Model Output Discussions 12z 03/05/2016

[knocker]

5 hours ago, knocker said:

The position and strength of the jet stream is determined by the atmospheric circulation and hence where surface low and high pressures reside.and this is influenced by many factors so no it wouldn't be correct to say that

It has been pointed out to me in no uncertain terms that my answer above is incorrect.Actually it's very badly worded which I will now correct and it would have been better worded thus.

The position and strength of the jet stream is determined by the atmospheric circulation and hence where surface low and high pressures reside.and this is influenced by many factors as well as ENSO so no it wouldn't be correct to say that the latter alone would be responsible for the position and strength of the jet stream.

Edited July 3, 2016 by knocker

[Frosty.]

17 minutes ago, Matthew Wilson said:

Yes Frosty would love to see this! When Knockers on your side too i think we'll be in the money. I might frame that chart, a nice one for the living room

I don't think knocker will ever be on my side, in winter he doesn't like cold and in summer he posts the coolest most unsettled charts he can find..  only joking malcolm. 

[Frosty.]

9 minutes ago, markyo said:

I would hang fire a bit longer,charts looking good for those after that heat as with winter charts showing cold have a habit of doing a vanishing act! Just need a firming up and cross model support to really have any chance of them coming off.

I should add some balance to my earlier post showing those peachy 6z perturbations, there were also some horrible charts showing cool, windy unsettled weather but since we have seen enough of that cr*p already this summer, i decided to post the eye candy instead

[northwestsnow]

Well gfs12z looks decent for most of next week with temps of 20 and slightly above for much of southern Britain.

Indeed, it sweeps the low through very quickly as hit the weekend and again much of Southern Britain escapes most of the wind and rain.

Its becoming almost cursory now that ukmo/ecm will be no where near as good, as in winter when coldies get a cold snowy gfs the others always seem to disagree.

Maybe its my paranoia but there rarely seems to be agreement between the models when its nice in summer or cold in winter!

[markyo]

21 minutes ago, Frosty. said:

I should add some balance to my earlier post showing those peachy 6z perturbations, there were also some horrible charts showing cool, windy unsettled weather but since we have seen enough of that cr*p already this summer, i decided to post the eye candy instead

I agree totally Frosty! This summer for warmth/heat lovers has been shocking in my opinion,fingers crossed something like these charts comes off....i'll just have to grin and bear it! Many thanks for the positive tone you always give,be it winter or summer,many on here appreciate it.

[Matthew.]

4 minutes ago, markyo said:

I agree totally Frosty! This summer for warmth/heat lovers has been shocking in my opinion,fingers crossed something like these charts comes off....i'll just have to grin and bear it! Many thanks for the positive tone you always give,be it winter or summer,many on here appreciate it.

Yes well said. Frosty is very positive. Knocker gives balance then with the Noaa, Eps etc.

[Cableguy]

Anybody brave enough to hazard a guess for next Saturday for the Swindon area? Going to Royal International Air Tattoo(RIAT)  at RAF fairford and slightly concerned it will be a bit soggy, shame it wasn't a few days earlier by the looks of it but I guess there is still time for things to change for the better.

[knocker]

The GFS is still running with a transient ridge Tues/Weds before fronts arrive from the west on Thursday mainly affecting the west and north. Then on Friday the NE trcking depression arrive bringing wet and windy weather, more focused to the north, but blustery everywhere, The is no consensus on the track of this depression as yet so in the pending file. 

Edited July 3, 2016 by ajpoolshark
removed reference

[knocker]

3 minutes ago, Cableguy said:

Anybody brave enough to hazard a guess for next Saturday for the Swindon area? Going to Royal International Air Tattoo(RIAT)  at RAF fairford and slightly concerned it will be a bit soggy, shame it wasn't a few days earlier by the looks of it but I guess there is still time for things to change for the better.

Well I'm certainly not and much rather depends on the track of above mentioned depression so best hang fire until the models firm up on this.

