Model Output Discussion - heading into April

[Frosty.]

This would be a decent day 10 mean from the ecm 0z if it was winter with good ensemble support for a Northerly but why am I even talking about day 10?..when most of the next 240 hours looks settled and warm / very warm under high pressure / strong ridging!!!

 

Edited May 27, 2020 by JON SNOW

[Methuselah]

The GEFS 06Z ensembles are nae bad, Nae bad at all!

        

           

Now down to just a couple of days' cack, before things quickly return to normal: nothing to see here, move along! 

[Tamara]

On 22/05/2020 at 13:02, Tamara said:

Based especially on seasonal wavelength changes to summer -  stable AAM is very counter intuitive to any sustained Atlantic retrogressed ridging and downstream trough. What a slight weakening of the eastern flank of the downstream ridging, and some attempted inroads of the trough to the NW might indicate is the completion of the robust intra-seasonal convectively coupled kelvin wave (CCKW) at present propagating eastwards across the tropics and due to pass through the Pacific in the remainder of this month.

It is at this completion point in the "mini ENSO cycle" that angular momentum edges back somewhat. Suppression of convection occurs across the Pacific as the convective element moves on towards the African continent and Western Hemisphere. As atmospheric stability returns to the ENSO region in the wake of the CCKW,  then this leads to trade wind inertia increase and a rise in pressure which in turn retrogrades the downstream wavelength and the sub tropical highs in the Atlantic (Bermuda and Azores)

NWP will be anticipating the wind-flow inertia switch at this time, but as previously, they may over-egg the fall in momentum budget tendency based on global overall parity of momentum. This means they may either overdo the degree of retrogression/ fall of pressure close to the UK and/or the time period any such indicated switch of pattern lasts.

As long as heat content across the tropics as a whole, and irrespective of the cooling in the Pacific, remains anomalously warm, this is conducive to further downstream summertime sub tropical ridging and the upstream pattern allowing the ridging to adjust back towards Western Europe.

Re-quoted this as it has become wholly relevant to the evolution into the extended period as anticipated c/o the cyclical tropical>extra tropical cycle and a useful guide as why NWP depicts an Atlantic ridge and fall of downstream pressure in the extended period.

There is no reason at all to change the thinking applied  in that post and especially also there is no reason at all to have any doubts at this time about the final sentence either

The tropical>extra tropical cycle has shown a remarkable timeline symmetry over the Spring months and easily identified by the timing intervals of the peaks and troughs of the angular momentum budget in the atmospheric circulation.

 

 It is an illustration of how the atmosphere "remembers" from its previous ebbs and flows-  and it is now useful to look at these intra-seasonal timelines and assess how seasonal wavelength changes (in this case Spring to Summer) may augment existing default patterns or alter the evolving pattern from that seen at the previous peak change-end of month periods. Both March and April saw attempted retrogressions of the longwave pattern when AAM was embarking on its down cycle with momentum being removed from the atmospheric circulation resulting in the downstream pattern backing westwards and leading to cooler/colder winds from the polar regions. Both months have subsequently seen the Atlantic ridge punctured by an advancing trough heading into the West/Central Atlantic and adjusted the pattern eastwards once more

Rinse and repeat of this switch once more as May hands over to June and we say "hello" to summer.  Does this reasonably mean that following any northerly incursion,  should we see one, there is a return to the dry and warmer spells during June which dominated April and we are now enjoying once more at this time in May?

Extrapolation of suggested new patterns requires great caution with NWP suites at extended ranges and always a good time to check back on the diagnostics that are driving the numerical models to gauge whether they are on the right track and/or whether they are over egging a new signal. As often repeated by me in the past - signals lead models, models do not lead signals. This means that at distance they often (not always) latch on to a correct signal, but then they overstate its duration and can subsequently backtrack or smooth down the prognosis. Based on the above captioned summary, it remains sensible in my opinion to keep this in mind and rather than make assumptive extrapolations out in time of more of the same, continue to assess how the atmosphere applies its "remembering" . The weather patterns are not wholly chaotic all the time as they are often believed to be.

In basic weather terms - overall good reason in my opinion to expect verification of the Atlantic ridge and an attempt to introduce cooler and more changeable weather from the NW approaching ahead of the second week of June. This, not before some excellent summery conditions for the bulk of the coming ~ 7 days which should be enjoyed as much as possible without dwelling too much on the suggested breakdown approaching after this time. Also, it is not yet clear as to how this eventual encroachment from the NW pans out across the UK in terms of sensible surface weather detail and as ever, regional variations may well be the case.

These numerical model forecasts of AAM are not to be taken too much at face value and often enough there is a certain degree of correction of suggested trends moving forward in time.

 

However this forecast looks reasonable value and reflects accurately enough the direction of travel with momentum loss occurring as the CCKW now propagating across the Pacific Ocean (as discussed in previous post) leaves convective suppression in its wake and the negative wind-flow torque processes that arise as westerly winds associated with the convective passage of the CCKW are scrubbed out and replaced by increased easterly trade winds. This atmospheric induced wind-flow switch sets up up the longwave retrogression of the downstream pattern at the inflection point of where the negative torque occurs upstream.. The CCKW at that time having moved on through the Western Hemsiphere ahead of the next tropical>extra tropical cycle beginning in the Indian Ocean

So this lull phase of the "mini ENSO cycle" is totally consistent with the advertised synoptics.  It is during this lull phase time that the last sentence of the previous post has relevance and is the watching brief over the coming week to 10 days to look for clues as to when the next warm settled summer spell arrives and will be the subject matter for the next summary. 

