Spring Curveball Delivers Biggest East Swell Of The Year
'If there's a cyclone on the charts, all bets are off.'
It's not so much a saying as a sentiment among weather watchers. The rapid development of Hurricane Otis, which struck Mexico last week, was an example of the capricious nature of cyclones and how they can defy the best of weather forecasters, and even the computer models powered by decades of historical data. If there's one on the charts, stay alert.
After yesterday, we may have to include ex-cyclones too.
You see, despite being fully aware of ex-Tropical Cyclone Lola (which was downgraded to a tropical depression on October 26th) and its merging with a surface low, most weather watchers and computer models were caught flat-footed by the swell it delivered, particularly in the zone from Port Macquarie to the Shoalhaven.
Dawn on Tuesday 31st revealed leftover NE windswell plus occasional east sets largely in line with Swellnet's own written forecast ("A strong new pulse of E swell is due through the morning" peaking mid-afternoon, sets to 4-5ft+).
By mid-morning, however, it was clear something else was happening as long-period six foot sets built to eight, then at select spots, ten feet, all under blustery offshore winds. It was the biggest east swell all year, with blue sky and the sun glinting off wind-brushed lips adding to the already impressive picture.
Despite the Swellnet staff getting amongst it, it's hard to unconditionally celebrate the swell as we're in the game of weather prediction.
Steve Shearer wrote the written forecast and observed the early developement of TC Lola and the low it was absorbed into.
"It first showed on the charts October 21st," said Steve, "a deepening easterly trough off the North Coast which then formed a broad surface low in the Tasman, spraying the East Coast with easterly quadrant swell - a classic late autumn or early winter pattern. Added to the dynamism of the charts was an October tropical cyclone."
"All indications pointed to a large to extra-large easterly swell event with the Mid North to North Coast of NSW getting the largest swells," observed Steve.
However, the system was dowgraded through the week, the data reading true between all global forecast models and also between succesive six-hour runs. Model consenus being one of the factors that influence weather forecasters.
Considering the downgrade, Steve thought "we'd get good to great surf, but not in the initial size range expected for Northern NSW."
Over the weekend ex-TC Lola was absorbed into the surface low drifting towards New Zealand. The positioning was good, being just north enough for the fetch not to be bisected (at least at first) by New Zealand's North Island, and the evolution of Lola from a warm core to cool core system appeared on track. There appeared no reason to lift the forecast and so Monday's Forecast Notes were posted as 4-5ft+.
In hindcasting the event, two distinct yet connected events happened that caused the swell to be undercalled. The first was that ex-TC Lola retained more moisture and energy than any model was able to calculate, meaning the process of extra-tropical transition was coming off a higher base level - call it a headstart - which ultimately made for stronger winds.
The other factor was the ASCAT reading and the piecemeal way it's delivered. Short for Advanced Scatteromteter, ASCAT graphs supply real time readings of wind speed at sea level. However, it's not comprehensive, reading some slices of ocean while missing others. The graph below is the best ASCAT reading of the low's core - around which the wind will be strongest - and it signifies nothing outstanding. Other ASCAT readings miss the core altogether. We can only assume the winds that caused yesterday's pulse were present during those passes.
The take home message, says Steve, is, "to watch all Tasman lows for signs of rapid intensification that fly under swell model radars."
On the plus side is that, after two year's work, we're just a few months away from releasing our new wave model. Amongst other things, this includes a better source of wind data leading to more accurate output.
Stay tuned as we finalise it.