Wintertime easterlies

[Thundery wintry showers]

During the current zonal spell, I thought I'd share my recent analysis of easterly types with the rest of the forum.

The main considerations are:

1. How much moisture will be picked up off the North Sea. This is determined by a number of factors. Broadly speaking, dominant high pressure, anticyclonic curvature of isobars, short track over the North Sea, limited contrast between air source temperature and sea temperature and very dry airmass origin will reduce the chance of significant moisture being picked up.

2. What happens to the moisture that is picked up off the North Sea. This appears to be largely dependent on the stability of the atmosphere (in general, the greater the temperature decline with height, the less stable). In stable conditions, a region of sinking air develops, and moist air is unable to rise and form cumulonimbus cells, instead spreading out into stratocumulus layers. In unstable conditions, the moist air rises readily to form cumulonimbus cells.

Now for the TWS Categorisation of Easterly Types:

1- stable atmosphere, 2- unstable atmosphere

a- low moisture, b- medium moisture, c- high moisture.

1a: Some patchy stratocumulus picked up off the sea in eastern areas, but most places seeing at least some sunshine. No snow.

1b: Dry and cloudy for most with stratocumulus layers, sunshine confined to western Britain, possible drizzle (or snizzle) and soft hail in east. If cold enough, slight accumulations of snow possible in east.

1c: Cloudy, with stratus and stratocumulus giving patchy moderate drizzle/snizzle in the east, penetrating at times to the west. If cold enough, slight to moderate accumulations of snow are possible, but may aggregate to large amounts in the east if this pattern persists for a number of days.

2a: Sunny, with isolated showers in the east. If cold enough, slight accumulations of snow possible in the east.

2b: Sunshine and showers in the east, scattered showers in the west. If cold enough, significant accumulations possible in the east, slight to moderate accumulations possible in the west.

2c: Prolonged heavy, locally thundery, showers in the east, penetrating to western areas at times, limited sunny intervals. Troughs may sometimes move west off the North Sea bringing long spells of precipitation, and in these circumstances, if cold enough, the whole of Britain may end up with a deep snow cover, eastern areas particularly likely to be heavily hit.

You can get an idea of the atmospheric stability by looking at the 850hPa temperatures, and detecting whether there is a significant "cold pool" aloft, relative to the surface air temperatures. If there is, a type 2 easterly is likely to result.

Finally, some examples of charts producing easterlies fitting the categories I described:

1a: http://www.wetterzentrale.de/archive/ra/20...00120030220.gif

http://www.wetterzentrale.de/archive/ra/20...00220030220.gif

1b: http://www.wetterzentrale.de/archive/ra/20...00120021210.gif

http://www.wetterzentrale.de/archive/ra/20...00220021210.gif

1c: http://www.wetterzentrale.de/archive/ra/19...00119960121.gif

http://www.wetterzentrale.de/archive/ra/19...00219960121.gif

2a: http://www.wetterzentrale.de/archive/ra/19...00119961225.gif

http://www.wetterzentrale.de/archive/ra/19...00219961225.gif

2b: http://www.wetterzentrale.de/archive/ra/19...00119931121.gif

http://www.wetterzentrale.de/archive/ra/19...00219931121.gif

2c: http://www.wetterzentrale.de/archive/ra/19...00119910208.gif

http://www.wetterzentrale.de/archive/ra/19...00219910208.gif

[MAF]

thanks TWS, of course 1C and 2C would be my favoured scenarios

[J10]

I remember the event in 1991, dry snow falling in sub zero daytime temperatures, and 3 days without a thaw, hasn't even come close since. [Note 2c above]

[Gregg Lamb]

I remember the event in 1991, dry snow falling in sub zero daytime temperatures, and 3 days without a thaw, hasn't even come close since. [Note 2c above]

Ahhhh ! Feb 1991. I remember the first snow flake that hit the ground, or should I say, " Bounced off the ground " Tremendous memories from that period.

