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Model Output Discussion 17th Dec.2013-12z onwards.


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Incredibly difficult to forecast the detail with these shortwaves appearing and disappearing between outputs, somewhere could get lucky over Xmas. Still any snow looks transitory especially on lower ground but given the overall NH pattern at least perhaps a little interest which could deliver the odd surprise.

Edited by nick sussex
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steve have we ever seen snow fall at 1300 dam before?

 

yes mate - its not the only parameter to measure-  however 1290 is usually the benchmark -

 

Probability:............................90%.....70%.....50%.....30%.....10%850-1000 hPa(gpm)..............1279.....1287....1293....1297....1302 (un-adjusted)

 

If we could bet 1293 then that's 50/50- obviously the scaling on the NAE would be better at intervals of 5 instead of 10- however at least it shows roughly where the risk is-

Theres also an adjustment for height-  so elevated areas can see snow in higher thicknesses.

 

S

 

S

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I quite enjoyed that run of the GFS. A fairly weak and divided PV, a southerly tracking jet and low pressure over Southern Europe. As far as trends go, that's one I am pretty happy with.

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Also, that Asian stratospheric warming shows up again and is still slowly moving closer in time rather than being kept at bay. One to watch for later on in January methinks

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yes mate - its not the only parameter to measure-  however 1290 is usually the benchmark - Probability:............................90%.....70%.....50%.....30%.....10%850-1000 hPa(gpm)..............1279.....1287....1293....1297....1302 (un-adjusted) If we could bet 1293 then that's 50/50- obviously the scaling on the NAE would be better at intervals of 5 instead of 10- however at least it shows roughly where the risk is-Theres also an adjustment for height-  so elevated areas can see snow in higher thicknesses. S S

nice one mate . fingers crossed for thursday evening then!!is the blue colour on the thickness chart 1290 or 1300 cos I can see a streak of blue on the border of east anglia?
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yes mate - its not the only parameter to measure-  however 1290 is usually the benchmark -

 

Probability:............................90%.....70%.....50%.....30%.....10%850-1000 hPa(gpm)..............1279.....1287....1293....1297....1302 (un-adjusted)

 

If we could bet 1293 then that's 50/50- obviously the scaling on the NAE would be better at intervals of 5 instead of 10- however at least it shows roughly where the risk is-

Theres also an adjustment for height-  so elevated areas can see snow in higher thicknesses.

 

S

 

S

An excellent summary there from Steve

 

For newbies or less experienced model watchers, John Holmes (in his days as NW senior forecaster) compiled this excellent thread on how to try and forecast snow synoptics, with detailed synoptic parameter analysis (3 reasonably long words in succession there, must have swallowed a dictionary...lol)

 

link ------> http://forum.netweather.tv/topic/27989-how-to-try-and-forecast-snow/

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I am really confused still.

So this shortwave pops up on thursday in between stormy periods and the area around and behind the shortwave is colder than say further north where the wind is still feeding in and mixing out the colder air?

What causes this shortwave to suddenly appear?

 

sorry for being slow!

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Seeing as we are discussing the chances of snow, here is some supplementary info regarding various parameters and their relationships with the likelihood of snow... (all from the Met Office, for educational purposes)..Probability of snow________________90%____70%___50%___30%___10%Surface temp ( C ) _________________+0.3____+1.2___+1.6___+2.3____+3.9Height of 0C isotherm agl (hPa)______12______25____35_____45______611000-500hPa thickness (gpm) ______5180___5238__5258___5292____5334 ______ (the "528 line" falls around 40%)(mutually exclusive)Based on WHEN precip begins, as mentioned above latent cooling of the atmosphere will lower the freezing level once the snow starts falling/melting.The wet-bulb freezing level is very important as well...   Mainly snow Readily turns to snow   Mainly rain   Snow very rareHeight of theta-w freezing level agl < 1000ft (~300m)               <2000ft (~600m)             >= 2000ft       >= 3000ft  (~1000m) It's also important to watch out for cold surface air undercutting warm air.A general rule of thumb for the UK is that an 850hpa theta-w of 2C or below can allow for snowfall down to penetrate down to all elevations (with a large number of caveats obviously). Heavy instability and persistent precip with an initial wet-bulb temperature level of as high as +3C can allow snow to reach the surface. 

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You know it's a bad winter for cold and snow when folks are getting excited about the prospect of some transient wet stuff falling but not settling ...roll in 2014. 

