On 13 January an Atlantic depression began developing in the western Atlantic, before moving quickly eastwards to affect the UK overnight of 14/15 January.
The storm, named 'Rachel' in Europe, brought strong winds and heavy rain across the UK and Ireland, with many places reporting gusts of more than 70 mph and rainfall in excess of 25 mm.
by Ian Mills (EUMETSAT) and Ivan Smiljanic (DHMZ)
This intense cyclone, with the lowest values of central surface pressure of less than 940 hPa, produced strong winds around the occluded area. And strong winds were also observed behind the low centre, in the cold sector, where the cold outbreak occurred from the north-west direction.
The animations, 13 January 12:00 UTC– 15 January 06:00 UTC, show the development of this storm as it moved quickly across the Atlantic.
The airmass RGB sequence (MP4, 10 MB) shows the white cirrus cloud just to the west of Newfoundland.
This frontal cloud mass marks the boundary between the cold air (purple-brown) to the north, and the warm air (blue-green) to the south.
As the sequence progresses the change in colour becomes more apparent north of the white cirrus cloud, indicating a marked temperature change across this frontal system.
At the start of the water vapour sequence (MP4, 4 MB) the cirrus shield can be seen to the east of Newfoundland. By 00:00 UTC on 14 January (Figure 2), as the system moves east, a dark area begins to develop to the north of the frontal cloud.
This dark area becomes darker and more pronounced as the system moves quickly eastwards. This dark slot is an area of very dry air which is being pulled down from the stratosphere and is a signal that rapid development of a depression is taking place at the surface.
The boundary between the dark and light area along this frontal system marks the position of the core of the jet stream which, in this case, was extremely strong, driving the depression very quickly across the Atlantic.
At 02:00 UTC on 15 January the jet stream can be seen stretching from Newfoundland, curving over the Atlantic Ocean and then northeastwards across the southeast tip of Ireland across the border between Scotland and England, before beginning to curve to head south eastwards across the tip of Norway and continuing into Denmark.
The jet stream can also be seen on the infrared animation (MP4, 7 MB). From around 04:00 UTC on 14 January (Figure 3) transverse bands (or fringing) can be seen in the white cirrus cloud along the front.
This pattern is indicative of strong jet streams and the fringing indicates areas of turbulence, usually severe. This pattern continues for some time but is most prominent at 11:00 UTC.
Later, at 17:00 UTC on 14 January, fingers of cloud can be seen extending southwards from the hook of cloud to the west of Ireland. This pattern is indicative of a sting jet where very strong gusty winds extend down to the surface from aloft.
Model v Reality
Looking into the 10 m wind fields of 12-hour ECMWF model forecast and comparing it to ASCAT instrument readings (Figure 4) it is obvious that the model data did a good representation of the dynamic in the lower atmosphere around the low pressure centre.
The NWP 10 m wind field is overlaid on the SEVIRI 6.2µm water vapour channel, which shows the black stripe of dry intrusion following the contours of the cold front and the occlusion spiral.
This area can also be seen in the water vapour loop above. The brightest areas represent the high level moisture content, i.e. high reaching clouds that are forming cloud bands along the baroclinic zones.
The high resolution visible (HRV) image, 14 January 14:30 UTC (Figure 5), reveals the cloud streets and open and closed convective cells that were present in the cold sector, behind the system.
Strongest winds overnight 14–15 January 2015 (Met Office News Blog)
Intense Storm Battered U.K. with High Winds, Heavy Rain, and Snow (The Weather Channel)