[Polar Continental]

Thanks TWS,

Think I will settle for 1a if we have to endure the present conditions for much longer, anything for a change.

From 1b onwards, is for the time being completely out of reach, and set-up 2c looks so far out at the moment, it seems almost like a fantasy.

Paul

[Thundery wintry showers]

I've always wondered from an early age as to why individual easterly incursions brought completely different weather, the above analysis stems from my ongoing efforts to try and understand why.

Above all, though, it's to try and avoid bias creeping in when I evaluate potential weather from easterlies shown on the models. I freely admit that, while I still think easterlies are generally over-rated as a snow source, I used to go to the other extreme and under-rated them. For example, I used to assert that January 1987 probably didn't deliver much away from SE England, and Kevin's stats showed that up to be completely false.

And yes, I'd definitely classify last December's easterly as a "2b", though as Tamara said, we were very much on the end of the line.

[D KARLSON]

good post there tws, as you say easterlies can vary so much form not being cold enough or haing too little moisture, i think 1987 january was one of the best with snow and frost for about 6 days, and what amazes me is when theres only the odd flurry at sea level a drive inland into the east of the pennines and youve got about five inches of powdery snow

[Snow Queen one]

I apologise for using this thread to ask this question but it seemed the nearest appropriate one for my question to put on. Has anywhere in europe had any snow this winter so far? (excluding the usual places that normaly have snow whatever the weather at this time of year elsewhere)

[Thundery wintry showers]

Possibly some temporary snow over Scandinavia and Denmark/Holland/Germany during the northerly of 11-12 November, and also there was some in Austria on 1st November, but since midmonth I don't think so.

Certainly off-topic, as both snowfalls occurred from northerlies!

[jethro]

Hi folks, I live in the South West (Wells, Somerset); what kind of synoptics would lead to decent snowcover here? I've been here for 9 years and with the exception of one dumping in early April 2000, the only snow we've had has been a dusting. Is this usual for this area? Comparitively speaking I'm on reasonably high ground for round here 192m asl. Easterlies run out of steam before they get here, as do NW's, everything else delivers rain. Any clues as to what to look for on charts?

[Summer of 95]

Thanks TWS, an excellent guide- it's only relatively recently I've realised that there is so much more at play than bare synoptic charts alone. What still seems to be unpredictable though is how much if any precipitation will reach to central and western parts- last December's easterly was dry except in the far eastern parts, while the similar one of late January 1996 gave a decent snowcover around here.

Also I notice in the guide the recurring phrase "if cold enough"- it seems to me that since 1996/7 (the last time we got a decent cold and snowy easterly here) all too often they haven't been even when they've come off a cold continent- that of December 2002 gave scarcely any frost let alone snow while in that of Feb 2005 even with temps of freezing or just below what snow fell melted in a few hours when it managed to settle at all.

[Nick F]

Re: Easterlies in winter and watch out for: quite large localised snowfalls can occur uninfluenced by fronts due to coastal convergence of low-level airflows. This effect occurred during evening and overnight periods of 11-13 Jan 1987 across E Anglia/SE, early December 1995 in E/SE, 1st Jan 1997 across East/Midlands/SE and more recently 8 Jan 2003 along the Thames Estuary.

Basically what happens is a cold pool (shown by the 1000-500mb thicknesses or 850mb temps) moves W or WSW over the relatively warmer North Sea picking up moisture, and the discontinuity in surface roughness from flat sea to raised land area along North Sea facing coastlines of UK results in upward motion, particularly when the wind approaches at angle, i.e. ENE. As a result of this shallow uplift of air near the surface along the coast, a convergence line/coastal front develops inland and produces more persistant snow over inland areas 20km or so from the sea. On 12/13 Jan 1987, and lesser extent 8 Jan 2003 convection was enhanced downwind of large bay areas when the wind backed ENE or NE, this created the infamous ‘Thames Snow Streamer’ another similar examples in Easterlies are the ‘Wash Snow Streamer’.

