Storm Frank brought gales and heavy rain and more flooding to the NW of Britain and especially for parts of Ireland. It was perhaps fortunate that the storm did not cross directly over the UK but swept north towards Iceland and the Pole. The northward track was due to a developing blocking high of 1047mb over Scandinavia which directed the jetstream and attendant depression north.
The “cause” of Storm Frank was an increasing temperature gradient over the Atlantic which increased the power of the jetstream.
Polar air pouring into the Atlantic from a cold Arctic Canada and Greenland met Tropical air moving up from the Gulf. This contrast was increased by a cooler Mid-Atlantic and a warm Gulf Stream creating a large pressure difference.
The ingredients for a perfect storm were created as the mother of a jetstream embraced her baby Frank in the Mid-Atlantic. Infant Frank lay underneath the left-exit of the jetstream and this caused surface pressure to “bomb” extremely rapidly, more than 24mb in 24 hours, creating an unusually low central pressure of 931mb on 30/12/2015. This process is known as bomb-cyclogenesis and occurred with St Jude storm.
— Stephane Gentile (@FrenchScotPilot) December 29, 2015
Cold Arctic air & warm tropical air colliding off Newfoundland, driving powerful jet stream (pink), spawning storms. pic.twitter.com/ewvMW5lcf4
— Liam Dutton (@liamdutton) December 30, 2015
Storm Frank is not a record breaking storm in terms of low pressure. The lowest pressure recorded for a North Atlantic cyclone was 913mb in the Braer Storm in January 1993. This storm skirted closer to NW Scotland.
Sub-930mb storms in the Atlantic are not unprecedented, especially in winter. Several storms have fallen below 930mb in the last 200 years, although only two have been recorded below 920mb.
However, Storm Frank is unusual because, with assistance from the high pressure over Scandinavia, it has squeezed unusually warm air far north into the Arctic causing a “heat wave” over the North Pole.
This process is called warm air advection and it has heated parts of the Pole to between 20 or even 30C above “normal”.
Svalbard, usually around -14C in December, has risen to +8.1C and people in Longyearbyen are revelling in summer clothes outside in the Polar night.
Even the North Pole, usually about -30C in late December, is expected to “melt” for a time even rising above freezing for a short period.
GFS analysis time series of North Pole temps: +0.71°C so above freezing for < 6 hrs ECMWF 12z analysis: -6°C (…) pic.twitter.com/5UPzimWYPd
— Ryan Maue (@RyanMaue) December 30, 2015
This is probably only the second time this has happened on record. It is especially remarkable considering, of course, that there is 24 hour darkness and the sun is not set to rise until March 21! Any warmth is therefore entirely due to warm air advecting north on the back of Storm Frank.
— Simon King (@SimonOKing) December 30, 2015
With the HIGH over Scandinavia pushing cold easterlies into SE Europe Athens and Istanbul are expected to be colder than Longyearbyen in Svalbard. Infact it is snowing in Athens and Istanbul while it is now raining in Longyearbyen on Spitsbergen in the High Arctic.
Unfortunately the weather is set to continue stormy with more depressions arriving through New Year. This time they might even impact the South of UK more with gales and heavy rain expected here too. The temperature is set to cool to average so the anticipated cold lurking out east is not expected to arrive soon. Northern blocking is expected but this does not seem to be leading to any cold break-outs imminently. Nevertheless, the weather is telling some extraordinary stories at the moment.