June Forecast Thread

[Glacier Point]

May's overall, and, anomalously warm pattern, was highly correlated to the spatial distribution of SSTA, not just those associated with La Nina, but also the now obvious -PDO signature in the North Pacific and the arrangement of warm and cold anomalies in the North Atlantic.

Bursts of westerly winds in the western Pacific and seasonal trend to decline have weakened equatorial SSTA (region 3.4) to neutral ENSO conditions according to latest data.

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

Easterly surface winds remain in place however and cold subsurface anomalies persist although to shallower depths. Anticyclonic flow is evident across large parts of the Tropics and, allied to a strong recent mountain torque event within the last two weeks, easterly wind anomalies are shifting equatorwards dropping GLAAM into deeply -ve values consistent with the Global Wind Oscillation (GWO) moving through phases 1-3 and increasing Nina signal.

http://www.bom.gov.au/bmrc/clfor/cfstaff/m...Last40days.html

At the same time, there is a corresponding increase in westerlies in the sub-tropics likely to weaken sub-tropical ridges within the next 7-10 days. This is likey to continue for the next 7-10 days with the net result of persisting, indeed strengthening, La Nina conditions and impacting on global weather patterns. The Global Wind Oscillation is likely therefore to orbit within the Nina quadrant of phases 2/3:

http://www.cdc.noaa.gov/map/images/gcm/gwo_90d.gif

This teleconnects to low pressure over the NE Atlantic, similar to that experienced in late April and late March and likely to influence week 1.

Moving further forward into June, tropical convection (-ve OLR) is anticipated to increase in the Indian Ocean and move eastwards in line with a more amplified Asian Jet pulse and SSTA in the equatorial Indian Ocean. There is already evidence of this process beginning to occur and this will likely result in an increase in GLAAM (westerly winds) signalling a challenge to the rejuvenated Nina and generating meridionally orientated Rossby Wave Dispersal forcing ridge development over Scandinavia and pulling the trough in the Atlantic westwards.

This is shown nicely for composites for the MJO in phase 3 - which is where the centre of the convection may form which also corresponds with a movement of the GWO into phase 3/4 which mirrors the periods late March into April and late April into May - which as we know, was a signal for trough development further west in the Atlantic and a very warm pattern.

The recent mountain torque event, which was mostly attributable to storminess over Asia, serves to illustrate the close relationship with stratospheric temperatures. Following this event, wave breaking of the stratospheric jet took place resulting in rising stratospheric temperatures and slackening zonal winds in the upper layers. This could have likely implications for July's weather although strangely it could have a perverse response in the forecast timeframe by trending the AO more positive.

http://ds.data.jma.go.jp/tcc/tcc/products/...e/pole30_n.html

The best guidance however comes from SSTA analogues for the base period March-present.

Base period:

Analogue SSTA:

Analogue H500 pattern:

There is a risk here that GLAAM falling low will stay low bringing about a renewed Nina signal for a large part of the month. If that happened, then a much more unsettled and cooler pattern would be the case:

But this relies on continued supression of tropical convection and the Nina signal being strong enough to over-ride the SSTA signal in the rest of the Pacific and Atlantic, especially those cold anomalies really motoring in the SW Atlantic.

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

Allied to this, we have a warm and slightly unstable airmass now becoming established over central Europe. This is going to be very difficult to shift and any hints of tropical convection in the Indian Ocean (or MJO moving through phases 1-5) will set up a strong ridge over Scandinavia (through Rossby Wave Dispersal) also suggested by SSTA analogues.

To summarise, an unsettled start to the month with a trough or closed low to our immediate west but generally warm and humid. This trough to get pulled westwards leading to increasingly warm and drier conditions but never totally settled due to the instability of the airmass so convective showers cannot be ruled out. Overall above averge temperature wise and average to slightly below rainfall (although a lot of uncertainty in this respect).

[Glacier Point]

Spot the difference....

CPC week 2:

SSTA analogue:

[johnholmes]

thanks for both inputs GP.

Still trying to understand some of the links but those I do seem to support my view of a rather unsettled and rather warmstart to the month and that fairly high probability of it settiling down into the second week of June.

[Glacier Point]

Comparison of OLR data confirms the orginal forecast thoughts of a growing core of thunder activity and tropical convection in the western Indian Ocean shifting eastwards and influencing hemispheric weather patterns.

These -ve OLR anomalies have, and will continue to, lead to an increase in westerly winds across the equator over the Indian Ocean. This development is evident in a clockwise rotation of the GWO into low amplitude phase 3 (phase 4 now very imminent) and likely brings us to a point similar to where we were in late April into May:

http://www.cdc.noaa.gov/map/images/gcm/gwo_90d.gif

Whether this activity will be associated with the MJO is unclear, but the point is that tropical convection in this position is very likely to add extra zip to the atmosphere which is already primed towards an Atlantic trough c/o the SSTA signature.

http://www.bom.gov.au/bmrc/clfor/cfstaff/m...Last40days.html

We should however see an increase in the MJO towards phase 2/3 soon.

