144
FXUS65 KREV 131902
AFDREV

Area Forecast Discussion
National Weather Service Reno NV
1202 PM PDT Mon May 13 2024

.SYNOPSIS...

Afternoon showers and isolated thunderstorms return today and
Tuesday with best chances in the Sierra and south of Highway 50.
Otherwise, this week will feature well above average temperatures
and dry conditions before cooler weather and increased shower
chances return this weekend.

&&

.DISCUSSION...

Key Points:

* Best overall thunderstorm chances for this afternoon with
  lesser chances on Tuesday afternoon, then dry thru Friday.

* Above average temperatures prevail this week, with lower
  elevation highs possibly reaching 90 by Thursday-Friday.

* Shower/thunder chances may return by the weekend, with start of
  a cooling trend that likely becomes more established next week.

A small deformation zone to the northeast of an upper low off the
CA coast will produce conditions favorable for scattered showers
and thunderstorms this afternoon through early evening, with
initial storms firing up by 1-2 PM and ending by 8-9 PM. The most
favored area for this activity has been rather consistent,
covering the eastern Sierra south of Tahoe, and western NV south
of US-50 and west of US-95. A few storms with more sparse
coverage and shorter-lived activity could pop up a bit farther
north to near I-80 within this late afternoon time frame. Any
mature thunderstorm will be capable of brief downpours, small
hail, gusty outflow winds to 45 mph and cloud to ground lightning
strikes.

For Tuesday afternoon, the upper low is projected to drift
southeast, limiting the thunder chances to the Sierra crest of
Alpine/Mono counties, with most activity then pushing west of the
crest. The risk for storms drops to below 10% from Wed-Fri as a
high pressure ridge builds into CA-NV, suppressing convective
growth despite well above average temperatures.

Warm late spring and even early summer conditions prevail this
week, with highs mainly in the lower-mid 80s each day through
Wednesday for lower elevations, and 70s for Sierra communities.
the location/strength of the upper ridge, some of the warmer
valleys could touch 90 degrees on Thursday-Friday, with ensemble
probabilities ranging from 50-70% for Reno, Lovelock and Fallon.
The average date for Reno`s first high of 90 (since 1990) is May
30, so this would be a couple weeks ahead of schedule. Be sure to
stay hydrated this week, and limit strenuous outdoor activities
especially if it takes a while to acclimate to these warmer
conditions.

This warm spell is not projected to be prolonged, as a more
active storm track currently near/north of the US-Canada border is
showing signals of dropping south to the Pacific NW. The early
stage of this transition should arrive by the weekend, with
temperatures coming down a few degrees from their Thurs-Fri peaks.
One other area to watch is the potential for another closed low
off the CA coast, which could bring a return of PM shower/thunder
chances in similar fashion as today or Tuesday.

By next week, additional weather systems moving across the
northwest US could lead to either a flatter west flow or even some
troughing for the western US. This would lead to additional
cooling (may fall back to near average), periods of increased
winds and chances for more PM showers/thunder--especially if the
trough interacts with the potential upper low offshore from CA and
draws it inland toward the Sierra. MJD

&&

.AVIATION...

* Primary focus is t-storm chances through this evening and then
  again Tuesday, otherwise conditions next 1-2 days should be
  pretty typical with westerly breezes, afternoon thermal bumps,
  and VFR flight categories.

* Latest HREF guidance shows an axis of isolated to possibly
  scattered showers/t-storms along and east of the Sierra between
  roughly 21z-03z today. Roughly 30-50% probabilities mostly
  between TVL-MMH/BIH-HTH, but some 10-30% odds of cells farther
  north to RNO/TRK. Erratic outflows to 40 knots probably the main
  hazard, along with lightning and brief MVFR flight categories.

* Storm coverage Tuesday appears to decrease, but more focused
  along and just west of the Sierra crest. Still looking at a
  10-25% chance of a storm near TVL/MMH in the afternoon-evening.

-Chris

&&

.HYDROLOGY...

Unseasonably warm daytime temperatures and mild nights, combined
with higher solar angles, longer days, and limited cloud cover,
will continue to increase snowmelt rates this week. Even the
higher elevation deeper snow areas will begin to melt in earnest.
This rapid melt will lead to elevated flows with rivers and
streams running cold and fast, most notably in areas draining
significant terrain over ~7500 feet from Lake Tahoe south through
Mono County.

While these high and cold flows can be hazardous to recreationalists,
flooding is very unlikely. Remember the highest flows are
significantly lagged from the heat of the day and can often occur
at night.

More steady high flows will also continue along the Lower Humboldt,
with additional rises likely in very late May or early June.

Please use extra caution around local rivers and streams which
will be running fast and cold. Bardsley

&&

.REV Watches/Warnings/Advisories...
NV...None.
CA...None.
&&

$$