July Forecast Thread

[Glacier Point]

Our current La Nina is enduring death by a thousand cuts, but it was still potent enough to assist in a largely average June. During July, the indications are that further decline will occur in negative SSTAs across the equatorial Pacific although an overlying -PDO will likely prevail.

Current data suggests we are in ENSO neutral conditions with Region 3.4 showing weekly values of -0.21:

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

Negative sub-surface anomalies in the Pacific have also declined and tropical convection is starting to develop more widely indicative of westerly wind development further removing cold surface waters:

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

In line with a poleward burst of easterly winds (typical of La Nina) during June, levels of global momentum fell to values of around -2 SD:

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

Moving forward into July, the indications are that there will not be any such widescale development of easterly winds with more neutral conditions set to prevail.

EWP and CFS tropical convection forecasts suggest the development of conditions likely to spawn tropical convection moving in an organised band across the central and eastern Pacific:

This will likely put pressure on La Nina and continue to generate bursts of westerly winds helping to keep global momentum at neutral values overall. Tendency of global momentum shows no -ve (easterly) wind development in the equator or tropics - in fact the tendency is for westerly (+ve) wind development propagating poleward and easterly winds sinking towards the equator:

All of this is likely to lead to the Global Wind Oscillation (GWO) moving slowly through Phase 5 (El Nino transition) and then decaying before a La Nina type phase towards the last third of July as westerlies are removed from the atmosphere by mountain torques. Note how we are at very similar stages in the GWO as to late April into May and this provides an excellent analogue given the more or less every factor (bar a weaker La Nina) remains the same:

If the May type pattern were to be repeated, then low pressure in the Atlantic and higher pressure to our east would prevail:

(note the amount of high latitude blocking going on there)

Compsosite reanalysis for the SSTA captures nicely the residual Nina and -PDO in the Pacific and arrangement of cold and warm anomalies in the Atlantic. This would suggest the low pressure to be closer to the UK than in May:

Composite years:

SSTA response:

Also worth mentioning, stratospheric wind anomalies remain fairly weak and overall wind flows are relatively quiet or even easterly allowing further episodes of high latitude blocking:

http://ds.data.jma.go.jp/tcc/tcc/products/...lat_u30_nh.html

...which also tallies with the GWO and SSTA composites.

Putting this together:

• low pressure to be centred over and west of the UK to start with showers
  
• low pressure to remain centred west of the UK with rising pressure into week 2 - showers becoming fewer and becoming warmer
  
• pressure falling low over Iberia week 3 and warmth over the UK - fine and settled
  
• tendency for breakdown week 4 although not marked by any very unsettled pattern.
  

Overall I would suggest a high probability of above average temperatures (though not as great as May) and average levels of rain.

Edited June 27, 2008 by Glacier Point

[johnholmes]

tks for that GP another thought provoking read.

[Glacier Point]

I will extend this July thread to cover August and September to keep some continuity in the topic area.

The recent strong burst of tropical convection across the Indian Ocean has left its mark with SSTs below average over large parts of the area. Allied to this, a strong poleward projection of zonal wind anomalies which started in June has begun to retreat southwards in the northern hemisphere. This, added to the underlying -PDO / residual Nina signal are reinforcing a sharp decline in global angular momentum which will likely dominate the weather patterns for the first 2/3 of August.

Declining angular momentum will drive the global synoptic dynamical model (GSDM) through phases 1, 2 and 3 at medium amplitude. This has a remarkable symmetry to the pattern of the summer so far with nearly three orbits of the GSDM in the last 90 days. AS can be seen, phases of settled weather have tended to occur with the GSDM moving from phase 4 to 5 and unsettled weather associated with (the La Nina type) phases 1,2,3.

Looking at NH SSTAs, I can't see any reason for supporting a swift orbit through to phase 4. Composite reanalysis for phases 1, 2 and 3 of the GSDM bear a resemblance to current modelling into the extended period showing an upper low located over Scandinavia following a brief ridge.

This looks very much like a sustained period of unsettled and largely cool weather with showers or longer spells of rain.

In terms of the promise of more summery weather, I would be looking beyond 12 days right now based on previous experience in May and July.

On a more positive note, the upper atmosphere is changing subtly with the peak in the the west phase QBO now reached and +ve zonal wind anomalies likely to increase the polar field into September offering the prospect of an Indian Summer.

[johnholmes]

tks for that input GP an Indian summer would be great if it came off