Technical Forecast Discussion

455
FXUS65 KREV 202232
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
232 PM PST Wed Nov 20 2024

.SYNOPSIS...

* Gusty winds will maintain impacts to fire, aviation, and
  recreation through this evening with strong ridgetops winds
  lingering into the weekend.

* Periods of rain and mountain snow are expected into early next
  week with Sierra travel impacts likely over the weekend.

* Colder storm likely Sunday through Tuesday ahead of the busy
  holiday travel.

&&

.DISCUSSION...

A prolonged period of unsettled weather is well underway across
the Sierra and western Nevada as a robust, compact cyclone directs
a belt of amplified, moisture-rich flow into the West Coast. Main
impacts thus far have been from strong and gusty southwest winds
areawide, which are supporting critical fire weather conditions
from the Sierra Front into the western Nevada Basin and Range per
2100 UTC surface obs. Winds are also impacting aviation travel and
recreational activities today, making for bumpy flights into and
out of regional airports and choppy waters on area lakes. Gusty
winds will continue into the evening before weakening across lower
elevations, although strong winds of 60-90 mph will likely
continue along the Sierra crest into the weekend.

Light snow showers have reached parts of the Tahoe Basin such as
Truckee according to latest radar/webcams obs, but has yet to
arrive to southern Tahoe communities such as South Lake Tahoe and
Stateline. This is expected to change this afternoon and evening
as coverage in showers expand southward towards Alpine and far N
Mono County. Ptype will likely begin as snow or a rain-snow mix in
the Sierra through the afternoon before snow levels climb to over
7500` and limits snowfall to the higher Sierra peaks through
Thursday morning. The main band of showers will recede northward
late Thursday morning, ending showers around Tahoe by the
afternoon and maintaining rain and high elevation snow in NE
California. Persistent rainfall in Lassen and Plumas County will
result in some flooding concerns into the weekend (see Hydrology
section below for more details). Given the high snow levels,
heaviest snowfall will be limited to elevations above ~7000` from
Tahoe northward there is a 30-60% chance of exceeding 8" of snow
by Thursday afternoon. Much of western Nevada, including the
Greater Reno-Carson City-Minden area, will stay dry during this
first round of precipitation as showers are shadowed out.

Our second of three waves of showers will arrive on Friday and
last through much Saturday as the branch of moisture dives
southward. This round will feature more widespread precipitation,
now reaching western Nevada and Mono County, as well as greater
rain and snowfall totals compared to the first round. Snow levels
begin above 7500- 8000` at the onset of precipitation as the
region remains situated in the warm air regime, once again
confining snowfall to the High Sierra with rainfall elsewhere.
This is until early Saturday when we transition into the colder
side of the storm, lowering snow levels down to 6000-7000` in a
north-to-south fashion. While exact timing is still uncertain,
Saturday morning will likely mark the beginning of snowfall down
to Sierra passes and when travel impacts through the Sierra arise.
A foot or two of snow is possible along the Sierra crest during
this second round with a 20% chance of highest peaks receiving 3
feet or more of snowfall. Meanwhile, only an inch or less is
expected for Sierra communities near lake level.

The finale of this storm is set for Sunday into early next week
as the cyclone finally dislodges and passes through the western
states as an open wave feature. Expect periods of rain and snow
showers in the days leading up to Thanksgiving. Snow may fall down
to valley floors early next week as Reno has a 10-15% chance of
1" of snowfall by Tuesday morning.

-Salas

&&

.AVIATION...

Precip: Light snow at KTRK and KTVL has begun this afternoon, and
will remain intermittent throughout tonight. No snow
accumulations at Sierra terminals are expected through Thursday.
KRNO/KCXP/KMEV will remain shadowed by the mountains and not
receive any rain through Thursday. The band of precip will stay in
place through Thursday afternoon, then shift far enough north
Thursday night to where KTRK will just be OVC.

Winds: FL100 winds are southwesterly at 55kt this afternoon, and
will be around 50-60 kt through Saturday morning. Expect mountain
wave turbulence and 40-55kt LLWS across the Sierra through
Thursday. Sfc winds for Sierra terminals this afternoon will hover
around 30kt before decreasing slightly to 20kt after 0-3Z.
Western NV terminals are going to maintain their gusts of 20-25kt
out of the south through Thursday. Expect to see Sierra ridgetop
gusts near 70-85kt through at least Thursday.

By Friday into the weekend, we`re looking at a stronger, colder
and wetter storm. Liquid water storm totals are nearing 2-3" for
along the Sierra crest, with a 30-40% chance of 2+ inches of snow
for KTRK/KTVL/KMMH. In addition, look for mountain obscurations
during periods of MVFR/IFR conditions in rain/snow for Sierra and
far western NV terminals.

-Justin

&&

.AVALANCHE...

An atmospheric river will bring gusty winds and periods of snow
to the Sierra through early next week:

* Snow levels rise to 7000-8000` this evening before falling to
  near 6000-6500` on Saturday.

* First round of snow totals (through Thursday): 8-12" above 7000`
  north of Tahoe, little to no accumulation elsewhere.

* Second round of snow totals (Friday and Saturday): 12-24" along
  the Sierra crest with a 20% chance of highest peaks reaching 3
  feet of snow in Mono County.

* Third round of snow totals (Sunday through Tuesday): Colder storm
  with a 30-40% chance of 6+" of snow at lake level, 50-70% chance
  of a foot or more along the Sierra crest. Confidence is low on
  this, however.

* Strong ridgetop gusts of 60-90 mph will continue into the weekend
  before weakening.

-Salas

&&

.FIRE WEATHER...

A Red Flag Warning remains in effect until 7 PM tonight across
the Sierra Front and western Nevada Basin and Range where
southwest gusts of 30 to 50 mph and humidity in the teens exist.
Winds will subside below critical thresholds this evening in
tandem with rising humidity, mitigating the fire threat after
sundown.

While periods of gusty winds will continue through the weekend,
higher humidity and precipitation will preclude any fire concerns.


-Salas

&&

.HYDROLOGY...

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

Flooding is of the greatest concern in areas of Lassen and Plumas
counties, where the combination longer duration rainfall and
lower elevation watersheds are likely to lead to minor flooding of
the Susan River as well as area small creeks and streams.
Flooding concerns diminish somewhat to the south, and no other
mainstem rivers are forecast to flood, but minor flooding of small
creeks and streams is possible 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 Wednesday 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.

-Bardsley

&&

.REV Watches/Warnings/Advisories...
NV...     Red Flag Warning until 7 PM PST this evening NVZ420-421-423-429.

     Lake Wind Advisory until 7 PM PST this evening NVZ002.

     Wind Advisory until 7 PM PST this evening NVZ001-003.

CA...Winter Weather Advisory until 10 PM PST this evening CAZ071.

     Flood Advisory until 12 PM PST Saturday CAZ071.

     Red Flag Warning until 7 PM PST this evening CAZ274.

     Lake Wind Advisory until 7 PM PST this evening CAZ072.

     Wind Advisory until 7 PM PST this evening CAZ073.

&&

$$

NWS REV Office Area Forecast Discussion