There are other examples of heavy snowfall from coastal convergence as well, 7th Dec 1995 the wind changed from a cold E'rly to an equally cold SE’erly and a snowfall giving up to 15cm was reported in the Brighton area due to convergence of air across the Sussex coast. On the 1st Jan 1997, a convergence zone moved inland across Lincs and Norfolk as far WSW as Warwickshire and Staffordshire giving falls between 5-15cm, the same day saw a convergence zone move inland across Kent from the Thames Estuary giving 24cms near Ashford, 14.5cm at Hastings and 12cms at Herstmonceux on the Sussex coast.

Hi folks, I live in the South West (Wells, Somerset); what kind of synoptics would lead to decent snowcover here?

I'd say a potent NNW'erly bringing convective snow showers down off the bristol channel or further North from the Irish Sea, or a rarer beast - the channel low with cold enough just to the North, are your best bet where you are.

[Weather-history]

I'm glad you posted this TWS, too often since 1987, people compare an easterly in the charts with January 1987, thinking that every easterly is like this and when it doesn't turn out like this asking why it was a poor easterly The fact is that easterly was exceptional in terms of its coldness, therefore is the exception rather than the rule.

I think northerlies could also be categorised in a way, not every northerly is potent or snowy.

[Thundery wintry showers]

Northerlies could well be categorised, though I think the categories would have to be more generalised as much of it depends on synoptic-scale features. I'd say that in general, for a straight northerly without trough activity, assessments would vary depending on wind direction:

NNW: showers over N, NE and W Scotland, Ireland, Cheshire/Wirral, most of Wales, Cheshire Gap feed,

N: showers over North and West Wales, N & E Scotland, coastal areas of NE England and East Anglia

NNE: showers generally over E Scotland and NE England, penetrating to SE areas if on a vigorous flow

However, troughs can result in showers being more widespread, and a polar low can generate prolonged snow. Surface heating from the sun can lead to homegrown showers surprisingly early in the season, e.g. I remember late February 1993 being quoted, and then seeing it happen for myself in late February 2004.

Synoptically, I think a 36-48 hour toppler usually delivers little except when troughs, polar lows and/or solar heating comes into play, while a northerly that can sustain upwards of two days is more likely to deliver.

As for whether it's cold enough for snow, with both northerlies and easterlies, there's some good indicators been put up elsewhere on the forum (I can't remember if it was Nick F or John Holmes who put them up).

[BLAST FROM THE PAST]

Very much so Kev, Dec 81 and Jan 87 were exceptional events in the history of weather recording in Britain. Those of old enough are fortunate to have lived through them.

Indeed and the way Jan 87 came about was unusual too, after a very zonal and mild Dec

BFTP

[Stargazer]

Slightly OT, but can anyone recall the synoptics that led to an unusual snowstorm in March 1987 (I think it was around March 10 and remember it was around midweek, but am not completely sure of the date)? A band of snow only a few miles wide zipped overnight from the Bristol Channel area across to the Isle of Wight, dumping about 6 inches of snow on Southampton (I remember having difficulty getting out of the house first thing, and Southampton is usually notably snow free). The temperature was well above freezing throughout, and the snow melted by mid morning. Portsmouth and Bournemouth (20 miles either side) both had minimal snow, but the Isle of Wight apparently had 10 inches plus in places. Any takers?

[Nick F]

As for whether it's cold enough for snow, with both northerlies and easterlies, there's some good indicators been put up elsewhere on the forum (I can't remember if it was Nick F or John Holmes who put them up).

