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Model Output Discussion - The Start Of Winter


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Its certainly going to be an interesting week for the forecasters. Not sure how the fax charts from Exeter are going to place the wave development in the cold flow by Thursday Mid day.

The jet streak looks to power up over Southern England for a period with the formation of a fast moving warm sector. So I would expect it to run across SW Ireland and the South Midlands before making a left exit into the North Sea. A very active cold front with the 528 dam holding it location close to the front. I am sure this will be a rapid scenario, but the exact track of the low may be causing a fair amount of uncertainty for the chief forecaster. Will it be further north or south. I think it will hold the line as explained above. Should be great to watch developments this week and see how the models turn out.

C

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A few of you have asked today about rationale for the (tail-end of) UKMO latest 30-day outlook. There is, of course, a dedicated thread elsewhere for this but I'll just confirm that GloSea5 output rem

Thanks for this Barry   You are however wrong on both counts you suggest :   1) I have explained in extended detail, as much as I am able, the progress of the patterns thus far from autumn (see firs

Hopefully a timely interjection, but UKMO caution (again) that medium range NWP may not be adequately reflecting ultimate broadscale outcome. First tentative signs emerging, they suggest, of *possible

Posted Images

^^^ 

 

We have the Russian high to the east and the US ridge to the west with the Azores directing the jet towards the East; in layman's terms it is hard to squeeze another HLB in the Atlantic with no slack in the system (Asia/Pacific). So IMO this will dictate our medium term outlook.

I have to say I agree with the above and I can see this pattern lasting for some time with nothing major in terms of cold.

I expressed yesterday a concern that the models looked quite flat after next weekend's northerly and today not only they continue to look flat but the northerly has been downgraded substantially so a poor day of model watching.

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it is fluctuating a tad but in general, it is forecast by the extended ens to be weakly positive from the 9th to around the 20th.

Oh ta ba  about 2 weeks ago the AO was expected to go v negative for a fair while, havnt seen the prediction chart for a while so mustve changed a fair bit. soz.

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Thank you Mark. I doubt if there is much to support any such development in the current output. I have certainly seen similar happening on past occasions but, as they say, each situation is unique in some way. Anyway, here's hoping!

 

It is pretty much how UKMO would develop but not supported elsewhere as yet but there may be very good reasons for this.

ECM looks much too progressive and GFS, as I alluded to earlier, is phasing the trough and flattening the pattern which is also the same mistake it made in previous output. Reasonable reasons to think both these models may be wrong and modified through tomorrow.

We will know more tomorrow and see whether UKMO develops the theme and ECM moves toward it while GFS gets tog rip with the upstream pattern but I think there is some support for what you suggest if one reads between the lines of the output.

Fingers crossed.

Edited by Mucka
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the ecm extended are 'as you were' but at the back end they seem rather strange. days 14 to 16, the mean east alaskan ridge throws itself further into the arctic and a small upper ridge appears between iceland and e greenland. very odd for a mean chart at days 15 and 16 ?  almost looks a bit retrogressive upstream.  cant draw any conclusions from output at such a range but to see a mean upper ridge appear ....... (its not a strong feature but then why should it be in 360 hours). 

 

EDIT: a more detailed look - it maybe just be the way the lowest heights are drained whilst a lobe is left just northeast of iceland. hard to tell - will see tomorrow i guess. naefs look like it wants to repeat next weeks more amplified scandi trough in two weeks time

Edited by bluearmy
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A quick update regarding the overnight snow potential for mainland UK.

 

If you are up and about at 0500 hours tomorrow morning, the following 0c isotherm chart and equivalent snow risk chart, illustrate the current thinking.

 

post-7183-0-28377100-1417989555_thumb.pnpost-7183-0-90774100-1417989578_thumb.pn

 

By rush hour, well by 0800 hours tomorrow morning, the following is suggested by the NetWx-SR 12z model run.

 

post-7183-0-34208700-1417989687_thumb.pnpost-7183-0-16308500-1417989694_thumb.pn

 

Please bear in mind, these are NW Extra chart offerings so I won't be posting these too often, otherwise I'll be unpopular with the bosses.

 

I won't go any further with these, but as mentioned on the BBC forecasts today, there is potential for overnight wet snow even down South, but more specifically with the focus across parts of the mainland with elevation on their side and prone to the relatively breezy NW'rly winds. In fact, a generally windy ole week lies ahead of us and we can certainly expect travel disruption at times, so do take care out there. 

 

Added to the above, my general view of the week's weather from yesterday evening remains as stated, despite today's modelling ups and downs which tended to revolve around next weekend onwards.

 

Regards

 

gottolovethisweather

Edited by gottolovethisweather
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Not a bad run from the 18z GFS for coldies. This run used to get a lot of stick but Lorenzo (I think) posted recently saying it

was, along with the 00z, the best of the GFS suite in terms of accuracy.

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The latest fax chart from the UKMO looks  wintry for Friday,occluded front moving se within 528 dam. Indeed it looks a lot more wintry then just viewing it on the normal SLP charts.

 

The big question is will we at some point see a UKMO output verify at T120hrs because over the last few weeks its been just one backtrack after another from it. The UKMO owes coldies in here, please deliver this time or be downgraded into the same category as the NAVGEM!

 

attachicon.giffax120s.gif

 

You can compare its view of Friday with the ECM postage stamps to see how much support it has:

 

attachicon.gif120.gif

Yes, we need to save this one. We remember the one for Friday 5th Dec!!!

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The latest fax chart from the UKMO looks  wintry for Friday,occluded front moving se within 528 dam. Indeed it looks a lot more wintry then just viewing it on the normal SLP charts.

