Tornadoes USA?

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

Very cold mainland USA but much warmer in the Gulf.

The clash could well mean tornadoes at the end of this week.

Jack

[Stephen Davenport]

On the contrary - it's been abnormally mild or warm in most of the US in the past week or two (except California and parts of the Southwest), with record high temperatures in places, notably the Midwest. There were indeed tornadoes, and the warmth helped - notably the first February tornadoes ever recorded in Wisconsin. You just don't get winter tornadoes in the northern States when it's even seasonably cold.

Stephen.

[Paul Knightley]

It should also be noted that a ‘clash of airmasses’ does not cause tornadoes (pet peeve!).

[Graham Easterling]

Well, they are associated with supercell storms which are in turn associated with a vigorous cold front.  Are we just talking semantics here or am I missing something?

Graham

Penzance - Where it's been a beautiful day

[Freddie]

You don't need a front to get supercells.  You do need an abundance of instability that gets suddenly released, and the correct shear profile through the troposphere - not to mention an plenty of moist air to feed the cells.  All of which you can have without a cold front.

[Len W]

Instability in terms of a shear and buoyancy mix and often embedded in a mesocyclone can lead tornadoes.

There is a clash and feedback of scales here so ruling out fronts is incorrect.

Len

Wembury

[Paul Knightley]

Supercells form when the right mix of instability, lift, moisture, and shear is present. Sure, frontal systems can include supercells, but the notion that the violent tornadoes of the USA are caused by a ‘clash of airmasses’ is incorrect. 

[Len]

The concept of airmasses is an oversimplication. Frontal features and the dynamics of supercells on the smaller scale are not.

[Graham Easterling]

I probably didn't word it very well, I didn't mean supercells were always associated with a cold front. However the conditions just ahead and during the passage of a cold front can be conducive, and I think that was what Jack was getting it. There's been many severe storms along the north Cornish coast associated with the onset of a sea breeze, effectively a cold front. The last Boscastle storm being an example, though there seems some dispute over whether it was a supercell or not.  A north coast storm I witness develop.  http://penzanceweather.atwebpages.com/SrMerrynStorm.PDF

The 200mm+ that fell at Cadgwith was very interesting event (David Braine left a zero off when he reported it)

Sorry, I'm drifting, I just like extreme weather.

Graham

Penzance