Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 202142

Area Forecast Discussion
National Weather Service Reno NV
242 PM PDT Tue Apr 20 2021


A low pressure system will continue to move through the region
today and Wednesday bringing increased winds, cooler temperatures,
and chances for showers. A few thunderstorms are also possible
near the Sierra and northeast California along with some blowing
dust across western Nevada. Drier and warmer conditions return
Thursday and Friday before windy conditions usher in a potential
wet storm this weekend into the early part of next week.



Low-level, cumulus clouds have increased in coverage early this
afternoon mainly north of I-80 with some clouds beginning to
pop up over the Truckee Meadows region. North to easterly winds
have also picked up for these locations with some isolated shower
activity taking place in northern Lassen County. There remains a
slight chance for some thundershowers to form during this
afternoon into the early evening hours with the best chances along
the Sierra and the Lassen convergence zone. All of this indicates
our low pressure system starting to make its way through the
region from the north-northwest.

For locations south of US-50 today, expect the breezy southwest
winds continue with dry conditions in place until later this
evening. High-res guidance (NAM 3km & HREF) from this morning
shows a deformation band on the backside of the low developing
around 4-5 am Wednesday morning near southern Alpine County, making
its way through Mono County, and falling apart after sunrise.
Higher passes, including Carson, Ebbetts, and Tioga, could see a
coating to 1/2" of snow from this band with ~35% chance for 0.1"
for Mammoth Lakes. As the low begins to pull away towards the east
Wednesday morning, northeast flow will result leading to upslope
flow along the eastern Sierra. Chances for rain and higher
elevation snow showers will continue for this area throughout the
day on Wednesday with a slight chance for thunder in the
afternoon. By the end of the day, there is a 25% chance that the
Sierra Crest from southern Alpine through Mono could see roughly
over an inch of snow.

Northerly flow relaxes overnight Wednesday into early Thursday
before northwest winds become breezy by the afternoon hours as
high pressure builds back in from the west. Dry and warm
conditions with some afternoon breezes for Friday with the axis of
the ridge directly overhead at that time. Temperatures are
forecast to be their warmest on Friday with highs in the mid 70s
across western Nevada and northeast California and low to mid 60s
for the Sierra communities.

A pattern change is looking more likely for this weekend with the
potential for a late season, Pacific storm to impact the region
with strong winds, colder temperatures, and widespread
precipitation. For specifics on winds and fire concerns during
this time, please see the Fire Weather Discussion below.

As for overall timing and strength of this potential system, both
GEFS and EPS guidance currently show a longwave trough pushing
through the west beginning early Saturday and lasting until
Monday. The WPC Clusters Days 4-6 are onboard with this synoptic
pattern as well, so confidence in an active weather pattern for
this weekend is moderate to strong at this time. However,
discrepancies exist in terms of system strength and moisture
availability as well as the possibility for this system to split
in two pieces of energy (one towards Pacific Northwest and the
other over California and Nevada) which would result in a weaker
system overall. As of now, there is a 25% chance that this system
could split as it makes its way inland by Sunday.

Saturday will most likely feature the winds with an initial wave
of energy possibly generating showers for northern Lassen County
and locations near the Oregon border. The bulk of the moisture
looks to arrive by Sunday morning with the main push Sunday
evening into early Monday morning. Both the GFS and ECMWF show a
Pacific moisture tap during this time with the GEFS probability of
IVT showing some inland penetration for northern California and
the Sierra. As of now, the forecast stands at up to 2 inches of
QPF total along the Sierra Crest with up to 0.50 inch of liquid
for communities around the Tahoe Basin. Shadowing looks to be a
factor for western Nevada, but should see some measurable rainfall
(up to 0.10") when it is all said and done. There still remains a
25% chance for a wetter scenario to pan out according to the WPC
Cluster QPF tool.

Temperatures will be chilly from Sunday through Tuesday of next
week with highs 4-8 degrees below average for the end of April.
Ridging looks to build back over the region by mid-next week with
warmer temperatures returning. -LaGuardia



LLWS chances have increased and were added to primary TAF locations
as a frontal boundary moves through the region this afternoon
through this evening. Expect a period of LLWS mainly in the 22-03Z
time frame for Sierra Front and Tahoe/Truckee terminals. LLWS was
extended for KMMH where terrain will drive westerly flow while winds
aloft switch from SW to N through the evening and overnight hours
into Wednesday morning. Surface winds will remain breezy this
afternoon into the overnight hours with gusts generally less than
30kts with flow switching from southwesterly to northerly this
evening. Northerly flow will remain brisk for Wednesday, but gusts
will generally be 20-25kts.

High resolution models are now defining snow bands overnight into
Wednesday morning over and SE of the Tahoe Basin as the upper front
pushes through the region. Best potential remains in the vicinity of
TVL and south of MEV through northern Mono County. Any accumulations
would be very light given the minimal moisture associated with the
low, but a dusting would be possible for this area.

Otherwise, the forecast remains similar to the previous iterations
with mainly minor adjustments. Thunderstorms will remain a
possibility this afternoon mainly north of the I-80 corridor.
However, forcing parameters for thunderstorm formation could be
strong enough to continue about a 10% chance into the overnight
hours along the Sierra crest. For Wednesday, best chances remain
west of the Sierra crest given steering flow. Other flight weather
hazards include marginal reductions to visibility due to blowing
dust and mountain obscuration due to cloud cover. Boyd



Low pressure continues to push into the region with cloud cover
expanding and breezy winds for the region. These winds couple with
lower RH are resulting in localized fire weather concerns mainly for
lower elevations along the US-6, US-95, and US-395 Highway
corridors. Gusts in these locations will generally be around 30 mph
but could see some peak gusts up to 40 mph this afternoon.

RH today has been lower than forecast and more similar to yesterday
for some locations. The most notable locations affected have been
southern Lassen and the Plumas National Forest. This was largely due
to a slightly delay in cloud cover allowing for warmer temperatures;
coupled with efficient mixing, RH has been running 10-15% for lower
Lassen and the PNF through the early afternoon. With cloud cover on
the rise and showers beginning to develop, expect RH to rise through
the late afternoon hours.

While temperatures will be cooler for Wednesday and RH generally
higher, the Chalfant/Hammil Valleys and the Swall Meadows area could
still experience near critical fire weather conditions. Northerly
flow will likely channel here with gusts topping 30-35 mph.

Stronger winds are possible again this weekend ahead of a more
potent wave. Saturday`s wave has to potential to bring gusts 30-45
mph area wide and could bring another round of fire weather
concerns. Boyd


.REV Watches/Warnings/Advisories...




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