Last summer I predicted a “wet, windy and wild” winter. That prediction has been somewhat true, but for the last two weeks we have seen a “mid-winter dry spell” – something that happens just about every year. Even in wet winters.
In fact, fall and winter have been wet. December, with more than 10 inches, was particularly rainy. Since September, we (mid-Willamette Valley) are about 2 inches ahead of our long-term average, or “normal.” It hasn’t been particularly cool, due to the lack of serious Arctic outbreaks, but active winters tend not to be especially cool. Even January, which was rather dry overall, had flooding in mid-month.
You have probably noticed that the Midwest and East Coast have been having a rough and very snowy mid-winter, even as we have our dry period. Again, this is not surprising. Consider this:
A “ridge” is an elongated area of generally high air pressure, while a “trough” is an area of low pressure. Generally, a ridge is associated with dry (and often warm) weather, while a trough tends to produce stormy, and often wet and cool, weather.
If you had a weather map of the US tracing lines of equal air pressure, you would see a pattern that looked like waves. The crests, or high points, are ridges; the low points are the troughs. That pattern would continue around the world, with four or five ridges separated by four or five troughs. If these are evenly spaced, that means that if a ridge is over the western US, a trough is affecting the eastern half of the country.
And that is exactly what is happening now. While we enjoy dry weather, the eastern half of the country is getting pummeled by very active weather systems. And it’s as if things are stuck in place: “our ridge” for the last two weeks and “their trough” over that same period.
And not just in the Midwest and East. In the Southwest, “Frigid weather in the U.S. Southwest choked natural gas supply on Friday as frozen wells starved pipelines. Analysts estimated that between 3 billion and 5 billion cubic feet per day of natural gas supply was shut in due to the weather over the past couple of days.” (Reuters, Feb. 24, 2011)
In Dallas, Texas: “An overnight snowstorm brought air and ground traffic to a near standstill in the Dallas area Friday, frustrating thousands of fans trying to get into town for Sunday’s Super Bowl.” (Washington Post, Feb. 4, 2011)
Farther north, in places that EXPECT cold winter weather, it has been brutal. In Chicago, “Snowmageddon 2011 has brought much of Chicago’s business to a standstill.” (BusinessWire.com, Feb. 4, 2011). “Snowmaggedon” even had unusual “thundersnow” (thunder is rare during big snowstorms).
Cleveland’s WKYC said this: “The month of January 2011 will go down as the second month in a row where temperatures were below normal across northeast Ohio, unlike much of 2010. The month of January was 2.6 degrees colder than normal with a daily average temperature of 23.1 degrees. In comparison, during an El Niño spell when ocean waters are warmer, January 2010 was 1.4 degrees above average.”
Then they said “A stronger La Niña pattern in the Pacific Ocean is partly to explain on how the tide of fortune has changed in our weather since November, according to meteorologists who study climate change.”
Exactly! Just as La Niña is responsible for our generally wet winter, so it has brought extreme cold and plenty of snow to places like Chicago and Cleveland. And last winter was mild and relatively snow-free in the Midwest, and dry here, due to El Niño.
Now let’s look ahead. The Climate Prediction Center’s forecast for the Northwest for February-April calls for cooler than average temperatures and average precipitation (see below). Mine is different: above-average temperatures and below-average precipitation. A far cry from last year’s cool-wet spring.
In a few months we’ll know who is right!