NWS Reno, NV wrote its last area forecast discussion before the first event at Feb 7, 4:08 AM PST, 1 hour 52 minutes earlier.
Area forecast discussion, Feb 7, 4:08 AM PST, 1 hour 52 minutes before
Area Forecast Discussion
National Weather Service Reno NV
408 AM PST Tue Feb 7 2017
.SYNOPSIS...
Moderate to strong atmospheric river is currently making landfall
bringing heavy rain, strong winds, and high elevation snow.
Flooding is expected on small streams in the Sierra and Sierra
foothills as well as the Pit, Middle Fork of the Feather and Susan
Rivers. An additional storm Thursday into early Friday will bring
another round of heavy rain, mountain snow, and strong wind to
the region.
&&
.SHORT TERM...
Only minor adjustments where made to the QPF and snow level
forecasts through Wednesday night.
**CURRENT CONDITIONS AS 0F 3:30 AM**
Current radar returns this morning show moderate to heavy rain
from the coast of California to the Sierra filled in from the
northern California border south to Fresno. To say there is some
moisture coming our way today would be an understatement. GEFS
IVT magnitude values between 37-38 degrees longitude have risen to
700-850 for the next 12 hours. Those values have continued an
upward trend the last few days and now put this atmospheric river
in the moderate to strong category.
Snow levels have just started to rise over the last few hours and
currently look to be around: 6000-6500 in the northern Sierra,
6500-7000 feet around the Tahoe Basin, and 7500-8000 in Mono
County. The profiler at Colfax showed a dramatic jump in snow
levels between 11:30-12:30 from 6000 feet up to 8500 feet as the
warmer subtropical moisture moved into the west slope of the
Sierra. In the next few hours snow levels throughout the Sierra
are expected to follow this trajectory, rising to around 8500
feet today.
A Flood Watch also remains in effect in the Sierra and the Sierra
foothills for flooding along creeks, streams, and poor drainage
areas through tonight. For more detailed information on flooding
refer to the plethora of flood products our our website.
**HEAVY PRECIPITATION AND SNOW LEVEL FORECAST**
The heaviest precipitation is expected to occur over the next 12
hours with moderate precipitation along the Sierra crest
continuing for an additional 12-18 hours. The moisture plume from
the AR is currently wider than the Sierra is long. The strongest
moisture transport is expected to shift south slightly through the
day, being aimed from the Tahoe Basin into Mono County by late
afternoon. Then the AR plume is expected to weaken and narrow.
Late tonight the narrower plume will move north reaching northern
California tomorrow eveining before the next storm Wednesday
night into Thursday strengthens the plume and moves it back into
the Sierra - See the long term dissertation for the last storm in
the series.
In the last 24 hours the Sierra crest has received around 1-2
inches of precipitation, with localized areas up to 3 inches. The
forecast for the next 48 hours is for the Sierra crest to receive
an additional 4-6 inches of precipitation, with a major portion
of that coming in the next 12-18 hours. The extreme depth of the
moisture coupled with strong 70+ knot ridgetop winds are expected
to push significant precipitation into the Sierra foothills and
light to moderate precipitation into western Nevada valleys
today. The spillover precipitation is evident on radar and
expected to intensify through midday before stronger shadowing
again takes over this afternoon.
**STRONG WINDS**
Last night winds calmed down some, only gusting to around 85 mph
along the Sierra crest. This 'lull' in the winds has come to an
end as ridgetops are again gusting to 110 mph. 700 mb winds are
forecast to become much stronger today reaching 70+ kts this
afternoon. Winds today along the Sierra crest could reach extreme
speeds potentially gusting upwards of 150 mph today. The wind
forecast for western Nevada remains a bit tricky. Current thinking
is that when spillover precipitation begins to recede to the
foothills during the early afternoon this will allow the stronger
winds to begin to reach the surface. There will likely be lulls in
the winds at the surface today with occasional very strong gusts.
High Wind Warnings and Advisories will continue through today.
For more details of specific wind speeds see those products. -Zach
.Long Term...Thursday through Monday...
