15th Mar – Storms are brewing


SST analysis for Tuesday 12th March – RAMSSA
SST anomaly for Tuesday 12th March – RAMSSA

This week’s SST anomaly map shows that some regional anomalies have eased however there are still significant warm patches along the east coast. Firstly, off southeastern Tasmania there is a large region over 2 deg C above normal. On the temperature map we  can see the mild Tasman Sea water (the green colour at 18 to 19 deg C) pushing well south of its usual position.

SST Analysis and Anomaly for southeast Australia, with black arrows and line showing extent of Tasman Sea water southwards – RAMSSA

The warm patch does not seem to be directly related to a warm-core eddy (although there is a weak one to the southeast of the anomaly, see SLA here), but rather it seems indicative of the continuing Tasman Sea heatwave.

Thunderstorm fuel in NSW

In NSW a warm-core eddy is a culprit. Positive SST anomalies persist due to a large eddy moving southward and pulling hot EAC water down the coast with it. That effect, combined with a break in the northeasterly winds and associated upwelling, means that temps have climbed again along the beaches. The Sydney buoy is at 25 deg C, and even Eden has recorded 23 deg C this week.

SST Analysis and Anomaly for NSW – RAMSSA

Local swimmers must be wondering at the temperature yo-yo over the last few months, and choosing what to wear when going swimming may feel like a game of roulette.

Of more concern, the extra heat is helping fuel severe thunderstorms along the NSW coast. Low level moisture from the warm ocean is feeding into the coastal atmospheric trough. The instability then spirals that air upwards rapidly, forming hail and then the upper level  winds push the thunderstorm cells eastward again.

MSLP analysis at 12 UTC on the 14th of March, and the severe thunderstorm warning issued by BOM
Trace from Williamtown at 03 UTC on 14th March – showing low and mid-level instability, with rotating wind directions near the surface – BOM
Radar image and resultant hail stones in Western Sydney on 14th March – ABC/Cimmaron Sanders

but wait, there’s more

The trough is expected to move offshore over the weekend and most models expect it to deepen and form an East Coast Low (ECL). This could have major impacts over southern NSW on Sunday and Monday.

ECMWF forecast for 17th of March at 06, 12, 18 and 00 UTC. Block colours are OceanMAPS SST, hashed coloured areas show 3hourly rainfall, and wind barbs are the 10 m winds – BOM – click to expand

In the NSW regional forecasting centre, the checklist for ECL development includes warm SSTs as a precondition, and warm eddies as an intensification factor.

East Coast Low forecasting checklist, with red circles around ocean temperature factors – BOM NSW

From what we know about the SSTs, we can safely say there are at least two ticks on the ECL checklist. You can keep an eye on the NSW warnings here.

TC Savannah in the wide open sea

A tropical cyclone was named by the TC Warning Centre yesterday morning when a low pressure system intensified near Cocos Island. ‘TC Savannah’ developed over 28 deg C water and is expected to track south then west.Depth of 26 deg C isotherm for 14th Mar, with annotated forecast TC track – OceanMAPS

The 26 deg C isotherm plots of the area show that in the vicinity of Cocos Island the water is warm down to 5o to 70 m. However past 18 S there is a sharp ocean front and the TC-friendly water abruptly runs out.

TC Savannah seems to know this, and is due to take a right hand turn and head west just before the cold water. Maybe the TC is living up to its name, and ‘roaming the savanna’ of the ocean for favourable conditions!

TC Savannah forecast track on 14th March – BOM

Cocos Island has been experiencing strong to gale force winds from this system in the past two days. The tide gauge has recorded a couple of large spikes on the 13th that look erroneous. Is this due to wave action (overtopping?) in the tide gauge location?

Tide gauge readings from Cocos Island – Aggregate Sea Level Viewer

The tide gauge on Cocos is located on the jetty of Home Island – the eastern side of the lagoon.

Google Earth image of Cocos Keeling Islands, with location of tide gauge shown

A phone call with the local BOM weather observing team confirms that the island was affected by northerly swell on Wednesday and Thursday this week (13th and 14th). The worst sea state was the 14th when the lagoon was “closed” to all activity by local authorities.

Even with large swell hitting the outer reef, it would be hard to image waves of 2.0 m coming through the lagoon into the jetty. Also note that the tide gauge readings are hourly averages. This seems to rule out wave action as a cause.

The BOM’s local observer speculated that the strange readings may have been caused by the commotion of the ferries that were hurriedly brought into the jetty to take shelter at short notice. Perhaps…

Luckily the TC has now passed to the south and the ferries are back up and running for the locals’ daily commute across the lagoon.

more thunderstorms and cyclones

Thunderstorms are also developing in the northwest. Large convective storms have formed over the waters between northern WA and Timor, in the same area as very warm sea temperatures of 30 to 32 deg C (see the image at the top of this post, or the latest image here). SIGMETS have been issued to divert aircraft from the area.

Himawari visible image from 00 UTC 15 Mar, with dots showing GPAT lightning strikes. Red box indicates an aviation SIGMET – BOM

If that’s not enough storms for you … next week the atmospheric models are playing with more TC formation. The ECMWF model has two TCs to the northwest and northeast of the country (below left), while the ACCESS-G model has a TC in the Gulf of Carpentaria (below right).

The divergence in the models is striking but not usual. While we can’t rely on one particular model this far ahead, we can see that conditions are ripe for cyclone formation.

ECMWF and ACCESS-G forecasts for the 21st of March – BOM

‘Tis the season

All of the storms brewing this week remind us that although air temperatures have started to cool in southern states, we are still firmly in the convective season further north. In fact, March is one of the busiest times of the year in Tropical Cyclone climatology.

Monthly frequency of tropical cyclones in Australia by region 1973-2016: BOM



2 thoughts on “15th Mar – Storms are brewing”

  1. Looking at the Cocos Island traces at the NTC site (http://ntca/NRTM/HTML/co_index.html) there is some noise/interference in the primary sensor but there is no residual of note in the other working sensor.

    I think part of the problem here is that the aggregate sea level viewer can make noise look like a real signal due to providing 1 hour averages instead of 1 minute raw data.

    1. Hi Rob,

      I spoke to an observation technician who said that the erroneous tide gauge readings could have been caused by water getting into the instrument. For this particular type of tide gauge, in conditions of strong wind and heavy rain (eg. a cyclone) a water drop can get into the environmental tube and cover the calibration hole. This would cause zero readings on the gauge.

      The Cocos Island AWS recorded no rain on the 13th of March, but 22.6 mm and 43 knots of wind on the 14th of March. The tide gauge spikes started at 1700 local time on the 13th and continued on and off for 10 hours. It is feasible, but as yet unproven, that the rain and wind caused the errors.

      On the raw data it is clear that the spikes are nonphysical, but as you say, when hourly averaging is applied it can make the spikes look like a real signal.

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