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Model output discussion - proper cold spell inbound?


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The way  I see it, it’s the pulling back of the Azores ridge later next week that makes the flow flatter and therefore faster = less amplification and less cold/blocked. I don’t see it being over modelled beyond midweek 

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Evening All  Ice Cold hope your still with us .... So the much touted SSW is coming into view & here’s my thoughts + timelines of when / where to expect it... We have seen lots of p

Well it's probably fair to say that I've been rather quiet on these forums for the last few years really, especially since 2013. After 15 years of internet weather watching, and more importantly,

Well - ho hum - let's have a sit rep. My eyes are hanging out on stalks.  The pacific first - engine of the climate system. How is our AAM looking in the face of the current high amplitude MJO?

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Fantastic charts! Widespread snow showing on the GFS again tonight not just over hills but even lower levels and coastal areas too in the south and west. West Wales and Cornwall for example could well see quite frequent snow showers or even longer spells of snow if these charts verify, at last some proper winter charts finally after several years   :yahoo: 

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16 minutes ago, Catacol said:

Hehe - feeling sore?

One run is a rather short sample... but my thoughts are still that the azores high may be being overmodelled. Why? 

 

Because the top chart here shows clearly the strong westerly input at the equator and the resultant strong negative anomalies around 30N. This will serve to weaken the azores high, but in addition it is my opinion, based on observation and nothing more, that this forecast itself may undershoot what is actually happening. GFS has a known bias towards a negative AAM state, and so my educated guess is that this chart undercooks the rate of decay over the next week to 10 days. The current MJO phase was also hugely underestimated by ECM - in fact GFS was much nearer to the mark... and so ECM forecasts may also be underplaying the loss of support for sub tropical high pressure belts.

Any evidence? As said - one run since my post is a bit short term... but the charts below, using ensemble data not op runs, shows the azores high in retreat. I think it may retreat on the faster rather than the slower side of the ensemble range, no matter what op runs have suggested this afternoon.

 

 

 

By the end of the run further withdrawn though we really want to see less of a flat Nina shape when the date comes around. If the MJO keeps working in our favour we should be able to hang on to raised anomalies in some shape or form to the NE.

 

In the end it is only an interpretation of the data and some observational instinct. If it end up wrong it ends up wrong and I'll hold my hands up for a bad call. 

Deleted the charts to save space.

Looking at those charts C and it shows something that other 500 mb anomalies have shown before this winter. That is an attempt to give the flow from the Greenland area at 500 mb but with any Atlantic blocking being well west. Not what we usually see connected with deep cold.

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20 minutes ago, Nicholas B said:

I would say that going off the first chart, those of us in Kent away from the coast would be best placed for a decent fall of nice 'dry' snow. 

Could be but also the Pembrokeshire Dangler is notorious for feeding in a real mixed bag of rain, sleet, snow or hail showers with even thunder & lightning from off the Irish Sea. This scenario can last for many hours or even days in a slow moving weather pattern which brings some impressive precipitation totals

Edited by Dafydd Tomos
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30 minutes ago, Catacol said:

Hehe - feeling sore?

One run is a rather short sample... but my thoughts are still that the azores high may be being overmodelled. Why? 

Not Complete Yet

Because the top chart here shows clearly the strong westerly input at the equator and the resultant strong negative anomalies around 30N. This will serve to weaken the azores high, but in addition it is my opinion, based on observation and nothing more, that this forecast itself may undershoot what is actually happening. GFS has a known bias towards a negative AAM state, and so my educated guess is that this chart undercooks the rate of decay over the next week to 10 days. The current MJO phase was also hugely underestimated by ECM - in fact GFS was much nearer to the mark... and so ECM forecasts may also be underplaying the loss of support for sub tropical high pressure belts.

Any evidence? As said - one run since my post is a bit short term... but the charts below, using ensemble data not op runs, shows the azores high in retreat. I think it may retreat on the faster rather than the slower side of the ensemble range, no matter what op runs have suggested this afternoon.

ecmwf-ens_z500a_nhem_5.png

ecmwf-ens_z500a_nhem_6.png

ecmwf-ens_z500a_nhem_7.png

By the end of the run further withdrawn though we really want to see less of a flat Nina shape when the date comes around. If the MJO keeps working in our favour we should be able to hang on to raised anomalies in some shape or form to the NE.

ecmwf-ens_z500a_nhem_11.png

In the end it is only an interpretation of the data and some observational instinct. If it end up wrong it ends up wrong and I'll hold my hands up for a bad call. 

