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10-11-2011, 17:52   #1
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A Beginner's Guide To Reading Winter Charts

For the newer members on here, it might be a good idea to give a brief guide to what to look for on the charts as we head into wintertime - a time when many boardsies come back to the feeding ground after 8 months wandering the plains. As people jostle for position around the trough (pun intended!), tempers can flare up, and some can get chased way, back to other grounds in such places as Coleraine.....

Firstly, there are several computer forecast models run by various agencies throughout the world. These models take in observational data from surface stations, ships, buoys, weather balloons, satellite and other sources, and run complex physical equations to determine the state of the atmosphere at various intervals into the future. Some models only deal with a limited area of the globe for a couple of days into the future (eg. HiRLAM (European Hi-Res Limited Area Model, NAE, RUC, etc), while other global models deal with the whole globe, for periods of several days to over two weeks (eg. ECMWF (European), GFS (USA), UK Met, GEM (Canadian), BOM (Australian), JMA (Japanese), etc.

Different models have different amounts of freely-available data to the public (eg. ECMWF - widely regarded as the best overall perfomer, run twice a day, but few freely-available data), while others have more. The GFS has the best array of data available, on a wide number of atmospheric variables, and is hence one of the more popular ones used on places like boards. It also runs four times a day, out to 384 hours (16 days), and so it the one used in the Fantasy Island thread.....

Here is one good source of models most people here use.....

When determining the potential for cold weather, there are some key variables to look at. The first is the "upper chart" (500 or 300 hPa), which gives an outline of the broad global pattern of upper waves in the jet stream (at around 5.5 km and 9 km, respectively). As pressure falls with height, each pressure will be found at a certain height (geopotential) as you go up in the atmosphere. In general, the main "pressure levels" are 1000 hPa (near gound level), 850 hPa (around 1450 m), 700 hPa (around 3 km), 500 hPa (around 5.5 km) and 300 hPa (around 9 km). The exact height of these pressure levels (geopotential) is a function of the temperature of the atmosphere at that time - the warmer the atmosphere, the higher the pressure level will be, as warm air is less dense than cold air.

An area of low geopotential corresponds to a cool airmass and is called a trough or depression, while and area of high geopotential corresponds to warmer airmass and is called a ridge or high. These areas of low and high geopotential form the familiar Rossby wave pattern in the atmospheric circulation around the globe. These upper features are a driving force for systems at lower levels, including surface storms, fronts, etc.

Linked to these pressure levels are corresponding temperatures. Cold upper temperatures makes the atmosphere unstable, meaning warmer, less dense, air from below finds it easier to rise to great heights, forming showers and thunderstorms. Warmer air at upper levels puts a "cap" on such convection, and usually leads to more stable weather, with layered clouds. For snow lovers, a favourite chart is the 850 hPa one, which gives geopotential (in decameters, dm) and temperature of that level. To get snow at sea level, we generally look for 850 hPa temperatures (T850) of at least -8 °C, but it can still snow at -4 °C if conditions are right in the lowest layer of the atmophere (boundary layer).

Another important chart is the 500 - 1000 hPa thickness chart, which shows the thickness of the layer of atmosphere between the 1000 and 500 hPa pressure levels. As described above, the colder the airmass, the denser the air, and hence the closer these two levels will be. For snow in Ireland, usually we need somewhere around 528 dm or lower, as this will ensure that the airmass is probably cold enough at all levels for snow to survive the descent to the surface.

Of course, we need to also factor in conditions at the surface. Snow can fall with surface temperatures of below around 4 °C, provided the wet-bulb temperature is at zero or below. We look at the dewpoint readings to figure this out, as the wet-bulb temperature is usually around one third of the way between the dewpoint and dry-bulb temperature. So if the dry-bulb temperature is +3 °C, and dewpoint is -2 °C, the wet-bulb temperature will be around zero or just below, and snow is pretty likely.


All too many times, conditions in Ireland can be "marginal", with a very fine line between rain and snow. That's when the "lamppost watching" takes place, with people desperately watching for those sleety drops to turn fluffier and fall slower as snowflakes......then they come on here and proclaim to the world that they have found the white gold!

Last edited by Su Campu; 10-11-2011 at 20:25.
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10-11-2011, 18:06   #2
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Thanks very much for that Su

I can make a little more sense of them now

Hoping to see good ones predicting plenty of snow and the likes over the next few months
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10-11-2011, 18:33   #3
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Thanks very much Su - will use this as reference in future.
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10-11-2011, 18:37   #4
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Pin this for winter i say.
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10-11-2011, 18:39   #5
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Thanks, that was a great idea. I been lurking around the weather forum for ages, and still couldnt quite get the charts. this I can study

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10-11-2011, 18:39   #6
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Well done Su. Don't know why this wasn't done two years ago.

