There is a new website for the FAQs of the Usenet's UK.sci.weather
group. I have just been trawling it and came upon this, which I
thought interesting enough to sha

What is a 'Polar low'?
Submitted by uk.sci.weather on Wed, 17/01/2007 - 9:56pm.

When Arctic-origin air in winter flows southward (northward in the
southern hemisphere) across (relatively) warmer seas, strong surface
heating acts both to enhance the degree of instability, and trigger
vigorous moist convective towers. This is sufficient alone to give
rise to heavy, wintry showers/cumulonimbus clusters etc., but often
marked troughing, or even a closed circulation in the isobaric flow is
found; the resultant low-level convergence/positive vorticity
enhancement, plus the localised concentration of the latent heat
energy released, enhances development within the system, and an
intense (but synoptically small) area of rain, hail, sleet or snow &
squally winds can result - a polar low (or polar depression or polar
meso-cyclone in some texts).

The dynamics of such systems are not fully understood, and it is only
with the (recent) arrival of very high-resolution satellite imagery &
sensors in a wide variety of spectral bands that the detail within
such systems can be studied. Even so, for operational meteorologists,
careful monitoring of all available data is required; Geostationary
satellite imagery has a rather course resolution at high latitudes,
and the visible channels are of little use in the winter season. Polar
orbiter passes (which give much higher resolution imagery) may not be
frequent enough to maintain a continuous watch on developments.

Numerical models also have difficulty with such events; they are born
in data-sparse regions, and most schemes 'paramaterize' convection
i.e. models don't explicitly forecast each individual convective
event, but rather indicate the degree of instability expected, its
areal extent etc., and thus have problems going one step further and
turning an area of disorganised (model) convection into an organised
self-sustaining polar low/trough, where upper troughs are not the
primary forcing mechanism. The one remaining Norwegian weather ship,
and a handful of research and fishing vessels may be the only clues to
developments taking place in, for example, the Norwegian Sea.

Polar Lows can develop, and move (in the prevailing flow) with
surprising speed, and lead to considerable dislocation of normal life
in regions directly affected. Preferred locations for genesis are to
the west of large, slow-moving occluded depressions - i.e. those with
a pre- existing rear-flank arctic flow. It may be that the geography
of the regions in question play a significant part in genesis of polar
lows - Dave Wheeler, who I am grateful to for checking much of the
above, suggests that vortices shed by high-arctic island groups (e.g.
Svalbaard) are enhanced by the land mass of Scandinavia (Norway) to
the east and Greenland and its ice shelf to the west.