Significant Pattern Change Wednesday Through Saturday

Synopsis:  The ridge over the western US will weaken and shift east over the next 48 hours, while a trough currently in the Gulf of Alaska moves toward the west coast to bring two rounds of mountain snow and valley rain though Saturday.  After a brief break, the active pattern may return by the middle to latter part of next week.

Current Conditions:  The latest GOES 17 image shows a weak low just off the southern California coast with a larger low and associated cold front in the Gulf of Alaska, with decent instability behind the front due to strong cold air advection.  The current upper air pattern at 300mb continues to show an amplified pattern of a ridge over much of the west and a trough in the eastern US and a sharp trough in the Gulf of Alaska.

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Short Term Forecast:  Over the next 24-28 hours, expect partly cloudy skies as high clouds pass over the area from the low off the southern California coast.  Temperatures will remain cool under valley inversions through Wednesday afternoon.  A look at the latest HRRR smoke model fields show the flow at the surface and aloft becoming southerly by Tuesday through Wednesday.  This will primarily keep smoke out of the western Nevada region.

By Wednesday afternoon, winds should increase out of the south-southwest at 15-25 mph to flush out the valley inversions.   Rain and snow will reach the Sierra and portions of western NV by late afternoon.  The current GFS (wettest) and NAM MOS (very dry) statistical guidance are currently at odds with how much precipitation will fall over the area from the first system. Both the MOS guidance have their bias.  At this time, it looks like it will mainly be around 1/10th to ¼ of an inch of rain with locally higher amounts near the Sierra front and the western Carson City area by Wednesday night.  Snowfall amounts in the eastern Sierra and the Carson Range will likely see 4-8 inches of snow especially above 7,000 feet.

For Thanksgiving Day, the first system will move east of the area, so any precipitation should shut off in the valleys with some rain and snow showers continuing in the Sierra.  Southwest winds will become rather strong and gusty over the mountains and possibly the western Nevada valleys ahead of the next potentially stronger system moving in Thursday night or Friday.

Medium Range Forecast:    The next system will move in quickly to bring another round of mountain snow and valley rain.  Currently, the forecast models are in good agreement on bringing abundant subtropical moisture with a modest atmospheric river.  Currently, IVT values suggest a weak AR event at best mainly focused on northern CA into the Lake Tahoe basin.  The latest model trends continue to keep this AR over the area for at least 12-24 hours.  Due to the warmer nature of this system, expect snow levels to rise to around 7,000 to 7,500 feet during the event.

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5-day Precipitation Forecast:  A look at the latest forecast precipitation from the various operational models continue to be in good agreement at a general area from at least Lake Tahoe north will see around 2-4 inches with some higher amounts to 5-6 inches near the burn areas around the Camp Fire where flash flooding may be possible.  For the eastern Sierra and the Carson Range from highway 50 north, expect amounts of 1 to 1 ½ inches.  Total snowfall accumulations above 7,500 feet will range from 12-18 inches.   In western Nevada valleys, the Carson City area will likely see total amounts of ½ to ¾ of an inch over the period.   Currently, forecast models are at odds as to how much precipitation will spill over into western Nevada, so some uncertainty exists.

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Long Range Forecast:  Latest guidance continues to indicate a ridge will build into the western US once again, but it currently looks to be short lived as another active pattern return to the western US by the middle or end of next week.  Currently the westerlies look to possibly undercut the western ridge to bring more active weather and another series of mild storms to the area.  There is still a lot of uncertainty this far out, but the long range CFSv2 for what it’s worth, continues to bring above normal precipitation to much of California and Nevada into the beginning of December.

2018-11-19_16-17-51.pngForecast graphics are courtesy of NOAA, Tropical Tidbits and Weatherbell Analytics.  NOAA’s GOES-17 satellite images have not been declared operational and its data are preliminary and undergoing testing.

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