Upper Western 12-month Rainfall Forecast

10 5 0
Apr 17
3
May 17
8
Jun 17
9
Jul 17
8
Aug 17
5
Sep 17
7
Oct 17
4
Nov 17
5
Dec 17
5
Jan 18
1
Feb 18
3
Rainfall deciles
10 Well above normal
8-9 Above normal
4-7 Near normal
2-3 Below normal
1 Well below normal

Issue Notes

ENSO status: Neutral.
Indian Ocean Dipole (IOD): Neutral.

Sea surface temperatures (SSTs) and atmospheric indicators across the equatorial Pacific Ocean remain within neutral ENSO boundaries.

The Nino3.4 value remained within neutral at 0.0 in February, though with a warming tendency. The Southern Oscillation Index (SOI) recorded a value of -2.2 during the same month, also within neutral.

Current consensus continues to suggest neutral conditions during the first half of 2017, with a gradual warming into El Nino side of neutral by winter. The Southern Annular Mode (SAM) continues to exhibit a mostly negative phase, though models suggest a possible positive trend towards the end of the month.

Climate forecasts are favouring drier than average conditions for the southern half of Australia during autumn. During this period, southern Australia sees a gradual increase of cold fronts brushing the south. However, as we head into autumn stronger than average high pressure over Australia is looking to keep these at bay.

Along the east coast, sea surface temperatures continue to be exceptionally high, increasing the amount of moisture in the atmosphere. Rainfall is looking to be close to average earlier in autumn east of the divide (during the later stages of the wet season), where extreme rainfall events could bring significant rainfall in short periods of time.

Models continue to favour a more active North Australian Monsoon over northern WA and the NT during the final weeks of the wet season. Moisture streaming south from the North West Shelf is likely to combine with fronts clipping southern Australia in the short terms, bringing average-to-above average rainfall across the northwest and central parts of the country.

In the longer term, models are hinting to a possible El Nino scenario, which increases the risk of drier than average conditions for winter and spring.

Issued Mar 9

Forecast Explanation

Notes on the concept of deciles

If all the data in a record are ranked from lowest to highest they can then be divided into 100 equal blocks. These blocks are known as percentiles. The values that fall into the lowest 10% range (from 0 to 10%) are said to be in the first decile, those in the group 10+% to 20% are in the second decile, and so on. Those in the group 90+% to the maximum value recorded are in the 10th decile. The 50% value is a special one known as the 'median'. It is noteworthy since there is the same number of records above and below its value.

Deciles have been found to be very useful for analysing rainfall in particular as its distribution is not the normal bell-shape distribution but is skewed towards many low values with only a few high values. The deciles can be described in qualitative terms. A table is provided in the accompanying results.

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