SACRAMENTO, Calif —
For nearly the entire last month, the atmosphere has been locked in a pattern that has dropped storm system after storm system from the north.
A strong area of high pressure in Gulf of Alaska squeezed cold air south down the west coast, producing conditions that have resulted in low snow levels and extraordinary snow totals recently.
These systems have been largely beneficial, although the extreme amounts of snowfall have prompted emergency declarations in many counties.
The cold storms dropping from the north haven’t had access to subtropical moisture like the upcoming atmospheric rivers will have, prompting renewed flooding concerns for many areas.
The origin and nature of the storm systems will alter with the upcoming pattern shift. These storms will be much warmer and wetter.
The jet stream, which dominates most of the weather in the mid latitudes, will shift south and will become mainly zonal (east to west). This means that the jet stream will be moving quickly across the Pacific in a nearly straight line from Japan to California.
The jet will take a slight dip south near Hawaii thanks to low pressure set up near the islands. The counterclockwise flow will help funnel abundant tropical moisture into the zonal jet stream and eventually towards California.
On Thursday, a swirling low pressure system will center itself off the coast of Oregon. The system will be able to tap into the subtropical moisture plume as the low gains access to the jet stream. It is currently expected to be a level 4 (strong) atmospheric river, according to the Center for Western Weather and Water Extremes.
With the jet stream roaring overhead, winds will be very strong as precipitation makes its way onshore. Winds of 40-45 mph are possible in Sacramento Thursday night and into Friday morning.
The air will certainly have a different “feel” to it compared to the chilly conditions that have dominated the last month or so. This air mass will be much warmer and moisture laden due to its Hawaiian origins and valley temperatures will be in the 50s and 60s by this weekend.
While the mountains have seen incredible amounts of snow, the valley hasn’t seen nearly the same level of precipitation. In fact, Sacramento received below average rainfall in February, receiving 2.62” compared to the average of 3.42”.
The upcoming storm cycle will certainly put Sacramento and other Central Valley communities well above average rainfall. Current forecasts have 1-3" of rain for the valley and 3-6" in the foothills.
Snow levels will start out around 3,000-5,000 feet before quickly rising with the onset of tropical moisture to 6,000-8,000 feet.
Areas in the lower foothills could see substantial flooding impacts due to the combination of heavy rain and snowmelt. In the higher elevations, the snow is cold enough and deep enough to prevent considerable melting. Rising creeks, rivers and streams will be a concern in the lower elevations.
Many have wondered if the upcoming warm storm could cause a catastrophic flooding situation similar to that of 1997 due to the massive snowpack in place. While flooding is possible in some areas, the National Weather Service in Sacramento has dispelled those rumors.
"This is not a repeat of 1997," according to the National Weather Service. "This is not as strong or warm of an atmospheric river. There is less rain forecast over a longer period of time and water is managed differently now."
It will no doubt be an impactful event, however. The Weather Prediction Center has a moderate risk of excessive rain for a large swath of Northern California.
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