Model Output Discussion - heading into April

[Man With Beard]

10 minutes ago, johnholmes said:

Hi MWB

Confirmed max for yesterday 22.6C at Hurn ... a 1.6C increase on ECM modelled temperatures for the same day - a bit of a lower increase than usual but still a bit of a lift on the predicted value

 

+/- 2C is, I believe, considered correct for a forecast by UK Met!

That seems entirely reasonable.

[Singularity]

2 hours ago, knocker said:

I would have to say I would interpret this a little differently. There is indeed a trough that exits the eastern seaboard which attempts to phase with main trough

But fails and from this point the main trough deepens quickly near the left ext of the jet

But hey ho that's model discussion

 

I'm afraid I must disagree on this one, I think we may be looking at different small lows attempting to phase. The one I'm referring to engages with the base of the trough at +168 and then proceeds to deepen rapidly as it interacts with the polar air and strong upper-level divergence (near the left exit of the jet, as you rightly say).

[Methuselah]

Some nice potential to end today's GFS 06Z; after a week-or-so's convective weather (plants need rain!), it'd be great were warm air from the east and west to fuse together, over Blighty. I really am the Archbishop of Canterbury...Honest!

    

 

Edited April 23, 2020 by General Cluster

[Man With Beard]

Interesting that the AROME has downgraded temperatures for this afternoon quite a bit in the past 18 hours, not just across the UK but across a lot of Europe.

  

This is very unusual for the AROME at such short range. Might the changes in vehicle related heat and low level pollution be causing trouble with the models? (Just a theory!)

[knocker]

21 minutes ago, Singularity said:

I'm afraid I must disagree on this one, I think we may be looking at different small lows attempting to phase. The one I'm referring to engages with the base of the trough at +168 and then proceeds to deepen rapidly as it interacts with the polar air and strong upper-level divergence (near the left exit of the jet, as you rightly say).

Okay fair enough

 

 

Edited April 23, 2020 by knocker

[Tamara]

16 hours ago, Singularity said:

 

I know this is at the not-very-reliable range but... I think this adjustment by ECM (and GFS as others have shown) to the broad trough stalling west of the UK makes good sense. Reason? This:

Barely has the recently concluded decline in AAM to neutral begun to move the pattern away from UK/European ridge developments, when AAM rises considerably again to bring those patterns back to the fore.

This is how a gentle Nino-like 'lean' to the atmospheric state can bring us repeating spells of fine weather. The rising AAM stages are very important. I've found in my research that rising into +AAM (GWO 4-5-6 progression) forces European weather patterns more than falling to neutral AAM (7-8-0). Similarly, falling to -AAM (8-1-2) impacts more than rising to neutral AAM (3-4-0).

Hi all - the lovely sun with its Vitamin D goodness, shining down from clear blue skies onto quiet streets has been a surreal yet comforting sight during  a very difficult time. I am sure many will be hoping to see plenty more sunshine make things a little more bearable in weeks to come.

This, above, is a very good observation from @Singularity   which I wholly agree with and one I have made previously in past summaries.

To take this theme even further: 

Its interesting to choreograph the weather patterns over the past month with the ocean>atmospheric angular momentum wind-flow pattern. Taking a look at the last week of March to the coming last week of April period, there is a discernible correlation to be made:.

 

                     AAM cycle :                                         Wind-flow pattern inertia related to tropical MJO forcing:

                    

It can be seen that the two falling momentum phases, the first one in late March and the second one upcoming in this last week of April, choregraph nicely with the tropical "mini ENSO cycle" periods where increased trade winds (blue) produced a deceleration in global momentum and where westerly wind bursts (red) during the first half of April quickly scrubbed out that easterly trade wind inertia and momentum strongly recovered. Now, to follow, we are seeing a reprise of the falling momentum sequence as seen four weeks back.

The effect of reducing momentum is to have a retrogressive "pull" on the hemispheric synoptic pattern. This means, related to our own part of the NH, that the mid latitude downstream trough/ridge response that @Singularitymentions under more buoyant momentum phasing starts to retrogress back to the Atlantic and allows a drop in pressure in its wake and cooler northerly influences developing on the eastern flank of the ridging.

We can put this process into practice by taking a look at the synoptic sequence that started the 30 day period.

