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Tracking the potential Polar Low - Thursday into Friday


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Sadly the snow only skimmed my location but I do have a slight dusting outside. Im not that bothered anyway because last night was very exciting because sometimes the thrill of the chase is more exciting than the actual weather it brings.

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Is it only me, or are the 'is it a/isn't it a Polar low' reamrks just plain old semantics.........as long as I get a dumping of snow, you can call it a lesser spotted chocolate teapot if you want    

Hi all.  Had great fun following this brilliant forum last night.  Just to introduce myself; I was senior forecaster at Manchester Weather Centre for 20 years, now retired.  I think that it was defini

so then....members in the west believe it'll track down the irish sea giving the action in the western regions...... members in the east believe it'll track though northern england and the SE giving s

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It was an ex-polar low as far as I can see. Or an extrapolar low if you prefer. Might have started off as the real Mc Coy, but true polar lows don't have mild sectors and don't drag in warmer air behind them. Like all those ex-hurricanes we get, this one was a bog standard Atlantic low by the time it got to Britain.

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Yesterday I made an analysis of the system near Scotland, saying that this system was in truth a polar low. After a short reanalysis, I think I was not completely correct on this. Quoting from my previous post:

 

Thermal structure

 

Zooming in on this system gives a surprising detail. This can be seen below:

 

attachicon.gifPolar_low_zoom.jpg

Eumetsat satellite imagery of Europe, zoomed in on the system to the northwest of Scotland. (image is from 17:00 UTC)

 

Note that the colours found at the center of the low are more red than the colours surrounding it (which are more purple). In general, purple air indicates colder air at 500 hPa as compared to red. This means that the low pressure system is actually warmer in its core than its surroundings at 500 hPa (about -32*C in its core compared to -34 to -37*C 500 hPa temperatures encompassing the low to the north, east and south).

 

Although not much is known about the thermal structure in general from polar lows, a warm core is one of the characteristics of them. This has to do with the fact that when an air parcel starts to rise in cold conditions (see explanation on stability here), the water vapour in the parcel condenses to form water droplets. This happens especially over sea because there is simply more water available over sea than over land. When condensation occurs, heat is being released, warming the atmosphere surrounding the rising air parcel. This is essentially what causes polar lows to have a warm core. The mechanism is known to be quite similar compared to the strengthening mechanism of hurricanes (tropical cyclones).

 

Summary

 

In short, the relatively warm air at 500 hPa therefore argues that this system is really a polar low nearing the shores of the UK. Exciting to say the least :smile: .

 

For this analysis, the 500 hPa temperatures of Thursday 18 UTC (from the 12UTC GFS run) are given.

 

15012918_2912.gif

500 hPa temperatures (colours) and heights GFS 12Z T6

 

The low pressure system is located where the low 500 hPa heights (contours) edge toward the west over Scotland, indicating low pressure activity.

 

What can be seen is that the low pressure system itself is associated with -35*C 500 hPa temperatures, while much warmer 500 hPa temperatures (up to -31*C) exist to the north of the system. According to Eumetrain, the maximum 500 hPa temperatures to allow for polar low formation are in the vicinity of -40*C. Quoting from their site:

 

 

Temperature at 500 hPa (T500 < -40°C)
Deep convection is indicative of a Polar Low. This deep convection is caused by very cold arctic air overrunning relatively warm sea. At 500 hPa this air more or less keeps its original temperature. From the statistical analysis and the literature a temperature of -40°C or less at 500 hPa (T500=< 40°C) is a good threshold value for the triggering of Polar Low development.

 

With a minimum of -35 to -36*C 500 hPa temperatures found at the center of the system, one could suggest that it does not meet that criterum of a polar low.

 

Conclusion

 

Of course judging from only temperature is rather tricky, but to my eye (in contradiction to what I said earlier) it is doubtful that this system really was a polar low. It could definitely have had some characteristics of one, and it was an exciting system nontheless.

 

Source:

http://www.eumetrain.org/satmanu/CMs/PL/navmenu.php?page=3.0.0

Edited by Vorticity0123
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I cannot really understand the recent lack of snow, the polar low last night bought rain here; melting the dusting of already lying snow, now it's really icy. Last night did not even bring 1cm, not a snowfall in my records.

