Yesterday, post-tropical storm Katia visited the UK, much to their chagrin. The highest reported wind gust was 86 mph at Glen Ogle, Scotland with numerous reports of 70-75 mph wind gusts from Ireland through the UK. Katia had a rather unique extratropical transition as the storm was so decoupled that the upper-level and mid-level vort maxes had fully separated from the low-level circulation before the system affected anyone. A strong westerly speed max at 500-600 mb ripped the mid-level circulation away from the storm, while a digging trough at 300 mb pulled high level moisture and the associated vorticity to the north-northwest, well away from the surface circulation. I have documented this structure change using the SEVIRI Airmass RGB product available from NASA SPoRT and CIRA.
In the above image, the approximate location of the surface low associated with Katia is identified with a red “L”. The blue line represents the approximate location of the baroclinic zone or separation (using the Airmass product) of a martime polar (north – blue and some red coloring) and subtropical airmass (south – green coloring). There is some purple/red bias to the left of the storm due to the boundary conditions of the product, therefore this is unrepresentative of the true airmass. Looking closer to Katia’s center, note the dry air (red coloring) which has just about overtaken the mid/upper levels. A strong burst of convection is indicative of this dry air interacting with the moist, tropical tongue just to the right of the blue line. This is a typical extratropical transition as Katia looks like half a storm at this time stamp.
About 11 hours later, the image above shows a fully decoupled post-tropical storm Katia with the mid-level center (black “L”) approximately 450 nautical miles downstream from the approximate location of the surface circulation (red “L”). Again, the blue line denotes the contrast in airmasses with maritime polar air to the north and subtropical air to the south. The black line indicates the approximate location of a negatively tilted trough which is diving towards Katia. The northern portion of the cirrus shield is already being drawn north ahead of that trough, while the jet streak at ~500 mb pulls the mid-level circulation described earlier towards the east.
About 14 hours later, this image shows a rather diffuse looking post-tropical Katia with very little indication that it once was a hurricane. The mid-level circulation (black “L) is now over 570 nautical miles to the east-southeast of the surface circulation (red “L”) which has continued moving to the east-northeast. The blue line still shows the martime polar airmass to the north and a deeper subtropical plume to the south aimed at Ireland. Meanwhile, the upper-level moisture is being pulled north-northwest away from the system as the upper trough (black line) catches up to the surface low. Meanwhile, a new upper-level low is forming near the tail-end of the blue line. At this time, strong west-southwesterly winds are racing across the North Atlantic towards northern Ireland and Scotland.
This final image shows post-tropical storm Katia racing towards the east, making landfall (surface low – red “L”) over the northern UK. As you can see from the image, the upper-level moisture is now strung out in a northwest-southeast band with some low-top convection (white clouds near the red “L”) helping to transport the higher winds to the surface. The blue line shows how the subtropical airmass is being advected more northward ahead of the developing system near 40N, 25W. This is an interested case study as Katia’s energy was seemingly split up at different levels during this ET transition. Had Katia remained a coupled post-tropical storm, it is possible the effects in the UK would have been far worse.
If anyone is interested in a loop of this transition, I can try to post one in the coming days. Thanks for reading!