Preliminary of momentum fall 22 March: Pressure high close to UK with Azores/Atlantic ridge strengthening                 

                                                                        

 

 Falling momentum: 27 March: Atlantic ridge becoming more dominant as pattern amplifies upstream due to deceleration of zonal winds (blue anomaly in top plot ) across the Pacific                                                                                                                                                                      

                                                                                                                          

 

  28 March: Retrogression of ridge complete

 

Now take a look at how the rise in AAM subsequent to this low point in momentum during early April re-instated the eastward displaced ridge and troughing replacing retrogressed ridge across the Atlantic flipping the pattern back around to warm southerly/south easterly flow. See how this correlates with the westerly wind  burst (shaded red circled red) on the Pacific wind-flow anomaly chart at the top of the post for the same time - and which creates an upstream Pacific jet extension and provides downstream "push" momentum to the longwave pattern

 With all this in mind, we can take a look at the tropical cycle for the past 30 days. Viewing the late March progression of the tropical cycle shows the MJO moving through the Indian Ocean towards the Maritimes (shaded green) This represents the suppressed (lower momentum) part of the tropical cycle in the Pacific

 

Similar to what is about to happen at this stage of April. Notice also the eastward progression of the MJO right through the Pacific that coincided with the subsequent rapid recovery of momentum earlier in April (shaded light blue in above plot)

Moving forward through April to this particular point in time we can see the suppression phase of tropical convection across the Pacific as shaded deep red/white in the following velocity potential anomalies.

 

This correlates to the latest forecasted increased trade winds across the Pacific to the dateline (shaded blue) and re-posted as per the top of the page (minus my scribblings!)

Which in turn is represented by steeply falling angular momentum tendency, from the peak in early to mid April. The drop closely mirroring the late March low-point, as per the following plot :

 

So synoptically we repeat a similar sequence to March with this:                             Leading to this today:                                       Leading to retrogressed pattern and fall of pressure across UK

                                                                             

                                                         

 

One remaining illustration of the jigsaw pieces is how the Global Wind Oscillation has interpreted this fascinating previous 30 days. Notice how the large swings in angular momentum, as depicted on the pull and push on the synoptic pattern in this time are depicted by the strong transitional orbits between high amplitude Phase 8 (rapidly falling momentum shaded blue) to high amplitude Phase 4 (rapidly rising momentum shaded red) on through the Nino attractor phases 5,6,7 (high momentum) and now back to Phase 8 and momentum rapidly falling back once again as it did late March

 

Its worth adding to all this analysis that the late March cooler retrogressive spell coincided in a preliminary drop in zonal winds across the polar field c/o initial weakening of the 19/20 seasonal polar vortex prior to brief uptick once more during the first half of April.  The upcoming cooler and less settled period from the start of next week coincides with a final splitting and demise of the remaining vortex that ends its long and formidable season of influence.

Looking to the late Spring and towards early Summer, much is going to hinge on how the tropical>extra tropical handles a potential switch to La Nina conditions later in the year. In this respect it depends whether a further westerly wind burst can occur during May to return fine conditions once more as tied into the tropical cycle heading back to the Pacific and rallying angular momentum as happened in the early to mid part of this month. Wavelength changes tied into the approach of summer are going to accentuate trend patterns either way (rising or falling momentum) so as often is the case at bridging periods of the year (much as November prior to winter) something of a crossroads is likely during the coming 3 to 5 weeks.

Another uncertain factor is the QBO transition to easterly. This process has been rather stalled since January and the evolution of this in coming weeks will play a part in how the weather patterns play out. Unlike last late Spring, it seems that absence of an abrupt final warming of the stratosphere, *should* an eQBO progression be further delayed, aid any further recovery of momentum during the first half of May into producing another fine spell. If that happens.

In that respect the AAM forecasts from the CFS and GFS provide signals for that rally. The latter model profile looking rather out of synch with actual proxy data however.

 

Should eQBO progression resume, this increases likelihood of a more sustained fall in momentum, and without the recovery seen in April, then the models are likely to latch onto a more supressed jet stream remaining with pressure staying higher across the NE Atlantic and lower pressure displaced between the UK and Scandinavia. This particularly for approach into summer if the CFS AAM trend to default longer periods of increased trade winds verifies in the immediate approach to summer.

There are no definite answers to these questions at this time - the analysis simply points out factors to look out for. On top of uncertain e/QBO progression timing, NWP volatility is likely to persist as a result of a combination of likely still ongoing volatile GSDM momentum patterns (as described in this post and earlier by @Singularity ) and also the issue of possible (?) data glitches related to the ongoing global crisis (non weather related).

