Before we get to Katie, it’s worth mentioning recent and current weather for Easter Sunday: the first convective day of 2016. A cold pool of polar upper air followed the impressive cold front yesterday which developed a very long squall line.
squall line cold front
cold upper air 500hPa
cold front moving east
Thundery conditions followed the cold front in an unstable Polar airmass with notable CAPE (convective available energy) and negative 2 Lifted Index for the time of year – both indicators of potential thundery heavy shower activity. The limiting factor on multicell thunderstorms locally was lack of wind shear and an anticyclonic jetstream, both subduing anything truly impressive or sustained. Nevertheless, some hail and thunderstorm activity is rife across the UK today in this unstable polar maritime airmass.
Storm Katie is riding a powerful jetstream and deepening rapidly over the Atlantic through today. She is some 600miles away to the SW but approaching and deepening rapidly as she undergoes rapid cyclogenesis (RaCy) falling from 999mb to 975mb: 24mb in 24 hours, qualifying as a “bomb depression”.
Katie emerged out of Canada earlier this week as cold continental air met humid sub-tropical maritime air. She engaged with an active jetstream to be launched across the Atlantic. Storms tend not to move fast and deepen rapidly at the same time, so her rapid progress across the Atlantic will now slow as she now expends more energy lowering pressure on her approach to the UK.
The evolution and track of this storm is similar to St Jude October 2013, but Katie is forecast to be less powerful, due in part to the slacker pressure rise on her departure into the North Sea.** (update: in the event Katie pipped StJude, see below!)
St Jude fax chart October 2013
October 1987 storm fax chart
Katie also has some similarities to the October 1987 but again is significantly less powerful than that rare 200 year event.
The water vapour sat pics below show the evolution of Katie and cirrus cloud associated with the jetstream.
Baroclinic leaf in the Atlantic, jetsream cirrus to the UK
dark slot shows rapid intensification stage as strat air descends into LOW, showers over UK
Storm Katie is forecast to deepen through the SW approaches to below 980mb on an expected track overnight Sunday-Monday from Biscay into the Bristol Channel through to exit around the Wash.
The pressure gradient between the Isle of Wight and the Wash could be as much as 12mb… the highest of our Wight-Wash Oscillations recorded, and greater than the 10mb WWO recorded in StJude, if this comes off.
However, the pressure rise behind Katie is expected to be less rapid and sustained than St Jude, so this critically reduces the potential for highest gusts, sting jets aside.
The highest wind speeds for Storm Katie will be south of the low core in the occluding warm sector in the small hours of Monday morning.
Max gusts of over 80mph are possible in the Channel, 70mph along Channel coasts and 60mph further inland. For Reigate this means a brief episode of gusts possibly exceeding 50mph and more over exposed hills. This could wake people up momentarily as gusts roar through trees and round houses.
Rainfall will be significant too and some hi res models put down >40mm over parts of the North Downs, showing the significance of small hills in generating orographic rainfall.
Rainfall more widely is more likely to be around 20-30mm, enough for local flooding. The MetOffice have issued a yellow warning for Storm Katie. Check the weather impact matrix showing moderate likelihood of moderate impact (low yellow).
Update: Amber warning issued for Storm Katie.
The shipping forecast is less equivocal and shows storm warnings right around the UK associated with Katie and other low pressure systems. Storm Force 10 is possible in the Channel for a while. http://bbc.in/1RtwvyS
Furthermore, evolution so far matching StJude and the Wight-Wash pressure gradient forecast seems to be matching or beating St Jude.
Below is a Meteosat picture (courtesy Dundee sat receiving station) showing Katie moving into the North Sea, deepening again slightly to 972mb,
Meteosat visible photo of Storm Katie moving into North Sea on Monday 28 March
Post-Katie we enter a broadly zonal westerly flow but turning cooler as slack winds swing northerly through the week as LOW pressure moves into North Sea/Scandinavia. Unsettled and showery activity continues through the week but with pressure slowly rising from the west by Thursday. No further significant storms are expected for the time being. Into the start of April, pressure over the Atlantic is expected to rise and progress over the south of the UK into Europe to give a settled period for the start of April.
RGSweather is off to East Iceland, so expect occasional tweets and updates from cooler climes.
Storm Katie: In the Event:
various meteorological impacts show Katie exceed StJude in power but both fall well short of October 1987 Great Storm.
- Max wind gusts Needles 105mph (St Jude 99mph)
- Reigate max wind gust 51mph, widely 50-60mph and 70mph on North Downs (Kenley) Gatwick 59mph
- Redhill aerodrome recorded 70mph gust at 04:50
- Lowest pressure: 971mb (St Jude 2013 976mb; Oct’87 958mb)
- Reigate lowest pressure 976mb (St Jude 980mb; Oct ’87 est
- Reigate rainfall 20mm
- Wight-Wash Oscillation 14mb (St Jude 12mb, Oct ’87 storm 20mb!)
October 1987 storm chart
- Rapid rise in River Mole catchment – no significant flooding but overtopped bankfull stage.
Note, compared with the Oct ’87 storm Katie and St Jude are found wanting. The Wight-Wash Oscillation for the 200 year storm of October 1987 was a whopping 20mb! See the chart below for the exit of the Oct 87 storm through the Wash on 16 October 1987.
Finally, which of these big SE storms are most powerful? Play SE storms TRUMPS to find out..