Weather Watch: The roaring 40's - winds from the west

Windy westerlies probably sum up the weather that's on the way for the rest of this week and into the weekend.  It's very much a weak spring weather pattern that's forming early across New Zealand with plenty of north west/south west wind patterns – something that's been remarkably absent for 2009 in my view.  The westerly flow – the wind direction that gave the name to the "roaring 40's" - is being fuelled by two solid air pressure systems: A string of deep lows which have joined together in the Southern Ocean and a large pool of high air pressure to the north that stretches all the way from Perth in the west to Fiji in the north east.  New Zealand is sandwiched in between in what we call the "squeeze zone"…that's where all the isobars on a weather map squeeze up together.
 
Much like the lines on a topographical map these isobars indicate the gradient…the closer they are, the steeper the gradient…and like a bike going down a steep gradient the air moves in a similar fashion, so the steeper the gradient (the closer the isobars are) the faster the wind moves.
 
The isobars will be closely packed over the South Island and lower North Island for the next week or so – I don't think anything severe is on the way but conditions will be gusty for many areas.  Localised areas may not even have westerlies…they may be northerlies or even north easterlies as the air moves around the Southern Alps and Cook Strait…. But for the most part the winds will come from the westerly quarter.
 
So that means warmer weather.  Not tropical.  But warmer.  It's a pretty safe bet that the frosts are now done for the rest of July – I guess there's a slight risk some pockets in the South Island may get some tonight before the winds pick up.  Overnight lows in the North Island, like around Auckland, will be up around 8 or 9 degrees compared to Monday.  Daytime highs won't surge too much but will probably be up a few degrees nationwide.
 
The weekend doesn't look so sunny and settled this weekend thanks to the westerly flow.  It looks as though a front will race up from the Southern Ocean on Friday bringing more heavy rain to the West Coast and possibly western parts of the lower North Island. 
 
It's quite possible that this new north west/south west flow will last well into the first week of August and beyond.   What does that mean for you?  Well it usually means more rain for those in the west, gustier winds for those in the east and lower North Island…and perhaps drier for those in the north east and east of the North Island.
 
To sum up – windy, warmer, weather for the next 7 days.
 
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