Tornado in Nevada, September 21, 2017

Area Forecast Discussion
National Weather Service Reno NV
104 PM PDT Thu Sep 21 2017

.SYNOPSIS...

Scattered rain and snow showers remain possible through Friday as a
chilly airmass overspreads the region. Localized snow accumulations
may occur in the mountains. Widespread frosts and freezes are likely
Saturday morning. A gradual warming is expected next week as high
pressure builds in, with minimal risk of storms or strong winds.

&&

.DISCUSSION...

* Autumn's first real cold front pushed through the region last night,
  bringing high elevation snows and brisk northwest winds today. Snow
  showers, some lake enhanced, developed today with localized snow
  mountain accumulations. Based on radar trends and HRRR, we expect
  these showers to continue into this evening, dropping gradually
  south from Tahoe into the eastern Sierra. Some localized road
  impacts are possible for passes above 6500-7000' especially with any
  showers after sundown. Isolated thunderstorms may occur as well.
  Continued lake enhancement is possible tonight into Fri morning off
  of Tahoe with NW flow, minimal shear, and solid instability.

* One change to inherited forecast was to add scattered showers to
  Friday. Continued weak instability plus passing short waves will
  provide the ingredients. Comparing NCAR high-res ensemble output
  today vs tomorrow, showers Friday look to be fairly light with less
  snow impact potential for mountains. It probably won't be as sunny a
  day as we earlier thought.

* Have trended temperatures down some for Friday and Saturday based on
  latest guidance. Especially Saturday morning when we'll likely have
  clearer skies and lighter winds, which puts many of our populated
  and agricultural areas in freeze risk. This includes Carson Valley,
  Smith Valley, suburbs of Reno/Sparks, Sierra Valley, Susanville, and
  around Fallon and Lovelock. So a Freeze Watch has been issued for
  these areas. Time to pick those last garden tomatoes, protect
  sensitive vegetation, and check above-ground irrigation.

* Into next week ECMWF/GFS ensemble guidance continues to show broad
  ridging overspreading the region. This will result in a warming
  trend, but a slow one given cool northeasterly surface flow
  persisting. No signals for storms through next week, though there
  are some hints of a trough developing off the west coast Sept 30
  weekend into early October, but no meaningful AR signals.

-Chris

&&

.AVIATION...

* Cold front pushed through the region last night ushering in brisk
  northwesterly winds. Those will continue into early evening before
  subsiding. Instability aloft in the wake of the front will yield
  scattered to numerous showers and isolated storms through this
  evening and then again Friday, especially over the mountains
  including TRK, TVL, and MMH.

* We're looking at a 60-80% chance of seeing a period of IFR
  conditions in snow showers at TRK, TVL this afternoon, MMH late
  afternoon into early evening. Some light accumulations are possible.
  These periods will last 30-60 minutes. For RNO, CXP it will be
  mainly brief rain showers, 30% chance of MVFR. Some cells have grown
  enough to produce lightning, so that's possible too.

* Friday snow showers will return to the mountain regions with a 30%
  chance of MVFR/IFR status from brief snow showers at TRK, TVL, MMH
  from late morning into the afternoon. Accumulation risk is lower
  than today. Into Saturday morning, with clearing skies and recent
  precipitation, dense freezing fog is possible in Sierra valleys
  especially TRK, possibly some W Nevada valleys but low
  predictability.

-Chris

&&

.REV Watches/Warnings/Advisories...
NV...Freeze Watch from late Friday night through Saturday morning
     NVZ001-003-004.

CA...Freeze Watch from late Friday night through Saturday morning
     CAZ071.

&&

For more information from the National Weather Service visit...
http://weather.gov/reno