[A.J]

posts removed for being off topic....if you want to chit chat, use the pm system............I'm surprised posters in question didn't already know this

[Frosty.]

40 minutes ago, Cableguy said:

Anybody brave enough to hazard a guess for next Saturday for the Swindon area? 

My guess is unsettled with a band of rain working across from the w / sw. timing difficult at this range but behind the rain it looks brighter with sunshine and then showers, some heavy and thundery. I would prepare for saturday being an unsettled day following the drier and brighter spell through most of the week ahead in the south. 

[Cableguy]

Thank you both for your replies.......Very much looks like keeping an eye on the models/keeping your fingers crossed.

[Frosty.]

3 minutes ago, Cableguy said:

Thank you both for your replies.......Very much looks like keeping an eye on the models/keeping your fingers crossed.

You're welcome, timing of the rain band is uncertain for now, it could push through on Friday evening / night leaving Saturday with sunshine and only a few showers but good luck with the weather anyway. :- )

[Man With Beard]

The GEFS 12Z follows on from the 06Z (as Frosty picked up nicely earlier) in producing some very encouraging runs for mid-month. There seems to be a clear opportunity for hotter weather coming up after D10. It seems highly likely there will be a deep trough to our W or NW. The key is whether this trough links up with another trough to the E. Those that do not link up allow heat to rise up from the south as with the following charts:

The remaining charts see a stronger Scandi trough gobbling up any troughs from the Atlantic, keeping us rather cooler:

It's about a 50/50 split between these two scenarios. But there is hope on the charts that ensemble means disguise. We can be buoyed by this coming week, which at one point looked a write-off but to be quite honest is now looking like a very reasonable summer's week in the southern half of the UK at least.

[A.J]

GooFus has it dry next weekend in Swindon, chance of a light shower but moderately warm (around 70F).......ECM looks ok in the short to mid time frames the further south one is.....warming up gradually this week (temps into the low to mid twenties maxing on friday in the south)) then an atlantic low steams in with attendant fronts, and then guess what, for the rest of the run it's groundhog day with troughing slap bang over the UK

[Frosty.]

The GEFS 12z mean shows high pressure settling in across the south for a time during the week ahead so it should be mainly dry with some sunshine and light winds across the s / se...good news for Wimbledon! Thereafter, the unsettled weather to the northwest becomes widespread by the end of the week and then it's a changeable outlook with some fine and warm spells across the south as the Azores high occasionally ridges closer to the southwest of the UK...so, in a nutshell, south has the best potential with central and northern areas progressively less so but one thing I noticed is the troughing to the NW looks like weakening / filling by mid July onwards so the second half of the month could be better, at least further south.

Edited July 3, 2016 by Frosty.

[knocker]

The GEFS anomalies still showing the trough dominating proceedings with the caveat that in the 3-8 time time frame the HP does push a little north thus systems will track NE bringing the wettest and windier weather to the north. In the later time frame the trough regains the ascendancy bringing unsettled to most of the UK

 

[Summer Sun]

High pressure taking over across Greenland in the latter stages of ECM...

Edited July 3, 2016 by Summer Sun

[Summer Sun]

GEM shows a deep low heading our way later in the week once it arrives its in no rush to leave

[knocker]

The ecm has the main depression south of Iceland at 12z Saturday with the fragmenting front lying across the UK it then has brief ridging before moving the next depression in at the beginning of next week Temps varying slightly around the average for the run

 

[Frosty.]

The week ahead is going to become noticeably warmer according to the Ecm 12z with temps into the low to mid 20's celsius across the southern third / half of the UK during the second half of the week, especially southern / southeast England as warmer uppers (T850s) gradually spill across most of the BI from the southwest. The other good news is a ridge of high pressure builds across the south bringing largely dry and fine conditions for a while although central UK has a few rainy spells to deal with early in the week before conditions improve for Wed / Thurs. The fine spell lasts longest across the SE.The NW of the UK becomes more generally unsettled later in the week and eventually the fine spell breaks down further south, possibly with thundery rain / showers as it still looks warm and humid in the south of the uk towards the end of the week. Another ridge attempts to build into the south at the end of the week but the end of the run shows a trough slowly drifting east over the north of the UK so a cyclonic end with sunshine and heavy, thundery showers with temps closer to average but for the south in particular, most of the next 5 days or so look much better than recently, at least feeling like summer again!