 

The big question is will we at some point see a UKMO output verify at T120hrs because over the last few weeks its been just one backtrack after another from it. The UKMO owes coldies in here, please deliver this time or be downgraded into the same category as the NAVGEM!

 

attachicon.giffax120s.gif

 

You can compare its view of Friday with the ECM postage stamps to see how much support it has:

 

attachicon.gif120.gif

How much support by those stamps Nick? Not being lazy honest, just can't zoom in on the individual stamps and

my eyesight is not what it used to be lol!

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How much support by those stamps Nick? Not being lazy honest, just can't zoom in on the individual stamps and

my eyesight is not what it used to be lol!

In terms of a flow from the nw or north its good but to get the coldest air depends on how quickly that flow is cut off as heights lower to the nw. It's a bit difficult to see in all honesty with those stamps because we don't get a good view of Greenland and the mid Atlantic.

 

If you look at the raw UKMO output the coldest air arrives on Saturday , we'll just have to see what happens in the morning and hope that we can squeeze just a bit more amplification from the models.

 

Eventually those heights will drop to the nw but its just really trying to make the best of the window before that happens.

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Certainly a propensity to follow the ECM op with regards to building heights to our east on the 18z GFS op an control runs.

I am hoping to wake up to this trend turning into a growing trend on the 00z :cold:


In terms of a flow from the nw or north its good but to get the coldest air depends on how quickly that flow is cut off as heights lower to the nw. It's a bit difficult to see in all honesty with those stamps because we don't get a good view of Greenland and the mid Atlantic.

 

If you look at the raw UKMO output the coldest air arrives on Saturday , we'll just have to see what happens in the morning and hope that we can squeeze just a bit more amplification from the models.

 

Eventually those heights will drop to the nw but its just really trying to make the best of the window before that happens.

Thanks Nick

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The only problem with northerlies is that they can be skewed by shortwaves which are only picked up a few days before the event (eg last year), you can see the GFS 18z does develop a shortwave, but currently the flow still brings in colder weather, if we see the short wave pushing further south we could see a more significant snow event if warmer isnt mixed in, but for now its just a word of a caution that the setup is far from simple!

18z left, 12z right

h500slp.pngh500slp.png

Edited by Panayiotis
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The undoubted priority this Wednesday is how destructive are these winds going to be in Scotland and N. Ireland.

Charts courtesy weatherbell

For you - perhaps. For any winter weather lover in Scotland? Unlikely. Personally it looks like quite a dangerous mix for those with any kind of altitude. Whiteout potential with gales and some horrific travel times for commuters you would envisage.

Here's hoping things don't get too rough, especially not at this time of year.

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Polar Low ingredients with -40c 500hpa.

 

gfs-13-132.png?18gfs-0-138.png?18

Just a quick question. If a polar low was to develop in that kind of setup. would this cause amplification and rapid change in weather modeling or would it be more straightforward.?? I have been thinking that this kind of scenario would be a good thing for cold lovers???

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For you - perhaps. For any winter weather lover in Scotland? Unlikely. Personally it looks like quite a dangerous mix for those with any kind of altitude. Whiteout potential with gales and some horrific travel times for commuters you would envisage.

Here's hoping things don't get too rough, especially not at this time of year.

To be honest, given the kinds of wind speeds we saw regularly through both last winter and much of 2011/12 (including some monster storms in the preceding Autumn and following spring) I'm not convinced this one will necessarily be any worse for most of us, perhaps aside from the potential for blizzards and the longevity of the strong winds. 

Gusts near 60mph for much of central and eastern Scotland on Tuesday, with sustained winds around gale force:

ukgust.png ukgust.png

 

 

The next, and more prolonged, pulse, hits Wednesday afternoon, with gusts widely 50-60mph and pushing towards 70mph for the Outer Hebrides:

ukgust.png

 

Sustained winds again around gale force for most of the mainland, but with some fairly worrying blasts of >60mph at points for North Uist and then Orkney:

ukwind.png ukwind.png

 

I imagine the biggest threat is probably to ferries, which will probably be cancelled anyway because of this:

http://magicseaweed.com/news/black-wednesday-swell-for-north-atlantic/7017/

Certainly not a day to be out in the Minch or the Pentland Firth.

Just for comparison, a few charts are attached belowfrom some of the more notable wind events. The worst in recent years for the central belt was January 3rd 2012, when something resembling a sting jet descended on the central belt giving gusts of 90-100mph in Edinburgh and Glasgow and sustained winds of 50+mph. Last winter we frequently had storms which registered 60+mph gusts across the central belt too, so while it might be quite an impressive storm it may not be notably windy given the frequency of severe storms we've seen in the last 3 years (the chart at the bottom of this shows just how windy it's been in recent years in a historical context http://www.metoffice.gov.uk/climate/uk/interesting/2013-decwind). However, combined with temperatures like this:

ukmintemp.png ukmintemp.png

and near constant precipitation for central Scotland, from the warm front Tuesday morning to the cold front by evening to an endless stream of likely wintry showers Tuesday night through to Thursday, I think even modest snowfall to fairly low levels would make this a very notable winter storm.

 

post-9298-0-61341500-1359494377_thumb.pn
post-9298-0-64914800-1359502624_thumb.pn
post-9298-0-29704900-1323456719_thumb.pn
post-9298-0-29329400-1323285290_thumb.pn
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Just a quick question. If a polar low was to develop in that kind of setup. would this cause amplification and rapid change in weather modeling or would it be more straightforward.?? I have been thinking that this kind of scenario would be a good thing for cold lovers???

 

A polar low would only have local effects being of a mesoscale nature.

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