Is any snow due next week? Cause that Artic blast looks dry and cold.

Edited by Cumulonimbus123
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Yesterday I made an analysis of the system near Scotland, saying that this system was in truth a polar low. After a short reanalysis, I think I was not completely correct on this. Quoting from my previous post:

 

 

For this analysis, the 500 hPa temperatures of Thursday 18 UTC (from the 12UTC GFS run) are given.

 

15012918_2912.gif

500 hPa temperatures (colours) and heights GFS 12Z T6

 

The low pressure system is located where the low 500 hPa heights (contours) edge toward the west over Scotland, indicating low pressure activity.

 

What can be seen is that the low pressure system itself is associated with -35*C 500 hPa temperatures, while much warmer 500 hPa temperatures (up to -31*C) exist to the north of the system. According to Eumetrain, the maximum 500 hPa temperatures to allow for polar low formation are in the vicinity of -40*C. Quoting from their site:

 

 

With a minimum of -35 to -36*C 500 hPa temperatures found at the center of the system, one could suggest that it does not meet that criterum of a polar low.

 

Conclusion

 

Of course judging from only temperature is rather tricky, but to my eye (in contradiction to what I said earlier) it is doubtful that this system really was a polar low. It could definitely have had some characteristics of one, and it was an exciting system nontheless.

 

Source:

http://www.eumetrain.org/satmanu/CMs/PL/navmenu.php?page=3.0.0

I think the polar low formed at around 11am - what were the temperatures then- both upper and lower to see what baroclinicity was involved? In all truth it looks like it started as an eddy from the main vortex situated to the NE. This was barotropic at inception and then became baroclinic as it headed south into a different airmass and hooked onto the jet stream. Hence the rain seen encompassed in the system. When looking at the very first visible satellite images at formation one could see the cluster of convection cells circling a core - similar to a hurricane. Extremely interesting to watch.

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Hi all.  Had great fun following this brilliant forum last night.  Just to introduce myself; I was senior forecaster at Manchester Weather Centre for 20 years, now retired.  I think that it was definitely a polar low. It showed all the characteristics on the sat pics. Also, 'mature' polar lows do acquire frontal characteristics, there are many examples. As circulation develops around the low, warmer air is entrained, with cold air to the north coming down the west side and a sort of warm front/cold front pair develops. Hence the areas of rain last night were in the warm sector, with snow to the north and east of the low - exactly as happens to the north of a low up the channel.  Another thing. Polar lows are not dependent on particular temperatures at 500mb on any other pressure level. They simply form (exact mechanism is up for debate) when sea temps cause instability to sufficient depth and a circulation develops - like hurricanes. Both hurricanes and polar lows need some upper encouragement to start, easterly waves/troughs for hurricanes and I think yesterday's polar low was encouraged to start with the old trough/occlusion extending west from the parent low or by some small PVA (see below).  But it was all unstable to sea temps hence the polar low.  This can happen in any season and I certainly saw them outside winter.  I'm in Macclesfield and we got 7cm snow from it overnight and Manchester airport closed twice (look at the METARS for EGCC) during the night to clear the snow. Another thing. MetO forecasters that produce the 'official analysis' are obsessed with putting fronts on charts along lines of cloud. The Norwegian frontal theory, still used for mslp analyses is still rampant, because it is a useful tool in forecasting , but don't take all those occlusions you see on charts too literally, many of them did not originate by a cold front catching up a warm front.  They simply mark troughs or just lines of cloud from satellite imagery, originating from various mechanisms Positive Vorticity Advection (PVA) being one of them. PVA is a term from fluid dynamics and is an area of differential flow in the atmosphere at any level. Of interest to the forecaster are areas of differential flow at high level (usually 500mb or above). Jet streams are classic examples where a change of wind flow from high speed to low speed, coupled with a change of direction in the flow is an area of vorticity (spin). This changes the pressure in that place (just like flow over an aircraft wing). Positive VA causes lower pressure and Negative VA causes an increase in pressure.  The atmosphere being 3 dimensional (4D if you are picky) then air moves vertically as well as horizontally to try and equate the pressure gradient.  PVA creates lowering pressure beneath it. NVA causes descending air away from the increase in pressure at that level and highs develop along with, usually, clearing cloud.  Anyway I've bored you enough now just though you needed another input.