[Vorticity0123]

June has been an interesting month for many parts fo Western Europe, featuring excessive rainfall and floods for several parts of e.g. Germany, Switzerland and the Netherlands. This was caused by relative weak pressure differences over mainland Europe, on average rather low pressure and relatively moist air. However, July has shown a turnaround in the pattern. A steady westerly flow has brought a series of troughs and low pressure areas towards Europe, resulting in mostly changeable weather. Will this westerly flow persist, or will we see a significant change in the weather patterns to come?

Westerly flow - but ridge to come

The westerly flow has also been dominating today with low pressure to the north of the UK and high pressure to the south. However, change is on the way if we look upstream - just south of Greenland.

GFS analysis of 500 hPa heights (colours) and surface pressure (white contours) for Sunday 12Z.

To the south of Greenland, a weak ridge can be identified mainly in the upper levels (orange colours pointing northward). Often such high pressure areas (or ridges) tend to stabilize the weather for some time.

- But for how long?

The ridge will pass by the UK around this Wednesday, but it does not seem to be a rather transient feature. This fits very well in the pattern which we have observed over the past week or so - with ridges and troughs quickly alternating in a meandering westerly flow.

GFS forecast of 500 hPa heights (colours) and surface pressure (white contours) for Wednesday 12Z.

From the west a new low pressure area (green colours) is again approaching the UK. But will it also reach the UK? The models agree that this will somewhere around the weekend. However, one can also see notable differences developing between these models on the details of the low pressure area.

(Un)certainty and phase differences

The further outlook is what one could call certain as well as uncertain - depending on the point of view. Although there is a lot that can be said about the general pattern - it is about nigh impossible to have a certain local forecast in 6 days out or more! And we can caputure this (un)certainty in just one plot.

GFS ensemble spaghetti forecast of 500 hPa height for Monday (8 days out) 12Z.

The image above shows an ensemble of GFS forecasts for 8 days out. Here one should focus on the bottom lines, indicating the height of the so-called 5760 dam (5.76 km height) surface. Each line represents one model run.

First of all, it can be seen that all lines are generally very close together for the 576 dam line, suggesting high certainty in the overall forecast. This forecast suggests low pressure to the north and high pressure to the south, with a potent westerly flow in between. One can also see this from all lines running west-east. See also the forecast of the CPC (Climate prediction center) for day 6-10.

However, there is one huge caveat if one wants to apply this to a local weather forecast. If you examine all the lines individually, you can see that there are slight meanders in them. The position of these weak meanders (which can be interpreted as ridges if they point northward and troughs if they point southward) varies from run to run. However, the exact position of such a small scale feature is very important for the weather on that day. For example, a ridge for a given day (a northward pointing line) would argue for settled conditions, and a downward-pointing line would suggest a trough and thereby unsettled conditions. This uncertainty is what one could call phase differences, as depicted in the figure below.

Illustration of phase differences between various model runs. Note how the location of the ridges (H) and troughs (L) differ strongly, an indication of a phase difference.

Summary

It appears that the westerly flow which has been present over the past few days will continue to persist for at least the next week (and probably thereafter). This will create changeable weather (with a possible drier interlude on tuesday-friday). Details at long range are hard to gauge due to the phase differences discussed above, but it is pretty certain that the main theme will be changeable weather.

 

 

 

 

Edited July 3, 2016 by Vorticity0123
Deleted obsolete images

[Frosty.]

The Ecm 12z ensemble mean mirrors the op with a general warming trend through the week ahead, Friday probably the warmest day in the south with mid 20s celsius and a good deal of fine, sunny weather across southern areas in the next five days. The northwest becomes more unsettled and then towards the end of the run, a trough in the north Atlantic drifts over the UK with sunshine and heavy, thundery showers and temps high teens to low 20s c from north to south.

