First snow #2

The polar vortex is finally starting to boil.  No, not our way … towards South America:



This is “the famous spag plot“.  It shows a US weather model prediction of the state of the southern polar circulation for later this week.  Simply put, we need at least the main spaghetti of red lines — preferably the blue lines — to sweep our alps for us to get snow.  There’s no sign of that in the current model runs (out to ~2 weeks), but given what’s shaping as our warmest May on record, it’s comforting to see that at least there’s activity somewhere.



The plot shows contours of geopotential height from the US National Weather Service’s GFS model.  The contours are of the height above sea level at which 500 mb (500 hPa) atmospheric pressure occurs.  That’s about halfway through the atmosphere by mass (average sea level pressure is ~1000 hPa and the pressure at the edge of space is zero).  Geopotential height reflects two main effects:

  • The prevailing pressure of the circulation pattern.  If a low pressure zone is passing (surface or upper or both), the geopotential height will be lower.
  • The average temperature of the atmosphere below.  The atmosphere is effectively a free column of gas bound by gravity.  It expands and contracts with temperature according to the universal gas law, so even without circulation, geopotential height would rise and fall with the average temperature of the gas column below.

The plot shows contours of two elevations: 5820 m in red and 5700 m in blue.  Both are heights to 500 hPa; the blue ones are closer to the pole than the red because it’s colder there. The green lines show the average positions of the two contour levels for the time of year. (The contour elevations chosen to make this plot are adjusted through the year to keep the average positions roughly constant as the atmosphere warms and cools with the annual cycle.)

There are lots of lines at each contour elevation because this is a plot of an ensemble of model runs.  The model has been run 42 separate times: 21 starting at 00z (midnight universal time) and another 21 starting at 12z (midday). The 21 runs comprise one using a best estimate starting condition (the “control runs”, plotted in brown for 00z and grey for 12z), plus another 20 starting from very slightly differing initial conditions.  This approach gives an indication of the chaotic nature of weather systems.  It’s particularly useful in long range forecasting.


The broad state of the polar 500 hPa circulation used to be referred to as the “LWT” (long wave trough), a concept which maybe shouldn’t quite die.  Basically if you filter the isobars at the 500 hPa level to show just the very broad scale pattern, it’s often possible to discern a multi-lobed arrangement that rotates very slowly clockwise around the pole, taking about 4 weeks to get right around (if it does at all).

So if you buy that theory, the action in Chile later this week might propagate to hereabouts around the week ending 14 June.  Maybe.

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