Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 210912

Area Forecast Discussion
National Weather Service Reno NV
212 AM PDT Mon May 21 2018


An active pattern with chances for showers and storms continues this
week as low pressure slowly moves across the region. Periods of
heavy rainfall and small hail are possible in these storms. Showers
and storms decrease in coverage Thursday and Friday before another
low pressure system approaches and provides additional chances for
storms by the weekend.



The upper low was closing off as it dropped south through central
CA this morning and will be centered over the southern Sierra by
00Z this evening. This will result in bands of showers and
thunderstorms across the Sierra and western NV with emphasis today
being south of I-80 and in particular the eastern Sierra. Brief
heavy rain has already occurred with the stronger cells this
morning around Fallon and Mammoth Lakes with some lightning
between Fallon and Fernley as of this writing.

Very little has changed with the track of the low, lifting it
slowly northward into eastern NV/western UT by late Tuesday and
ejecting it to the northeast Wednesday as the next upstream
trough approaches the west coast. Chances for showers and storms
will spread farther to the north Tuesday before we see an overall
decrease in coverage Wednesday. While there will be fewer storms
Wednesday, plenty of instability will be present along with weak
steering flow aloft to promote very slow moving storms capable of
heavy rain/local flash flooding.

Temperatures will be cooler today given the extensive cloud cover
and showers with more seasonal temperatures returning Wednesday.

.LONG TERM...Thursday through Sunday...

The 21/00 UTC numerical model guidance cycle saw fairly significant
changes to the evolution of the next low pressure area to affect the
region at the end of this week and into the weekend. Therefore...
fairly substantial changes were made to the inherited forecast.
Normally... we would be a little reluctant to make substantial
changes this far out...but the deterministic models and most
ensemble members are trending toward a more consolidated solution.
That is not to say there are not some differences...but these
differences are not as great as previous runs.

The GFS has started trending toward the previous ECMWF solution and
is now bringing the upper low closer to the California coast late
Thursday before driving it inland late Friday into Saturday. The
ECMWF has slowed a little this cycle and is a bit farther north
than the GFS...but overall they are close. The GEFS ensemble
members are in fairly good agreement as well.

Differences do arise Sunday as the GFS is slower to push the upper
low east and also maintains a mostly closed low. Meanwhile...the
ECMWF is lifting and opening the upper low as a trough more to the
northeast Sunday with better forcing moving into Idaho.

So what does all this mean for the forecast? Well...instead of
warming and drying for Thursday and Friday we have reintroduced
increasing POPs for both days with the better coverage Friday. We
have also lowered temperatures and they are now more in line with
seasonal averages. For Saturday...if this solution holds...we are
likely to see more widespread showers and thunderstorms with
temperatures below normal. Highs may struggle to reach the lower 70s
in the western Nevada valleys and the lower 60s in the Sierra
valleys; potentially not the most pleasant start to the Memorial Day
weekend if you want warm and dry conditions.

For Sunday the forecast is a bit more uncertain given the divergence
in the track of the upper low. We will maintain some low end POPs
over the far northern parts of the forecast area north of I-80 and
begin to let the temperatures drift back up as the low lifts out.
Timing could be everything Sunday. If the low is slower to lift out
the day will be cooler with a better chance of showers and



Residual showers and a few thunderstorms early this morning are just
the precursor to an active day as an upper level low pressure closes
off just south of the forecast area and persists into Tuesday. This
will bring more widespread showers and a few thunderstorms to the
region today through Tuesday night. Bands of heavier showers are
likely to develop today that will result in lowered cigs/vsbys.
There is the likelihood of low level gusty winds near the stronger
thunderstorms along with small hail and turbulence.

Outside of the heavier showers and thunderstorms we should see VFR
conditions...but near the showers and thunderstorms MVFR to
occasional IFR conditions are possible. Mountain obscurations are
also likely.

The same type of activity is likely for Tuesday before the upper low
fills and lifts away to the east-northeast. By Wednesday there
should be less convective coverage. Another low begins to make its
way onshore late in the week and could cause poor flying conditions
for the early part of the Memorial Day weekend.


.REV Watches/Warnings/Advisories...


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