Re:Indicators for whether it'll snow or not:

For snow to fall to sea-level and settle:

1000-500mb thicknesses preferably below 522 gpdam

1000-850mb thicknesses preferably below 1290 gpdam

850 mb temps below -6C

Dps 0C or below preferably -1C and below

Wet-Bulb temps below 2C

Air temps of below 2C

I think just getting sub-zero day maxes across much of the UK during Easterlies has been a struggle in recent years post Jan 1997, it's even more of struggle with Northerlies away from Scotland. The late December '05 Easterly spell was probably the coldest Easterly since Jan 1997, day max temps remained sub-zero towards the end of the brief easterly spell in some areas, though only for one day. Easterly spells of the mid 80s, notably Jan '85, Feb '86, Jan '87 saw more than 7 days or more of sub zero day max temps across wide areas of the UK. Rare these days, and it seems the sources aren't cold enough, or SSTs moderate the air too much now.

[Hellboy]

I remember the 8 Jan 2003 Thames Snowstreamer that you mentioned Nick. We had an inch of snow the day before and it had frozen that night, then from around 7.30am onwards until about 4pm it snowed. It was heavy blizzard type stuff at around 10am. We got 6 inches in total. The heaviest snow was very localised though as places 4/5 miles North-West up the road from me such as Northfleet/Greenhithe/Dartford recieved 8-9 inches, whilst 2 miles East to me at places such as Cuxton/Snodland barely recieved anything. I had a great day in the snow, and a day off of school! It stuck around for about a week as well.

I remember there was one patch of snow in Higham that never recieved any direct sunlight so the snow there stuck around for about 3/4 weeks.

[Weather-history]

Slightly OT, but can anyone recall the synoptics that led to an unusual snowstorm in March 1987 (I think it was around March 10 and remember it was around midweek, but am not completely sure of the date)? A band of snow only a few miles wide zipped overnight from the Bristol Channel area across to the Isle of Wight, dumping about 6 inches of snow on Southampton (I remember having difficulty getting out of the house first thing, and Southampton is usually notably snow free). The temperature was well above freezing throughout, and the snow melted by mid morning. Portsmouth and Bournemouth (20 miles either side) both had minimal snow, but the Isle of Wight apparently had 10 inches plus in places. Any takers?

I think it was 18th/19th March 1987 and it may have been a polar low. The Gold Cup at Cheltenham was delayed by a heavy snow shower.

http://www.wetterzentrale.de/archive/ra/19...00119870318.gif

[Stargazer]

Many thanks for the information, Mr D. I'm sure that anyone who has lived in Southampton for a few years will agree with me that a significant snow event there tends to stick in the memory! I thought the track of the storm indicated that my post would probably be a bit OT, and suspected a NW airstream. What reminded me of this was Jethro's post - events like the one I described seemed to be ideal for snow lovers in Somerset (even if, in this case, only a small area of the county would have seen the full effect).

Edited December 7, 2006 by Stargazer

[Thundery wintry showers]

It's a fair point; I often find that severe cold spells have a habit of starting off with easterlies, followed by high pressure retrogressing from Scandinavia to Greenland and opening the back door up to northerlies and north-westerlies. March 1987 would appear to have been quite a good example. The most recent was earlier this year; an easterly in late Feb '06 was followed by a northerly in early March, then we had another easterly around 16th-20th, followed by another attempt at a northerly.

Of course it can also happen the other way; an initial northerly blast, followed by a rise in pressure to the north, and then easterlies.

[Altohumorous]

I think it was 18th/19th March 1987 and it may have been a polar low. The Gold Cup at Cheltenham was delayed by a heavy snow shower.

http://www.wetterzentrale.de/archive/ra/19...00119870318.gif

I recall what must have been the same period, but my hazy memory has it as March 23/24 (how can I find the charts for those dates?). The forecast barely mentioned a risk of snow, it was a chilly night, fairly calm with a slight air frost, a weak area of low pressure with a cold front crossed from the North West, then intensified and rain turned to snow here during the night, by the following morning there was about 3 inches on the ground. It melted fairly quick but the day was very cold & windy with scattered snow showers.

