MODELS AND RAINFALL - PM EDITION

The modelling is continuing to remain divergent in the short term and again in the medium term, creating headaches for the curation of forecasts for the coming days. This comes as no surprise with the guidance provided last week.


But the movement of weather systems over the southeast in particular carry large consequences. For example, Canberra had 150mm forecast over 2 days in the lunch update for Wednesday/Thursday, now it has 20mm. So some areas that are forecast very wet tonight on your favourite apps that derive data off the Euro model will CHANGE!! They are low confidence and need careful consideration and comparison with broader data sets.


Now for the remainder of the nation, the rainfall looks to be mainly thunderstorm based, so that means uneven distribution in rainfall is to continue for much of the inland of the country.


The northern tropics remain wet and stormy, after scattered falls of 50-150mm of rainfall overnight.


Lets take a look at the latest details.


SHORT TERM

The short term, very tricky forecasting this week, not only are we dealing with severe storms and uneven rainfall totals for many areas, but a small scale low and deep trough will see some locations through southern or central NSW and the ACT potentially copping 1-3 months worth of rainfall, but who gets what, remains to be seen. The storm activity over QLD will lead to heavy falls in random scattered pockets, but the coverage is contracting east this week. The south and southwest of the nation, through most of SA and into WA dry, with isolated showers and storms about. The northern tropics, wet and thundery, with the routine showers and thunderstorms rolling through every 18-24hrs.


UKMET - Rainfall Next 7 Days - MODERATE CHANCE

Rainfall lean across the south and non existent for the week ahead over SA. The north unsettled but the heavier rainfall which has been consistently modelled is over the southeast of NSW and the ACT extending south into Victoria and north into southern QLD. Locally heavy falls within this zone still yet to be determined. Isolated showers and thunderstorms possible for western parts of the nation.

MEDIUM TERM

The medium term, looking drier for the nation this evening, especially southern and eastern areas which would be a change. Hot and dry weather from WA will come east and hopefully bring a spell of settled weather with only isolated showers and storms. The wettest part of the nation may initially be over the northern tropics, but coming into the period from the 16-18th of December, a trough bringing the high heat will lay across the nation from northwest WA through SA into VIC and QLD producing showers and thunderstorms, which will likely become more widespread as we go through the period as the trough runs into deeper moisture and stalls over the east. That may lead into a wet and thundery, humid period as we move into Christmas. The signals from the monsoon now over the Coral/Pacific Ocean. I have enjoyed watching the social media pages and the talk of cyclones developing over the north.....not what the models are saying. The only time you get excited about tropical lows is when they appear in the very short term < 5 days from development, not 16 days out.


ICON - Rainfall Next 8 Days - MODERATE CHANCE

Starting to come into line with the other models with the heavy rainfall for the short term over the southeast and drier weather back west over much of central VIC through SA and into southern WA. Storms and showers most days through the outlook period over the east of NSW and QLD in the warmer sector and the tropics unstable. Note the heavier rainfall over the southeast is mainly over Gippsland.

CMC - Rainfall Next 10 Days - MODERATE CHANCE

This model has come into line with the other models and brings the heavier rainfall back to the southeast of the nation and extends it over eastern NSW and into QLD. The northern tropics are expected to be wet with showers and storms continuing and the west becoming more unstable. Mostly dry for SA and western NSW and VIC. Note the heavier rainfall over the southeast of NSW and near the ACT.

EURO - Rainfall Next 10 Days - LOW CHANCE

This is the outlier this evening, not often you say that about the Euro. But watching trends from the other models in the coming runs to see if the Euro is starting a trend or will revert back to the heavier rainfall further south over NSW and the ACT into VIC. The rest of the nation too dry.

ACCESS - Rainfall Next 10 Days - MODERATE CHANCE

Heavy rainfall for the southeast and east, with another event next week which remains lower confidence. The tropics more unstable with showery weather increasing. The west dry for a little while but the showers and storms will return mid week and again next week with inland troughs. The heavier rainfall over the east and southeast would result in some locations recording 400-500mm if this was correct.

KMA - Rainfall Next 12 Days - MODERATE CHANCE

Very similar to the other models in the rainfall spread across the nation but noting that this model is an outlier with the tropics becoming way more active with upwards of 500mm of rainfall. We will see. The rainfall over the southeast very similar to the ACCESS.

GFS - Rainfall Next 16 Days - LOW CHANCE

Still way too dry for the 2 weeks but the rainfall over the southeast is in line with other models. The model is reintroducing the rainfall back to the northern tropics.

ENSEMBLE DATA

The ensemble data sets offer more insight as to where the heavier rainfall in the shorter term will be and the area of concern where the flood charts have been drawn on this site, remains over southeast NSW and the ACT with surrounding districts. This area looks very unstable, but remember that the main impact will be near and south of this low as it pulls in the easterly flow. The rest of the nation becoming wetter as we go, as the hot dry airmass gives way to low pressure and humid air from the north will likely feed the troughs with a more organised trough building as mentioned above from about mid month through the nation, laying northwest to southeast.


GFS - Rainfall Next 16 Days (median of 35 members) - LOW CHANCE

Still way too dry.

CMC - Rainfall Next 15 Days (median of 30 members) - HIGH CHANCE

Been most consistent across the nation and remains so tonight

EURO - Rainfall Next 15 Days (median of 51 members) - MODERATE CHANCE

Still aligned more with the CMC but the tropics possibly a little too dry as well as QLD.

LONG TERM

This has been the most consistent feature on the modelling, the persistent wet signal as we go through the remainder of December and into January.


GFS - Rainfall Next 5 Weeks

Still a wet look for the north and east which is in line with La Nina and does not indicate the tropical influence and I do think the east will be wetter as will parts of the interior with tropical moisture dropping south.

CFS - Rainfall Next 6 Weeks

This has remained very consistent.

More details coming up from 7am EDT.

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