Thursday 8th January
Thanks to the shift in wind direction on Wednesday, to come from a southwesterly quadrant, the temperature lifted to a maximum of 5.7C at 13.02, still below average at -1.4C, but the second highest this month. The overnight moisture froze on smooth surfaces producing very icy conditions for a short while, as although the air temperature was above freezing at 1.3C, the ground surface was still frozen.
The continuous cloud cover overnight meant a frost free night, in fact the temperature rose from a low of 1.0C at 21.00 to 2.2C at 04.00 when the rise began to increase resulting in a temperature of 3.8C at 08.00 on Thursday.
The start to Thursday was very calm, no wind and the barometric pressure static. There is no possibility of sunshine today thanks to the advance cloud from Storm Goretti. This depression brought rain to western Cornwall by 06.00 and by 08.00 had reached east Devon and was knocking on the door of Exeter. The area of rain from Storm Goretti is extensive, due to reach our area around midday and continuing, very heavy at times, well into the evening, producing a substantial rainfall total.
It will be difficult, if not possible, to state the dominant wind direction today. The air flow at the start of the day is from the southwest, however, as the depression moves from west to east across southern England, during the next twenty-four hours, the wind will back into the south then east and eventually back to northwest.
The interesting weather will arrive, probably just after midnight. At that time the wind will have backed into the northwest bringing with it some of the residual cold air from the Midlands that will see the temperature here drop to around freezing. If that happens, the precipitation is likely to turn to sleet and possible snow. At the same time the wind will increase in strength to at least 30mph or higher.
Storm Goretti is a depression that has formed off the western approaches and is already showing signs of deepening rapidly. Over twenty-four hours the centre pressure will drop from 1008mb to a forecast low of 968mb when it arrives over southern England tonight. A change of 40mb over a twenty-four period is exceptional, which will produce very strong winds as there will be a substantial pressure gradient. The strongest winds for our area will likely be in the early hours of Friday.
In meteorology such conditions are referred to as ‘explosive cyclogenesis’ or “weather bomb’. The criteria being a drop of at least 24mb over a twenty-four hour period. Weather bombs can be a little unpredictable as they develop so fast. This time there is the complication of residual very cold air on the northern flank of the low pressure, thus a battle between the cold Arctic air and warmer air being carried in from the Atlantic.
We are just within the Met Office yellow warning for snow between 15.00 today and 08.00 Friday