That is a signal for the trough in the Atlantic to develop and remain for around 14 days - sustained in a metreological sense.

This will also pose a real and very interesting challenge to La Nina. The westerly burst of winds 'upstream' may become more organised and shift eastwards pushing the GWO potentially towards phase 5 which should crank up the polar jet and introduce a welcome bit of inertia to the North Atlantic.

Either way, I think a more SSTA dominated pattern for parts of western Europe is highly probable (see the orginal post for this implication).

Edited June 3, 2008 by Glacier Point

[Kiwi]

Either way, I think a more SSTA dominated pattern for parts of western Europe is highly probable (see the orginal post for this implication).

I don't pretend to understand all of that Glacier Point but;

"average to slightly below rainfall (although a lot of uncertainty in this respect)"?????!!!

Does that mean no more rain this month in soggy Hampshire?

[johnholmes]

tks GP, would agree with much of what you post

interesting times during the major transition of seasons for each hemisphere as it seems to me.

[Glacier Point]

Does that mean no more rain this month in soggy Hampshire?

Drying out for the middle parts of the month - I guess you could call it good growing weather.

[Glacier Point]

One other, not unrelated point is that the PNA is forecast to become significantly -ve indicative of deep troughing in the west and a ridge over the eastern USA:

http://www.cpc.noaa.gov/products/precip/CWlink/pna/pna.shtml

http://www.cdc.noaa.gov/map/wx/images/pna.gif

Reanalysis of June's where there was a PNA value of -1 or lower supports the development of a weakish Euro High:

...and crucially a deep upper Icelandic low extending a trough into the North Atlantic with also a suggestion that the -PNA drives a +AO.

Edited June 3, 2008 by Glacier Point

[johnholmes]

yes I would agree with that, a 500mb trough somewhere in the Atlantic, and most signals from most sources, suggesting it may stay there for a little while, at least several days possibly longer, with the high somewhere east of the Uk, se or ne I would not be too sure, possibly linking with the Azores high at times.

[Glacier Point]

The second week of the month has been marked by tropical convection remaining in the Indian Ocean (unexpected) and, as a result, we have seen the base Nina state being pepped up - even when official SSTA data suggests that ENSO neutral conditions have developed:

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

Monthly composites for a weak Nina confirm the broad arrangement of pressure anomalies we are familiar with now:

The focus for convection around India is strongly tied to a high pressure anomaly in the eastern Atlantic with long wave trough over Europe. Composite reanalysis for week 2 is a mirror image for analogues for tropical forcing centred over the Indian sub-continent:

This has lead to an (unexpected) fall in GLAAM and shallow orbit of the GWO around the La Nina (phases 2-4) side of the oscillation confirming a weak La Nina base state still in existence:

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

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

Moving forward, note how the negative global wind anomalies are moving poleward over time. This should tie into a peak in the next week followed by the development of mid latitude ridges as westerly wind anomalies begin to take effect and shift the GWO towards phases 4-5 and then neutral values.

The overall circulatory pattern remains one of a weak La Nina oscillating between weak Nina signal (last week into this) and neutral conditions (1st week). This will likely persist until tropical convection shows more of a tendency towards the eastern hemisphere and GLAAM rises towards neutral values.

We also must consider that the Atlantic SSTA is strongly signalling a -NAO and zonal wind anomalies throughout the atmosphere remain weak trending -ve (blocking). I think we will continue with this pattern swinging around and generally something between Nina and ENSO nuetral with no overall dominant and obvious signal. It is likely that models will struggle picking up any meaningful long term solutions given this pattern with the location of tropical convection being very important. For the last week to 10 days of the month we are likely to move away (possibly temporarily?) from the weak Nina signal more towards an SSTA pattern.

Putting some detail on this:

1) Trending towards mid latitude ridge last week with more settled conditions for a time.

2) Breakdown into 1st week July - but uncertainty on the timing of this.

3) No significant warmth, but no significant cold either with bands of rain pushing across the UK at times but feeling pleasant.

[johnholmes]

tks GP, yes I see neither warmth nor cold from the various signals, maybe we have to wait for the hurricane season to set some kind of pattern, which might not be what warm lovers, or at least dry and warm lovers want.

[BrickFielder]

Global Mountain torque has taken a bit of a nose dive during the later part of last week which would hint at a pick up global angular momentum.

Mountain Torque

MJO has just completed a fairly weak cycle and I expect we have about a week for the wave to come back round the globe through Africa.

MJO

Angular Momentum is still on the low side tending towards la nina conditions.

Global Wind

All in all it looks like a bit of a quietly unsettled period with south and east areas best for sunshine with a typical south westerly flow in contrast to the northerly flow which was around in May.

World wide then attention is drawn to Tropical storm Fengshen looks set to cause problems for Hong Kong and possibly Shanghei.

Fengshen

Hints of tropical storm development towards central america perhaps showing the hurricane season is beginning to take shape.

Key indicator for me is the SOI with daily readings tanking recently with some stormy weather down under towards tasmania and New Zealand. This will slow down the demise of la nina and may trigger a slight global pattern change.

SOI