The Extratropical Transition of Hurricane Debby (1982) and the Subsequent Development of an Intense Windstorm over Finland

[knocker]

On 22 September 1982, an intense windstorm caused considerable damage in northern Finland. Local forecasters noted that this windstorm potentially was related to Hurricane Debby, a category four hurricane that occurred just five days earlier. Due to the unique nature of the event and lack of prior research, our aim is to document the synoptic sequence of events related to this storm using ERA-Interim reanalysis data, best track data and output from Open IFS simulations. During extra tropical transition, the outflow from Debby resulted in ridge building and an acceleration of the jet. Debby did not re-intensify immediately in the mid-latitudes despite the presence of an upper-level trough. Instead, ex-Debby propagated rapidly across the Atlantic as adiabatic Rossby-wave like feature. Simultaneously, an upper-level trough approached from the northeast and once ex-Debby moved ahead of this feature near the UK, rapid re-intensification began. All Open IFS forecasts diverged from reanalysis after only two days indicating intrinsic low predictability and strong sensitivities. Phasing between Hurricane Debby and the weak trough,and phasing of the upper and lower-level potential vorticity anomalies near the UK was important in the evolution of ex-Debby. In the only Open IFS simulation to correctly capture the phasing over the UK, stronger wind gusts were simulated over northern Finland than in any other simulation. Turbulent mixing behind the cold front, and convectively driven downdrafts in the warm sector, enhanced the wind gusts over Finland. To further improve understanding of this case, we suggest conducting research using an ensemble approach.

debby.pdf

https://journals.ametsoc.org/doi/abs/10.1175/MWR-D-19-0035.1

Edited November 5, 2019 by knocker

[Severe Blizzard]

I remember the passage of this depression over southern England on Tuesday 21st September 1982. Having reached 26 deg.C 3 days earlier in a period of lovely anticyclonic warm autumn weather, this was quite a shock to the system. Like this autumn, the ex-Debby depression was the beginning of a long wet autumn, starting around 20th/21st September, with plenty of interesting weather including several large rainfall totals (eg. 2nd October, 27 mm; 22nd October, 30 mm or so), violent / dramatic cold fronts and a memorable gale on Friday 12th November associated with a very active cold front. 1982 was a more boisterous and wetter autumn than 2019 but the pattern is similar. Stormy weather continued in December with a gale on the 9th uprooting a tree resulting in a train getting de-railed near Fleet, Hants. 

I remember that following the passage of the ex-Debby depression on the 21st, where there had been strong and gusty winds in the morning, 9 mm rain including another active cold front around 11 am, the afternoon was quieter with a light northerly wind and sunny periods. The low was quite shallow, 1000 mb, when passing over the south steered by a larger low over or near to Scotland.   A cold night followed with a minimum temperature of 2 deg.C. on Wednesday 22nd.