Why do deep Lows favour moving to NW of UK?

[Jo Farrow]

From Nigel L on Facebook

What is it that makes the deep low nearly always sweep up to the left of the UK instead of going straight over the top of us ??

[Carl46Wrexham]

Cant answer that question, but the deep lows in February 2014 certainly did not track to the North West, they plowed straight into the UK. Here is just one example :

 

[Lettucing Gutted]

Deep lows usually sweep or stall to the Northwest of the UK because of the steep temperature gradient which exists between the very cold Labrador Current and the frigid Greenland Ice sheet and the warm North Atlantic Current.  This is reinforced by the deep Cold Air Advection within the prevailing frigid Polar Vortex/Trough over Greenland and the deep Warm Air Advection within the warm upper level ridges associated with the Azores High and/or Bartlett High.  This temperature gradient extends throughout the troposphere and often extends through the GIN Corridor aka "Greenland Iceland Norway Corridor" in association with a powerful upper level southwesterly Jetstream.

 

 

Cant answer that question, but the deep lows in February 2014 certainly did not track to the North West, they ploughed straight into the UK.

The deep lows in winter 2013-2014 ploughed through the UK due to the main southwesterly Jetstream being displaced further south than usual due to the upper level Greenland Polar Vortex/Trough and associated deep Cold Air Advection extending all the way down the eastern seaboard of Canada and the U.S.

Edited January 5, 2015 by Craig Evans

[Jo Farrow]

Cant answer that question, but the deep lows in February 2014 certainly did not track to the North West, they plowed straight into the UK. Here is just one example :

 

Nigel did say 'nearly always'

[johnholmes]

I will try and give my ideas on this when I get time. Not sure how long that may be. Nice to see a topic like this being discussed.

Perhaps the best source might be what may be, not sure, found on UK Met web site?

jh

[Jo Farrow]

Lots to think about, the balance of forces acting, coriolis (with the turn of the earth) Friction over the open waters of the Atlantic, 

This question's interest relates to deep lows, the RACY rapidly deepening ones which cause stormy weather for the UK, mostly in NW UK but occasionally trauma-ing the south. So have to look at the jetstream patterns. Left exit development of the low , the temperature gradient that Craig mentioned, adding to the Oomph in the jet. 

What else?

[johnholmes]

Picking up on what Jo commented about, and the following may help, not my words but those of an ex colleague on this site; it has lots of interesting and accurate explanations of things in meteorology

http://weatherfaqs.org.uk/node/144

 

What is the significance of the position of the jet stream?

The position of the PFJ governs the overall weather type at all seasons of the year. If it sweeps from SW to NE between Iceland and Scotland, then the BI/NW maritime Europe is generally mild/windy with occasional warm anticyclonic periods; If it comes due west to east around 55N, then a stormy run of weather, with frequent, but well distributed rainfall; if it comes 50 N or below, then generally a cold or cool type, with storms running along the Channel or over northern France etc., with wintry/or cold weather to the north. Further, the wave patterns in the upper westerlies, in very large measure, govern weather events on a variety of time scales ranging from a day to weeks, or perhaps even months.

Why does the jet stream change?

The position/orientation of the jet stream, and its strength/continuity is governed by such things as the distribution of Pacific SST anomalies; Pacific/North American temperature contrasts; injection of energy from active tropical disturbances etc. (These are deeper matters which are inappropriate for this article.) Why the PFJ in some years runs well to the north (mild winters/potentially very warm summers) or well to the south (cold winters/unsettled summers) or becomes blocked is still a subject of debate. My own opinion is that a primary trigger must be Pacific SST anomalies allied to the strength/depth of the Polar vortex, which in turn is coupled to the Stratospheric flow and the switch from summer to winter circulation type. If you talk to someone else, you will get other answers.

 

