The south midlands region is an interesting area to observe during such conditions.
Lets set out the stall:
* Long land fetch to the northern coasts
* 1025hPa pressure in a slack airflow
* Clear sky
* Nocturnal cooling underway several hours
Here in south Laois, just on the eastern side of the small town and on the western banks of the river Nore as it flows south. The map below shows current temperatures and I've highlighted upland areas in brown and enhanced the rivers in blue lines. I will demonstrate subtle airflows and cold pooling and how they are affected both by the stall above and the geography to be explained in tandem with observations of other stations nearby:
My own station at Durrow has yet to drop below 0c (currently 0.3c) and under a WSW airflow coming down off the hills to my SW. The airflow at 10m is running a subtle 4mph to 5mph. I'm at just under 90m elevation.
To my SW is the Johnstown station in north Kilkenny County. At 9.30pm they're at -0.9c ( a 1.2c difference over 17km - they're at almost 140m elevation). The wind is calm.
Further north from Johnstown is the tiny village of Galmoy on the border with Laois and Kilkenny, not as high up as Johnstown at almost 130m and they're recording -1.4c. The wind is calm.
Just east of Castlecomer is another station high up on the pleatau at almost 240m elevation. Again, at 9.30pm they're recording -0.9c. Wind direction is NNE at 6mph.
Finally, Kilkenny city is also below freezing at -0.3c with a persistent N breeze blowing from the north. Elevation 63m.
Conclusion - geography (topography) is playing an important role in determining wind direction and temperature at this stage of the evening. All elevated ground is gone sub-zero, areas to my west are calm, to my east and southeast there is a northerly element to the wind and both are sub-zero. But why is Durrow the odd one out with a positive temperature reading?
Here is why: The hills to my SW and slightly higher grounds to my W have "bled out" their daytime heat and will now cool slowly. As heat rises, cold air sinks to take it's place and this process has largely completed at these locations and while further cooling will take place, it will be at a much slower process.
Castlecomer is still loosing daytime heat and cold air is sinking down off the higher slopes neatby into the river valley that flows down to the Nore just north of where the N77 and N78 roads meet on the map.
In Kilkenny, that cold air coming down the valley is pooling, but still coming down the valley and generating a breeze. This will continue for another while yet.
Similarly, in Durrow the breeze should start to die out in a couple of hours and then the natural cold pooling will commence with probably a sharp drop in temperatures, for the moment, I'm stuck with alot of cool, but not cold air using the Nore valley gap to "get out".
Let's predict this further: Johnstown and Galmoy will probably get close to or just below -3c tonight. Both are on semi-dry to dry soil types and should naturally loose heat over the coming hours, but there may be weak northerly breezes keeping a little airmass mixing going on owing to elevation.
Over Castlecomer side - again there should be a close to a -3c recorded there, but I would expect more air mixing from the northerly breeze than at any of the other locations, so they might end up with the highest minima out of the four stations tonight.
In Kilkenny City, by the banks of the Nore, only the small UHI effect might stop them reaching below -4c.
Here in Durrow, I expect a sharp drop in temperatures, probably a few fluctuations throughout the night - yo-yo effect as cooler air follows the river south, but by morning all that should have stopped and I wouldn't be surprised to see a low value of almost -4.5c and likely to end up with the lowest minima out of the four stations.
All the above predictions could put egg on my face in the morning, but I've seen this happen before many times.
Let's see!
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