 

In the nearer time frame and both the GFS and the ECM have the South Coast battening down the hatches and putting out the sand bags for Boxing day. 

 

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GFS
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Plenty of time for change, but almost every single run over the past week has been showing something extreme for the 3 days of Xmas and especially so for the 26/27th

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Yes still too far out in the extended for Boxing Day, but some very strong consistence ,intra model, extra model and within ENS. Movement north is probably going to happen (but that's not a good thing). So far sustained winds(for several hours) of 60mph being shown by ECM. And these are widespread low level speeds, so the current models hinting at maybe 70-80mph sustained over exposed or higher areas. This is obviously not including gusts.

 

We still have 3 days imho for this to change though before the red lights start blinking on and off.

 

Transient snow for Thursday loos quite good for some (m4 corridor as always), with maybe a few hrs of the stuff, maybe even settling under heavy precip (if it occurs).

Nothing to be sniffed at. But the cyclogenesis is still the most amazing thing with the current models

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The posts from Steve, Mark Canning and Nick S are all worth bookmarking giving information on additional data that forecasters always are aware of for snow to fall when perhaps least expected. Some time, perhaps 2-3 years back I did post the same table as SM and MC posted, used even now by forecasters. The link given to Net Wx Guides is also well worth bookmarking as it does include the above issue.

Snow forecasting is extremely complex and is well worth knowing what is taken into account both by the models and the human forecaster.

 

this is another link well worth bookmarking, it was started by a senior forecaster with UK Met and is stuffed full of information about every meteorological feature you can think of, be it storms, snow, whatever, just trawl through the Glossary and the additional links

http://weatherfaqs.org.uk/node/60

Edited by johnholmes
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Just when you thought things couldn't get any wilder and wetter, guess what?...They get wilder and wetter.

 

As Icebreg said the details will clearly change at the kind of range still involved, but charts like this suggest Xmas 2013 may well be remembered for all the wrong reasons meteorologoically.

 

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Edited by shedhead
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00z GFS  this morning has a severe storm for Xmas day, compared to the 80mph winds forecast for later this evening up in Scotland, if this came off im sure winds would be topping 100, hope it doesnt happen as shown

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Edited by Arron B
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You know it's a bad winter for cold and snow when folks are getting excited about the prospect of some transient wet stuff falling but not settling ...roll in 2014. 

 

In the nearer time frame and both the GFS and the ECM have the South Coast battening down the hatches and putting out the sand bags for Boxing day. 

 

Posted Image

GFS

Posted Image

 

Plenty of time for change, but almost every single run over the past week has been showing something extreme for the 3 days of Xmas and especially so for the 26/27th

That saved me a post, yes that very deep LP is still showing and has been for a while, timing is the issue.  I'm going for Boxing Day as a punt

 

BFTP

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Morning Folks. Well my my its looking like we are in for a pretty blowy Christmas holiday. That ECM chart for boxing day is actually a tad concerning, is that really a 955mb low over the north midlands? going forward I was wondering what the views were on the ECM, is it as shocking for coldies as last nights? it looks to me as there were some heights building way to the north.

 

Cheers

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I think the next few weeks are looking about as far removed from settled weather as you could get. The Gfs 00z op run looks extremely unsettled with storm after storm battering the uk and all of them take a slightly different track so everyone gets a blast at some point, on at least a few occasions. It also looks like becoming generally rather colder between christmas and new year and carrying on into 2014 which brings a higher risk of sleet and snow. In the short term, tomorrow is looking cold and windy with wintry showers but most of the showers being concentrated across the northwestern half of the uk, only a few showers making it further south and east with more shelter and a mainly dry and bright day, more wet and windy weather sweeping in off the atlantic during friday and then the weekend gradually becoming colder and showery with the showers becoming wintry on hills as far south as wales, again most of the showers across the northwest with the more sheltered south and east having most of the dry and sunny weather, another storm system early next week with severe gales and heavy rain followed by colder and showery weather again with the northwest again in the firing line for most of the wintry showers and then even more stormy weather during the christmas period, no let up in the disturbed pattern and with colder air coming into the mix, the risk of snow increases.

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Edited by Frosty.
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Ensemble suite for the 00z is at least beginning to show some fragmenting on a number of the Perturbations to the PV to our North and Northwest in FI, with large chunks spinning off.

 

Gives us some hope at least of seeing the end to our current constant battering from the huge LP spawner PV

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Edited by Arron B
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