Virtually a week of that 10 days comprises the remainder of our current fine spell anyway

 

 

Edited May 27, 2020 by Tamara

[MATTWOLVES 3]

Awesome post as always tams, always great to read. Just taking a look at the 6z mean... Lovely conditions for the next 7 or so days, before perhaps a lull to something a bit cooler, I'm not so sure about washout conditions though.. It looks to me like the South will become generally more settled again beyond a several day blip. Pressure looks to be on the high side even come the end of the run, which is deep FI... So pinch of salt required. I'm still not seeing any major Atlantic breakdown in the traditional sense.. So most definitely enjoy the next week and we will worry about a breakdown come closer to the time.. Suncream... And beers at the ready..

[Zak M]

12z ICON looks settled all the way through once again!

[shaky]

4 hours ago, Zak M said:

Lovely 06z GFS today.

Looks to be very warm or even hot in some places on day 7!

Icon looks fantastic again!!cut off low going through west off spain and extending the warmth!!

[Timmytour]

A northerly plunge in seven days time?

I have to say I would welcome it getting cooler, but the track record of such plunges over the last six months materialising when forecast at such a range has been nothing short of tragic, so I for one am not buying it!

 

[Djdazzle]

GFS already miles apart from its 6z run as early as 120. This is getting confusing!

[Zak M]

I am liking the 12z UKMO @120

[Djdazzle]

GFS looks like producing another outrageous outlier!

[Methuselah]

Going by the GFS 12Z, cooler (and hopefully) less-settled weather looks to arrive by Day 8. And with all mowed grass looking as brown as ever, a wee drop (100mm?) of rain is needed. Pronto!

    

[northwestsnow]

Breakdown coming into view on UKMO 144...

Exeter also suggesting a switch,moreso for the north in early June.

No complaints from me its been a gorgeous spring this year.

[Guest]

2 minutes ago, Djdazzle said:

GFS looks like producing another outrageous outlier!

What do you expect after two runs almost producing the same   The end will surely be an outlier, screaming northerly from the North Pole to Western Sahara......yuk no thanks. 

[chrisbell-nottheweatherman]

3 hours ago, Tamara said:

In basic weather terms - overall good reason in my opinion to expect verification of the Atlantic ridge and an attempt to introduce cooler and more changeable weather from the NW approaching ahead of the second week of June. This, not before some excellent summery conditions for the bulk of the coming ~ 7 days which should be enjoyed as much as possible without dwelling too much on the suggested breakdown approaching after this time. Also, it is not yet clear as to how this eventual encroachment from the NW pans out across the UK in terms of sensible surface weather detail and as ever, regional variations may well be the case.

 

 

 

Always useful to have the background to the NWP explained, even if I still haven't learned what all these teleconnections are and how to understand them...virus shutdown has been busier than I expected!

[LRD]

If only it were Dec/Jan/Feb*

 

 

*although if it were winter, a possible snowstorm would get watered down

[mb018538]

I also personally think the end of the very long dry and settled spell is coming to an end towards the middle of next week. No complaints here, been the best spring I can ever remember.

[Methuselah]

10 minutes ago, chrisbell-nottheweatherman said:

Always useful to have the background to the NWP explained, even if I still haven't learned what all these teleconnections are and how to understand them...virus shutdown has been busier than I expected!

I think that's about spot-on CBNWF...Last summer, @Tamara's musings meant that I was neither surprised nor disappointed, by the way the season turned out.

Edited May 27, 2020 by General Cluster

[Guest]

While hopefully it doesn't happen the 12Z GFS ops is the perfect example of the trough becoming anchored over the UK, absolute worst case scenario, nowhere for it to go and you lot moaning about your brown lawns will be moaning about the flooding......

[Djdazzle]

1 minute ago, Alderc said:

While hopefully it doesn't happen the 12Z GFS ops is the perfect example of the trough becoming anchored over the UK, absolute worst case scenario, nowhere for it to go and you lot moaning about your brown lawns will be moaning about the flooding......

Yes, you’re right. But it’s so extreme, it’s unlikely to come off. Don’t worry though, when do charts at that range ever come off?

[LRD]

A grim GFS 12z run from about 180 onwards. Will likely be at the bottom end of the ensemble pack. Still have a gut feeling that this breakdown is being over-played but we'll see

[Methuselah]

1 minute ago, Djdazzle said:

Yes, you’re right. But it’s so extreme, it’s unlikely to come off. Don’t worry though, when do charts at that range ever come off?

When they show 'mild' in winter?

[mb018538]

GEM also turns much cooler and unsettled through the middle of next week - though most of the rain just misses.

[Fourty Point Three Degrees]

Said it yesterday and ill say it today, its an echo of 2012 in FI. Exactly what happened then when a trough squeezed in between two areas of high pressure and became stuck. 

My hopes for a 2018 repeat or at least a decent continuation of the warm and dry is eroding away at great pace. 
 

[northwestsnow]

2 minutes ago, 38.7°C said:

Said it yesterday and ill say it today, its an echo of 2012 in FI. Exactly what happened then when a trough squeezed in between two areas of high pressure and became stuck. 

My hopes for a 2018 repeat or at least a decent continuation of the warm and dry is eroding away at great pace. 
 

I think,with all due respect, a bit of perspective is needed,we have had virtually 7 to 8 weeks of dry settled weather.We don't reside in Spain or Greece afterall ..

[Djdazzle]

GFS mean does not support the op, although does turn cooler.