Thread needs to be stickied for all the new people this winter.
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10-11-2011, 18:51   #7
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Another thing I should have mentioned is Lake Effect snow. This gets its name from the Great Lakes in the US, but we are affected by the same phenomenon here too.

As a cold airmass moves over a relatively warm sea, the sea warms the lowest layers of air, which then move up through the atmosphere to produce showers, as I described above. If the difference between the water temperature and the T850 is 13 °C or more, and wind does not vary in direction by more than 60 ° between the surface and 700 hPa level, then heavy bands of showers can form, and if the airmass is cold enough (so T850 around -8 °C or lower, then these showers will most likely fall as snow.

This is what happened last winter, especially in the run up to Christmas, as countless flights were cancelled at Dublin Airport, with "streamers" of lake effect snow showers continuously tearing in off the Irish Sea.

Last edited by Su Campu; 10-11-2011 at 20:05.
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10-11-2011, 20:10   #8
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wow SU your wealth of knowledge has me stunned

all i know is warm front mild and cold front cold

clouds rain blue sun.

well i know a bit more but jaysiz it all makes perfect sense now
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11-11-2011, 00:44   #9
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Here are some juicy historic set-ups for snow (note that the maps are for 00z and sometimes the historic snowstorm would be on the day ending rather than beginning at that midnight, or overlapping for two or more days):

This final set-up gave huge blizzards in southern England, perhaps somebody could research what happened in Ireland:


In general, for heavy snow that is more extensive than just "sea effect" streamers, it is good to see this combination on the chart:

-- 850 mb temps below -5 C (ideally -8 C) ... can tolerate higher values because of next factor

-- surface pressure gradient better than 12 mbs from Dublin to Valentia ideally a closed low near south coast

-- system feeding on long-fetch easterly to north-easterly with typical air mass temps in -5 to -10 C range

Weaker set-ups can still be potent on a meso-scale or local scale but then sea-effect is the primary driver ... these historic cases seem to be a blend of synoptic scale and enhancement from sea-effect.

The best snowfall events have been associated with lows either developing "in situ" over south coastal regions with a drift to either east or southwest, or, travelling lows that move almost due east along 50 to 51 N towards the Channel. Good set-ups for southeast England may include lows in northern France that could head northeast or even north. While similar set-ups are possible for Ireland, with lows moving north from Brittany, the problem is usually too much mild air advection in a southeast flow, but a set-up like that could dump heavy snow on higher terrain even though it could be raining or sleeting near sea level.

Last edited by M.T. Cranium; 11-11-2011 at 01:00. Reason: checking links
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11-11-2011, 00:49   #10
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Can't get any of them links to work.

EDIT Links working perfect now,thank you.

Last edited by Redsunset; 11-11-2011 at 01:44.
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11-11-2011, 01:20   #11
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Meanwhile, here are some cases where heavy snow developed in some parts of Ireland and the UK from polar northwest to northerly flows:

(note: links are now fixed and should be working for you)

If you would like to see the correpsonding 850-mb charts (which have been reconstructed back to the 19th century, of course they didn't actually measure upper air values until about 1942 ... when you have the map on your screen, go to the address link and change the 001 before the year to 002, that will give you map 2 for the same date and that's the 850-mb chart. These have been back-cast from surface data correlations in the older maps. Check out that 20 Jan 1881 chart for its 850-mb values. Juicy. Now if you want to see a sequence, you could go to the source (wetterzentrale) or you could change the map dates, same general idea, boot up the map you want and change the last two numbers which are the date, and the two before them if you need to change the month as with 19781231 then 19790101 but if you change the year you have to go back further in the address and change it twice.

Last edited by M.T. Cranium; 11-11-2011 at 01:29.
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11-11-2011, 01:32   #12
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Severe cold without much snow:

Last edited by M.T. Cranium; 11-11-2011 at 01:34.
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11-11-2011, 03:37   #13
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Great posts also MT, I've been looking at old charts for the last hour!
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11-11-2011, 09:51   #14
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great post guys - you should write a book between you all as I hve yet to find one that explains things a simply with examples.
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11-11-2011, 10:08   #15
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Thank you for this thread. I have been lurking in here since last winter. Now I might be able to make some sense of all the rigamarole. The charts are like double dutch to me
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