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3rd-4th October When ex Hurricanes affect the UK -an object lesson in forecasting
The synoptic background to the depression which developed and crossed the
UK overnight on the 3/4th October 2004 is one of the most complicated for some time.
A depression had been forecast to cross the UK on Sunday for some days as picked out by some of the reliable Medium
Range Guidance, GFS - (the USA' s National Centre for Environmental Prediction -or NCEP's- Global Forecast System), the UK
Met Office's (UKMO) Global Model, and the T168 ECWMF (the European forecast centre's model). This had been well forecast,
however all was not straightforward due to the volatile nature of the airmasses involved.
On Friday 1st October, a complex shallow area of low pressure lay to the
south of Newfoundland. This area of disturbed weather contained the remnants of an ex Hurricane 'Jeanne' which had been
pounding Florida the previous weekend. To the south east of the area of low pressure lay Hurricane 'Lisa' with 65kt winds,
this system moving slowly north, gradually becoming entangled in a complex low that lay to the south of Newfoundland. To its north a powerful cross Atlantic jet was present, with cold upper air troughs
moving SE from Greenland. The tricky question was how would 'Lisa' interact with all these other elements and were conditions
in place for intense cyclogenesis to occur over the weekend?
Charts
UKMO (UK Meteorological Office) final charts for 00Z 2nd October showing Hurricane Lisa in the SW corner, near 39N, 12W; 00Z 3rd October showing the remnants of Jeanne developing (1001mb low SW of Ireland and, now,
TS Lisa) and 00Z 4th October showing ex Lisa as a 993mb depression over Dublin and finally at 00Z 5th October as a deep 970 mb low near the Faeroes.
Model guidance during the 24 hours from 00Z Saturday was conflicting, with
several different (but feasible) solutions being generated by the various NWP model guidance. During Sunday, a breakaway depression developed, approaching SW Ireland at 06Z; 'Lisa' had by this time become
extratropical, however the system still contained siginificant amounts of latent heat energy. Heavy rain fell through the
morning across Wales and the South West of the UK. Developments to the
SW were still rather elusive, for instance the 00Z run of the UKMO Global Model developed an intense low in the subsequent
24 hours lying over Northern Scotland, however the 00Z run from NCEP's GFS only produced a relatively shallow wave feature
running NE. This clearly gave forecasters a major 'headache' with respect to the days weather.
During the afternoon and evening major cold upper troughing was approaching
the thermal zone from the NW, water vapour imagery indicating the upper forcing was interacting with the thermal zone
and a slight dry wedge and 'Baroclinic leaf' appeared (often indicative of major deepening that is about to occur -an
elongated pattern with well-defined borders on both sides associated with mid-tropospheric frontogenesis). IR Loops showed a large area of cold cloud tops moving up from the southwest, PVA associated with the trough releasing
the latent heat from 'Lisa' and generating heavy rain.
Most of the ingredients appeared to be in place for an explosive deepening
phase. However, overnight only modest deepening took place, with the centre lying over Northumberland as a 987mb feature by
06Z the next day, Monday (see synoptic chart below). Closer inspection of the upper air analyses indicate that the jet
was not in quite the right place over the depression at the time it crossed the UK for the kind of intense explosive
deepening to occur that had been thought possible.
Swanland, 7 miles to the west of Hull in East Yorkshire got a squally storm
at around 8:15am on Monday 4th October. It had been raining for quite some time in its usual 'drizzly' and miserable fashion
but there was a sudden and awesome downpour and severe winds for about 10 minutes before it calmed and the rain even stopped
(I left the house to get in my car and drive to work following the storm to the east).
The 'line' of
weather was very clear as I drove along the A63 (edge of the River Humber) and extended to the south over in
a SW 'angle' across north Lincolnshire. An aeroplane coming into land at Humberside made final approach only to be 'waved
off' at very low altitude and returned to the 'stack' for another attempt, I presume the squally wind was too much to allow
a landing attempt. I got to see it all because they had decided to open Myton bridge (over the River Hull) so
stopping all traffic in and out of the city and causing a stationary queue of traffic for 35 minutes.
I was out in
the Bahamas this summer when Hurricane Alex developed and we witnessed some serious storms there, the downpour and winds witnessed
were of similar severity, although obviously not over a prolonged period of time as they are in the Caribbean -and also without
the heat and humidity. It was a pretty impressive weather event in my opinion...
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Martin Dennison wrote this about
a scenario from
the event in his
area near the Humber in the
UK Weatherworld forum
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Above- Synoptic map 06Z October 4th 2004
Below-Rainfall amounts (mm) from 18Z-06Z October 4th 2004
Charts courtesy Paul Blight (C)
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Left above- Synoptic map 06Z
October 4th 2004
Left below-Rainfall amounts (mm)
from 18Z-06Z October 4th 2004
Click on them for enlarged charts
Charts courtesy Paul Blight (C)
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Overnight Sunday and into Monday, the depression moved under the right entrance
region of the Atlantic jet (around 130 kts). This area of the jet provides convergence aloft and can provide the mechanism
for some modest strengthening but it is the left exit of the jet that provides the most influential development criteria for
a 'RACY' (rapid cyclogenesis) event to occur. However, the depression did not move under the left exit of the jet until it
crossed into the North Sea, where more rapid deepening did finally occur as it raced north-north east.
Very heavy rain was reported overnight nevertheless; over 40mm at St Bees
Head, with nearly 13mm falling in just one hour. Capel Curig reported a 24 hour total of over 70mm, whilst radar estimates
from Mid Wales indicated upwards of 85mm falling in the same period (the Environment Agencies raingauge data will reveal
more in the months to come).
Another interesting feature of this system was the impressive line convection
which developed in the early morning hours. This line convection was associated with a marked wind veer (SSW to W) and some
very strong gusts. Wainfleet (Lincs) reported a 68mph wind gust, Mumbles (near Swansea, S Wales) and Wittering, Lincs
(well inland) both reported a 65mph wind gust. Damage to trees was reported in some locations. The depression continued to deepen in the hours after leaving the UK, taking severe gales and heavy rain into South
Norway.
In summary then, a very complex and difficult situation with more than one
wave depression containing tropical moisture affecting the UK. Numerical Guidance was mixed, with the models failing to represent
the true nature of Lisa and her vast expanse of tropical Heat and moisture. In the end the 24 hour prognosis from the
NCEP issued at 06Z on Sunday and the Midday GM from Sunday presented forecasters with the most accurate guidance; however
it was a close call. Had the jet been positioned slightly differently with a slightly different speed then a major storm could
well have developed across the UK. A good example of just how far we have come in forecasting, and also how much further there
is still to go.
Paul Blight (c) Oct 2004
Above: Regional radar at 06Z on October 4th (with thanks to Paul Blight).
A thread discussing this storm is available on the UK weather forum 'UKww' just
here.
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