saturday.

Aside from a few lingering showers, we`ll likely have a brief break
in the rain and snow late Saturday through Sunday afternoon, which
may be the opportune time to travel in the Sierra before the next
storm arrives Sunday night. This third and final round of showers
will last through Tuesday and will be the colder than its
predecessors. It`s likely that the Sierra crest sees another foot
(50-70% chance) or two (20-40% chance) of snowfall through Tuesday
with a few inches likely down to lake level (40% chance of 4" or
more). With snow levels falling to valley floors late Tuesday,
there`s a 10-15% chance that the Truckee Meadows sees a light
dusting of snow by Wednesday AM.

Ensembles continue to favor dry conditions for Thanksgiving Day, but
temperatures are looking to be rather chilly. There`s still plenty
of time for the forecast to change being a week out, but as of now,
the holiday is looking dry and cool. Stay tuned for any
forecast changes!

-Salas

&&

.AVIATION...

Precip: Light rain at KTRK and KTVL is starting to move north and
away from the Tahoe Basin this morning. Precip will be minimal
through Friday mid-day, when the next wave comes in. Snow levels
will be around 700-7500` Friday afternoon/evening so KTRK/KTVL will
stay rain Friday. KRNO/KCXP/KMEV will remain shadowed by the
mountains and not receive any rain through Friday morning. Friday
evening is when we may start to see the rain make it over the
mountains and into the vicinity of KRNO/KCXP/KMEV. We`ll have a
brief break in the precip until Sunday evening.

Winds: FL100 winds are southwesterly at 50kt this afternoon, and
will be around 45-55 kt through Saturday morning. Expect mountain
wave turbulence and 40-55kt LLWS along the Sierra and eastern Sierra
through Friday. Sfc winds for Sierra terminals this afternoon will
hover around 30-40kt. Western NV terminals are going to maintain
their gusts of 25-30kt out of the south through Friday. Expect to
see Sierra ridgetop gusts near 70-85kt through at least Friday.

-Justin

&&

.AVALANCHE...

A series of storms from an atmospheric river will bring periods of
gusty winds, rain, and mountain snow through early next week. The
main weather highlights through Tuesday are:

* Snow levels will hover around 7000-7500` through Friday before
  falling to near 5500-6000` on Saturday morning.

* Upcoming round of snow totals (Friday-Saturday): 18-24" along the
  Sierra crest with a 15-30% chance of highest peaks reaching 3 feet
  of snow in Mono County. There`s a greater than 60% chance of 1" of
  SWE over a 24 hour period for elevations above 7000`.

* Final round of snow totals (Sunday through Tuesday): Colder storm
  with a 30-40% chance of 4+" of snow at lake level, 50-70% chance
  of an additional foot or more along the Sierra crest.

* Strong ridgetop gusts of 60-90 mph, sometimes in excess of 100
  mph, will continue into the weekend before subsiding.

-Salas

&&

.FIRE WEATHER...

Winds have trended upwards in latest guidance across the Sierra
Front into Mono County, Mineral County, and portions of Churchill
County Friday afternoon and evening. Southwest winds of 15-20 mph
with gusts of 30-35 mph will prevail during the afternoon,
increasing to 20-25 mph with gusts of 45-55 mph after 7 PM PST.
These winds continue through midnight before subsiding early
Saturday.

The extent of Friday`s fire threat is uncertain given: 1) the timing
of strongest winds (late in the evening) are temporally offset from
the period of lowest humidity (mid-afternoon) and 2) minimum RH will
only fall to 15-20% Friday afternoon. However, there may be a 1 to 3-
hour period of critical conditions in the late afternoon across the
portions of the Sierra Front and western Nevada Basin and Range
before humidity rises and light rain showers arrive in the
evening/overnight. Overnight humidity will be good (widespread RH >
50%) Friday night into Saturday AM, but the main concern will be
strong SW winds in the evening into Saturday AM that may compensate
for the rising humidity, especially if there are any ignitions in
this area before Friday evening.

Otherwise, periods of rain and heavy mountain snow are expected
through early next week, which will limit fire concerns through
the middle of next week

-Salas

&&

.HYDROLOGY...

An atmospheric river is bringing heavy rainfall across northern CA.
This will result in water ponding in poor drainage areas and rapidly
increasing flows along rivers and streams throughout the area today
through late Friday or early Saturday.

Flooding is of the greatest concern in areas of Lassen and Plumas
counties, where the combination of longer duration rainfall and
lower elevation watersheds may lead to minor flooding of the Susan
River as well as small creeks and streams in the area. Stream
response on the Susan River has been slow so far, but may still
respond quickly to additional rainfall Friday and Saturday morning.

Flooding concerns diminish somewhat to the south, and no other
mainstem rivers are forecast to approach flood levels, but minor
flooding of small creeks and streams can`t be ruled out from Lake
Tahoe north.

Recent burn areas, especially north of Lake Tahoe, are a concern for
minor flooding, exacerbated by the transport of ash, sediment, and
debris that may obstruct drainage areas.

There is considerable uncertainty in the hydrologic response due to
(1) range in the precipitation forecast and (2) snow levels with
varying precipitation type and transitions from snow to rain and
back to snow again from today into the weekend. Additional
consideration should be given to the fact that at storm onset soils
will be dry with generally low streamflows, which will delay and
mute streamflow responses initially. Keep an eye on river forecast
updates at cnrfc.noaa.gov. These forecasts will be updated by 9pm
Thursday evening and again by 3am and 9am Friday.

-Bardsley

&&

.REV Watches/Warnings/Advisories...
NV...Winter Storm Warning from 10 AM Friday to 10 PM PST Saturday
     NVZ002.

     Wind Advisory from 10 AM Friday to 1 AM PST Saturday NVZ001-003.

     Lake Wind Advisory from 10 AM Friday to 1 AM PST Saturday NVZ004.

CA...Winter Storm Warning from 10 AM Friday to 10 PM PST Saturday
     CAZ071-072.

     Winter Storm Warning from 4 PM Friday to 10 PM PST Saturday
     CAZ073.

     Flood Advisory until 12 PM PST Saturday CAZ071.
&&

$$