Thank you, great first post and welcome. Far better explained than I ever could.

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Polar lows are not dependent on particular temperatures at 500mb on any other pressure level. They simply form (exact mechanism is up for debate) when sea temps cause instability to sufficient depth and a circulation develops - like hurricanes. Both hurricanes and polar lows need some upper encouragement to start, easterly waves/troughs for hurricanes and I think yesterday's polar low was encouraged to start with the old trough/occlusion extending west from the parent low or by some small PVA (see below). 

 

Thanks for your detailed analysis and welcome to the forum.

 

I guess the 500mb temp is not so critical then rather, from what I've read, it's the difference between sea surface temperature and T500 that triggers deep enough instability for the convective cells aided by area of high PVA to form a tight circulation of a polar low. Also, from literature, it appears polar lows are characterised by a deep warm core too, from the latent heat release of the deep convection, last night's low appears to have had this too.

Edited by Nick F
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I think the polar low formed at around 11am - what were the temperatures then- both upper and lower to see what baroclinicity was involved? In all truth it looks like it started as an eddy from the main vortex situated to the NE. This was barotropic at inception and then became baroclinic as it headed south into a different airmass and hooked onto the jet stream. Hence the rain seen encompassed in the system. When looking at the very first visible satellite images at formation one could see the cluster of convection cells circling a core - similar to a hurricane. Extremely interesting to watch.

 

The archive of the airmass satellite shows what looks like a baroclinic leaf development at 5am - quite distinctive to the convective mass of showers ahead of it.

 

An archive of the satellite images from yesterday.

 

http://brunnur.vedur.is/myndir/seviri/2015/01/29/

 

Some of you will remember this well, it gives a good run down on the post event analysis.

 

http://onlinelibrary.wiley.com/doi/10.1002/wea.740/full

 

Conclusion - "a reverse-shear polar low." - there are multiple types to confuse the situation even more!

 

seviri_nat_airmass_20150129_0500.png  seviri_nat_wv_high_20150129_0600.png

 

The 500 mb temperature chart for 06Z  

 

 

 

gfs-2015012900-13-6_szw8.png

Edited by Nouska
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Hi all.  Had great fun following this brilliant forum last night.  Just to introduce myself; I was senior forecaster at Manchester Weather Centre for 20 years, now retired.  I think that it was definitely a polar low. It showed all the characteristics on the sat pics. Also, 'mature' polar lows do acquire frontal characteristics, there are many examples. As circulation develops around the low, warmer air is entrained, with cold air to the north coming down the west side and a sort of warm front/cold front pair develops. Hence the areas of rain last night were in the warm sector, with snow to the north and east of the low - exactly as happens to the north of a low up the channel.  Another thing. Polar lows are not dependent on particular temperatures at 500mb on any other pressure level. They simply form (exact mechanism is up for debate) when sea temps cause instability to sufficient depth and a circulation develops - like hurricanes. Both hurricanes and polar lows need some upper encouragement to start, easterly waves/troughs for hurricanes and I think yesterday's polar low was encouraged to start with the old trough/occlusion extending west from the parent low or by some small PVA (see below).  But it was all unstable to sea temps hence the polar low.  This can happen in any season and I certainly saw them outside winter.  I'm in Macclesfield and we got 7cm snow from it overnight and Manchester airport closed twice (look at the METARS for EGCC) during the night to clear the snow. Another thing. MetO forecasters that produce the 'official analysis' are obsessed with putting fronts on charts along lines of cloud. The Norwegian frontal theory, still used for mslp analyses is still rampant, because it is a useful tool in forecasting , but don't take all those occlusions you see on charts too literally, many of them did not originate by a cold front catching up a warm front.  They simply mark troughs or just lines of cloud from satellite imagery, originating from various mechanisms Positive Vorticity Advection (PVA) being one of them. PVA is a term from fluid dynamics and is an area of differential flow in the atmosphere at any level. Of interest to the forecaster are areas of differential flow at high level (usually 500mb or above). Jet streams are classic examples where a change of wind flow from high speed to low speed, coupled with a change of direction in the flow is an area of vorticity (spin). This changes the pressure in that place (just like flow over an aircraft wing). Positive VA causes lower pressure and Negative VA causes an increase in pressure.  The atmosphere being 3 dimensional (4D if you are picky) then air moves vertically as well as horizontally to try and equate the pressure gradient.  PVA creates lowering pressure beneath it. NVA causes descending air away from the increase in pressure at that level and highs develop along with, usually, clearing cloud.  Anyway I've bored you enough now just though you needed another input.

