Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 200944

Area Forecast Discussion
National Weather Service Reno NV
244 AM PDT Sun May 20 2018


Another low pressure area will settle over California and western
Nevada, bringing chances for showers and thunderstorms through
mid week. Some of these storms will produce locally heavy
rainfall, especially Monday and Tuesday.



An upper trough will continue to drop southward into California
today and then close off over the southern Sierra/southern NV
Monday. The low then moves little Tuesday, maybe lifting slightly
north as it begins to fill/weaken. The main change we made this
forecast package was to cool temperatures for Monday-Tuesday,
especially south of Highway 50. We also lowered snow levels a bit
for these areas although they will still be quite high (above 8000

Winds aloft increase some today which will allow for convection
to focus more across western NV as well as the Lassen County
convergence zone across northern Lassen County into northwest NV.
The Tahoe Basin will likely escape the threat today as storms
develop along the Carson Range early and quickly push off into
western NV by late afternoon. Shear and low level convergence
will only be slightly improved today. So storms will be similar to
the ones on Saturday with a few stronger storms producing small
hail and heavy rain.

The flow will begin to turn back to the southeast as the upper low
closes over the southern Sierra tonight. Moisture will push back
to the west with showers increasing across the area overnight and
Monday morning, especially south of I-80 and along the eastern
Sierra of Mono County. It is looking more rainy and cool for these
areas and we have most notably lowered temperatures Monday. It
could even be cooler where persistent rain bands form, but more
likely along the Highway 395 corridor through Mono County where
the flow will have an upslope component. By Tuesday, things become
a little more uncertain as the upper low weakens but remains over
eastern NV. So the atmosphere will remain unstable and supportive
of scattered showers and thunderstorms. Some locations will likely
see some impressive rainfall totals (possibly up to 1") while the
highest elevations of the Sierra in Mono County could be looking
at several inches of snow as the snow levels fall to near 8500
feet at times Mon-Mon ngt as the core of the upper low passes
nearby. Hohmann

.LONG TERM...Wednesday through Saturday...

Little has changed with the extended forecast reasoning this
morning. The only significant change was to increase POPs somewhat
for Wednesday as this looks to be the day with the best chance for
widespread showers and a few thunderstorms.

A filling low lifts northeast Wednesday with residual moisture being
acted upon by daytime heating to produce showers and thunderstorms.
The showers are likely to be more widespread over the northern half
of the forecast area.

There is residual moisture Thursday as well...but the deterministic
model solutions are starting to diverge and there is little in the
way of forcing that would support higher POPs for that day. The best
chance for showers and thunderstorms will be later in the day as
heating produces instability.

The model solutions really diverge by Friday and that continues into
Saturday. This makes the weekend forecast a very low confidence
affair. The GFS is taking the next low up the coast toward Oregon
while the ECMWF brings the low slowly to the east...parking it over
central California late Saturday night and early Sunday. They both
have support from the GEFS ensemble members.

The ECMWF solution would produce more in the way of showers and
thunderstorms and cooler temperatures. The GFS solution would favor
warmer highs and less chance for convection...mainly in favored
areas along the mountains driven by differential heating.

Given the lack of agreement in the model solutions...we have opted
to maintain slight chance to low end chance POPs over much of the
forecast area Friday and Saturday.



Fog will reduce vsbys to 1/2 mile or less for KTRK this
morning... but burn off shortly after sunrise. Showers and
thunderstorms are in the offing again today...but the focus of the
convection appears to be mainly northeast California and western
Nevada as weak low level southwest flow keeps the northern Sierra
from experiencing as much instability as the rest of the forecast
area. Any storms that form today could have gusty surface winds
and turbulence along with small hail and brief moderate to heavy
rain with lowered cigs/vsbys.

The presence of a developing closed low will cause more widespread
showers with a slight chance of thunderstorms Monday. There is a
better chance for bands of rain with lowered cigs and vsbys
Monday afternoon and evening that would produce MVFR conditions
over a broader area. This low persists into Tuesday with continued
chances for showers and thunderstorms.


.REV Watches/Warnings/Advisories...


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