厂辞…测辞耻鈥檙别 planning a ski trip over the holidays. You鈥檙e probably curious about the snow outlook. Well, the good news is that after a parched summer and fall throughout much of the West, El Ni帽o has brought fresh powder to a number of ski areas in the Rocky Mountains鈥攁nd forecasts predict the precip will keep coming.
An El Ni帽o occurs when sea surface temperatures in the eastern Pacific Ocean near the equator are warmer than normal for several consecutive months. (A La Ni帽a is the opposite of an El Ni帽o, occurring when these听waters are cooler than normal for the same period of time. The standard temperature anomaly used to define an El Ni帽o or La Ni帽a is half a degree Celsius above or below normal, respectively.)
Over time, we鈥檝e come to recognize these oceanic temperature anomalies as an important influence on global weather patterns; it鈥檚 hard to talk about a hurricane season or the potential for a snowy winter without discussing temperatures in the Pacific Ocean.
The latest report, issued听December 13, from NOAA鈥檚 Climate Prediction Center showed of an El Ni帽o forming in the Pacific this winter. Sea surface temperatures in the all-important eastern equator region of the Pacific have been above normal since July and the water has steadily gotten even warmer this fall. We could soon be in a full-fledged El Ni帽o if these trends continue.
El Ni帽o and La Ni帽a can have a significant effect on weather patterns in certain parts of the world. These two phenomena are pretty cool examples of teleconnections鈥攈ow different parts of the atmosphere can influence one another even if they鈥檙e separated by thousands of miles. It doesn鈥檛 seem like water temperatures fluctuating a degree or so either way could have that much of an effect on weather patterns. But just like we鈥檙e seeing with climate change, it doesn鈥檛 take much of a temperature difference over a long period of time to disrupt the atmosphere鈥檚 delicate balance.
It helps to understand how the jet stream forms in order to understand how El Ni帽o affects winter听in the U.S. The jet stream is a fast-flowing stream of air in the upper atmosphere that typically meanders around the cruising altitude of most听aircraft. Almost all of our exciting weather in this country is caused by the orientation and speed of the winds in the jet stream鈥攖hese dips and streaks in the upper-levels lead to rising and sinking air near the surface, which can create everything from intense storms to clear blue skies. 听
The atmosphere is fluid, always churning and bubbling. Some of these disturbances are as small as a neighborhood鈥攔ising air forms a puffy cloud over your house鈥攁nd some can turn into enormous storm systems that sprawl across the continent. We also have extremely large air circulations throughout the atmosphere known as 鈥渃ells.鈥 The cells are roughly oriented north-south and each take up about 30 degrees of latitude. In the northern hemisphere, the jet stream forms from the sinking air in these cells around 30掳N and 60掳N. The southern one is known as the subtropical jet stream, while the northern one听is the polar jet stream.
Most of our weather systems in the U.S. are caused by the polar jet stream, or the one that lingers around southern Canada, occasionally dipping south of the border to create storms that sweep across the country.听But El Ni帽o can disrupt that. The warmer-than-normal sea surface temperatures over the听Pacific add extra heat to the atmosphere, helping to bend and twist that subtropical听jet stream to cause unusually active winter weather across parts of the country.
The from the Climate Prediction Center line up pretty well with the graphics you鈥檇 see in the El Ni帽o chapter of a meteorology textbook. The agency expects warmer-than-normal temperatures to persist across the West, as well as the classic uptick in precipitation across the southern half of the country.听
Forecasters expect a weak El Ni帽o if current trends hold up throughout the rest of the winter. A stronger El Ni帽o has a better chance of affecting weather conditions in the U.S., but even a weak one听can still lead to stormy, wet conditions.听So what does that mean for your ski trip?听When it comes to snow out West, El Ni帽o really only tends to help if you鈥檙e in southern California, the desert Southwest, or the southern Rockies; expect those places to get near- or above-average snowfalls. El Ni帽o patterns typically leave the Pacific Northwest and northern Rockies warmer- and drier-than-normal鈥攏ot completely without snow,听but less than they鈥檇 expect during a normal year.
Whatever happens this winter won鈥檛 solely be the result of El Ni帽o, but the weather pattern听could certainly leave its mark on a number of states in the coming months. Areas in the Northwest won鈥檛 go completely snowless this year, while听many areas in the south听probably won鈥檛 have more snow than they鈥檒l know what to do with. Seasonal snowfall totals depend on lots of factors. Think of El Ni帽o听as听an added bonus.
In short: Go听book a trip to the Rockies, then cross your fingers.听听