[thestixx]

29 minutes ago, Vorticity0123 said:

June has been an interesting month for many parts fo Western Europe, featuring excessive rainfall and floods for several parts of e.g. Germany, Switzerland and the Netherlands. This was caused by relative weak pressure differences over mainland Europe, on average rather low pressure and relatively moist air. However, July has shown a turnaround in the pattern. A steady westerly flow has brought a series of troughs and low pressure areas towards Europe, resulting in mostly changeable weather. Will this westerly flow persist, or will we see a significant change in the weather patterns to come?

Westerly flow - but ridge to come

The westerly flow has also been dominating today with low pressure to the north of the UK and high pressure to the south. However, change is on the way if we look upstream - just south of Greenland.

GFS analysis of 500 hPa heights (colours) and surface pressure (white contours) for Sunday 12Z.

To the south of Greenland, a weak ridge can be identified mainly in the upper levels (orange colours pointing northward). Often such high pressure areas (or ridges) tend to stabilize the weather for some time.

- But for how long?

The ridge will pass by the UK around this Wednesday, but it does not seem to be a rather transient feature. This fits very well in the pattern which we have observed over the past week or so - with ridges and troughs quickly alternating in a meandering westerly flow.

GFS forecast of 500 hPa heights (colours) and surface pressure (white contours) for Wednesday 12Z.

From the west a new low pressure area (green colours) is again approaching the UK. But will it also reach the UK? The models agree that this will somewhere around the weekend. However, one can also see notable differences developing between these models on the details of the low pressure area.

(Un)certainty and phase differences

The further outlook is what one could call certain as well as uncertain - depending on the point of view. Although there is a lot that can be said about the general pattern - it is about nigh impossible to have a certain local forecast in 6 days out or more! And we can caputure this (un)certainty in just one plot.

GFS ensemble spaghetti forecast of 500 hPa height for Monday (8 days out) 12Z.

The image above shows an ensemble of GFS forecasts for 8 days out. Here one should focus on the bottom lines, indicating the height of the so-called 5760 dam (5.76 km height) surface. Each line represents one model run.

First of all, it can be seen that all lines are generally very close together for the 576 dam line, suggesting high certainty in the overall forecast. This forecast suggests low pressure to the north and high pressure to the south, with a potent westerly flow in between. One can also see this from all lines running west-east. See also the forecast of the CPC (Climate prediction center) for day 6-10.

However, there is one huge caveat if one wants to apply this to a local weather forecast. If you examine all the lines individually, you can see that there are slight meanders in them. The position of these weak meanders (which can be interpreted as ridges if they point northward and troughs if they point southward) varies from run to run. However, the exact position of such a small scale feature is very important for the weather on that day. For example, a ridge for a given day (a northward pointing line) would argue for settled conditions, and a downward-pointing line would suggest a trough and thereby unsettled conditions. This uncertainty is what one could call phase differences, as depicted in the figure below.

Illustration of phase differences between various model runs. Note how the location of the ridges (H) and troughs (L) differ strongly, an indication of a phase difference.

Summary

It appears that the westerly flow which has been present over the past few days will continue to persist for at least the next week (and probably thereafter). This will create changeable weather (with a possible drier interlude on tuesday-friday). Details at long range are hard to gauge due to the phase differences discussed above, but it is pretty certain that the main theme will be changeable weather.

 

 

Thankyou for taking the time to out together such an informative post. 

 

 

[knocker]

The ecm and NOAA still in the same ball park and indicate remaining unsettled for the next 14 days Of course there will be day to day variations on the smaller scale when the warm and colder air interact, or as Vorticity more correctly puts it the phase difference, within the long wave pattern as mentioned previously when discussing the general outlook. Thus a continuation of westerly upper flow with temps around average initially and then perhaps dropping a tad below.