Atmospheric river inbound, heaviest rain on Sunday

January 7, 2017 – Light to moderate precipitation with snow levels down to the northern Sacramento Valley in the morning, rapidly rising in the afternoon.

Very heavy rain and flooding concerns arrive Sunday into Monday.

Another wet storm Tuesday into Wednesday with continued flooding concerns and significant mountain snow.

Flood Watch from 4 PM this afternoon through Wednesday afternoon.

• 

  Precipitation begins Saturday morning and continues through Monday
  • heaviest Sunday, intensity will decrease Monday, but there may not be much of a break before the Tuesday storm
• Rivers and Streams
  • rising water levels Sunday – next week
  • some points reach flood stage by Sunday. River forecasts at http://www.cnrfc.noaa.gov/
  • streams and creeks that are free-flowing (no dams) will bear close watching; we could see impacts in those areas unseen for years
  • additional rises expected from Tuesday’s storm
• Snow levels
  • ~500 feet northern mountains (including Redding) and 4000-5000 feet in the Sierra this morning expected to rise above 8000 feet this evening
  • drop back below pass levels early Monday
• Winds
  • Valley gusts 30-45 mph
  • Mountain ridgetop gusts 45 to 60 mph, locally higher over ridgetops
• Wet pattern continues past Monday
  • Another wet storm system Tuesday–Wednesday, this time with slightly lower snow levels. Another storm expected by the end of the week.
  • Continued flood threat.

Discussion

Moderate precipitation has spread across the forecast area. Rain amounts so far are generally around a third to three quarters of an inch. Valley rains will decrease in intensity this afternoon, then increase again this evening and overnight. Heaviest rain is expected during the day Sunday. Snow levels in the Sierra are rising, with snow falling down to 4000 feet, and reported to be already rising to round 6000 feet.

Northerly winds in Shasta County have brought cold air drainage and along with evaporative cooling have brought low snow levels this morning. A mix of wet rain and snow briefly reached as low as Red Bluff, around 350 feet, with some snow accumulating on the grass around Cottonwood (elevation 420 feet). Moderate accumulating snow is falling in around Redding. Snow will continue through the morning, with snow levels rising rapidly this afternoon. A Winter Weather Advisory remains effect, and will be updated to expand the area southward.

Gusty winds have remained mainly over the northern San Joaquin Valley this morning, with sustained winds up to around 26 mph at Stockton at 8 am. Winds gusting to 20 mph at Mather, while Sacramento Executive and International Airports have remained calm. Will likely cancel the wind advisory later this morning. Stronger winds are expected Sunday.

Extended discussion (Wednesday through Saturday)

Westerly flow aloft will continue to bring moisture through northern California going into the extended period. Model agreement starts to fade but there is a general agreement in this westerly flow and orographics should ensure at least moderate precipitation over the Sierra. Temperatures should come in around normal for this time of year.

Models really begin to diverge by next Thursday. ECMWF digs a low in over northern California while GEM puts upper ridging over the Great Basin. GFS keeps a more zonal flow over the region. Any of these solutions could allow for continued precipitation over the CWA but only the ECMWF could bring a continuation of the heavier precipitation that is expected earlier in the week. More amplified ECMWF solution would bring a break in precipitation threat Friday while GFS and GEM continue to feed moisture into the north state. Chance pops continue to be best bet at this time. Although wet pattern may continue into next weekend, at this time models are hinting at at least a reduction in precipitation amounts. Snow levels remain moderate through the extended period.