FXUS65 KREV 210108

Area Forecast Discussion
National Weather Service Reno NV
608 PM PDT Thu Jul 20 2017

Zephyr winds once again brought smoke from the Detwiler Fire into
the Tahoe Basin and western Nevada. Air quality readings are
lower than yesterday, but starting to nudge into the unhealthy

Latest smoke plume simulations show the worst air quality could
filter into Mono, Alpine and Douglas Counties tonight. For the
Tahoe Basin and Reno-Carson area, smoke should diminish 7-10 pm
then may return again by early morning.

Will hold off on any dense smoke advisories for now as satellite
data shows the thick plumes off the Detwiler fire are stretching
north and south along the western slopes. Brong



Areas of smoke from the Detwiler Fire will produce periods of
unhealthy air quality across the Sierra and western Nevada for the
next couple of days. Dry conditions will continue with
temperatures warming a few degrees by this weekend. Low pressure
off the west coast may bring a chance for showers and
thunderstorms Sunday through the middle of next week.


.PREVIOUS DISCUSSION... /Issued 243 PM PDT Thu Jul 20 2017/


The main weather issue through Saturday will be the smoke and haze
from the Detwiler Fire. At a minimum, hazy conditions are likely
to continue through at least Friday especially from Reno-Tahoe and
I-80 southward. If the fire generates large smoke plumes each
afternoon, the risk for higher smoke concentrations and poor air
quality would be highest across Alpine/Mono counties and western
NV mainly south of US-50.

Otherwise, we are expecting dry conditions through Saturday with
near average temperatures mainly in the 90s Friday, then rising to
around 100 degrees in lower elevations Saturday as high pressure
ridge over the southwest US expands across most of the Great
Basin. Warm temperatures aloft will inhibit thunderstorm
formation, but some afternoon cumulus buildups are probable
especially south of US-50 on Saturday.

By Sunday and Monday, the potential for showers and thunderstorms
will increase as an upper low approaches the California coast
while moisture rotates around the periphery of the ridge over the
southwest US. The latest guidance seems a little more favorable
for increasing instability both days, with some forcing associated
with the low, especially on Monday as it ejects inland across far
northwest NV. The location/track of the low remains in question,
but overall it appears that enough favorable factors for
convection will come together, so we continued a mention of
thunder across the region both days with some lingering showers
possible during the overnight/morning hours. Initially there could
be an increased risk for dry lightning strikes on Sunday, then
transition to a mix of wet and dry storms with potential for
strong outflow gusts.

By Tuesday and Wednesday, the overall trend appears to favor
decreasing thunderstorm chances as the western portion of the
ridge weakens a bit in response to a trough brushing the Pacific
Northwest coast, with drier southwest flow aloft over the Sierra.
The evolution of this transition could occur slowly so we opted
for a more gradual reduction of the areas with thunder chances
(limited to west central NV southward to Mono County by
Wednesday). Temperatures both days are expected to ease back to
near-seasonal highs mainly in the 90s for lower elevations.

By next Thursday, a drier and more stable air mass appears more
likely to spread across west central NV so we did not include
thunder chances. The concern could shift to increasing winds,
although at this time the stronger southwest flow looks to remain
north of the Sierra, so we are currently anticipating zephyr-type
breezes with gusts 25-30 mph. MJD


The main aviation concern for the next couple of days will be the
smoke and haze from the Detwiler Fire in Mariposa County,
California. So far today the push of new smoke from this fire
appears to be spreading east toward the central Sierra, and may
reach the northern half of Mono County by early evening. This new
smoke could bypass the main terminals, resulting in only limited
reductions to surface visibility. Slantwise visibility will be
lower across the region as long as haze areas are present and
especially during the morning hours. Outside of the smoke and haze
areas, VFR conditions will prevail with typical afternoon breezes
(gusts near 20 kt) today and Friday, then lighter winds Saturday.


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