Thanks for the reply. I do hope you are on the right lines here. No need for anyone to feel sore, too long in the tooth for all that tosh :)

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3 minutes ago, Steve Murr said:

The models tonight ( note 18z @120 ) only need to be a smidgeon progressive & we’re back in a snow front -

Remember the UKMO 12z & now the moreover 18z IKON have a better ridge ahead of the 144 low-

UKMO 168

4295C273-2899-4DA7-8537-6988BC879DA5.thumb.png.6e9633a60391d90fe090f855a263cba3.png

18z IKON 120 - note the arc of the high to the North of the UK pushing energy NE

398AFA38-AC86-4B05-A1E8-C1A2FC09A0D9.thumb.png.cef10276bc7e399a6984751945e5b50d.png

whilst it hasn’t been the very best of days it’s far from over because some are taking 144-168 charts as Gospel when infact there’s plenty of scope for change...

The More immediate 72-96 charts present a rare example of frontal snow for the UK that will bring a dusting for many - as well as some convective snow in the SE corner ..

S

 

Good post Steve ?. What's your insight into what's gonna happen around mid month with the predicted SSW and all that ? TIA

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Guys. It's like Deja vu when people were writing off next and this weekend. Catacol is right no way should the models be this progressive with the current AAM state. Also models notoriously show milder air winning far too soon. Thursday will be snow in my opinion and then we will see a renewed build of heights to the North East. I think Steve is right with his instincts 

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28 minutes ago, bluearmy said:

The way  I see it, it’s the pulling back of the Azores ridge later next week that makes the flow flatter and therefore faster = less amplification and less cold/blocked. I don’t see it being over modelled beyond midweek 

The flip side to that though is the danger of the Azores ridging east and not north east and we would end up in boring nothingness. What a paradox! 

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It's hard not to feel a bit disheartened given the change in model output and the change in tone from the pros today. Especially as it can give a feeling of deja vu. Just have to take solace in the fact that features like Tuesday's front aren't yet resolved, other small features that may increase snow risk in places can't really be modeled until very close range and the only real milder stuff is at least a week away, which is in la la land at the moment. Plus the apparent signs for potential further cold weather further onto into February and maybe even into March. Some disappointment but plenty to cling to. It wouldn't be a UK winter without the usual ups and downs!

Edited by MattStoke
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3 minutes ago, ICE COLD said:

Good post Steve ?. What's your insight into what's gonna happen around mid month with the predicted SSW and all that ? TIA

I think the first step is to actually get it to verify as the GFS vertical resolution is not the best in terms of forecasting ‘correctly’ SSWs it’s always to extreme - we also need one that gets over the line properly rather than just creeping over the 0M/S line....

Also history perhaps indicates that a wave 2 split is better than a wave 1 displacement - 

The reality with the lead times is though people expecting a quick response may be a little over expectant when the lag time is often 10-14 days -

so really it’s late Feb early March if it follows the normal downwelling process...

mid to late Feb if we get a QTR ( quick tropospheric response ) 

S

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2 minutes ago, Richard Fisher said:

The cold snap ends Wednesday according to the pros. They have lots more data than we can see. Lack of posts this evening says it all.

I see cold snap after cold snap from the look of the 18z tonight, plenty of wintry potential going forward through most of this month possibly.:)

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4 minutes ago, Richard Fisher said:

The cold snap ends Wednesday according to the pros. They have lots more data than we can see. Lack of posts this evening says it all.

I've not seen a single forecast say that it's ending on Wednesday. Sure, the BBC are pointing towards less cold conditions by next weekend because the models are suggestive of that, but there's also the potential & support for the cold returning rather quickly, if it even gets pushed completely away in the first place.

So to say "Yep cold snap ending Wednesday. Fact. Pros said so" just seems silly. 

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4 minutes ago, Richard Fisher said:

The cold snap ends Wednesday according to the pros. They have lots more data than we can see. Lack of posts this evening says it all.

They can forecast based on probability but not with certainty remember that. Let's just wait and see. They will probably be right they are the pros but I'm with BA wait till tomorrow's runs

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