Area Forecast Discussion
Issued by NWS Reno, NV

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000
FXUS65 KREV 231006
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
306 AM PDT Wed May 23 2018

.SYNOPSIS...

Low pressure systems will continue to affect the Sierra and
western Nevada this week and the first part of the Memorial Day
weekend. Locally heavy rain, small hail and gusty winds will
accompany stronger storms. Temperatures will generally be near
seasonal levels except for Friday and Saturday when extensive
showers and clouds will keep readings below normal.

&&

.SHORT TERM...

One upper low exits while another approaches the west coast today.
The flow aloft will turn back to the southwest by this afternoon.
As a result, it is going to be a bit more active along and just
east of the Sierra as abundant moisture and instability will be
in place for thunderstorms. While shear will be weak, westerly
winds will develop from convective outflow over the higher terrain
and act to increase low level convergence as storms push off into
the lower valleys. Best coverage will be along the Highway 395
corridor from Reno-Carson City northward with more isolated
coverage elsewhere. A few stronger storms will produce small hail
and heavy rainfall. Any storms that can work a little farther east
toward Highway 95 will have a better chance for strong outflow
gusts to 50 mph as lower levels are drier across the Basin and
Range.

As the upper trough approaches, a low will close off the central
CA coast and move inland Friday along a track that will take it to
the southern Sierra. This track is a bit farther south and brings
the potential for more widespread showers and thunderstorms to
the Sierra and western NV. Locally heavy rain and accumulating
hail will accompany stronger storms with a few very intense storms
possible on the leading edge of the cooler airmass that develops
along the east slopes of the Sierra and far western NV. Sufficient
low-mid level shear will exist for these storms to sustain strong
updrafts capable of producing larger hail (around 1 inch) and
stronger outflow winds (gusts 50-60 mph). After high temperatures
near seasonal levels today and Thursday, much cooler conditions
will prevail Friday, especially for the Sierra and extending
into the Highway 395 corridor. Hohmann

.LONG TERM...Friday Night through Tuesday...

The reasoning for much of the extended portion of the forecast has
changed little in the past 12-24 hours...but there were some changes
made to the forecast this morning to reflect subtle changes in the
model guidance.

First...both the ECMWF and GFS are bringing the upper low for the
weekend into California and Nevada a little farther south than
previous runs indicated. This has some support from the GEFS
ensemble members as well. This would mean a better chance for rain
with embedded thunderstorms north of the low covering more of the
forecast area than previously thought and slightly cooler
temperatures for Saturday. To trend toward a wetter scenario we have
increased POPs and QPF for Friday night into Saturday evening.
Temperatures had already been lowered for Saturday...so few changes
were made there. With the increase in QPF it is possible that areas
above 8000 feet could see up to 3 inches of snow by Saturday
evening. Accumulating snows are even possible on the passes above
7500 feet by late Saturday evening.

Second...the forecast models are a little slower at moving the upper
low away the region Sunday and Monday. The presence of the low to
the east may be just enough to induce showers and a few
thunderstorms Sunday afternoon...with the best chances in far
eastern Churchill and Pershing counties and also along the Sierra
from Alpine County south into Mono County where northeast upslope
flow will be maximized. While less likely...it is not outside the
realm of possibility for convection to redevelop Monday. Thus we
have included very low end POPs for Monday afternoon and evening
in the central Sierra where differential heating and upslope flow
will be maximized.

Lastly...Tuesday is becoming more uncertain. The GFS is now more
aggressive in trying to push another trough into the coast by late
Tuesday while the ECMWF builds a short wave ridge. There is support
for both of these solutions...and others...in the current crop of
GEFS ensemble members. While there is quite a bit of uncertainty...
and confidence in any solution is low...we will err on the side of
caution and introduce slight chance POPs for parts of the Sierra for
late Tuesday. There is little flow in the mid and upper levels and
that generally translates to short lived convection caused more by
low level heating acting on residual moisture with some boost from
differential heating in the terrain.

&&

.AVIATION...

Mid clouds rotating around upper low have been in and out of KTRK
this morning with fog trying to develop. Probably best to count on
it being there more than not through about 16Z. Otherwise VFR
conditions will prevail today outside of shower/storm activity.

Showers and thunderstorms will likely fire along the Sierra early
to mid afternoon and then push off into the lower valleys of
northeast CA and western NV by evening as the flow aloft turns
southwest and a low level westerly zephyr/outflow develops just to
the lee of the Sierra. Storm coverage will be best from the Tahoe
Basin and Sierra Front northward with more isolated coverage
elsewhere.

It will be mostly quiet late tonight into Thursday morning except
for fog around KTRK. Another low pressure area approaching the
west coast will once again increase shower and storm chances
Thursday afternoon through Saturday with activity not just
confined to the daytime period but also likely to persist
overnight. Hohmann

&&


.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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