Model output discussion - hot spell beckons but then what?

[Polar Maritime]

The 6z into the run and towards Month end certainly shows a more settled warmer outlook compared to the 12z. With High Pressure over the UK bringing a warm S/E flow.

 

[summer blizzard]

GFS6z another interesting run in FI trying to retrogress the high.

[MP-R]

Assuming retrogression of the high towards Greenland, the question is whether a trough slides in underneath from the southwest, potentially bringing a wetter but still quite warm start to September, or low pressure over Scandinavia helps usher in more of northerly flow i.e. drier but much cooler. Interesting indeed.

In the short term, however, perhaps Friday this week will be worse in terms of rain than the weekend itself (I certainly hope so given outdoor plans on Saturday), then an improvement beyond the weekend. Next week could be very decent indeed, leading to a 2017-esque Bank Holiday weekend.

[Methuselah]

Not a bad set of 06Z ensembles:

   

   

And, even right at the very end, the 06Z looks okay...which is in-tune with the thoughts of Those Who Cannot Be Named?

And a lovely low in mid-Atlantic too!                                                      

[JeffC]

25 minutes ago, summer blizzard said:

GFS6z another interesting run in FI trying to retrogress the high.

lets hope it can stay in this type of pattern until the end of December!

[knocker]

There was no great agreement between NOAA and the EPS this morning with the former not showing much interest in any ridging close to the UK in the 5-10 range. Although one has to take a punt on where the surface features reside the clusters at the end of the period would not suggest much amplification of the subtropical high either.

And, although I accept that taking one day is really a no no, no particular signs of it in the ext period either. But not surprisingly the Atlantic is positively anonymous so nothing nasty lurking in the woodshed

Edited August 14, 2019 by knocker

[MATTWOLVES 3]

There is finally light at the end of this unsettled tunnel... All the signs and signals are now pointing to a decent fine spell. Just how warm is still open to question but I would say a very nice spell of late summer weather looks a good call. The 6z literally plants high pressure over us... And it's been a while since we have witnessed such charts.... Fingers crossed for a beauty of a Bank Holiday! 

[Downpour]

Models shaping up here. BH weekend looking promising indeed. 

[Tamara]

On 10/08/2019 at 16:04, Tamara said:

In the here and now it continues to question the retention of La Nina type downstream flow advertised by NWP in the medium/longer term range. Albeit there is an eventual very slow relinquishing of the trough solution between the UK and Scandinavia.

 There is no route back to heat that can yet be justified, but any lingering cool air advection looks most likely linked to the transitional relaxation of the upper trough as ridging takes over from the south. This in turn though, an eventual bridge to warmer and settled conditions to return as thicknesses rise and any cooler uppers get mixed out.

.However, based on the subsequent return of the low frequency signal in the CPAC and further westerly winds added back to the atmospheric circulation, there is not really support for a persistent Nina type low angular momentum response long term c/o of a Scandinavian/UK trough and amplified Western Atlantic profile either...

The suspicion remains that NWP is being too slow in the extended period to acknowledge the upstream pattern and obfuscation to a ridging transition is overly procrastinated.

Excerpts from last Saturday post as means to try to help link and document progress since that time

Since that post, NWP has duly responded by focussing away from any notion of a persisting trough between the UK and Scandinavia, and towards return of ridging next week - as the upstream changes in wind-flow pattern discussed in the previous post(s) create our downstream improvement in conditions, initially most especially further south.

As proposed in the quoted post, it remains the case that no instant heat looks likely as the changes get underway, but every sign that by the bank holiday the cooler air left in the wake of the weekends deep trough will have been mixed out with time - and also with warmer air starting to be drawn up from the south as the High migrates close by.

Using Atlantic/European views, both ECM and GFS day 10 means are very closely aligned - look set fair for a weekend that hopefully will be some compensation for the two decidedly poor weekends that are preceding it.

