Extratropical Storm Laura Approaches UK - Oct 4/5th

[martinastro]

This is from Sam Jowet from the Ukweatherworld. Looks like one to watch!...

Time to kick off some detailed discussion about the potential evolution of the extratropical remnants of (currently) subtropical storm Laura I think. This storm is expected to be picked up by the westerlies in a few days time, then driven towards the UK. This storm is currently acquiring tropical charactersitics in terms of DMC and will therefore carry a good deal of warmth and moisture with it. As this cuts through the mid Atlantic ridge on Friday and Saturday it could receive a gulp of cold polar air by virtue of high pressure over Greenland (if this is being correctly modelled). As the baroclinic gradient steepens markedly across the system there will be fuel for strengthening. GFS currently suggests a 100kt jet left exit will run across the top of the system as it crosses Scotland on Sat/Sun 4th/5th October, providing further grounds for expecting cyclogenisis. Given the potent ingredients, if they phase as currently forecast there could be some rather severe weather across the UK on the weekend, including some sting jet potential. GFS currently suggests gusts in excess of 50kts may be possible across much of the UK.

The pattern also looks reminiscent of past instances where we've had powerful squally cold fronts, bringing sharp convection and the risk of tornadoes etc... something to be examined closer to the time.

For reference, here are the relevant parts of the NHC's discussion on Laura. Of interest is the SHIPS model forecasting intensification to a 80kt ETS on approach to the UK. I'm unsure how good this product is at modelling ET depressions though?.

SUBTROPICAL STORM LAURA DISCUSSION NUMBER 2
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL122008
1100 AM EDT MON SEP 29 2008

LAURA WILL LIKELY BE PICKED UP BY THE MID-LATITUDE TROUGH
CURRENTLY OVER THE CANADIAN MARITIMES BY DAY 3 AND THEN ACCELERATE
EASTWARD WITHIN THE MID-LATITUDE FLOW...POSSIBLY APPROACHING THE
BRITISH ISLES AS A POWERFUL EXTRATROPICAL LOW.

MOST OF THE DYNAMICAL GUIDANCE IS
RATHER UNIMPRESSED WITH THE POTENTIAL FOR INTENSIFICATION WHEREAS
THE SHIPS MODEL SHOWS LAURA STRENGTHENING TO 80 KT BY DAY 4 OR
5...PRIMARILY DUE TO A STRONG CONTRIBUTION FROM COLD UPPER LEVEL
TEMPERATURES.

FORECAST POSITIONS AND MAX WINDS

INITIAL 29/1500Z 37.4N 47.8W 50 KT
12HR VT 30/0000Z 38.1N 48.3W 55 KT
24HR VT 30/1200Z 39.8N 48.3W 60 KT
36HR VT 01/0000Z 41.8N 47.7W 60 KT
48HR VT 01/1200Z 44.4N 46.6W 55 KT...EXTRATROPICAL
72HR VT 02/1200Z 51.5N 41.5W 50 KT...EXTRATROPICAL
96HR VT 03/1200Z 55.5N 30.5W 45 KT...EXTRATROPICAL
120HR VT 04/1200Z 55.0N 15.0W 45 KT...EXTRATROPICAL

FORECASTER BERG/PASCH

As ever, the key will be in the phasing of the jet stream, but also how accurate the modelling of the input of cold air to the system is... and we all know how reliable the modelling of air around the Greenland plateau is!

[martinastro]

Here's the projected path of Laura for the weekend. Thanks to Stuart Robinson from the UKWW for the graphic.

[jgs001]

Looks like Scotland is in for a bit of a battering

[Roman White]

Is it a joke?! tropical storm at 55N latitude in October.   

If the forecast is true you must be prepared for some action! 

[martinastro]

It 's true  , and your absolutely correct about the latitude, very unusual. It will probably weaken alot over the next few days over the cold Atlantic. It will still hit the UK soon, somewhere. I'm waiting for updates.

[brianb]

It 's true , and your absolutely correct about the latitude, very unusual. It will probably weaken alot over the next few days over the cold Atlantic. It will still hit the UK soon, somewhere.

And probably very few people will notice - it will look & feel like a normal Atlantic depression, and those are common enough at this time of year.