Area Forecast Discussion
Issued by NWS Reno, NV

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FXUS65 KREV 172037

Area Forecast Discussion
National Weather Service Reno NV
137 PM PDT Thu Aug 17 2017


Thunderstorms remain a possibility each day through the middle of
next week. Storms today and Friday will be mainly over the Sierra,
potentially becoming more widespread this weekend into early next week
but predictability remains below average. Flash flooding is the main
threat from storms due to slow storm movement. Temperatures through
the weekend will be near to slightly above normal, with weak westerly
winds each afternoon.



* Thunderstorms Today and Friday: Following reliable NCAR high- res
  ensemble guidance, light north/northeast steering flow will confine
  best storm risks to over the Sierra and points west today and
  tomorrow. Similar to what happened Wednesday - mainly non- severe
  heavy rainers. Flow aloft Friday is very light meaning that outflow
  boundary interactions will dominate storm evolution, movement.
  Instability further east over W Nevada is very sketchy, so while we
  can`t rule out some cells, coverage should be minimal for W Nevada

* Thunderstorms This Weekend: This is where things get interesting.
  Flow aloft goes from light westerly to light southeasterly Saturday
  to Sunday as upper low re-deepens off SoCal coast. Unresolvable
  shortwaves, plenty of moisture, and weak-moderate instability will
  surely trigger storms each afternoon, with Saturday looking the most
  promising of the two weekend days for widespread stronger storms.
  Flash flooding could be the bigger issue with light steering flow
  each day and precipitable waters in the "sweet spot" between 0.6"
  and 0.8". Too much higher than that and we tend to cloud cover with
  little convection.

* Thunderstorms Next Week: Upper low looks to remain off SoCal into
  Tuesday before ejecting northeastward on Wednesday or Thursday, but
  these pattern changes are often very low predictability this far
  out. So we`ve kept low-end chances for storms in the forecast each
  day through Wednesday. Hard to tell this far out if any of these
  days could go big with storms or flash flooding.

* Eastern Sierra Smoke: Latest GFS transport wind forecasts and USFS
  BlueSky model keep northeast flow going into Saturday over the
  Eastern Sierra. This will keep the thickest smoke from Yosemite-
  area fires away from the east side. Some haze is possible right
  along the crest however.

* Eclipse Weather: High confidence remains in us having a solar
  eclipse Monday morning, but lower confidence on cloud cover
  potential. Time lag ensemble of recent GFS runs is 60/40 for us
  having scattered-broken convective debris clouds in the region
  versus sunny skies. Whether or not this would be enough to obscure
  the eclipse is unknown. We should have a better sense this weekend
  on cloud cover potential for Monday, and may issue an unofficial
  Eclipse Cloud Watch if needed.




* The main aviation impact over the next couple days will be from
  mountain thunderstorms. Isolated storms will develop after 21z today
  and Friday mainly over the Sierra including around MMH, TVL, TRK.
  About a 20% chance of storms within vicinity of those airfields.
  Steering flow today is light NE taking storms mainly to west slopes,
  but Friday steering flow is very light meaning that random outflow
  boundaries will dictate storm movement. Slightly higher chance of
  seeing storms at TVL, TRK, MMH Friday. Storm chances for RNO, CXP
  <5% today and about 10-15% Friday afternoon.

* Otherwise, conditions will be VFR with light morning winds followed
  by low-end zephyr westerlies 10 kts or less. If areas like TRK end
  up getting rainfall, dense morning fog Friday or Saturday mornings
  is a good bet.



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