Regarding the "cold enough to snow" discussion, I've felt for years that cold incursions, while overall rarer, are also "less cold than they used to be in my day", due to the factors ,mentioned already (sources are less cold, sea temps warmer, incursions less penetrating and less persistant etc)

The great snow-storm of end Feb/start March (cant recall exact date) 2001 (2002?) is a good example, it was as good an incursion of Arctic Northerly (originally a plunge from north Greenland) as you could wish for, coupled with a deep low pressure slipping slowly south east through these islands, resulting in about 24 hours of heavy snow here, but it never really stayed on the ground. In previous eras I'm sure it would have been that degree or so colder.

This all begs the depressing question - (OT but on topic regarding northerlies from Greenland), if this warming continues apace (and increasing in pace), how long before all the factors accelerate a combined effect, Greenland gets greener and our weather will be vastly warmer, and so much less interesting, in a short space of time? Will we soon be talking about a westerly gale and a cool high pressure as being events of great excitement?

[BLAST FROM THE PAST]

This

Here

went to

Here

and that is why it was shortlived....sadly

BFTP

Edited December 7, 2006 by BLAST FROM THE PAST

[Thundery wintry showers]

I recall what must have been the same period, but my hazy memory has it as March 23/24 (how can I find the charts for those dates?). The forecast barely mentioned a risk of snow, it was a chilly night, fairly calm with a slight air frost, a weak area of low pressure with a cold front crossed from the North West, then intensified and rain turned to snow here during the night, by the following morning there was about 3 inches on the ground. It melted fairly quick but the day was very cold & windy with scattered snow showers.

Regarding the "cold enough to snow" discussion, I've felt for years that cold incursions, while overall rarer, are also "less cold than they used to be in my day", due to the factors ,mentioned already (sources are less cold, sea temps warmer, incursions less penetrating and less persistant etc)

The great snow-storm of end Feb/start March (cant recall exact date) 2001 (2002?) is a good example, it was as good an incursion of Arctic Northerly (originally a plunge from north Greenland) as you could wish for, coupled with a deep low pressure slipping slowly south east through these islands, resulting in about 24 hours of heavy snow here, but it never really stayed on the ground. In previous eras I'm sure it would have been that degree or so colder.

This all begs the depressing question - (OT but on topic regarding northerlies from Greenland), if this warming continues apace (and increasing in pace), how long before all the factors accelerate a combined effect, Greenland gets greener and our weather will be vastly warmer, and so much less interesting, in a short space of time? Will we soon be talking about a westerly gale and a cool high pressure as being events of great excitement?

You are probably thinking about 27/28 February 2001, when an area of heavy persistent snow moved south-westwards and dumped a fair covering on Belfast. Parts of south-west Scotland had 50-60cm of undrifted snow. In NE England, snow accumulated only inland, though east-coast areas then went on to pick up significant accumulations from the snow showers that followed behind.

As for March 1987, it's unlikely that it was the 23rd-24th; too warm. It could have happened on the 21st though:

http://www.wetterzentrale.de/archive/ra/19...00119870321.gif

[Altohumorous]

You are probably thinking about 27/28 February 2001, when an area of heavy persistent snow moved south-westwards and dumped a fair covering on Belfast. Parts of south-west Scotland had 50-60cm of undrifted snow. In NE England, snow accumulated only inland, though east-coast areas then went on to pick up significant accumulations from the snow showers that followed behind.

As for March 1987, it's unlikely that it was the 23rd-24th; too warm. It could have happened on the 21st though:

http://www.wetterzentrale.de/archive/ra/19...00119870321.gif

Thanks for that, it could easily have been another late March in that era (havent time to search for it right now) or else my dates are confused. The main thing I recall is that it was very much un-forecast by the main Met services.

Regarding the 2001 snowstorm, the LP system moved south/south east, with a very cold northerly & rising pressure from Greenland behind it resulting in a NNE flow wedge across Ireland, Scotland, North & West Britain on that day. In the hillier parts of Co Down & Co Wicklow here the drifts were magnificent