May Forecast Thread

[Glacier Point]

Strong indications that May will see a quite dramatic turnaround in pressure and temperature anomalies for NW Europe.

Negative sea surface temperature anomalies persist across the central and western Pacific consistent with a mature La Nina, partcularly strong within ENSO Region 4 where temperature anomalies of -0.87 exist:

http://www.bom.gov.au/climate/enso/indices.shtml

http://weather.unisys.com/surface/sst_anom.html

These temperature anomalies persist to some depth and easterly wind anomalies across the western Pacific remain in force, consistent with a -ve GLAAM and well-established Nina:

http://www.cdc.noaa.gov/map/images/aam/glaam.gif

All of this means that Nina conditions will remain in place for May and influence the overall circulation pattern. It should also be remembered that Nina has been in place for some considerable time now bringing about a strong Nina base state.

Composite reanalysis of mature (fading) Spring Ninas and strong -ve anomalies for ENSO Region 4 suggest the following patterns:

Both show a trough into Alaska and NW Canada extending southward into the northern Central Plains with a fairly zonal pattern over the US, blocking over the Hudsons, negative height anomalies over Greenland and Russian upper low. The key difference is in the placement of any low over Iberia. Both solutions tend to suggest an Icelandic low and High towards the UK.

A well organised band of tropical convection in the eastern Indian Ocean has become apparent in the last week. This is responsible for a burst of westerly winds across the equator and slight increase in GLAAM (see above link). The development of such convection is highly likely to influence the development of down-wind anticyclones. These are already starting to be evident in surface wind reanalysis leading to Rossby Wave Dispersal. Although the lates MJO wave is indeterminant, this convection and associated Rossby Wave Dispersal is likely to lead to extratropical ridges. Ed Berry certainly is going for a more meriodanally-orientated dispersal in the next few weeks:

http://weatherclimatelink.blogspot.com/

In this current position (MJO phase 4-6), tropical convection teleconnects to a ridge over NW Europe.

There is some doubt about just how far east this convection will get. My hunch based on the strength of -ve SSTAs to the western Pacific suggest not that much further east than the Philipines which would tend to support the notion of a ridge over NW Europe.

Stratospheric winds over the pole remain in a very low base state coming on the back of an early break up of the polar vortex:

http://ds.data.jma.go.jp/tcc/tcc/products/.../heit_u_nh.html

http://strat-www.met.fu-berlin.de/cgi-bin/...xes&alert=1

This is likely to persist for some time, equating to a stronger Arctic High. Stratospheric temperatures have however cooled over the last month. These -ve temp anomalies have started to permeate downwards into the upper troposphere and will likely lead to a slight increase in stratospheric zonal winds in due course helping to set up lower heights over Northern Greenland and Russia.

http://strat-www.met.fu-berlin.de/cgi-bin/...mps&alert=1

http://www.cpc.ncep.noaa.gov/products/stra...AMJ_NH_2008.gif

Reanalysis of similar years for the current observed 30hPa zonal wind anomalies suggests the the following zonal wind and 500 hPa patterns (note, these analogue years contain a high sample of early Spring Sudden Stratospheric Warmings):

Observed:

Predicted zonal and 500hP pattern:

Reanalysis of the seasonally lagged SSTA pattern across the Pacific and Atlantic (perhaps reflective of years with similar Nina and QBO signatures) suggests the following:

Composite years SSTA:

This year Feb-April so far:

Predicted baroclinic response:

Note the same messages emerging here - indications of trough in the western Atlantic and ridge towards NW Europe, but with Iberian High not low.

Current surface and lower tropospheric temperatures over the North Atlantic reflect the cold March and April with significant cold anomalies towards Newfoundland. These will likely persist in a negative feedback driving a -NAO type pattern with cut off lows and troughing in the Atlantic.

Putting this together, I think there are solid grounds for assuming a high pressure anomaly for the UK and NW Europe with a trough in the western Atlantic with an increasing trend towards stratospheric westerlies towards the end of the month as the impact of stratospheric temperatures works through to drive a +AO.

Weatherwise, a high probability of drier and warmer than average conditions so feeling very pleasant in comparison to previous months.