 

 

Edited April 23, 2020 by Tamara

[Methuselah]

With some folks talking about next week being 'very cool', let's see what the GFS 06Z ensembles show:

    

    

So, not exactly 'very cool', but given a prolonged, torrential downpour and heavy cloud-cover, I suppose it's possible...But, overall, maxes between, say, 12 and 17C still look reasonable, to me...?

[Zak M]

High-five!

[ChannelThunder]

1 minute ago, Zak M said:

High-five!

That's a Spanish Plume just waiting to happen, shame it's well outside of reliable timeframe!

[Frosty.]

The Gfs 12z operational turns into an unsettled run for the most part following a fine and warm start, there’s some complex troughs in there too but again longer term there’s glimmers of hope that high pressure might return but the main news is that following this gorgeous sunny spell things are going to change soon.

PS..Great post as usual from Tamara, sorely missed her mega high quality input in recent months!

Edited April 23, 2020 by JON SNOW

[Methuselah]

I was getting all excited when I saw that plume coming, only for it go the wrong side of the left-hand upright! He should have done better there, Alan?

   

Never mind though, we've got plenty of time yet!

[Methuselah]

Not surprisingly, the GFS 12Z operational run is a 'tad' on the cold side of the ensemble:

    

    

It's also rather out-on-its-own regarding SLP. So, maybe even less definitive than usual?

[MATTWOLVES 3]

The ECM is a bit of a dogs breakfast, and obviously shows a more unsettled spell beyond this weekend.... On a positive we see some signs of High Pressure building from the SW by day 10...but good lord the difference at day 10 from the 0z and 12z is ridiculous.. Perhaps a lack of balloon data... Or someone is having a laugh... Stay safe. 

Edited April 23, 2020 by Mattwolves

[stodge]

Evening All

Looking at the output, the one word that comes to mind is unsettled. It will be a marked contrast to the glorious weather of the past few days with largely cloudless skies and low humidity.

ECM, GEM, GFS OP and GFS Control respectively at T+240.

I'm struck by how GFS seems keen to maintain blocking over Greenland and a weak mid-Atlantic ridge. Pressure is trying to rise over SE Europe leaving the British Isles dangerously close to the trough which threatens to align into Scandinavia.

ECM and GEM offer a different evolution and it's interesting to see ECM trying to bring the Azores HP closer via a small LP over the islands sending the HP nearer us. We've yet to see the Azores HP play its hand but there's more than a hint from GFS, GEM and some earlier comments on this thread that we might see the trough dig well south between the Azores and Iberia which would be good news for summer fans as that would allow for a S'ly flow and of course potential instability. We aren't thee yet - the LP is too close and we end up with an unsettled SW'ly flow which will be damp for many though warm for the SE. 

ECM's evolution is very different and is more traditionally mobile though the strong LP coming out of Newfoundland at T+240 might encourage the mid-Atlantic ridge to push up in front - we'll see.

[damianslaw]

1 hour ago, stodge said:

Evening All

Looking at the output, the one word that comes to mind is unsettled. It will be a marked contrast to the glorious weather of the past few days with largely cloudless skies and low humidity.

ECM, GEM, GFS OP and GFS Control respectively at T+240.

I'm struck by how GFS seems keen to maintain blocking over Greenland and a weak mid-Atlantic ridge. Pressure is trying to rise over SE Europe leaving the British Isles dangerously close to the trough which threatens to align into Scandinavia.

ECM and GEM offer a different evolution and it's interesting to see ECM trying to bring the Azores HP closer via a small LP over the islands sending the HP nearer us. We've yet to see the Azores HP play its hand but there's more than a hint from GFS, GEM and some earlier comments on this thread that we might see the trough dig well south between the Azores and Iberia which would be good news for summer fans as that would allow for a S'ly flow and of course potential instability. We aren't thee yet - the LP is too close and we end up with an unsettled SW'ly flow which will be damp for many though warm for the SE. 

ECM's evolution is very different and is more traditionally mobile though the strong LP coming out of Newfoundland at T+240 might encourage the mid-Atlantic ridge to push up in front - we'll see.

A resurgent jet is on its way, and the trough is going to become squeezed direct over the UK - a westerly feed as we enter May, first significant rain for over 6 weeks to many NW parts.