from jh

Much of the answer lies in the large differences between north and south. Think of how warm you find it all year round in the Caribbean and how cold it is in winter over much of North America. This occurs both at the surface and high up in the Troposphere, that bit we all live in. This temperature contrast is at its greatest in winter, hence the jet stream is often at its strongest. The flow is governed in its direction by the differences between land and sea temperatures, by mountain chains, especially those lying north-south. Sometimes it runs pretty much west to east, at other times it has a more sinuous pattern, the long term wave patterns or Rossby waves. Minor changes in the upper flow also occur. Any of these can develop a surface low pressure system and the overall pattern is usually between Iceland and Scotland, two mountain ranges probably help create this ‘preferred’ path, those in Iceland and those in northern Scotland tending to funnel weather systems between them. Of course at times other factors overrule this general pattern. Intense cold air moving south and meeting very warm air to the south will intensify (increase the speed) of the jet, then minor upper troughs developing in this flow may generate rapidly deepening surface low pressure systems. Due to a number of things including Coriolis (see the link above for an explanation) and entry and exit regions of the jet stream itself these can deepen very rapidly, sometimes now referred to as ‘bombs’. Sometimes these low form far enough south in the North Atlantic that, in spite of generally turning to the left of their track as they deepen, the upper air pattern is such that they move more into the UK sometimes even they are forced south east from their development area around Iceland and become the ‘trigger low’ you here folk on here talk about. This is because, in certain circumstances, they allow a large rise of pressure to develop behind them in the Iceland/Greenland area with deep cold air and winds from a northerly point flooding south over the UK. This ‘block’ in the upper atmosphere then tends to prevent further Atlantic systems travelling on their preferred track for a few days, sometimes longer.

 

Enough for now I am sure others will add to this and may well find some errors of mine, so please feel free to correct any you find. 

jh

Edited January 5, 2015 by johnholmes

[chionomaniac]

Lots to think about, the balance of forces acting, coriolis (with the turn of the earth) Friction over the open waters of the Atlantic, 

This question's interest relates to deep lows, the RACY rapidly deepening ones which cause stormy weather for the UK, mostly in NW UK but occasionally trauma-ing the south. So have to look at the jetstream patterns. Left exit development of the low , the temperature gradient that Craig mentioned, adding to the Oomph in the jet. 

What else?

Don't forget strat vortex positioning and strength, Jo!! The stronger the strat vortex the greater influence on the AO and the pull of the jest stream northwards. Also upstream and downstream ridge positioning linked to the Continents - this leaves the predominant jet stream track across the Atlantic SW to NE - you should be able to demonstrate this with PSD map room composites 

http://www.esrl.noaa.gov/psd/data/histdata/

[Jo Farrow]

Don't forget strat vortex positioning and strength, Jo!! The stronger the strat vortex the greater influence on the AO and the pull of the jest stream northwards. Also upstream and downstream ridge positioning linked to the Continents - this leaves the predominant jet stream track across the Atlantic SW to NE - you should be able to demonstrate this with PSD map room composites 

http://www.esrl.noaa.gov/psd/data/histdata/

Loving it, we're going up and up in the atmosphere. A one line question from Nigel and so many facets to an answer

[knocker]

My six Ps worth. Does not the position of the Atlantic Polar Front have a role to play here. The following from Atmosphere, Weather and Climate, Barry and Chorley.

 

The major zones of frontal wave development are naturally those areas which are  most frequently baroclinic as a result of airstream confluence This is the case, for instance, off East Asia and eastern North America, especially in winter, when there is a sharp temperature gradient between the snow-covered land and warm offshore currents. These zones are referred to as the Pacific Polar and Atlantic Polar Fronts, respectively (Figure 7.20). Their position is quite variable, but they show a general equatorward shift in winter, when the Atlantic Frontal Zone may extend into the Gulf of Mexico. In this area, there is convergence of air masses of different stability between adjacent subtropical high-pressure cells. Depressions developing here  commonly move north-eastwards, sometimes following or amalgamating with others of the northern part of the Polar Front proper or of the Canadian Arctic Front. Frontal frequency remains high across the North Atlantic, but it decreases eastwards in the North Pacific, perhaps owing to a less pronounced gradient of sea-surface temperature.

Edited January 5, 2015 by knocker

[knocker]

Actually on reflection the title of fhe thread can be a little misleading. Shallow depressions are mainly steered by the direction of the thermal wind in the warm sector and hence their path closely follows that of the upper jet stream. Deep depressions may greatly distort the thermal pattern, however, as a result of the northward transport of warm air and the southward transport of cold air. In such cases, the depression usually becomes slow moving. I think we have seen a bit of that of late. I could of course be completely wrong.

Edited January 5, 2015 by knocker

[Su Campu]

Depressions will initially track parallel to the isobars in the warm sector, changing to parallel to the isobars just ahead of the warm front as the occlusion process takes place. This means a curve to the left (north), away from us.

The Rosenbloom Rule states that rapidly-developing lows generally track to the left (north) of the model forecast track. Well this used to be the case, but I'm not sure if the latest updates have tackled this.