Temperature Fluctuation Question.

[Graham Easterling]

West Cornwall normally gets it's snow with a depression running into France, and yesterday was no exception. 

It's the degree (excuse the pun) of temperature fall, relative to just before & after the event that surprises me, and as yesterday, tends to be understated by the models as well.

Taking yesterday as an example, even at 9am (when away from the immediate coast it was snowing heavily) the forecasts (BBC/MetO/) were showing around 4C and heavy rain,  for areas west of Truro, throughout the morning. It was then set to turn colder as the clearance spread in. 

In practice the lowest temperatures were late morning, 0C in Camborne, around 1C even at Lands End. Snow settled down to around 50m even as far west as Cape Cornwall for the first time since the Beast from the East. I know, after my daughter had a swim at Cape Cornwall Club around 11:00. They'd sat in a hot tub glued to watching the snow. (It's a bit of a novelty here!)

Then, as it all slipped away the temperature rose to 5C in Penzance, and even under the clearing skies and a NE wind aligned with the peninsula, the temperature failed to drop below 1.5C, which it reached around dawn. (A similar, slightly less pronounced pattern away from the coast at Camborne).

Obviously the models didn't expect it quite so cold during yesterday's event, and there was plenty of stirring of the air off the sea with an SST of near 12C (12.2C Scilly, 11.7C Porthleven & Perranporth) So even this morning with the light/moderate wind coming down the peninsula & a clear sky, no frost.

So why does the temperature drop so much, normally for just a short period,  in these events, despite mixing off the surrounding sea. It caught out the models which insisted it was still rain not snow throughout the morning. 

Graham

Penzance

[Freddie]

The precipitation was heavier than the models anticipated, hence there was more in the way of latent heat processes occurring (temperature drop due to the latent heat requirements of the phase change from solid to liquid as the precipitation falls towards the ground and melts).  This was the primary reason why the lowest temperatures occurred when they did.  This coupled with the ambient temperatures being colder in reality than in the models - although only by a degree or so - contributed to a lowered melting zone and consequently more snow down to low levels.  Forecasters are not kidding when they speak of "marginal" situations such as these - it only takes small errors in modelled temperature and precipitation rate for the reality to be totally different to the forecast.

[Graham Easterling]

Thank you Freddie, I was forgetting about the latent heat processes.

Graham