BOM warns of crop-damage frost to -4
In short
The Bureau of Meteorology has issued a severe frost warning for Victoria's North East, with temperatures forecast to fall to -4 degrees and threaten crops.
Alpine stations have already dropped past -5 degrees, and the same clear, cold air is delivering the season's sharpest frosts across the southeast.
What to watch
The bitter mornings bring dangerously cold water, so the calm conditions that follow a frost are far riskier on the water than they look.
A pool of bitterly cold air has settled over southeastern Australia, and the Bureau of Meteorology has responded with a severe frost warning for Victoria.
The warning, issued Thursday, forecasts severe frosts and temperatures down to -4 degrees across the North East district on Friday morning, cold enough to damage crops.
Frost warnings also cover the Wimmera, Northern Country, North Central, South West, Central and East Gippsland districts, sweeping in most of the state's inland.
Up in the alps, the cold is sharper again, with Falls Creek dropping to -5.4 degrees and Mount Hotham to -5.0 degrees during the outbreak.
What is driving the freeze
The cold snap is the work of a large high pressure system parked over southern Australia, dragging a cold, dry airmass up from the Southern Ocean.
Under that high the sky stays clear and the wind falls light, which is the perfect recipe for a hard overnight frost.
With no cloud to trap it, the day's warmth radiates straight back to space after sunset, and temperatures plunge in the hours before dawn.
Light winds let the coldest, densest air pool in valleys and low ground, which is why inland and alpine sites read colder than the coast.
The Bureau notes that ground-level temperatures can sit several degrees below the official station readings in these calm, clear conditions.
"A frosty, glassy dawn is the most inviting sight on the water and, right now, one of the most deceptive, because the sea is as cold as it gets all year."
Why this matters on the water
The same high that freezes the paddocks flattens the wind and the sea, serving up the calm, glassy mornings that tempt paddlers and anglers out early.
That is the trap, because winter water around the southeast is now at its coldest, and cold water is far more dangerous than cold air.
Royal Life Saving Australia warns that sudden immersion in water below 15 degrees triggers cold water shock, with an involuntary gasp and rapid, uncontrolled breathing.
That gasp reflex, not hypothermia, causes most cold water drownings, and it strikes in the first few minutes before the cold even reaches your core.
Hypothermia follows if core temperature falls below 35 degrees for half an hour or more, sapping the strength and coordination you need to self-rescue.
For boaters and paddlers
A fall from a kayak, ski or stand up paddle board into single-figure water can steal your breath and your grip in seconds.
Wear a lifejacket every time, because cold shock can incapacitate a strong swimmer long before they reach the shore or the boat.
Dress for the water temperature rather than the air, with a wetsuit or thermal layers, and always tell someone your plan before you launch.
Stay close to shore on the calm mornings, keep a means of calling for help on your body, and paddle with a mate where you can.
For anglers
Rock and beach fishers face the same immersion risk, made worse by slippery, frosted platforms and numb hands that fumble a grip.
The fishing itself slows in a cold snap, as fish drop into deeper, more stable water and feed less in the chilliest part of the day.
Work the warmer middle of the day over the frosty dawn, fish deeper holes and channels, and slow your presentation right down to match sluggish fish.
How long it lasts
The frosts should ease once the high drifts east and cloud or a change returns to blanket the overnight sky.
Until then, expect several more cold, still mornings, with the frost risk sharpest in the inland valleys and the alps.
The cold water lingers longest of all, staying near its winter low for weeks after the air has warmed back up.
Common questions
How cold is the water right now?
Southeastern coastal and inland waters are near their winter minimum, cold enough to trigger cold water shock on sudden immersion.
Is a calm, sunny morning safe to paddle?
The conditions look benign, but the water is at its coldest, so treat a glassy frosty dawn with more caution, not less.
Why is the frost worse inland and in the alps?
Cold air sinks and pools in valleys and high country under clear, calm skies, so those areas fall well below the milder coast.
When will it warm up?
Once the high moves on and cloud or a front returns, the overnight frosts will ease, though the water stays cold much longer.
Plan around the calm mornings with the Seabreeze Melbourne wind forecast , and check the latest Victorian warnings before you head out.
