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Friday 28 April 2017

First cold blast of winter arrives

Summer turned to winter around Anzac Day, as it so often seems to, with an active trough, rainband and front moving through the country's southeast.

The initial trough brought heavy rain leading to a few dozen new April rainfall records across SA, VIC, NSW and QLD for rainfall stations with shorter histories, and, because the rainband took several days to slowly work its way through, these occurred in the 24-hour periods to 09.00 on 25 April, 26 April and 27 April. There were a number of low temperature and high wind speed records set on those days, too, including a wind gust of 89km/h at Mudgee Airport at 13.27 on 26 April as the main cold front passed through, an all-time record in the station's 12-year history.

In the Melbourne area, Anzac Day was preceded by heavy pre-frontal rain with 20 to 30mm across the metro area, but higher around Geelong where 49.0mm was recorded in the 24 hours to 09.00 at Barwon Heads SE of the city (an April record) and 53.6mm at Gnarwarre to the west. The SES fielded 340 calls, of which 244 were from Geelong and the Bellarine Peninsula, mostly for water entering houses but some for drivers in trouble on flooded roads [The Age].

The front that heralded the truly cold air arrived in Adelaide in the early hours of 26 April bringing gusty wind and causing minor damage [News.com.au]. It worked its way quickly through TAS, VIC and NSW into QLD. Wintry showers with small hail occurred in Melbourne and there was a dusting of snow on the TAS Central Highlands, the VIC and NSW Alps and even Mt William, highest peak in the Grampian Ranges in western VIC. The cold airmass ushered in by the change subsequently combined with clear skies and little wind to produce two or three nights that were the coldest since last September or October in SA, TAS, VIC, NSW, the ACT and QLD and a record cold day for many places in all five states over 26 April and 27 April.

Australian weather briefs

• Tropical Cyclone Frances was named around midday Thursday 27 April when it was NW of Darwin. Since then it has strengthened and been given a nudge by the surge of E to SE flow developing behind the cold front mentioned above. At 08.00WST today TC Frances was at Category 2 and located midway between the Kimberley coast and Timor Leste, heading WSW into open waters. It is expected to briefly reach Category 3 status tomorrow morning before weakening slowly into a tropical low by Sunday evening, 30 April. Keep up to date with its movements on the BoM current Tropical Cyclone page.

Recent weather briefs - NORTH AMERICA

• California's drought-breaking rains have been phenomenal, and dismissed the long-term drought comprehensively in most of the state. By NASA's calculation, this year's snowpack in the Tuolumne River Basin in California's Sierra Nevada, the major water source for San Francisco and California's Central Valley, is greater than the past four years combined.
• On the other hand, and at the other side of the country, drought in Florida is so bad that airboats in the Everglades are getting stuck [Weather Underground]. Nearly half of the state is officially in drought and wildfires have caused the evacuation of 7,000 and the loss of about 10 houses [Reuters, Accuweather].
• On the US Atlantic coast, unprecedented amounts of rain for April have brought widespread flooding to Virginia and the Carolinas in the past few days. Raleigh, North Carolina, has been worst hit with Raleigh-Durham International Airport receiving 153.4mm in 24 hours over 24/25 April, its heaviest April daily fall in a 130-year history. Other 24 hour totals have exceeded 200mm with some over 220mm. Charleston, South Carolina, was also badly flooded due to a combination of heavy rain and a high tide. The heavy rain, brought by a slow-moving low pressure system passing up the east coast, has caused major flooding to unprecedented levels in places, flooded buildings, closed roads, cut power to over 60,000 properties, shut down businesses and schools in many areas and caused two deaths. [Floodlist, Accuweather, Weather Underground 24 April, 25 April, 27 April]
• Also in the past few days, a broad strip of the Central US running from the deep south to around Chicago has suffered flooding and severe thunderstorms bringing golfball to baseball-sized hail and at least one tornado. [Weather Underground, Accuweather, WeatherNation and a good explainer from Bob Henson on Category 6]
• In Canada, parts of Quebec have suffered river flooding causing 150 evacuations while Ontario has seen record heavy rain, flash floods and mudslides [Floodlist and video via Weather Underground]. Over on the east coast, St John's, Newfoundland, had sixty hour straight of freezing rain. Freezing rain happens when precipitation turns to rain as it falls through a warm layer in the atmosphere, then hits freezing surface levels quickly turning to ice [Weather Underground]. Meanwhile, crop-growing areas of the Canadian Praries seem likely to give farmers a double whammy. In Alberta alone, 500,000 to 600,000 hectares of crops had to be left in the ground to harvest in spring when wet weather made harvesting impossible last autumn. Now, late snow and rain are combining to prevent a spring harvest [Reuters].