Atmospheric River Late Thursday..........

Atmospheric River Late Thursday – Saturday: Update 3

A fairly significant precipitation event is still on the to northern California, and confidence has increased in that double-digit precipitation totals in some places are likely through early next week as a series of storms travels along a belt of very moist air. This belt of very moist air, also known as an atmospheric river, will drive precipitation amounts up significantly compared to what we may normally see from these systems without the significant plume of moisture.

The first in the series of systems traveling along a raging west to east jet will traverse into the Pacific Northwest & British Columbia late Thursday, spreading precipitation into northwestern California through the afternoon, and into the rest of northern California early Friday. The belt of moisture ahead of the system’s cold front will remain aimed at northern California through the day Friday, in fact increasing in intensity through the day into the evening as a another system moves upstream, strengthens, and slams into the west coast late Friday night into early Saturday, intensifying lift and increasing precipitation intensity across the region.

Lift weakens as this system moves inland and the moisture plume sags southward into central and even southern California through the day Saturday; meanwhile, cooler air behind the front, still packed with decent moisture, swings inland during the day Saturday. This cooler airmass will lead to a more unstable environment across northern California during the day Saturday… thus, some convection appears possible. All layers of the jet remain aimed at northern California through Saturday, thus there’s good mid/upper-level support for thunderstorms to form and perhaps get their act together… that is, if some pockets of sunshine can break out and boost instability/buoyancy further to increase potential for more robust updrafts given the presence of decent shear. This will be looked at closer in the coming days.

Saturday’s forecast thunderstorm potential.

A brief break in the action appears to be a good bet come Saturday night and (very) early Sunday before the next system arrives. This next wave, by the way, will consist of a fairly deep mid-latitude cyclone off the California coast riding the jet (stream) on northeastward into the Pacific Northwest, spreading a frontal shield of rain across northern California once more by Sunday morning along the coast, and into the interior by afternoon into the evening. The main frontal band scoots southward fairly quickly compared to the previous firehose event Thursday – Saturday, but showers will remain possible through Monday as another bout of cooler, unstable air slides overhead… thus, can’t rule out more thunderstorms Monday morning/afternoon. The front will dump a decent shot of precipitation across the region to add to Thursday – Saturday’s accumulations. But after this storm, it appears we’ll be done for a while once more, unfortunately.

Wind

Fairly tight pressure gradients between the train of surface lows off the west coast and the surface high(s) in the Great Basin and southwestern U.S. combined with periodically heavy precipitation late this week, periods of gusty winds are likely… with the strongest winds expected over the sierra. The strongest winds are currently expected during the day Friday into Friday evening, as a surface low intensifies off the west coast. 850mb winds are currently forecast in the 55 – 75kt range from the valley into the sierra Friday afternoon into the evening, which also when precipitation is expected to reach peak intensity across a good chunk of the region. Thus, believe some gusts in excess of 40mph are likely in the lower elevations Friday, with >60mph in the sierra.

Wind gust forecast based off a mean of model solutions.

Orographics/terrain enhancement

Given the strong south to southwesterly flow slamming into the mountains, the higher elevations will pick up substantially more precipitation than the lower elevations. South and southwest facing slopes will pick up the most, as the flow parallels these slopes and precipitation becomes maximized. The great flow stacking up against the mountains will also create a banking effect that extends into the east part of the Sacramento valley, where precipitation amounts will be higher compared to the west side due to banking of moisture against the west slope of the sierra/foothills.

Precipitation

Essentially every major global model except for the ECMWF indicates pretty substantial precipitation amounts across northern California through Monday. A blend, or mean of these solutions including that of the somewhat less-wet ECMWF generates fairly generous amounts of precipitation, and these amounts are as follows: 2 – 5″ for much of the Sacramento valley with upwards of half a foot or so in the north end of the valley and parts of the eastern valley, 1 – 3″ in the northern San Joaquin valley, 4 – 8″ across much of the foothills & mountains, 2 – 5″ along a good chunk of the immediate, 4 – 8″ in the coastal mountains (locally higher amounts possible along south/southwest facing slopes), near the same in the northern mountains, and upwards of 8 – 12″ in orographically favored slopes inland, such as the higher elevations of Butte, Plumas, Yuba, Sierra, Nevada, Shasta, and Siskiyou counties.

Rainfall amounts based off a mean of model solutions.

Next update should be out later Thursday, or Friday if no significant changes are needed.......