The late week storm is still on track with winds increasing again on
Thursday, with another round of high winds possible Thursday morning
through evening. Projected peak gusts are similar to the early week
wind events, ranging from 55-65 mph for most lower elevations, up to
75 mph for wind prone areas and 100-120 mph for Sierra ridges.
The first round of precip with this storm moves into northeast CA
and the Sierra crest Thursday morning, then increases in intensity
as it moves south during the day to the Tahoe basin. We are keeping
most of western NV shadowed out through the afternoon, although the
strong 700 mb flow across the Sierra will allow some rain to spread
across the foothills west of Reno and into the urban areas at times.
Snow levels Thursday will likely remain quite high, generally 8000-
8500 feet from Tahoe northward and around 9000 feet for Mono County.
By Thursday night, there are mixed signals for spillover potential
into western NV among the medium range guidance. Given that the main
atmospheric river plume appears to be directed straight toward the
Tahoe basin with added forcing associated with a cold front passage,
we did increase the potential for rain spreading into western NV
Thursday night. The front will initially not bring that much
cooling, with snow levels only edging down to 6500-7000 feet north
of Tahoe, 7000-7500 feet around Tahoe and near 8000 feet for Mono by
early Friday morning. This rain will likely produce another round of
rises on area creeks and rivers, aggravating flood risk in the poor
drainage areas already impacted from the early week storms.
One final low pressure then moves south across northern CA and
spreads rain and snow to most areas Friday-Friday night, then
finally pushing south of the region on Saturday. This system will
bring more substantial cooling with snow levels dropping to lake
level around Tahoe during the day, and could dip below 5000 feet in
far western NV by Friday evening. Mono County could be under the
atmospheric river influence for a nearly continuous period from
Thursday evening through early Saturday which would enhance their
precip and snow totals (current snow projections call for 1-2 feet
above 7000 feet with up to 3 feet near the crest). The lower snow
levels and less intense precip rates with this system will help
reduce the risk for additional flooding near the Sierra, but with
many rivers and creeks still running high from this ongoing series
of storms, it wouldn't take much additional rain in lower elevations
to aggravate flooding risk near these smaller creeks.
After the low pressure moves out Saturday, we are expecting a few
days of dry weather into early next week, with temperatures near
seasonal averages as ridge builds into CA-NV. East winds may
increase on ridges Saturday night-Sunday, otherwise winds should
generally be light. MJD
&&
.AVIATION...
Significant wind and/or weather-related impacts to aviation will be
present with few breaks in storm activity between now and Friday.
Prepare for flight delays, cancellations, diversions, or a rough
ride in/out of the main terminals if planning air travel this week.
For lower elevation terminals, winds will be the primary impact
today, with gusts increasing to 40-55 kt this morning and continuing
through this afternoon. Mountain wave turbulence and rotor activity
will produce rough flying conditions, with LLWS possible at times
during rotors or lulls in winds near the surface. Rain will also
accompany these winds, with MVFR conditions most likely from mid-
morning through early afternoon and mountain obscurations continuing
into tonight.
For the Sierra terminals, heavier rain will prevail today with
rainfall rates likely intense enough to keep IFR conditions going
through much of the day. Gusty winds around 30 kt will prevail near
the Sierra today through this evening, with short periods of
stronger gusts likely.
Conditions will improve a bit tonight and Wednesday with surface
winds easing up. Rain will return to the Sierra terminals on
Wednesday, bringing MVFR conditions at times while for western NV,
occasional light showers are possible with mountain obscurations
likely.
Active weather returns Thursday and Friday with another round of
very strong winds and heavy rainfall expected for Sierra terminals
and light to moderate rainfall for western Nevada terminals. Snow
levels could fall to western Nevada floors by Friday evening. MJD
&&
.HYDROLOGY...
Snow levels were finally on the rise and moderate rain was
beginning to spill over into the foothills. So while there has
been a slow start to heavier rain and rising snow levels,
temperatures were beginning to warm and rain was intensifying.
Conditions are coming together for an increased flooding
potential this week. The Pit River, the Susan River and the Middle
Fork of the Feather River have shown good responses to the rain
we have received so far despite the delayed snow level rises.
A new river flood warning has been issued for the Susan River
which will crest just below flood stage tonight - from today's
rainfall. The Susan River will then likely surpass flood stage by
Thursday evening as additional heavy rain affects its basin.