 

 

With the present unsettled pattern in mind, its worth looking at how the GSDM diagnostics represent the changes. On this 2 day lagged data, notice how far down -ve torque tendency ( strong -ve wind-flow anomalies at 20 to 30N) has led to considerable -ve momentum transport in the extra tropics and supressed the present pattern in our downstream location - due to sudden amplification upstream.

The Global Wind Oscillation, a key plot phase indicator of extra tropical wind-flows, has responded with a rapid aborting of the Phase 4 +ve tendency orbit late last week to -ve tendency Phase 8 through the weekend and into this week.

This type of rapid orbit momentum phasing, is a signal of an unstable GSDM momentum budget within the atmosphere, and often red flags high impact weather events due to the rapid shifts they force on the Jetstream. as the laws of conservation of angular momentum rapidly re-apportion wind-flow inertia to fulfil Mother Natures need to fill a vacuum.

Its not a surprise that this spooked the models in their longer term programming - before seeing that the mechanisms responsible for it are set to hit reverse afterwards....

In the very extended period - as for any hint of retrogression (which is 2 weeks away on operational GFS output)….very little point in speculating at that range, much as just recently has been illustrated with the modelled over extrapolation ahead of the UK/Scandinavian trough - and very slow to dissolve it right to the Bank Holiday period itself as analysed above..

For what little it is worth discussing at this time, a late summer/early autumn 2017 full-on La Nina style collapse, in angular momentum (as opposed to this temporary -ve momentum phase) would steer towards a major retrogression outcome.  GFS -ve momentum tropical bias would be catalyst for tendency to default to retrogression in lowest resolution (and quite possibly its a trend one or two observers would like to see such as an outcome which is a factor here as well )

The upstream Pacific pattern this August is different to 2017 and with a +ve PDO pattern in evidence that was not there then - but regardless, at this range, and based on the persistence/re-occurrence of the low frequency convergence zone in the Pacific, such sustained relapse, at present at least, is highly questionable in terms of support.

Notice the strong suppression signal over the Maritimes (shaded orange) on the velocity potential chart....this is hallmark of return of a standing wave in the Pacific..

 

…so ensuring the entire Pacific basin is not dominated for the time being by trade winds, as occurred heading into autumn 2017.  

Instead, classic wind anomaly convergence conducive to deep thunderstorm development,occurring at the dateline with -ve wind anomalies east of the dateline and +ve westerly winds at, and just to the west of the dateline. 

All c/o the split SST distribution of warmth to the west of the dateline and (relatively) cool to the east. The latter influenced by trade winds upwelling cooler sub water as triggered by intermittent -ve phasing of the South Pacific Oscillation and associated high pressure zone ( cool SST anomalies in South Pacific below the Nino 1.2  ENSO zone).

Its important here to differentiate between the low frequency walker cell circulation and the cyclical high frequency MJO cycle. Effects on momentum transport and related angular momentum tendency require key considerations of different proxies.. A reliable assessment of tropical convection and its potential effect on synoptic patterns cannot be made by simply referring to an MJO forecast alone. Extratropical propagation of momentum transport from the tropics has key variables, especially at a time when the Hadley cell is being skewed poleward by the latent heat processes of convection created by additional warming in the tropical oceans.  Then, factor in the amplification mechanisms created by the disturbingly low arctic sea ice, and it is no surprise to keep seeing some unprecedented scenarios cropping up within extra tropical hemispheres from one season to another.

More than enough to dwell on....At this stage its a case of seeing out the current unsettled spell and seeing in a highly welcome change to more settled conditions through next week

 

[knocker]

Still rather too much influence from the trough and weak jet from the gfs in the 5-10 period which tends to restrict any substantial amplification of the subtropical high and thus very much a tendency towards a N/S split over the UK

[Frosty.]

A very nice trend from next midweek onwards on the GEFS 12z mean with summer gradually returning following this autumnal wet, cool occasionally windy spell!

 

 

 

Edited August 14, 2019 by Jon Snow

[Cheshire Freeze]

Just viewed the 12z GFS. What struck me is the ongoing tendency for heights wanting to pull west. It's something we've seen all summer thus far. 