Welcome Icedust and gald to have your input. It was an exciting event

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I cannot really understand the recent lack of snow, the polar low last night bought rain here; melting the dusting of already lying snow, now it's really icy. Last night did not even bring 1cm, not a snowfall in my records.

Is any snow due next week? Cause that Artic blast looks dry and cold.

 

It ironically tracked too far East, even North Kent saw rain over snow because we were on the wrong side of marginal.

 

Yesterday we were all arguing the track, I don't think many people predicted the main band of PPN to be as far East as it was, East of London certainly wasn't expected, some were worried whether or not it would even reach London let alone go the other side of it.

 

Still, may not have seen any snow from it but it sure was fun tracking it, the forum exploded into life!

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The archive of the airmass satellite shows what looks like a baroclinic leaf development at 5am - quite distinctive to the convective mass of showers ahead of it.

 

An archive of the satellite images from yesterday.

 

http://brunnur.vedur.is/myndir/seviri/2015/01/29/

 

Some of you will remember this well, it gives a good run down on the post event analysis.

 

http://onlinelibrary.wiley.com/doi/10.1002/wea.740/full

 

Conclusion - "a reverse-shear polar low." - there are multiple types to confuse the situation even more!

 

seviri_nat_airmass_20150129_0500.png  seviri_nat_wv_high_20150129_0600.png

 

The 500 mb temperature chart for 06Z  

 

 

 

gfs-2015012900-13-6_szw8.png

The evidence of that baroclinic leaf does make me want to think again. Hadn't noticed it before and the low does seem to spawn right out of it.

 

Whoever said meteorology is easy!

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The evidence of that baroclinic leaf does make me want to think again. Hadn't noticed it before and the low does seem to spawn right out of it.

 

Whoever said meteorology is easy!

 

This series of water vapour images in this link show something similar in the Gulf of Alaska with a dry slot becoming entrained into a baroclinic leaf, the motion is west to east in this case rather than the intial east to west last night - http://cimss.ssec.wisc.edu/goes/blog/archives/410

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So in my opinion the low will hit NW England (Lancs) then move SE hitting S Yorks/N Midlands then E Midlands and finally heading off towards Essex.

My prediction last night wasn't far offf and especially pleased with the forecast for Essex.

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So it had warmer uppers north and west of it even miles up at 500mb, not just stuff that got mixed in low down. Maybe we should call it a "subpolar low" because it initially looked like a polar one, but formed in the wrong conditions. Like those "subtropical storms" that at first look like hurricanes but form over water too cool for true hurricanes to develop.

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Thanks all for the in-depth analyses of the system - I have learned more about polar lows today than I ever knew, a great experience to say the least.

 

However, one thing remains a little bit puzzling to me. Taking a slice from airmass satellite imagery of the system as of Thursday 17 UTC:

 

post-20885-0-65071600-1422657780_thumb.j

Airmass satellite imagery as of 17 UTC Thursday.

 

There appears to be a warm core at the center of the low, as the colours in the center are red (warmer air) compared to cooler air (purple) surrounding the polar low. If the temperature signature of a polar low is in general not present at 500 hPa, would it be associated with a warm core of the system developed by condensational heating, or would a different mechanism apply here? (baroclinicity etc.)

Edited by Vorticity0123
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The evidence of that baroclinic leaf does make me want to think again. Hadn't noticed it before and the low does seem to spawn right out of it.

 

Whoever said meteorology is easy!

....its not.

 

It's all down to the Chaos Theory.

 

Ian

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