WeatherWhys Blog

Tropical Moisture Hits California

Every winter, for at least a short time, the sub-tropical jet stream combines with its northern counterpart, the polar jet stream, to bring a “Pineapple Express” to our state. Extreme rainfall with very high snow levels can deliver either beneficial rains or extreme rainfall resulting in major flooding.

The upper atmospheric river of air, known as the “jet stream”, ribbons its way around the globe in both the northern and southern hemispheres. The term dates back to World War II when bombers flying at high altitudes from California to Hawaii discovered head winds so strong that they had to turn back for more fuel. Several distinct types of jet stream exist, each affecting different types of air.

A primary type is known as the “Polar Jet Stream”. This strong wind can develop dips in its flow from west to east that driving surface low pressure areas and their associated cold fronts. Another type is the “Sub-tropical Jet Stream”. During winter and transitional seasons this feature transports high cloudiness northward, but also allows copious amounts of mid level moisture to stream northward. Meantime, the polar jet possesses an extended ability to work the atmosphere with strong dynamics. Both jet streams are created by large temperature gradients from north to south. The greater the temperature differences, the faster the jet stream winds become.

When both the polar and sub-tropical jet streams converge, there is usually a major weather phenomenon that takes place. When this happens, meteorologists watch the best of both worlds combine their power to produce strong storms. Such is the case right now, as we flip the calendar over from November to December. In the eastern Pacific deep low pressure has strengthened and is bringing about the scenario mentioned above, a meeting of the two jet streams.

The greatest amount of available moisture mixed into the atmosphere is always at or near the equator. Over the next couple of days, some of that considerable moisture is expected to be transported northeast into California. The polar jet will apply dynamics to all this moisture that will force the air to rise, thus causing the water vapor to condense into liquid water and squeeze out excessive rainfall. A final element to this recipe for flooding is the north-south long mountain ranges in our state. As all the moisture moves northeast, the wall of mountains along the coast and the immoveable object that is the high Sierra will force that moisture up and demand even more rainfall from the air.

In the end, 8-14” of rain may fall in some of the most vulnerable spots in California along windward facing slopes. In the high country 2 to 4 feet of snow will pile up, generally above 9,000 feet elevation. This is all very good news for California, which currently finds itself in a moderate to severe drought. We need the rain desperately in order to take us through summer 2013. Unfortunately, Bakersfield and the south valley will be on the short end of this rain stick (we usually are). It is all due to our orographics, i.e. the mountains around us to the east, south and west. Whenever a strong southerly wind takes place out ahead of a big low pressure system, that southerly wind produces a rain shadow for Bakersfield and environs. Long time residents of the south valley are aware of this obstacle to our sharing in the rainy bounty most other Californians get. This time we will probably see minor amounts of rain. However, as the system plods eastward our wind will shift coming from the northwest. Then the south valley will be in an upslope zone and hopefully receive more rain. Upslope enhances rainfall. It is the opposite of how downslope winds hinder rainfall.

In the end I expect around a half inch of rain for the south valley- with upwards to an inch or a little more for windward facing Kern County Mountains. Let’s hope it rains cats and dogs, because opportunities are rare for heavy rain in this part of the world. This is especially true this winter, when the hope for a generous El Nino event first predicted last summer has now been dashed by the latest computer models. We should enjoy our rainfall- whenever it comes.