Archives For snow showers

The cold easterlies predicted have created interesting and unusual cloud patterns across the country. The satellite photo from today shows ripples in the breaking cloud to the west of the hills in Wales and the SW.  The easterlies driving the cloud are part of the cold sinking dry air created by the anticyclone to the north east of the UK.  The easterlies are  forced to rise over the hills and then sink the other side (the “lee-side”), creating this wavy cloud pattern.
Another feature of this cold snap is illustrated in the video below which shows how temperatures have changed over the last few days.  Notice how inland temperatures drop significantly lower than anywhere near the sea. The sea stays warmer for longer during winter as it takes 5x longer to cool down (and warm up) than land surfaces.  The video shows temperatures rapidly falling to low temperatures inland but coastal areas are moderated by the warmer seas surrounding the British Isles.

The cold easterly winds and some light snow flurries will continue over the weekend as a weak short-wave trough (a “blip” in the isobars surrounding the HIGH) passes to the north of our region.  The very cold pool of upper air is drifting away to the south, so temperatures will recover very slowly through to Monday.  However, spring is certainly on-hold at the moment and, despite being a mostly dry week,  temperatures will remain lower than average in the continuing easterly air flow.  Watch next weekend as a low passes to the north of the UK which could bring a “last hurrah” for real winter conditions as a cold northerly is possibly on the cards.  Longer range forecasts give little cheer for us in Reigate and the South East as HIGH pressure seems to stick around to the North and bring in Channel lows to the south of the country bringing wet and cool weather.

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Thursday update: situation on Sunday – Tuesday is mostly GRIM, wet, miserable and cold but details are still uncertain regarding snowy-ness here in Reigate: could be lots or just sleet and rain mix.  Snow up north definitely but SMALL changes in location and speed of LOW centre and wind direction will make a BIG difference to what weather we get in Reigate. Stay tuned for latest! (nb re-reading my post here: “mild” means “relatively mild” at Tmax 4°C Sunday, Monday “cooler” means Tmax 2°C: so cold, wet and miserable right through from Sunday am!)

The Arctic wind built threatening, angry stratocumulus clouds over Reigate early today (pictured) and some built into perky sleet showers by the end of the day.  The rest of this week should be brighter, clearer, frosty and less windy but sadly no sign of Spring yet – models show cool conditions continuing next week. Light snow is possible in Reigate on a decaying occluded front over Thursday night.  The Arctic wind will shut down in the next few days as a HIGH pressure builds from the SW.  Significant snow is a possibility in the first part of next week anytime from Sunday night through to Tuesday for Reigate and SE England as winds swing Easterly associated with a trough slipping SE. It’s tricky to forecast whether it will be snow, sleet or rain and where exactly it will fall at this stage but ,despite this vague detail, models do agree that a SLIDER LOW will move down from the NW and bring, initially warmer southerlies and rain on Sunday, but then drag in some cool Easterlies which could cause a spell of snow, possibly for the rush hour on Monday.  Early next week will be cold and snow / sleet showers threaten anytime. It’s a tricky forecast because the temperature of these Easterly winds is now creeping up as Spring approaches and surface temperatures will hover at the upper threshold of snowfall (2° or 3°C) so that it could easily fall as sleet or rain. Places further east and higher up on the Downs are more likely to get snow rather than sleet: it is that close. Follow 1-10 on the slides to check the causes of this potential snowy event: it is quite different from our Skyfall last month. 

showers over east coast 03-02-2013Today was bright and clear with a chilly northerly breeze, as forecast. Clouds developed throughout the day over Reigate but remained benignly flat anf fluffy, eventually joining up to form a thin broken blanket of stratocumulus through which the sun set spectacularly. Typically, in a northerly winter Arctic airflow, snow showers reach the East coast but fizzle-out and disappear inland. Today was an excellent example of this: watch the video. Places near the Kent coast and East Anglia saw wintry showers whilst places further from the sea, like Reigate, saw none at all.  A simple explanation goes like this:
Clouds form when air rises, cools and water vapour condenses into water droplets (clouds are tiny water droplets and/or ice crystals, not cotton wool!). The main driver of rising air is the heating of an airmass by a relatively warm surface. The North Sea is still quite “warm” at a balmy 7°C (note the nearby higher surface temperature in Margate 4.5°C) and this was sufficient to warm the chilly Arctic airmass travelling across it.  This warming caused convective uplift of warm bubbles of air (thermals) which formed shower clouds over the North Sea.  Think of the bubbles formed when a cold pan of soup is heated on the stove: they rise up, just like thermals rising through the atmosphere.  Rising thermals initiated cloud formationover the North Sea produced the shower clouds visible on the video marching across the East coast. The showers died-out inland because the land in January is colder than the sea (note the lower surface temp in Reigate 2.5°C) so warming is reduced and thermals are much less frequent.  Fluffy clouds may form but do not rise to any great height: these are called “fair weather cumulus”.   Showers and clouds tend to fade away in the evening because thermal activity is reduced.  (n.b: April showers occur for similar reasons: the stronger Spring sunshine warms the ground quickly and sets off rising thermals through airmasses which are still cool from the winter.  We must wait a few more months for the sun to get powerful enough for showers of this nature to kick-off inland: something to look forward to!)  Temperature changes with height are called lapse rates. Lapse rates control cloud formation and precipitation.  Try this site for more info.

The Arctic air mass will be replaced by warmer, wetter westerly winds tomorrow.  But don’t despair, if you enjoyed the fluffy cumulus clouds and outstanding visibility today then you will be happy to learn there is more to come this week!  Models agree that another long cold snap is due to return, initiated by an Arctic airmass on Wednesday.  This cold snap could last into mid-February with the possibility of an especially cold dip in temperatures next weekend.