Slight difference in opinion . .

[Graham Easterling]

Penzance Forecast - MetO

BBC

The BBC forecast is almost always significantly low (typically 2-3C), but the MetO forecast for Tuesday would shatter my highest ever May maximum of 24.4C (20/05/98 & 24/05/01).  It is a 'soft' record, but 27C would be 12C or so above the inshore SST.  Still, looking at the forecasts there may be enough of an ENE gradient wind down the spine of the peninsula to prevent any sea breeze, which is presumeably what the models are predicting.

Graham

Penzance

[Keith (Southend)]

Doesn't the BBC use Metoegroup (ECM), whereas the Met Office use their own?

Keith (Southend)

[Metman2012]

That's correct but I don't think Graham was suggesting they're the same...

[Freddie]

The models will struggle a bit to get the pressure pattern precisely right, and I think wind direction will be key to your temperature, Graham.  I don't think you will see a "classic" sea breeze, due to the very subsided nature of the airmass which would prevent development.  From Monday night there does seem to be the potential of an easterly or east-southeasterly gradient developing, which would pull in air off the sea with (no doubt) a thin cooler surface layer, so the BBC temps look better for Tuesday.  I think the Met Office values look okay before this.

Two caveats: the pressure pattern is very likely to change shape between now and Tuesday as we are looking 4 days ahead; and I haven't looked at any forecast low-level temperatures so am making a few assumptions about low-level inversion heights!

[Graham Easterling]

Not much difference this morning, both are slightly warmer overall.

For Sunday / Monday / Tuesday / Weds

BBC 20C, 21C. 21C, 20C  (was 19C, 21C, 21C 19C)

MetO 23C, 26C, 27C, 23C (was 21C, 24C, 27C, 24C)

MetO does show the wind ENE, backing ESE during the hotter part of the day, which is off the sea. However the origin of the air will probably be off the land to the east, rather than the open sea. This has fooled the postcode forecasts in the past when a light SE wind in Penzance has had minimal affect on temperature, The MetO have been very good for quite a while now following a significant upgrade, The air will be very stable, preventing convection generating a proper sea breeze bringing in air that has been sitting over the open sea.

Talking of sea breezes, when there are genuine sea breezes, Scilly is often a fair bit warmer than the north coast of Cornwall. THe Islands are so small they can warm up without a sea breeze. 

Graham

Penzance

[jack.h...@gmail.com]

I don't think you will see a "classic" sea breeze, due to the very subsided nature of the airmass which would prevent development.

A rule of thumb I used—and it worked when I did gliding forecasts (can't remember where I got the idea)—is that for a sea breeze to develop, the convection must reach between 5,000 and 10,000 feet.  So, subsiding air would not allow convection to reach 5,000 feet.

Equally, very deep convection, e.g. showers normally, but not always, suggests no sea breeze.

Jack

[Metman2012]

My Forecasters' Reference Book 2nd edition, 1993 agrees with you  Jack, or is it the other way round.

[Graham Easterling]

A remarkably consistent fore cast from the MetO. Peak temperature forecasts for Monday & Tuesday still 27C.

Met Office Forecast for Penzance

Forecast Date     Sun  Mon  Tues

21/05                  21C   24C     27C

22/05                  23C   26C    27C 

24/05                  24C   26C    27C

Given that it was 22C at Camborne & Culdrose around 11am, 24C is certainly likely.

I know 27C might be nothing special in May inland in the south, but in this most maritime of locations, in spring! (Current SST at the Penzance bouy 14.7C) 

Assuming 27C is reached in Penzance, which is now seeming increasingly likely, it's well in a league of it's own. To demonstrate this, a few statistics from my station in Penzance. Standard Stevenson screen, sheathed maximum and minimum thermometers.

Period of Records  1992-2025 

Only 27 days over this 34 year period have reached 27C.

Earliest date for 27C -  27th June 1995

Highest ever recorded in May 24.4C (20/05/98 & 24/05/01)

A very interesting spell!

Graham

Penzance