[Gavin P]

Thanks GP. A very nice month. B)

Any hints as to where this is going for summer? My own view would be warm for early summer, and then rapidly going downhill later on as high pressure pulls away into the mid Atlantic.

[johnholmes]

tks GP another very well presented analysis and summary from that.

Lots to read in depth.

a huge amount of work and effort into that.

a question from a newbie in this area?

The NAO is progged to go -ve but then suggets rising again although with a fair amount of spread?

Do you agree with this?

Can you say why or why not please GP?

How far ahead does a signal on the NAO give guidance which can be used sensibly?

tks if you can answer

Edited April 28, 2008 by johnholmes

[Glacier Point]

tks GP another very well presented analysis and summary from that.

Lots to read in depth.

a huge amount of work and effort into that.

a question from a newbie in this area?

The NAO is progged to go -ve but then suggets rising again although with a fair amount of spread?

Do you agree with this?

Can you say why or why not please GP?

How far ahead does a signal on the NAO give guidance which can be used sensibly?

tks if you can answer

The upper atmosphere reanalysis suggests an Icelandic low conistent with a +NAO as the blocking over the past month is turned round, but, SSTA and Nina reanalysis suggests something more of a neural/-NAO for May. I would tend towards a troughing solution for the Atlantic which equates to something of a -NAO pattern (but remember with a +AO or below heights over central / Northern Greenland).

In so far as the guidance, NAO and AO ensembles will be useful in informing a trend but I wouldn't trust this aspect of GFS modelling too much in this circumstance because I think the GFS has inherent problems when it comes to modelling longwave troughs in the Atlantic.

[mountain shadow]

Excellent post GP, however I'm in a quandry as to whether a predicted -NAO would support warmer than average conditions in the UK.

With a ridge centered north of the Uk, an Easterly may not necessarily be warm depending on actual positioning, Eastern coasts would be particularly cool I would think, with western areas warmer.

[johnholmes]

The upper atmosphere reanalysis suggests an Icelandic low conistent with a +NAO as the blocking over the past month is turned round, but, SSTA and Nina reanalysis suggests something more of a neural/-NAO for May. I would tend towards a troughing solution for the Atlantic which equates to something of a -NAO pattern (but remember with a +AO or below heights over central / Northern Greenland).

In so far as the guidance, NAO and AO ensembles will be useful in informing a trend but I wouldn't trust this aspect of GFS modelling too much in this circumstance because I think the GFS has inherent problems when it comes to modelling longwave troughs in the Atlantic.

tks GP for that

[summer blizzard]

Excellent post GP, however I'm in a quandry as to whether a predicted -NAO would support warmer than average conditions in the UK.

With a ridge centered north of the Uk, an Easterly may not necessarily be warm depending on actual positioning, Eastern coasts would be particularly cool I would think, with western areas warmer.

Mountain, at this time of year with the sun so strong, it is not so much where the ridge is centred, but how high pressure is over the UK, with above average pressure and almost 18 hours of sunshine you are more likely to see above average temperatures and also the CET has a western bias anyway, negating the effects of an easterly.

[kold weather]

According to the Aussie mets La nina has gone now and looking at the SSTA in the region there is barely enough cold water present to say we are in a la nina...I wonder whether this could skew the results a little esp in the 2nd half of May and into June?

[Glacier Point]

Surface values have weakened somewhat, but subsurface values in the western/central Pacific still show -1 to -2 C anomalies to 50-100m deep.

Easterly winds still in evidence for the central / western Pac.

Tropical convection popping up in the east Indian Ocean and eastern Pacific, but strongly +ve OLR over the central / western Pac.

http://www.cdc.noaa.gov/map/images/olr/olr.anom.gif

....and GLAAM continues to bump along in -ve territory (remember it was -ve when last year's El Nino collapsed).

http://www.cdc.noaa.gov/map/images/aam/glaam.gif

My view is that this is normal of a mature Nina event for the season but needs monitoring closely.

One of the factors for this Summer which makes me think weak Nina is a very weak westerly QBO.

Edited April 30, 2008 by Glacier Point

[azores92]

Surface values have weakened somewhat, but subsurface values in the western/central Pacific still show -1 to -2 C anomalies to 50-100m deep.

Easterly winds still in evidence for the central / western Pac.

