Melbourne 12-month Rainfall Forecast

10 5 0
Mar 17
Apr 17
May 17
Jun 17
Jul 17
Aug 17
Sep 17
Oct 17
Nov 17
Dec 17
Jan 18
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 on the cool side of neutral at -0.3 in January. The Southern Oscillation Index (SOI) recorded a value of 1.3 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. With no large scale climate drivers, the current forecast is being driven the Southern Annular Mode (SAM) and Sea Surface Temperatures (SSTs). The SAM continues to exhibit a mostly negative phase.

Current climate forecasts continue to favour drier than average conditions for eastern Australia during for the remainder of summer and the very early stages of autumn. During this period, a stronger than average high pressure ridge continues to reduce the chances of significant cold fronts moving up the east coast. However, as we head into the later stages of summer and moisture values increase in the atmosphere, extreme rainfall events along the east coast can bring significant rainfall in short period of time.

Models continue to favour a more active North Australian Monsoon over northern WA and the NT, compared to Qld in the shorter term with below average tropical cyclone activity over the Coral Sea. Moisture streaming south from the North West Shelf is likely to combine with fronts clipping southern Australia, bringing average-to-above average rainfall across the northwest, central parts of the country and to the southern coasts.

In the longer term, models are hinting to drier than average conditions for winter and spring as ENSO values return to El Nino side of neutral, with warming of the equatorial Pacific.

Issued Feb 10

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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