WeatherWhys Blog

Hadley Cells and You

Hadley Cells and You

Kern county and all of California have been on a “Spring Break” of sorts from the wet winter lately.  But the wet part of our year is still in its ascendancy.
 

Two weeks from Friday (March 5th) is the zenith of the rainy season. And El Nino is still a vibrant phenomenon is the central equatorial Pacific. High pressure has been present in the west for about a week now, while the eastern US continues to hang on through an historic winter, with epic snow and cold. But, as is usually the case, when one side of the country is one way the other is the opposite, weather wise. High pressure over the west will amplify over the next couple of days until it cuts off as a high latitude ridge over western Canada.

 
George Hadley was a pioneering British meteorologist of the early 1700s who discovered that low pressure typically resided in a band near the equator, as well as at about 60-degrees north and south latitude. High pressure, on the other hand, normally found a home at 30-degrees north and south latitude. He devised the concept of rising air near the equator along the Intertropical Convergence Zone (ITCZ) and also near 60-degrees latitude, while sinking air developed around 30-degrees latitude (and, to some measure, near the poles). These came to be known as “Hadley Cells”, which many a high school earth science student has studied. But while the 60th parallel may be home to some of the lowest pressure on the planet, that is not always the case.
Several times each winter, an interesting twist of atmospheric physics occurs. Hemispheric long waves in the jet stream become either too long or too short- and anarchy reigns. The river of air at the top of the troposphere which drives weather systems begins to buckle. Small eddies form and spin south as cut-off lows. But often in February, ridges of high pressure essentially do just the opposite of those cut-off lows. A ridge surges north until a stable large cell of high pressure forms. This is known as a “Blocking Ridge”. It is somewhat unusual since high latitudes are the abode of low pressure, not high. But when these blocks develop they typically persist for about a week and move slowly from east to west, contrary to the movement of transitory storms moving from west to east. These are often anchored by companion low pressure systems either straight to their south (as in a “Rex Block”) or to the southwest and southeast (as in an “Omega Block”). Blocks are most common over Greenland, Siberia and western Canada or Alaska. Skies become clear in the far north and nighttime temperatures in low valleys can fall well below zero. But perhaps the most important characteristic of these blocks is the effect they have on the polar jet stream, namely to consolidate and force them south. As a consequence, the Pacific jet stream is shoved south to latitudes much closer to the equator than usual. And when combined with an El Nino event already in progress, a block in Canada can end up producing a very wet period for California.

By late Friday the Pacific jet stream is forecast to once again become active and undercut the Canadian blocking ridge. Rainfall in the south valley from Friday evening through mid-day Saturday is expected to range from about .15” to almost half an inch. Snowfall in the Kern County mountains will accumulate about 3-6” above 5,500’. After a short break for most of Sunday, another wet system will douse the region with rain late Sunday through early Tuesday. We are in for longer dry break Wednesday through Friday, but more El Nino rains will arrive the following Saturday as we zero in on the rainy season zenith during the first week of March.