Westmere hits -2.5 in a spring frost

Frost inland, then a 26° northerly

The Bureau of Meteorology forecast for Melbourne, issued at 5:55am, put the morning in four words.

The chance of frost early this morning.

One high, two faces

The high pressure system sitting near Bass Strait is doing two jobs at once, and both of them turn up in the same set of observations.

Overnight it stripped the wind, which let heat radiate away from the paddocks west of Ballarat until Westmere's thermometer reached -2.5°.

By the middle of the same day that calm centre leaves Port Phillip variable about 10 knots with seas below 0.5 metres and mostly sunny skies.

Frost at dawn and a glassed-off bay after breakfast are not two stories, they are one 1031 hPa high twelve hours apart.

Where the cold pooled

Westmere recorded the coldest minimum among the Victorian sites listed in the 8:51am observations, and it was still calm when those numbers were issued.

Mortlake Racecourse fell to -1.0° and also reported calm conditions, with humidity at 88 per cent and pressure at 1031.2 hPa.

Omeo, at 690 metres in the east, bottomed at -1.0° before a northerly at 11 km/h arrived, with pressure at 1032.0 hPa.

Horsham Aerodrome dipped to -0.3° with no rain in the 24 hours to 9am, an east-southeasterly at 13 km/h and pressure at 1030.6 hPa.

Ararat Prison showed the lag, still reading just 0.2° at 8:51am with humidity at 98 per cent and a dew point of -0.1°.

Ballarat Aerodrome managed a minimum of 0.8° before a north-northwesterly at 9 km/h began scouring the cold layer off the surface.

Hamilton Airport at 1.7° and Geelong Racecourse at 2.6° show how quickly the frost risk drops away once you move off the higher inland plains.

The city stayed warmer than the paddocks

Melbourne Airport recorded a minimum of 3.3° and a west-southwesterly at 7 km/h, with pressure at 1031.5 hPa.

Viewbank fell to 2.8° and Laverton to 3.1°, with Laverton still reporting calm air when the observations went out.

Cerberus dropped to 1.8° with pressure at 1031.7 hPa, a colder start than the airport despite sitting beside the water.

The rest of Wednesday is sunny with light winds and a city maximum of 19°, with 20° at Frankston, 19° at Geelong and 21° at Tullamarine.

Scoresby and Watsonia are both on 21°, Yarra Glen on 22°, Laverton 19° and Mount Dandenong 18°.

UV is moderate at 5, with sun protection advised from 9:30am to 2:50pm even after a morning that started below zero inland.

Port Phillip today

The coastal waters forecast has winds variable about 10 knots on Wednesday, seas below 0.5 metres and mostly sunny conditions across the bay.

For anyone launching at St Kilda, Werribee or Geelong, that reads as a flat, low-energy day with no single direction to plan a session around.

Variable means shifty, so a breeze that fills for twenty minutes off one shoreline can fade and return from another quadrant entirely.

Sailors, rowers and paddlers get the better end of the deal, while anyone chasing wind will find the numbers thin until the pattern shifts.

Western district water

Lake Bolac sits near Westmere, so inland lake users started the day in the same calm, sub-zero air that produced that -2.5° minimum.

Ramps, trailers and banks at that altitude hold frost longer than the coast, and surfaces stay slick while the air sits close to the dew point.

Ballarat's forecast has mostly sunny skies, the chance of morning frost, light winds and a maximum of 20°.

Light winds inland today mean lake surfaces should stay soft, which suits small craft but leaves little for anyone rigging a sail.

Thursday's turn

The high is forecast to move across the Tasman Sea through the rest of the week, directing a warm north to northeasterly airstream over Victoria.

Melbourne's Thursday is partly cloudy with a northerly at 20 to 30 km/h, a minimum of 10° and a maximum of 26°.

That is a seven degree jump, from a 19° city maximum today to 26° tomorrow, driven by where the high sits rather than by any new system arriving.

Ballarat follows the same path with a range of 6° to 23° and a northerly at 20 to 30 km/h.

On the bay, Thursday brings north to northeasterly winds about 10 knots, seas still below 0.5 metres and partly cloudy skies.

A northerly across Port Phillip tends to leave the northern shoreline sheltered while fetch builds toward the southern end of the bay.

Into the weekend

Another high is forecast to move in from the west from Saturday, which keeps the broad pattern in the same family rather than breaking it.

Watch the overnight wind first, because calm air under high pressure is what decides whether inland minimums fall back toward zero again.

Why was Westmere so much colder than Melbourne?

It sat at 226 metres with calm wind and clear skies, so heat radiated away without mixing, while bayside and urban sites held onto more warmth.

What is a radiation frost?

It forms on calm, clear nights when the ground loses heat to the sky faster than it gains any, letting air just above the surface fall below zero.

Will Port Phillip stay calm all week?

Wednesday is variable about 10 knots and Thursday is north to northeasterly about 10 knots, with seas below 0.5 metres on both days.

How warm does Thursday get in Melbourne?

The forecast maximum is 26° after a minimum of 10°, with a northerly at 20 to 30 km/h and partly cloudy skies.

Before you head out

Check the current Victorian warnings summary on our Victorian warnings page before you commit to an inland dawn start or a bay session.

If today's flat bay has you looking further afield for breeze, compare the Hobart wind forecast before you lock in the rest of the week.