Area Forecast Discussion
Issued by NWS Reno, NV

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417
FXUS65 KREV 301029
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
329 AM PDT Fri Sep 30 2016

.SYNOPSIS...

A cold trough moves into the West Coast this weekend bringing
gusty winds and temperatures well below seasonal averages. More
widespread precipitation and even some Sierra snow showers are
expected Sunday and Monday.

&&

.SHORT TERM...

The overall picture for this weekend hasn`t really changed from
the previous forecast. Blowing dust was added mainly for locations
near Gerlach and for western Pershing County. Other locations
could see some more intermittent blowing dust if downwind from a
dust source.

Strong low pressure moves into the Pacific Northwest today and
Saturday before dropping south over the Western Nevada and
Northern California Sunday into Monday. The pressure gradient will
ramp up today ahead of the initial cold front that is expected to
pass through the region this evening into Saturday. The latest
model trends bring winds back up higher than the 12z runs from the
past two days. This may be an artifact of the 00z model runs, so
there is still a little uncertainty in how strong afternoon winds
will be. As a result, only bumped winds up marginally across the
region for this afternoon with gusts generally around 40 mph with
some wind prone locations reaching 45 to 50 mph. This compromise
is still below the average of multiple models which favor gusts
around 45 mph especially towards the Oregon border.

Temperatures fall behind the front Saturday and Sunday by about
10 degrees each day. Winds will also lessen Saturday with gusts
generally around 25 mph. However, another front will push through
the region Sunday with winds gusting 30-40 mph during the
afternoon hours. Sufficient moisture for precipitation will
accompany the second wave as well as the cold-core of the low as
it moves over the Sierra Sunday night into Monday. Winds aloft
will be somewhat perpendicular to the Sierra favoring a slightly
stronger rain shadow than previously implied. Chances of
precipitation were trimmed back along the Virginia Range a bit as
a result. Meanwhile, QPF totals along the Sierra Crest were
increased marginally. All said and done, some locations in the
Sierra could see 2-3 inches of snow Sunday evening into Monday
morning.

Implications...There will be fire weather concerns today despite
humidity being rather marginal. For more details, please see the
Fire Weather Discussion below. Expect hazardous boating
conditions on area lakes for much of the weekend. We are not
expecting much subsiding in winds overnight with this amount of
cold air advection. While there will be a slight decrease in winds
Saturday, expect rougher conditions to return Sunday. Also, early
season snow could catch travelers off guard over Sierra passes.
Prepare for winter driving conditions and carry chains for the
chance of slick roads over Sierra passes and as low as 5500-6000
feet; includes the Tahoe Basin. Boyd

.LONG TERM...Monday through Thursday...

Minimal changes were made to most of the extended forecast. Pops
were lowered a bit for Monday as the upper low begins to lift out of
the region. Temperature guidance that day may be a bit on the warm
side so only a few minor tweaks were made with regard to highs. The
ECMWF is a bit faster lifting the low out of the region. This would
support slightly warmer temperatures...but GFS has been a bit more
consistent and cooler.

The question then becomes whether pops should be increased or
decreased for Tuesday as weak warm advection develops in west
northwest flow. The GFS shows some potential over the far northern
CWA while the ECMWF remains dry. Little...if any...mention of pops
was in the inherited forecast. Given the lack of consistency between
the major models...a dry forecast was favored for Tuesday through
Thursday with rising temperatures. The EMCWF starts to dig another
trough into the region at the very end of the forecast...but this is
not supported by by the GFS or most of its ensemble members...so
this was discounted for now. 20

&&

.AVIATION...

Gusty winds develop today with surface winds gusting to near 30-35
kts at times north of Highway 50. Stronger gusts in excess of 45 kts
are possible over the ridges. Turbulence and LLWS are a distinct
possibility through this evening. Luckily...VFR conditions should
persist through the bulk of the weekend.

Winds decrease a little for Saturday with less turbulence aloft. An
approaching upper level area of low pressure will usher showers into
the region by late Sunday. Falling snow levels and increased
moisture will result in the likelihood of obscured mountains by late
Sunday afternoon into the evening hours. Fortunately...the winds
should not be very strong ahead of this system on Sunday morning. 20

&&

.FIRE WEATHER...

Gusty winds develop today ahead of an approaching cold front. Along
with the gusty winds there will be a period of lower humidity values
into the western Nevada valleys. This lowered humidity may not last
more than 3-5 hours...but with the winds possibly reaching gusts of
40-45 mph...humidity in the range of 15-20 percent is low enough to
cause fire weather concerns.

For the Tahoe basin we do not feel the humidity will dip below the
20-25 percent range and could be even higher on the mid slopes.
Therefore we have dropped the Fire Weather Watch for that area. The
Red Flag Warnings for the Sierra Front and the Basin and Range will
be continued.

The most problematic areas are far northwest Nevada and northeast
California. Model guidance has been a little slow to lower the
afternoon humidity for the past day or so. There is a dry slot seen
moving into the area...but it really does not interact with that
region until this evening. If the models are too slow locally
critical conditions are possible by late afternoon. We will upgrade
zone 458 to a Red Flag for today and add zone 270 as well. Winds are
expected to gust to near 45 mph making for critical conditions
enough with humidity around 20 percent. Zone 278 will be left out of
any warnings although there could be very localized critical
conditions near the Nevada border north of Doyle.

Winds drop off a bit for Saturday then increase slightly Sunday...
but the humidity will also be rising by Sunday. 20

&&

.REV Watches/Warnings/Advisories...
NV...Red Flag Warning from 11 AM this morning to 8 PM PDT this
     evening NVZ458.

     Lake Wind Advisory from 11 AM this morning to 8 PM PDT this
     evening for Pyramid Lake in NVZ004.

     Lake Wind Advisory from 11 AM this morning to 11 AM PDT Saturday
     for Lake Tahoe in NVZ002.

     Red Flag Warning from 11 AM this morning to 8 PM PDT this
     evening NVZ450-453.

CA...Red Flag Warning from 11 AM this morning to 8 PM PDT this
     evening CAZ270.

     Lake Wind Advisory from 11 AM this morning to 11 AM PDT Saturday
     for Lake Tahoe in CAZ072.

&&

$$

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



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