Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 122317
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
317 PM PST Sun Feb 12 2017

.SYNOPSIS...

Flood waters continue downstream today with flood impacts
continuing in several locations. Dry conditions with seasonable
temperatures are forecast through at mid week. The pattern becomes
wet again Thursday, possibly lasting into the early or middle part
of the following week.

&&

.SHORT TERM...

Flood impacts continue across the region, but the current dry
period is allowing conditions to improve. Please see the hydrology
section below for additional details.

Otherwise, a northeast gradient remains across the region, with
further weakening expected overnight. This is leading to Sierra
ridge gusts of 30 to 40 mph with light chop on Lake Tahoe;
especially from midlake to west and southwest shores. As winds
weaken overnight and skies remain mostly clear, patchy freezing
fog is likely to redevelop tonight through many Sierra valleys.
With significantly less rainfall in western Nevada than in the
Sierra, chances for fog are low with the greatest risk through the
Carson Valley.

Winds will remain light through Tuesday with mild temperatures and
increasing clouds heading into midweek ahead of the next series of
systems. See the long term below for additional details. -Dawn


.LONG TERM...Wednesday through Sunday...

After the much needed and appreciated (at least by forecasters)
break in the wet weather through Wednesday, the storm door re-opens
on Thursday with a moderate but brief atmospheric river moving into
northern California. From Wednesday night through Thursday night, 1
to 2 inches of liquid or liquid equivalent is possible near the
Sierra crest from Tahoe north, with 0.5" to 0.75" for the rest of
the eastern Sierra. With the atmospheric river (AR) and an upper
disturbance running into ridging over the Intermountain West, the
inland (into western NV) penetration of the AR is in some doubt as
simulations and AR forecast tools show weakening as the AR drops
south and east through the region. With this in mind and with the
fast movement of the upper disturbance, spillover precipitation
amounts for western and west-central NV look rather unimpressive at
this time, especially in comparison to recent events.

Snow levels do not look to be overly high per current simulations as
colder air aloft works into the region with the upper wave later on
Thursday. Snow level are currently projected to be between 6500-7500
feet Thursday morning dropping to between 5500-6000 feet in
northeast CA and the northern Sierra by Thursday evening. IF there
are any hydrological impacts, they are likely to be in the lower
elevations of northeast CA (Pit/Feather River Basins). See the
hydrology discussion for more details.

The next system after Thursday is expected Friday afternoon into
Saturday. That system is looking more uncertain as far as precipitation
amounts as it is trending towards southern California with signs
of splitting as it comes inland. This trend is backed up by the
GFS/EC ensembles which show a large height deviation to the south
of the system (implying digging in a southern direction). Also, AR
probabilities from the GFS ensemble show the best shot south of
35 degrees latitude by Friday afternoon-evening.

It is possible that western NV north of highway 50 will receive
little or no precipitation, with the Sierra seeing a glancing shot
of precipitation favoring Alpine and Mono Counties. POP was kept
modest for the region, especially east of the Sierra, with the
uncertainty moderate to high. With forcing and deepest moisture
directed south of the region, QPF projections are light.

The stormy pattern continues next weekend as another wave approaches
Sunday. At this time, the flow looks zonal with a cooler mid-
latitude source for the system so snow levels should remain fairly
low with flooding concerns on the low end. -Snyder

&&

.AVIATION...

Drier easterly flow and some higher cloud cover tonight and Monday
morning should keep fog concerns localized, with patchy FZFG
possible in the Martis and Sierra Valleys and VFR conditions
elsewhere. -Snyder

&&


.HYDROLOGY...

We will be in a dry period through Wednesday night which will be a
crucial time period to allow for flood waters to largely subside.
It`s also a good time to improve or strengthen flood mitigation
efforts by removing obstructions from culverts and drains that
have been blocking water flow.

* Middle Fork of the Feather River: The river continues to recede
  and is currently near moderate flood stage near Portola. Flood
  waters will continue to fall, but flood impacts are likely to
  continue into the middle of this week.

* Carson River: The flood wave has already passed Dayton with
  minor flooding continuing along the river from Fort Churchill to
  Lahontan through tonight.

* Pit River: At Canby, the river has fallen below flood stage
  earlier today. Flooding will continue on the Pit River and
  through Big Valley in northwestern Lassen County into the middle
  of this week as flood waters will take a long time to drain out
  of Big Valley.

* Walker River: The East Fork of the Walker River south of Mason
  Valley is experiencing some minor flooding at this time as water
  moves out of the mountains. There has been minor flooding reported
  just below the Strosnider area with water on low points of
  roads along the east Walker to Highway 338.

* Truckee and Susan Rivers continue to have elevated flows due to
  continued runoff, but no flood impacts expected through at least
  mid week.

* Humboldt River: The Humboldt is rising with flooding along the
  mainstem in eastern Nevada. This wave will continue to progress
  downstream towards Imlay and Rye Patch this week. Currently,
  flooding is not projected, but flows are increasing.

* New flood impacts are lessening through the Sierra and mountains
  of northeast California, but several roadways still have water
  and/or debris on them.

Aside from ongoing flooding, the next period of concern will
begin Wednesday night into Thursday when another atmospheric
river and interacts with several systems moving into the west.
The region could possibly see 4 to 5 systems over a week long
period. This atmospheric river is on the weak to moderate side of
the spectrum, but this could change as we get closer to the
event.

The first wave appears to have snow levels in the 6.5-7.5 kft
range with 1 to 2, locally 3, inches of QPF likely. Despite there
not being significant moisture, very wet antecedent soil
conditions will lead to rivers, creeks, and streams responding
quickly. Luckily, the systems following into the weekend appear
to have lower snow levels, helping to mitigate additional flood
risks. Snow levels are less certain into next week. We would
recommend keeping all flood mitigation efforts in place. -Dawn

&&

.REV Watches/Warnings/Advisories...
NV...None.
CA...None.
&&

$$

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



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