Monday, October 2, 2017

Going Out On A Limb - Predicting The Weather

Last night I turned up the thermostat for the first time this fall.  In another week or so I will light the stove and it will burn until April.

The question now is, how cold will this winter be, and how much wood will I need? 

Predicting the weather over a long period has always been tricky, since there are so many variables that change from hour to hour.  Climate change makes it more difficult as the rules are now changing. 

I checked the reports from NASA and NOAA, the two government agencies that study the weather, to see what trends they are seeing.

One indicator is the ENSO, the El Niño-Southern Oscillation.  Normally the winds in the tropics blow from east to west – the easterly trade winds.  This pushes tropical warm ocean water away from the west coast of the US, allowing cooler water from the ocean bottom to well up along the coast.  The result is the cool, dry, weather and the great fishing of southern California.  (It also blows hurricanes from Africa to North America.)  

If during an El Niño the winds slow down, the warmer water sloshes back towards California.  This warmth and resulting low pressure will pull the jet stream southward, giving wetter weather in the south and drier weather in the north.

If the trade winds speed up and push the warm ocean water further from North America, a phenomenon called La Niña, the opposite occurs.  The jet stream will be pushed up by the high pressure over the colder water, bringing wetter, warmer weather up to the north.

Currently it appears that the El Niño winds are slowing down, but with a only small chance of a full La Niña (stronger trade winds) developing.  NOAA thinks that there is only a 62 percent chance of La Niña development during November-January 2017-18. The official outlook indicates a likely return to ENSO neutral conditions by next spring. Based on that, there are some long-range predictions that might be made.  

With a slowing of the El Niño winds, the average temperatures in the US could be slightly warmer than normal.  The precipitation (as of today) is expected to be normal.  Unfortunately that combination could mean more sleet and ice.  

Remember that ENSO is only a part of the prediction, but as of right now it looks like pretty much like slightly warmer and slightly wetter than normal this winter in New England.

Then there is the polar vortex.  There are several high-speed winds that circle the earth;  you have probably heard of the jet stream.  These upper atmosphere winds are caused by the difference in temperatures between the equator and the poles. 

Normally the polar vortex traps the coldest air at the north and south poles.  But (as formally denied by the best politicians that money can buy) as the earth’s atmosphere heats up, and the poles are warming at a faster rate than the equator, there is less and less difference in temperatures and the polar vortex weakens.  The cold arctic air is not being contained at the dark north pole as the vortex slows down.   It comes down to visit us. 

As of right now there are no specific warnings about the polar vortex for this winter, so my prediction remains at very slightly warmer and wetter than average for New England.

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