447
FXUS65 KREV 150956
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
256 AM PDT Sun Sep 15 2024

.SYNOPSIS...

* Gusty southwesterly winds yield choppy lakes and localized
  critical fire weather conditions today.

* Well-below average temperatures expected Monday, in addition to
  widespread rain showers and high elevation snow.

* Another cold system expected Tuesday into Wednesday, bringing
  additional chances for rain and high elevation snow.

&&

.DISCUSSION...

Slightly cooler temperatures are expected today as cloud coverage
increases ahead of a cold frontal passage this afternoon. Increasing
southwesterly winds will also precede this frontal passage. Upper-
level winds are already increasing across the region, as current
mountain observations show ridge wind gusts nearing 50 mph at time
of writing. 700 mb winds are expected to peak late this afternoon
around 40 kts, translating to wind gusts peaking around 80 mph at
ridge level later today. Expect winds to increase late morning
through the afternoon and evening hours for valley locations, with
gusts up to 30-35 mph. These winds will yield choppy lake conditions
in addition to localized critical fire weather concerns, mainly
across the Sierra Front. See the fire weather section below for
additional details.

The associated upper-level trough deepens into northern California
later today, before cutting off into a closed low and ejecting into
northern Nevada on Monday. This system brings a much colder air mass
to the region, in addition to widespread rain showers and high
elevation snow from Monday morning into Tuesday morning. Snow levels
begin around 7500 ft at the onset of precipitation on Monday
morning, increasing to 8000-8500 ft through Monday afternoon.
Confidence is high for greatest accumulations to areas north of US-
50 and east of US-95. However, blended guidance suggests wetting
rains (>0.10") are likely across the entire region, with a 70-80%
chance.

There still remains a good amount of variability between models in
regards to precipitation amounts in the Sierra, mainly during the
early Monday morning timeframe. High-resolution models are split
between two scenarios per recent guidance. The first scenario
includes convective banding developing over the central Sierra
including the Tahoe Basin early Monday morning. This would bring
much higher QPF values to the region, and the convective nature
could lead to lowering snow levels to around 6000-6500 feet under
heavier cells. The second scenario keeps the convective band
contained over the Basin & Range, with almost no precipitation
accumulation over the Sierra on Monday morning. Given the potential
for a higher impact scenario that brings accumulating snow to Sierra
passes on Monday morning, make sure to keep an eye on the forecast
while details are nailed down.

The low quickly exits the region on Tuesday, but is trailed by
another another low pressure system Tuesday night into Wednesday.
This system will dig farther south and west than the previous,
indicative of more uniform widespread precipitation chances across
the region. We can also expect a continuation of cooler than
average temperatures through the end of the week.

A general troughing pattern remains over the western US through the
end of the work week, before being kicked eastward as a ridge
develops across the eastern Pacific. As such, we can expect a
gradual warming and drying trend through late week and into next
weekend. Whitlam

&&

.AVIATION...

Increasing clouds and winds expected today ahead of a cold frontal
passage this afternoon. SW`erly FL100 winds increase to sustained 30-
40 kts, yielding concerns for mountain wave turbulence and LLWS
after 15z today. All regional terminals are expected to see gusts
25-30 kts between roughly 18z-03z.

Rain and high elevation snow showers move into the region early
Monday morning and continue through Monday evening. Recent guidance
shows a 20-30% chance for snow showers to impact KTVL-KTRK-KMMH
between 12z-16z Monday. However, warm ground temperatures and the
mid-September sun angle should help rapid snowmelt on runways should
snow fall. Mountain top obscuration is expected under heavier
showers, especially in the Sierra. There is also a 10-20% chance
for isolated thunderstorms, primarily south of US-50 and near/east
of US-95, between 06z Monday-06z Tuesday. Whitlam

&&

.FIRE WEATHER...

Minimal changes from previous forecast:

* Winds ramp up out of the southwest today ahead of a cold front
  slated to shift through the area later today. While speeds do not
  look as intense or widespread as the event that unfolded this past
  Wednesday, it still bears watching with solid chances for 30-40
  mph gusts late morning through late afternoon, especially along
  the Sierra Front, including the US-395/I-580 corridor. Sierra
  ridge gusts will be in the 60-80 mph range ahead of the front.

* RH values are much higher than the last event as well, generally
  25-40% in valley locations. Localized critical conditions are
  expected, but no watches or warnings are planned at this time.

* Breezy NW winds with gusts of 20-25 mph to follow on Monday.

* Widespread wetting rains (0.10"+) are likely (70-80% chance)
  Monday and again Wednesday as a pair of cold weather systems
  drop into the region. These storms are cold enough to bring up
  a few inches of snow to high elevation terrain. -Dawn/Whitlam

&&

.REV Watches/Warnings/Advisories...
NV...Lake Wind Advisory from 11 AM this morning to 8 PM PDT this
     evening NVZ002.

CA...Lake Wind Advisory from 11 AM this morning to 8 PM PDT this
     evening CAZ072.

&&

$$