Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 201023

Area Forecast Discussion
National Weather Service Reno NV
323 AM PDT Thu Jul 20 2017


Areas of smoke and unhealthy air quality will continue across
portions of the Sierra and western Nevada the next day or two as
a result of the Detwiler Fire in Mariposa County. Dry conditions
continue with temperatures warming a few degrees over the next
few days. The next chance for showers and thunderstorms may occur
by late in the weekend and into early next week.



The main story for at least the next couple days is smoke from
the Detwiler Fire in Mariposa County. Looking at available air
quality monitors around the Tahoe and Reno-Carson City-Minden
areas this morning, the highest concentrations of smoke particulates
appears to be in the Carson Valley, with readings in the unhealthy
to very unhealthful range. Based on visibilities and UNR fire
cameras, South Lake Tahoe may also be experiencing poor air
quality. Finally, for the Reno-Sparks area, the UNR fire cam from
Peavine Peak indicates a smoke layer. However, the layer may be
slightly elevated as Truckee Meadows air quality is currently much
better than in the Carson Valley.

An air quality alert remains in effect through mid to late
afternoon for the Reno-Tahoe area south through Mono County, with
the potential for the extension of the alert into Friday. While
air quality is currently much improved in Reno-Sparks, the
Detwiler Fire appears to be burning actively (per GOES-16 imagery)
and westerly winds are not expected to develop until this afternoon
so smoke could mix down into the valley this morning. As we go
into Friday and Saturday, which locations see badly degraded air
quality will depend on the intensity of the fire and subtleties in
wind direction over the Sierra and western Nevada. Unfortunately
for the Reno-Tahoe area, GFS and NAM simulations indicate a turn
to more southerly flow aloft with lessening westerly afternoon
zephyr winds so the potential for continued periods of degraded
air quality remain into the weekend.

As far as temperatures, highs will remain seasonably warm and within
a few degrees of average through Friday before a bump to 5-7 degrees
above average on Saturday as an upper ridge strengthens overhead.
No thunderstorms are expected through Saturday, although a few
afternoon cumulus buildups are possible over Mono and western
Mineral Counties on Saturday. -Snyder

.LONG TERM...Sunday onward...

The greatest challenge is how moisture advecting northward around a
large area of high pressure will interact with an area of low
pressure off the Pacific Northwest coast. Over the past several
days model solutions have bounced around with the location of
these features from looking like a monsoonal surge to dry
southwest flow remaining over the region, leading to lower
forecaster confidence. Latest guidance would suggest remnant
monsoonal moisture slowly moving northward into Nevada starting
Sunday and lingering through the week. Less of a surge and more of
a slow and steady increase in moisture through the week. This,
however, is dependent on the low sitting off the coast and how far
south it extends later in the week. Farther south would lead to
deeper southwest flow across the region and a drying influence,
but if it stays north, flow is unlikely to be strong enough to
scour moisture out of the region.

The other interesting features are the multiple shortwaves moving
through the region early next week, which could help to boost
thunderstorm chances as they interact with the moisture. Will
maintain the low end thunderstorm chances for now each afternoon and
early evening Monday through Wednesday, with Monday possibly having
greater coverage due to the shortwave passage. Nocturnal convection
remains a possibility, especially Tuesday night as an upper wave
interacts with a jet streak moving through the region, however best
chances may be more central into eastern Nevada. Thunderstorm
chances are likely to wane toward the end of the week as deeper
southwest flow becomes established across the west.

Otherwise, temperatures will remain near average with typical
afternoon breezes possibly strengthening toward the end of next
week. -Dawn



The greatest aviation concern continues to be the widespread smoke
across the region from the Detwiler Fire in Mariposa County,
California. This is causing reductions to visibility (1-4SM), smoke
ceilings (025-045 AGL), and terrain obscuration. The worst
conditions are across the Sierra into the Tahoe Basin and the Carson
Valley, with lesser impacts through the Truckee Meadows into the
central Nevada Basin. This includes KTVL, KBAN, KCXP, KMEV, KTRK,
and KRNO. Slantwise visibility will be especially poor, possibly
below a half mile, during the morning hours.

While the amount of smoke will be highly dependent on how actively
the fire burns, it is likely we`ll be dealing with impacts for at
least the next 36-48 hours. Conditions will be worst overnight into
the morning hours, with some improvement each afternoon
(approximately 21z-03z) due to mixing and the westerly zephyr
breezes of 10-20 kts. Even with the improvement, expect smoke and
haze to remain in the area with the least impact north and west of a
line from approximately KSVE-K1A8 (Susanville,CA - Empire, NV).


.REV Watches/Warnings/Advisories...
NV...Dense Smoke Advisory until 5 PM PDT this afternoon NVZ001>004.

CA...Dense Smoke Advisory until 5 PM PDT this afternoon CAZ072-073.



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