Northern Tablelands 12-month Rainfall Forecast

10 5 0
Feb 17
Mar 17
Apr 17
May 17
Jun 17
Jul 17
Aug 17
Sep 17
Oct 17
Nov 17
Dec 17
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) across the equatorial Pacific Ocean remain within neutral ENSO boundaries. Some atmospheric indicators however, continue to exhibit weak La Nina patterns.

The Nino3.4 value remained on the cool side of neutral at -0.3 in December. The Southern Oscillation Index (SOI) recorded a value of 2.6 during the same month, also within neutral.

Current consensus continues to suggest neutral conditions during the first half of 2017, although remaining on La Nina-side of neutral. With a La Nina event unlikely, the current forecast is being driven the Southern Annular Mode (SAM) in the shorter term. This continues to exhibit a mostly negative phase.

Current climate forecasts favour drier than average conditions for eastern Australia during mid-summer, returning closer to average later in February and early autumn. During this period, a stronger than average high pressure ridge is likely to reduce the effects of the south easterly trade winds responsible of bringing moisture onto the eastern seaboard.

Moisture streaming south from the North West Shelf is likely to combine with fronts clipping the southern coasts, bringing average-to-above average rainfall across the northwest, central parts of the country and the southern coasts.

Due to the elevated SSTs across the northwest, models are favouring a more active North Australian Monsoon over northern WA and the NT, compared to Qld in the shorter term.

The seasonal warming of the seas surrounding eastern Australia towards the end of summer could bring increased chances of wetter conditions to the eastern states towards the end of summer/early autumn. In the longer term, models are hinting to drier than average conditions returning in mid-autumn and early winter as ENSO values return to neutral values with warming of the equatorial Pacific.

Issued Jan 6

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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