Tropical convection popping up in the east Indian Ocean and eastern Pacific, but strongly +ve OLR over the central / western Pac.

http://www.cdc.noaa.gov/map/images/olr/olr.anom.gif

....and GLAAM continues to bump along in -ve territory (remember it was -ve when last year's El Nino collapsed).

My view is that this is normal of a mature Nina event for the season but needs monitoring closely. One of the factors for this Summer which makes me think weak Nina is a very weak westerly QBO.

so precipitation wise, what is more likely for the uk in summer?

[Glacier Point]

so precipitation wise, what is more likely for the uk in summer?

Too early to get a firm idea, but if La Nina conditions persist, a dry first half and average to slightly above 2nd half would be probable.

[kold weather]

Yeah no doubt the atmosphere is still acting in a la nina way but the surface temps only started to really rapidly rise about 4-6 weeks so there may be a lag effect with this I wonder.

subsurface temps in the western zone are very warm now but of course that makes no difference as long as its not upwelled, as you say GP you do see a weakening of the surface profile of the La nina as you head into Spring but this is a VERY rapid rise we've seen in the last 4-6 weeks somewhat above the usual rise we see. I suspect we will see overall an neutral summer with regards to the summer and the ENSO.

Once again its chicken and the egg stuff, does the surface temps respond to the changes of the atmosphere or does the surface temps dictate the atmospheric set-up, very hard to say!

Edited April 30, 2008 by kold weather

[BrickFielder]

El nino or La nina are caused by kelvin waves or in la nina's case the lack of strong kelvin waves. Normally trade winds along the equator of the Pacfic blow from the east to west which piles warm water up towards indonesia and causes cooler water to come up from the bottom in the east. As warm water builds up near indonesia it causes thunderstorms to bubble up in an eastward direction across the Pacific as the MJO wave circulates eastwards around the world. This, if strong enough reverses the trade wind for long enough for a wave of warmer water (kelvin wave) to cross the pacific to the east. Warmer water in the east cause thunderstorms which suppress the trade wind even more and allows more waves of warm water to come from Indonesia (El nino results). Eventually the warm water pool around indonesia is exhausted and the trade winds pick up again (la nina). A clue to what will happen is in the sub surface temperatures around indonesia. These do tend to warm up during Spring but appear to be very warm at the moment.

Warm water volumes have also increased markedly and this must spell the end of la nina during the summer and a quick transition towards el nino for late summer.

I have a suspicion though that el nino will be stopped in its tracks during the autumn and we will see a return to la nina. The reason for this is conditions in the Indian Ocean which has its own form of el nino/la nina. Subsurface temperatures to the south of Indian are low and will move eastwards to cool waters around Indonesia and damping down convection there.

The whole key to this is how kelvin and rossby waves work and their timing as water is moved about the surface of the ocean. My feeling is that the slower Indian Ocean cycle is providing a trigger for el nino conditions. This might suggest the asian monsoon along with winter snow conditions across northern asia play a key role in predicting el nino conditions over the long term.

MJO going through phases 6,7,8 along with mild la nina will most likely dominate Mays weather.

[johnholmes]

thanks for that insight Brick

[Glacier Point]

Bit short of time for a detailed update but the overall call for an above average May remains unchanged.

A band of intense tropical convection has shifted east over the Indo-Pacific region. This has introduced a westerly wind component to the global wind oscillation likely to cause a ridge building into the western States consistent with a +PNA pattern and bolstering of the sub-tropical highs (Azores High included) - which will galvanise the Polar Jet into action.

The MJO is in phase 6 and will likely continue in phases 6-8 which will result in some downstream changes for the UK over week 3 of May with more emphasis on less settled and less warm (but still likely average) weather.

Further on into week 4, when the MJO waves passes out of phase 8 and the centre of tropical convection moves back to the Indian Ocean, we will see the broad SSTA pattern take effect with a trough setting in the western Atlantic and sub-tropical ridge building into Iberia with pressure rising over the UK from the south (although some uncertainty here whether this will be a pressure build from the south or 'bridging' high over an Iberian low).

[johnholmes]

tks GP

[acbrixton]

El nino or La nina are caused by kelvin waves or in la nina's case the lack of strong kelvin waves. [...]