An improving picture, for sure, but how long it lasts is open to much debate.

[Methuselah]

A nice set of 12Z ensembles, and pretty much in line with Tamara's earlier musings...

   

   

Now here's a thought...Let's let the HP in before we start to retrogress it?

[Frosty.]

The Ecm 12z operational turns into a beauty, high pressure and much warmer!!

Edited August 14, 2019 by Jon Snow

[knocker]

The focus from the ecm for next week revolves around the battle between the subtropical high ridging north east and the energy crossing the Atlantic. Initially still unsettled but settling down a tad as the ridge gains some traction  But this remains a pretty fluid scenario so the EPS will be interesting later

[Frosty.]

The Ecm 12z ensemble mean longer term is great in terms of high pressure becoming the dominant feature!

Edited August 14, 2019 by Jon Snow

[knocker]

The North Atlantic500mb and surface analysis for midnight.(The weekend low can be seen in the western Atlantic)

Still some showers around in the west at the moment and these will filter east a tad before generally fading away as a transient ridge moves across the UK Resulting in a generally sunny and quite warm day, in comparison of late, for most areas. But by evening more persistent rain will arrive in N. Ireland and particularly western Scotland courtesy of the warm front associated with the aforementioned low which is moving rapidly east.

Through the night the rain moves east and intensifies to cover most of the north west by dawn

By midday tomorrow the low is 983mb NW of Ireland with a myriad of frontal systems crossing the country in the strengthening wind so a very wet and windy day for all

Over Friday night and through Saturday, as the low edges further east, the fronts do eventually clear away to the leaving the country in a strong westerly airstream, with gales in western coastal areas, and frequent squally showers. These could well be thundery in the vicinity of the trough.

By midday Sunday the center of the low 985mb over northern Scotland with the wrap around occlusion across N. Ireland and southern Scotland so still a risk of thundery showers here and possible on the trough lying across south west regions. Apart from this sunny intervals and still quite windy.in another rather cool day

On Monday the low is filling west of southern Norway but the occlusion is still draped across the north thus still a risk of thundery showers in the strong wind,in this region. Less windy further south with sunshine and showers but again the risk of the odd storm on any troughs floating about andit what will be generally another cool day

Edited August 15, 2019 by knocker

[knocker]

Continuing showery on Tuesday/Weds but less windy on Wednesday as the subtropical high starts to amplify to the west#

Over the next couple of days the amplification continues and nudges east over the UK as the next trough becomes neutrally aligned as it stretches south in mid Atlantic.

This is of course according to the gfs.

The GEFS mean anomaly in the medium term is supportive of the subtropical high becoming more influential

 

Edited August 15, 2019 by knocker

[I remember Atlantic 252]

unusual charts certainly, this more like May than Aug, warm seas though so less sea mist?

[knocker]

The weekend low and frontal system at 0600

[knocker]

According to the ecm this morning the eastbound energy, to some extent, curtails the amplification of the subtropical high and this allows frontal systems to track across the country Fr/Sat albeit weakening. This detail is of course quite academic at this range but it does highlight the problem,that remains to be resolved

[Methuselah]

This morning's GEFS 00Z ensembles clearly predict a sustained rise in SLP, though where the HP goes, as we approach Day 16, is anyone's guess...?

   

   

Drying-out time is coming!

[Summer Sun]

Once the deep low from this weekend clears away early next week we have continued signals for high pressure to build.

It doesn't look like we'll be dragging up any hot air, so I'd say temps ranging from around the mid 20s towards the south-east and upper teens/low 20s in Scotland with light winds and some sunshine it would be a huge improvement than we've had of late

 

Edited August 15, 2019 by Summer Sun

[Downpour]

25c and sunny skies now a real prospect in the SE quadrant over the bank holiday weekend. 

That will be welcomed by many. Fantastic charts, classic English summer conditions, not too hot but warm and lovely.

 

[Methuselah]

This looks tasty. Very very tasty!