Worth emphasising the word anomaly. Anomaly. Anomaly. There was that debacle a few years ago when anomaly charts were misunderstood. We don't want to go there again. 

Originally Posted by: Brian Gaze 

Worth emphasising the word anomaly. Anomaly. Anomaly. There was that debacle a few years ago when anomaly charts were misunderstood. We don't want to go there again. 

Originally Posted by: Brian Gaze 

I'd love to see the synoptics for that outlier/trendsetter on the ECM ensemble at the end. Max of -9C in London on the 18th? 

Originally Posted by: Rob K 

Just as an aside as we wait for the 12s....

Take a look at the upper air temps from the Wetter archive for these two notable snow events for southern England in early December 1981. According to discussions here, this would be far too marginal for lying snow with southerly tracking secondary lows but we know that wasn’t the case. Perhaps the shorter duration of daytime hours (as well as a lack of strong winds) makes lying snow with these 850s much more likely in December than say February? Snow cover and short daylight period led to record low temps at the end of the second week of Dec, day and night, under fairly benign 850s

http://www.wetterzentrale.de/maps/archive/1981/noaa/NOAA_1_1981120806_2.png

http://www.wetterzentrale.de/maps/archive/1981/noaa/NOAA_1_1981121112_2.png

And the 12” of snow in Birmingham on 8th December 1990, with gale force winds fell with 850s of around -5

http://www.wetterzentrale.de/maps/archive/1990/noaa/NOAA_1_1990120806_2.png



Jeff

Originally Posted by: Jeff 

Not much to misunderstand, is there? Anomaly = departure from the norm. Heights higher than normal over Scandinavia, indicating a greater likelihood of blocking in that region (which is indeed what is shown on well over half the ENS members).

Originally Posted by: Rob K 

Not much to misunderstand, is there? Anomaly = departure from the norm. Heights higher than normal over Scandinavia, indicating a greater likelihood of blocking in that region (which is indeed what is shown on well over half the ENS members).

Originally Posted by: Rob K 

Met going for a very mild Thursday down here with a frostless night to follow, then cool but nothing exceptional high of 5C Friday (prob end up higher) followed by warmer Saturday, and a wet Sunday which has temps pushing double figures by then.

WHERE IS THIS ARCTIC COLD SPELL THEN?

Originally Posted by: picturesareme 

I'd love to see the synoptics for that outlier/trendsetter on the ECM ensemble at the end. Max of -9C in London on the 18th? 

Originally Posted by: Rob K 

There is an issue with 850s but I don't think that's it. The issue seems to be that different scenarios have generally very different upper temperature thresholds. As a general guide these are the T850 threasholds for a few different scenarios:

Windless secondary low cold sector: -1C

Ana Warm/Occluded front: -3C

Convective snow Inland: -6C

Fast moving Ana cold front: -6C

Convective snow on coast: -8C

Kata front: -10C

The issue is that people use, rightfully, a benchmark of -6C or -7C for snow showers which works but then assume that its transferable to frontal snow events. It isn't. The vast majority of frontal snow events allow substantially warmer uppers than convective snow. The main reasons for this is humidity and wind strength. Convective snow is often associated with strong winds and a moist environment which means evaporational cooling is virtually non existent especially if you reside near the coast. However for the case of a windless secondary low with heavy precipitation falling into dry air; evaporational cooling is very significant indeed. Evaporation is ridiculously efficient at removing heat from the atmosphere; indeed snow is possible with positive uppers if the precipitation lasts long enough in a windless, increasing humidity, environment.

Originally Posted by: Quantum 

We had a situation where one or two people thought even a marginal positive height anomaly over Scandinavia / Greenland necessarily implied high latitude blocking. It led to quite a lot of misplaced excitement if I recall correctly. 

Originally Posted by: Brian Gaze 

Cherry picking charts from one run of one modle (of which some are nearly 2 weeks away) doesn't really show much.

Originally Posted by: picturesareme 

While we're being nostalgic, here's the conditions which delivered record snow drifts here in Guernsey in 2013 with 850hPA of circa -2/-4!
http://modeles.meteociel.fr/modeles/gfs/archives/gfs-2013031106-1-6.png?6

Originally Posted by: Karl Guille 

While we're being nostalgic, here's the conditions which delivered record snow drifts here in Guernsey in 2013 with 850hPA of circa -2/-4!
http://modeles.meteociel.fr/modeles/gfs/archives/gfs-2013031106-1-6.png?6

Originally Posted by: Karl Guille 

I sympathise with your frustration but there’s nothing to be gained in fretting about it - I should know as I’ve lived in coastal S Hampshire most of my life.

One glimmer of purple sun through a snow-laden sky I can give you is that, under certain special circumstances, we can get feet of drifting powder when other areas look on in envy. It’s rare - but it does happen.

Originally Posted by: nsrobins 

There is an issue with 850s but I don't think that's it. The issue seems to be that different scenarios have generally very different upper temperature thresholds. As a general guide these are the T850 threasholds for a few different scenarios:

Windless secondary low cold sector: -1C

Ana Warm/Occluded front: -3C

Convective snow Inland: -6C

Fast moving Ana cold front: -6C

Convective snow on coast: -8C

Kata front: -10C

The issue is that people use, rightfully, a benchmark of -6C or -7C for snow showers which works but then assume that its transferable to frontal snow events. It isn't. The vast majority of frontal snow events allow substantially warmer uppers than convective snow. The main reasons for this is humidity and wind strength. Convective snow is often associated with strong winds and a moist environment which means evaporational cooling is virtually non existent especially if you reside near the coast. However for the case of a windless secondary low with heavy precipitation falling into dry air; evaporational cooling is very significant indeed. Evaporation is ridiculously efficient at removing heat from the atmosphere; indeed snow is possible with positive uppers if the precipitation lasts long enough in a windless, increasing humidity, environment.

Originally Posted by: Quantum 

Aye 

For all intents and purposes, an anomaly chart could be just showing marginally less zonal winds, from the typical seasonal Zonal milarky.

Originally Posted by: Russwirral 

I sympathise with your frustration but there’s nothing to be gained in fretting about it - I should know as I’ve lived in coastal S Hampshire most of my life.

One glimmer of purple sun through a snow-laden sky I can give you is that, under certain special circumstances, we can get feet of drifting powder when other areas look on in envy. It’s rare - but it does happen.

Originally Posted by: nsrobins 

I have only ever seen 'inches' of snow in Portsmouth (1-4 inches at most, I think) and that was back in the 70s and 80s. I think it was from the 90s that snow became such a rare event for the coast. Of course, just a few miles inland is better - Cowplain, Clanfield, Petersfield etc (even Cosham versus Southsea!).

4-5c by day is about right for Pompey with a typical N'ly. A few charts showing the potential for an E'ly down the road and that will be much better for us. Maybe some of the younger ones can fret a little but I guess the likes of you and me Neil know the score when it comes to Pompey and snow 

Originally Posted by: Nordic Snowman