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Southwest WA winter rainfall

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Ken
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Southwest WA winter rainfall

Post by Ken »

Southwest WA continues to be an area that might be worth watching for late autumn and winter in terms of rainfall/temperatures. For the past few months, all models have been unanimous on wetter than normal (and warmer than normal) conditions for the waters off southwest WA and quite a few spread that signal over southwest WA itself. However, some only barely paint the extreme far southwestern coastal strip with the wet signal while others spread it over most of southwest WA. And as always, models generally have lower accuracy over far southern parts of the continent including this area so pays to keep an open mind.

The cause of all this is the extremely warm SST's that have been hanging around off western and southwestern WA for months now and progged by most models to persist for a little while longer. So it could be interesting to watch as the prevailing airstream turns from the offshore E/NE'lies during summer around to more onshore westerly directions in winter and starts flowing over these very warm waters. This is only provided the Southern Annular Mode cooperates and fronts don't stay too far south and the warm SST's persist as progged though. If these factors manage to line up, I think enhanced cold season thunderstorm activity with cold season tornadoes and heavy frontal rainfall from injections of unusually warm moist subtropical type air isn't out of the question. Would be a welcome break for them at least.

The WMO's multi-model ensemble forecast (using EC, UK, CFS, Canada, POAMA, MeteoFrance and JMA) showing the ensemble average of precip anomalies (mm/day) for May-Jun-Jul:
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The WMO's multi-model ensemble forecast showing the probability (the proportion of the 10 leading models) of wetter/drier than normal conditions for Apr-May-Jun (the red colours indicate a high proportion of models suggesting wetter than normal conditions):
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EC rainfall anomaly probabilities for May-Jun-Jul:
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POAMA's max temp anomaly probabilities (POAMA has a tendency to be "overconfident" but that's not necessarily a bad thing in some situations):
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Re: Southwest WA winter rainfall

Post by johnno »

True Ken but I have also read of late that where colder ssts move too the LWT also tends to dominate in that area with stronger fronts, colder air and better snowfall.. Currently we have cooler than normal ssts in the Bight and extending well South and off the SE coast of Oz and there is a tendancy thinking that the LWT may spend alot of its time in SE Oz this Winter leaving SW WA with below average rainfall once again (but perhaps not as bad as last Year with any decent fronts that do move through that area should provide worthwhile rain)
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Re: Southwest WA winter rainfall

Post by Ken »

It's actually the opposite johnno. Longwave troughing is usually associated with warm SST's (causing increased convection from the warmer waters and lower pressures) while longwave ridging (causing subsidence from the cooler waters and higher pressures) are usually associated with cool SST's.
e.g. compare http://www.bom.gov.au/cgi-bin/climate/c ... al&steps=3 with
http://www.osdpd.noaa.gov/data/sst/anom ... 7.2011.gif in the Northern Hemisphere for January this year.

Cold outbreaks can also often occur closer to the western border of warm SST's in winter because of the polar air that's dragged equatorward between the longwave troughing/low pressures over the warm waters to the east and the higher pressures over the cooler waters to the west e.g. longwave troughing close to or just off the east coast.

Also, you can often have an LWT near or off SE Oz and another one near southwest WA (two common regions for longwave troughing). In this setup, vigorous fronts arc up over southwest WA then develop a negative tilt and sink SE'ward through the Bight before arcing up again over SE Oz and the Tasman Sea. Longwave patterns are very hard to predict beyond a few weeks so there's no guarantees of anything yet.
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Karl Lijnders
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Re: Southwest WA winter rainfall

Post by Karl Lijnders »

Fantastic info Ken thanks a lot!!! :)

Enjoy the sun, perhaps, while it lasts!!!
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Re: Southwest WA winter rainfall

Post by johnno »

Well Ken warmer than normal ssts or not the highs this Year have been migrating and hovering around South of WA and as we move into May (tradtionally the month of where stronger fronts and rainfall increases in SW WA) it looks to remain in that pattern where High pressure looks to continue to migrate over that area leaving that part of the country generally fine, warm and dry anyway we shall see if that pattern changes as we move into June and Winter in general
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Re: Southwest WA winter rainfall

Post by Ken »

The position of the subtropical ridge axis certainly has been, as indicated by the average sea level pressure analysis for last month:
http://www.bom.gov.au/cgi-bin/climate/c ... th&area=sh - this has been a regular occurrence in the last couple or so decades contributing to the prolonged dry conditions there.

In terms of anomalies though, there's been somewhat lower than normal pressures in that region last month due to the warm waters:
http://www.bom.gov.au/cgi-bin/climate/c ... h&area=nat

Meanwhile the Antarctic Oscillation Index has climbed back up into positive territory so the likelihood for lots of vigorous fronts to surge further north is lower for now.

The WMO's latest multimodel precip output for the month of July only, shows the edge of the wet signal's strong probabilities only barely straddling southwest WA so it's cutting it fine for rainfall there:
Image
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Re: Southwest WA winter rainfall

Post by Ken »

Incidentally the unusually warm waters off southwest WA have also encouraged much greater than normal waterspout activity for this time of the year near the coast recently as the (relatively) cooler air flows over the warm waters and increases low level instability (six were observed within 90 minutes off Cape Naturaliste and Yallingup in far southwest WA with some very tall ones on Wednesday). An interesting feature of the precipitation in that region recently has been for convection to flare up as it moves over the warm offshore waters before dying just as it crosses the coast. Time will tell whether this lasts through the winter.
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Re: Southwest WA winter rainfall

Post by Ken »

EC's monthly forecast currently suggests interesting moderate to strong probabilities for lower pressures (dark blues) around southwest WA for the 1st week of next month (upper right panel) - this may be due to longwave troughing in the region or troughing extending from the north:
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Re: Southwest WA winter rainfall

Post by Ken »

Some reasonable rainfall amounts being suggested by the PME for coastal areas of far southwestern WA next week.

Although it's hard to associated any single event with the very warm waters off the coast, they may well have contributed towards the increased instability and moisture (encouraging favourable low condensation levels) that encouraged the damaging tornado which hit the Perth area a couple or so weeks ago.
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