[Zak M]

The GFS 12z vs the GFS 18z.

Just shows how quick things can change!

   

Edited April 23, 2020 by Zak M

[Fourty Point Three Degrees]

4 hours ago, Mattwolves said:

The ECM is a bit of a dogs breakfast, and obviously shows a more unsettled spell beyond this weekend.... On a positive we see some signs of High Pressure building from the SW by day 10...but good lord the difference at day 10 from the 0z and 12z is ridiculous.. Perhaps a lack of balloon data... Or someone is having a laugh... Stay safe. 

Theres nothing dogs breakfast about it. The 12z looks pretty much like standard weather for the UK for this time of year... The confidence for a return to a more standard weather pattern at the end of the April and into May has been growing for sometime now.

[mushymanrob]

23 hours ago, SteveB said:

I can't believe you think we need rain, have you not been living in the UK for the last 8 months? I averaged +100mm every month from August to February. And suspect many other parts of the Country did the same, if not more. If there is one thing you can guarantee in UK weather - it's rain, you can however, guarantee sunshine. I for one, wouldn't care less if it didn't rain again until Autumn. But, as previously mentioned,  it always rains in the UK.

And when it does, it has a habit of sticking around for minimum of week or more, which it looks likely will happen from Monday onwards.

Id suggest you get outside a bit more if you think we dont need rain!

The ground is rock hard, plants are becoming drought stressed, grass is starting to brown off on shallow soils.. all that rain we had between last june and march has gone... the aquifers may be full, reservoirs may be full, but gardens are dry... fields are now dry, growers, gardeners, farmers, need rain.

And we wont be disappointed by the look of it... a gradual slide to low pressure domination and "usual" westerly sourced weather is as good as certain.

[SteveB]

1 hour ago, mushymanrob said:

Id suggest you get outside a bit more if you think we dont need rain!

The ground is rock hard, plants are becoming drought stressed, grass is starting to brown off on shallow soils.. all that rain we had between last june and march has gone... the aquifers may be full, reservoirs may be full, but gardens are dry... fields are now dry, growers, gardeners, farmers, need rain.

And we wont be disappointed by the look of it... a gradual slide to low pressure domination and "usual" westerly sourced weather is as good as certain.

You miss my point mate. You don't need to wish for rain, we will get plenty of that,  what gardeners and farmers need is sun as well. I'm sure all those farmers whose land was flooded,  are happy to see some dry weather and sun, safe in the knowledge;  it will definitely rain again soon enough!

[Wynn D Woo]

1 hour ago, mushymanrob said:

Id suggest you get outside a bit more if you think we dont need rain!

The ground is rock hard, plants are becoming drought stressed, grass is starting to brown off on shallow soils.. all that rain we had between last june and march has gone... the aquifers may be full, reservoirs may be full, but gardens are dry... fields are now dry, growers, gardeners, farmers, need rain.

And we wont be disappointed by the look of it... a gradual slide to low pressure domination and "usual" westerly sourced weather is as good as certain.

It rained pretty much all day last saturday...

[Singularity]

Well, what can I say - Tamara took my comments and built a fortress out of them!  Big thanks for that .

An interesting point to note in relation - for over a week, the GEFS & EPS models had the MJO stalling in the western Indian Ocean. It's only in the last few days that further propagation eastward has been seen by those. They often seem to fall down when it comes to determining whether propagation will continue following a slow-down such as has recently taken place.

 

[mb018538]

A good soaking on the way for many areas over the next 10 days. Only exception is the far NW of Scotland if the GFS is to be believed:

[laddie]

1 hour ago, ¯\_(ツ)_/¯ said:

It rained pretty much all day last saturday...

It rained most of Saturday morning here made no difference, dry as bone again by teatime

[mushymanrob]

1 hour ago, SteveB said:

You miss my point mate. You don't need to wish for rain, we will get plenty of that,  what gardeners and farmers need is sun as well. I'm sure all those farmers whose land was flooded,  are happy to see some dry weather and sun, safe in the knowledge;  it will definitely rain again soon enough!

my bad... lol..

1 hour ago, ¯\_(ツ)_/¯ said:

It rained pretty much all day last saturday...

6mm all month here in Derby...

[knocker]

The SYNOP rainfall figures for this month speak for themselves

http://www.met.reading.ac.uk/~brugge/CURR.html

Edited April 24, 2020 by knocker