Foehn Effects 2022

[Cambrian]

This topic aims to be a focal point for discussion of the modelling, observation, retrospective recording and enjoyment of foehn effects and winds in 2022.

If you already know all about them, then let’s be ‘avin’ you as you are, but if not, please feel free to read this description.

The 'Foehn Effect' causes anomalously high temperatures to the lee of mountain ranges. It needs warm moist air, on a brisk breeze, to be forced to rise up the windward side of the mountains. As the air rises, it cools and condenses, forming clouds that “rain out” on the windward slopes, leaving much drier air to flow down the leeward slopes. The loss of moisture enhances its potential to.be warmed, and as a result, the air reaches a markedly higher temperature towards the bottom of the leeward slope compared to the temperature prior to passing over the mountains. 

The WMO defines a foehn wind as "a downslope wind in the lee of a mountain that is accelerated, warmed, and dried as a result of the orographic disturbance on the prevailing flow." 

Renowned foehn effects occur on the leeward side of the Alps, where the name originates, and where the foehn wind can raise temperatures as much as 20 to 30 degrees Celsius in just a few hours, and can cause avalanches. A similar effect is seen in the North American Rockies. However, foehn effects and associated high temperatures also appear to the lee of hills and mountains in the northwest Highlands, Grampian, northeast England, north Wales, and my beloved west Wales, to name but a few. 

Here’s a starter for ten. 

Post-heatwave, at day 7, from today’s 12z GFS operational run - a moist southwesterly feed into the west of Scotland, 14 degrees over the Isle of Skye, meandering its way over the mountains, 24 degrees in Aberdeenshire.

 

[Maz]

Nice topic, love a good Foehn effect, and the more subtle geography in this country makes it more interesting than the obvious Rockies and Alps cases, IMHO.

Isle of Moray, near Inverness and Llandudno are classic cases in a SW’ly.

Manchester in a SE’ly…gives an annoying rain (snow) shadow when a low pressure from SW bumps  into cold air.  Everything looks primed for a dumping and then a dry window appears on the downslopes of the Pennines.  Mind you, SE’ly in summer is great!

Is the air on Monday/ Tuesday too dry, or will there be little pockets of extra heat  where dry air is descending NNW of hills?

Edited July 16, 2022 by Maz

[Cambrian]

39 minutes ago, Maz said:

Nice topic, love a good Foehn effect, and the more subtle geography in this country makes it more interesting than the obvious Rockies and Alps cases, IMHO.

Isle of Moray, near Inverness and Llandudno are classic cases in a SW’ly.

Manchester in a SE’ly…gives an annoying rain (snow) shadow when a low pressure from SW bumps  into cold air.  Everything looks primed for a dumping and then a dry window appears on the downslopes of the Pennines.  Mind you, SE’ly in summer is great!

Is the air on Monday/ Tuesday too dry, or will there be little pockets of extra heat  where dry air is descending NNW of hills?

Nice one. These charts are for Tuesday at 1pm from today’s 12z GFS op. It looks like even with how exceptionally dry the air is over England that there’s higher humidity further west and north. There’s a southerly running up through Scotland following a flow up through the Irish Sea, with a pocket of lower humidity in Moray, north of the Cairngorms. The temperatures there are above 30 degrees, looks about 6-8 degrees higher than inland in Dumfries and Galloway before the rise and fall over the mountains.
  

With the wind direction, drying and temperature difference, I think we can count this one as a Scottish southerly foehn.

[SP1986]

One to watch out for as well is the flow over the Welsh mountains, I'm guessing there will be a transitory build up of humidity on the western and southern slopes of Snowdonia, which could result in transitory Fohn for places like Mold, Caergwrle and Hawarden. To a lesser extent, it may also affect Cheshire and Wirral and down to Shrewsbury way... that could raise the temperature near 40C

[Maz]

Foehn effects in place now:

Currently 25c  Exeter, 30c North Devon.  Also 19c western highlands, 25 along Moray coast.  

 

[Maz]

Foehn effect responsible for the new Welsh temp record (and I suspect a foehn effect probably involved in the record just beaten as well)

 

Gogerddan has reached 35.3°C so far today, exceeding the previous record high of 35.2°C, recorded at Hawarden Bridge, Flintshire on 2nd August 1990

 

Gegerddan is on the mid Wales coastline near Aberystwth.  With a S'ly the Preseli hills provide the foehn effect, if it is more SE, the Cambrian hills.

[The Northern Ramper]

On 16/07/2022 at 22:19, Maz said:

 

Manchester in a SE’ly…gives an annoying rain (snow) shadow when a low pressure from SW bumps  into cold air.  Everything looks primed for a dumping and then a dry window appears on the downslopes of the Pennines.  Mind you, SE’ly in summer is great!

 

Otherwise known as the Greater Manchester snow shield

[Maz]

West wales for some surprising high temps this week?  Assuming an easterly.    Abersoch in NW Wales, down  Cardigan Bay into north Pembrokeshire?

[Maz]

Resurrecting an old post:

Moist SW air, North coasts of NI, Wales and Scotland (classic Moray Firth effect).  Will there be some record breaking November temps in these areas?  Probably looking at high overnight minima?

[Maz]

..well, records were indeed broken.  Warm cloudy records in the cold half of the year, in remote parts of the UK, don’t promote too much excitement though!