3-day pricip forecast from the Hydrometeorological Prediction Center
November 29, 2012 - Water vapor imagery shows an impressive upper level low off the coast with moisture streaming into Northern California. A shortwave with a surface cold front is gradually spreading precipitation over the coast and inland. IR satellite indicates cold cloud top enhancement over the area. This system has a strong pacific moisture tap with the 18Z GFS Projecting PW values around 1.2 inches extending inland over the forecast area tonight. This is an atmospheric river pattern transporting moisture in from the central pacific. This pattern will continue through the weekend with multiple disturbances bringing periods of moderate to heavy rain.
There will be impressive quantities of precipitation through the day Sunday, especially with orographic lift in the Northern Sierra and northern mountains. This relatively mild system will have significant snowfall confined to above 7000 feet. Several feet of snow are possible for high peaks 8500 feet, with 8 to 12 inches total around pass level.
Precipitation has spread over the mountains and valley north of Stockton. The front was a little slower than expected which delayed the onset of rain by several hours. Radar and surface observations show the heaviest precipitation is currently over the Northern Sacramento Valley and Coastal Mountains. The HRRR and WRF models show precipitation intensity increasing overnight and spreading over the area. The Northern San Joaquin will see less precipitation than the valley to the north.
Main issue continues to be the threat of flooding and for debris flows in recently burned areas, so a flood watch continues through Sunday across the CWA. The heaviest periods of rainfall are forecast for overnight tonight and again Saturday night into Sunday.
Another impact includes windy conditions through the weekend with a wind advisory continuing into Sunday. There will be brief lulls in the wind at times but conditions will be windy/breezy much of the time.
A winter storm warning begins 4 pm this afternoon and continues to 4 pm Friday above 7000 feet for the West Slope Northern Sierra Nevada and the mountains of Western Plumas County/Lassen Park. Significant snow accumulation will be possible at the highest elevations of the Northern Sierra tonight into Friday before snow levels rise over the weekend.
The atmospheric river pointing at NorCal, courtesy NOAA
NESDIS issues Satellite Precipitation Estimates:
Satellite analysis and trends, blended TPW product continues to show atmospheric river stretched across the Central Pacific aimed directly into N Central CA with core of plume running in excess of 1.2", with a more general .9-1.1" over the discussion region.
Next Major S/WV impulse can be seen in water vapor imagery approaching SW OR/NW CA which should help increase lift/upper level divergence next several hours.
In addition the experimental satellite wind product depicts a powerful Pacific jet with a max of 135 knots nosing into NW CA providing additional dynamics to enhance rain rates through 6Z.
IR imagery has shown new enhancement ahead of short wave in NW CA as SW flow has interacted with terrain to produce heavier precip. Anticipate rates of .25-.5"/hr in heaviest bands with isolated .75"/hr rates possible, especially in orographically favored spots.
Help us bring you more news. Be a real reader:
By submitting a comment you consent to our rules. You must use your real first and last name, not a nickname or alias. A comment here is just like a letter to the editor or a post on Facebook. Thank you.