A discussion of the synoptic background and
development of the the 7th/8th Jan storm
During Thursday 6th January 2005 an active conveyer belt system
lay associated with frontal system across the North. Heavy orographic rain had been fairly stationary during the day across
Highland and Argyll - perhaps over 50mm fell in some higher locations.
However to the west of Ireland problems were developing that were
going to cause forecasters major headaches.
Models had been oscillating over the previous few days seemingly
unable to decide whether to bring an active or flat wave NE across the UK overnight into Saturday. On the 5/6th Jan there
was good agreement from the operational suite of NWP models for a fairly minor flat wave to run NE. However a few of the Ensemble
members preferred a deeper development. As Thursday progressed this solutuion seemed to be gaining support from the rest of
the operational runs.
Thursdays 12Z GFS, UKMO & French ARPEGE models all supported
a much more active wave running NE across N Ireland, Scotland and towards Norway. The 12Z GFS was the most intense and and
took the low down to around 951mbs by midday on the 8th (as it turned out, not too far from the truth)
A study of the upper air dynamics of the Atlantic revealed a very
complicated and highly sensistive upper pattern that was prone to errors and very difficult to forecast from.
At 12Z on the 6th a minor shortwave was moving SE across the N Atlantic
having emanated from the eastern seaboard of the USA earlier in the day. This shortwave supported a jet streak on its SW flank
and a large area of PVA on its ascending flank. It appears than the shortwave caught up with the elongated trough, creating
a rather larger & sharper upper trough to the west of Ireland. The jet streak (circa 120knts) rounded the base of the
upper trough during the day on the 7th (water vapour Loops confirm this). This happened at the same time as a wave on the
existing cold front was moving NE to the SW of Ireland. The wave interacted with the PVA and a low pressure development was
initiated and subsequent pressure falls began. Throughout the rest of the day on the 7th the development moved NE, this time
under the left exit of the developing jet over Ireland and more rapid deepening ensued. The jet strengthened across the UK
overnight (to around 150Knts) and the downstream trough amplified, becoming markedly confluent on its rear side. The low by
this time was deepening explosively under the diffluence aloft.
Fig 1 (see right) shows the WV image at 12Z on the 7th, the jet
streak had just rounded the base of the upper trough and was about to engage the cold front wave. All ingedients appeared
in place including a sharp thermal contrast across the NE Atlantic.
During the evening of the 7th the low was developing pretty much
as expected. Pressure falls in the 6-7mb /3hrs were occuring over Ireland, and
the centre appeared to be off Belmullet at 21Z at around 988mbs. Overnight it tracked across N Ireland and into S Scotland
(along the southern side of possible projection paths) whilst increasing pressure falls were being reported over eastern parts
and into the N Sea.
Fig 2 (right) shows the WV Image at 22Z on the 7th, by this time
marked cyclogensis was underway, the dry slot and attendent baroclinic leaf showing up in a "classic" shape- they don't
come much better than that with only a few similar occasions over the past ten years seeing such excellent
baroclinic leaf and dry slot presentations. The marked dry intrusion and cooling cloud tops on the baroclinic leaf indicated
marked cyclogenesis was underway with the colder polar airmass beginning to wrap around the depression's western flank.
Fig 3 (right) shows the WV image at 06Z on the 8th, the classic
swirl of cloud leading into the depression centre over the Borders. The back bent occlusion by this time coming across the
Irish Sea into Cumbria with gusts to over 100mph.
A plotted 06Z (on the 8th) analysis showed a rather elongated low
centre stretching from nr NE Scotland (where there appears to be a COG) to off the coast of Berwick upon Tweed, where another
centre appeared, both at around 968mbs. This elongation of the centre resulted in an exceptionally strong pressure gradient
through the North Channel, across N England with the strongest gusts in the strong post occlusion flow.
A gust at the exposed St Bees Head appears to be at the extreme end of
the scale (over 100mph), but does give an powerful indication of the gradient wind being dragged right down to the surface
in a typical sting jet scenario. Gusts elsewhere topped 70-80mph even at low level land sites.
By 12Z on the
8th the deep depression was sat off SW Norway and had dropped to sub 960mbs - Prob around 957mbs, Models were having difficulty
coping with the deepening and were generally a few hrs constantly behind when it comes to its depth. Maximum depth end up
close to 950-954mbs.
WV loops still showed marked forcing associated with the feature
with dry upper tropespheric air extending down to the the surface, ensuring very strong gusts to the SW of the back
bent occlusion.
Huge pressure rises occured during the morning over N England and
Southern Scotland in the strong confluence behind the upper trough (nr 20mbs
in 3 hrs - which is not common). Dunfalbin reported a rise of 19.3mb in the 3 hours up to 10Z, probably giving a return rate
of at least 50 years.
The strongest winds were now out over the North Sea, Oil platforms
reporting gusts to over 100mph and mean winds over Hurricane force 12 strength, Denmark was starting to feel the storms influence
and gusts to 80-100mph developed a few hrs later. The storm then spread its fury across the Baltic, before being progged to
slowly fill over the Baltic States on the 9th.
An excellent job of forecasting the storm was carried out by
the GFS model on Tuesday the 5th on the 18Z cycle. The model was more accurate on that one run than ANY of the subsequent
runs throughout the whole of Wednesday. From 00Z on Thursday all models produced some excellent results, but once again the
UKMO Global Model and the GFS were at the top of their peer groups in producing an excellent forecast. Credit must also go
to the UK Met Office at Exeter for timely warnings and a pretty accurate track and depth of the low.
The 7/8th Jan Storm was certainly a very fierce winter storm, developing
from a shallow wave on a cold front to a 'monster storm' in about 18hrs. A highly developmental North Atlantic upper pattern
produced some explosive cyclongenesis and some of the strongest winds over the UK for a few years. Unfortunately related
damage and destruction is the legacy both in the UK and for our European neighbours.
(C) PJ BLIGHT 2005 for WiseWeather
(Please be sure to e-mail Paul at pblightuk@yahoo.co.uk if you wish to reproduce
this or other articles on WiseWeather or contact him for related weather discussion).
)
 )
)