Flooding on the Middle Fork of the Feather River will get very
close to major flooding and thus we have upgraded the severity of
flooding. Water is rising across the Big Valley including the Pit
River and in the Sierra Valley where an areal flood warning will
remain in effect. Emergency officials should be preparing for
flooding problems through Friday and check the latest forecast
updates as well as warnings for the latest projections. Hohmann
Previous Discussion...Monday, February 6th 2017
A multi-wave atmospheric river system will bring copious amounts of
moisture to the region this week. The heaviest precipitation periods
will be this evening into tomorrow afternoon and again Thursday.
Precipitation will continue Wednesday and Friday as well just less
intense than Tuesday and Thursday. Here are current estimates for
liquid equivalents across the region:
Current Liquid Totals:
Sierra | Foothills | Sierra Front | Basin&Range
Mo PM - We AM: 3.0-7.0" | 0.75-2.00" | 0.50-1.00" | 0.10-0.50"
We - We Night: 0.3-1.0" | 0.10-0.25" | 0.05-0.10" | < 0.05"
Th - Fr AM: 1.5-3.5" | 0.50-1.25" | 0.25-0.75" | 0.10-0.25"
Friday: 0.5-1.0" | 0.15-0.25" | 0.05-0.15" | < 0.15"
Weekly Totals: 5.0-12.0"| 1.50-3.75" | 0.75-2.00" | 0.25-1.00"
These totals and higher-than-typical snow levels will create
hydrologic concerns. The main question will be: how much water can
the snowpack hold before excessive water is released via snow melt
and runoff into local river systems? The current consensus is that
the Truckee, Carson, and Walker River basins will be more resilient
since the snowpack is deeper and can store more water. Meanwhile,
the Middle Fork of the Feather River, the Susan River, and the Pit
River will respond much more readily due to these areas lying in
lower elevations and shallower snowpack. These rivers are already
showing decent rises from Sunday night's precipitation.
The first, heavy precipitation period through very early Wednesday
morning will bring rather sharp rises to the Pit River and Middle
Fork of the Feather River; subsequent precipitation will only
exacerbate flood conditions. The Pit River at Canby is expected to
reach flood stage Wednesday morning with continued impacts through
the 12th. However, expect a delay in the flood wave from Bieber to
Pittville which may occur from early Thursday morning through the
14th due to the slower response of the Pit River system. The Middle
Fork of the Feather River is expected to flood beginning early
Thursday morning, reaching Moderate flood stage by Thursday
afternoon, and continue flooding through the 14th.
The first wave of moisture will also ripen the snowpack for the
Truckee River system resulting in increased flows with many
locations reaching monitor stage. The Carson and Walker Rivers will
fair better and are projected to stay below monitor stage.
Subsequent rain and runoff will keep flows running higher through
the weekend, but flooding is not currently expected. We'll be
watching to see how the snowpack responds to the heavier
precipitation periods; the forecast may be adjusted based on the
response to these periods of runoff and snow melt.
Otherwise, the biggest impacts are likely to occur along smaller
streams, creeks, and areas where drainage is poor. This will be
especially true for the Sierra and foothill locations where snow
melt will be a more significant factor. Road wash-outs and ditch
failures will be possible. Minor home flooding in low-lying
neighborhoods that flooded in January will be susceptible to
flooding again this week from smaller creeks and drainages. Boyd
.REV Watches/Warnings/Advisories...
NV...Wind Advisory until 10 PM PST this evening NVZ002.
Wind Advisory until 10 PM PST this evening NVZ005.
Flood Watch through late tonight NVZ002.
Wind Advisory until 10 PM PST this evening NVZ001-004.
High Wind Warning until 10 PM PST this evening NVZ003.
Flood Watch through late tonight NVZ003.
CA...Flood Watch through late tonight CAZ071.
Wind Advisory until 10 PM PST this evening CAZ071-072.
Wind Advisory until 10 PM PST this evening CAZ070.
High Wind Warning until 10 PM PST this evening CAZ073.
Flood Watch through late tonight CAZ072-073.
&&
For more information from the National Weather Service visit...
http://weather.gov/reno