The whole key to this is how kelvin and rossby waves work and their timing as water is moved about the surface of the ocean. My feeling is that the slower Indian Ocean cycle is providing a trigger for el nino conditions. This might suggest the asian monsoon along with winter snow conditions across northern asia play a key role in predicting el nino conditions over the long term.

MJO going through phases 6,7,8 along with mild la nina will most likely dominate Mays weather.

BF many thanks. I am ashamed to say that I had not heard of Kelvin waves before...

regards

ACB

[Glacier Point]

Easterly wind anomalies are currently being added to the NH courtesy of a strong frictional torque event (also happening in the southern hemiphere):

http://www.cdc.noaa.gov/map/images/reanaly...ltauf.90day.gif

Outgoing longwave anomalies also show conditions for easterly wind flow across the equator with +ve anomalies across much of the Pacific and Indian Oceans but -ve anomalies (westerly wind flow) over the tropical Atlantic:

http://www.cdc.noaa.gov/map/images/olr/olr.anom.gif

All of this means that we are likely to see a fall in Global Angular Momentum and a return to Nina like base pattern. GLAAM is showing first signs of a fall, which is likely to accelerate in the next few days:

http://www.cdc.noaa.gov/map/images/aam/glaam.gif

Notice how easterly wind anomalies are shifting southwards towards the equator with a corresponding increase in westerly wind anoamlies over the sub-tropics. This is likely to continue as the Global Wind Oscillation pushes through Phases 2-4 in the next 7 -10 days:

http://www.cdc.noaa.gov/map/images/gcm/gwo_40d.gif

These are signals we should be very familiar with this Spring - particularly a strong southerly jet with tendency for cut off lows to develop. GLAAM correlations show nicely the relationship between -ve GLAAM and a ridge from the Azores towards a mid latitude high:

At the same time, hemispheric SSTAs continue to signal for a west Atlantic trough whilst the polar field is showing definate signs of a return to neutral or even positive values, supported by the shifting westerly stratospheric winds consistent wth the west QBO.

No surprise that both GFS and ECM are showing high pressure linking from the northward displaced Azores and surface Scandinavian ridge over a moderate strength longwave trough over Iberia. Current modelling looks good from my point of view for the last week of May into June:

http://www.wetterzentrale.de/pics/Recm1921.html

http://www.wetterzentrale.de/pics/Rtavn1921.html

Thereafter, I would be looking at a trend for the Azores high to weaken as the GWO moves through phase 8 and weakens and tropical convection gets organised over the Indian Ocean allowing a trough to reopen in the central Atlantic and strong ridge to build over western Europe with pressure steadily rising over SW Europe and western Europe week 2 of June.

Edited May 23, 2008 by Glacier Point

[johnholmes]

tks GP, I confess to still not being able to get my head around some of the links you quote, but from the little I've managed to pick up over the past 3 months I tend to largely agree with your closing paragraph.

Still not sure which way the ridge will build from but yes a ridge taking over again.

Edited May 23, 2008 by johnholmes

[Glacier Point]

Important to note that the spatial pattern of SSTAs across the Northern Hemisphere is playing a crucial role in determining our weather patterns.

May's predicted H500 anomaly pattern based on lagged SSTA pattern observed Feb - April was as follows:

Over the last 30 days, the following pattern has been observed with a high degree of similarity:

This captures not just the underlying Nina state, but also the arrangement of SSTAs across the western, eastern and northern Pacific consistent of the now very evident -PDO. This also gives confidence in rolling forward towards the underlying pattern for June:

I might tweak that expected pattern to reflect more of a trough in the western Atlantic and greater SW'ly flow but a good benchmarker for the next month.

.... and just for fun, rolling forward further into July and August....

[Gavin P]

Good June and July and a poor August then GP?

Edited May 25, 2008 by Gavin P

[kold weather]

It looks like that Gavin, July looks very interesting simply because of the higher pressure anomalies over Scandinavia and the lower pressure near the Azores.

One thing I've noted on both the tropical forecasts in the Atlantic basin and also now on the map that you have made GP using previous years is a distinct lack of any Azores high, which means we wouldm be totally dependant on any displaced blocking to help us, like it did during the first half of May.

[Gavin P]

I wonder if we're looking at a 1992 type scenario this year, with a very warm early summer giving way to a much poorer second half?