Archives For meridional flow

The “something and nothing” weather in the South East of the past week and the uncertainties in the forecast are set to continue for a while. Some met-people call these conditions “unforecast-able”. Models seem to be unreliable beyond a few days and even hours. Rainfall has been especially hit and miss to forecast in the SE: predictions have been varying wildly for specific days between torrential, heavy, some and then no rain arrives at all! The reason is possibly the lack of the usual “zonal flow” in the jet stream: i.e. west to east flowing jet.  The jetstream is meandering north-south and weather systems are more or less STATIC: the UK has been stuck in a low pressure trough for over a week.  The normal procession of low pressure systems (depressions) and brief sunny HIGH pressure ridges seems a distant memory: it simply hasn’t been a feature of our weather for ages. Forecast models seem to struggle with this.

meridional flow jetstreamThe overall synoptic weather situation remains the same. That is: a big blocking HIGH over the Atlantic and very weak westerlies with the jetstream in a North-South pattern (meridional) bringing down cool northerly winds direct from the Arctic which “pool-up” across Northern France and Southern UK creating a LOW pressure trough.  LOW pressure in Spring with a stronger sun can mean pleasant warm sunny spells but showers: it is the showers which, fortunately, have barely troubled Reigate.  However, (and this does look more certain!), a significant little LOW is set to spiral down the N Sea Thursday – Friday, deepen along the way and strengthen Northerly winds and bring rain, especially to the SE: some frontal rain and showers are predicted to accompany this LOW but again – it could be rather hit and miss depending on how close the LOW gets to the SE England and the strength of accompanying fronts.

storm risk thursThere is a 30% chance of thunderstorms over Reigate area through Thurs and Friday afternoons (as the sun heats the surface which creates bubbles of warm air through the day which convect upwards through the cool Polar airmass creating tall cumulonimbus clouds).  With upper air temperatures at 5000 feet as low as -12ºC later this week any vigorous showers may fall as hail.  Frontal rain attached to the LOW will certainly feel chilly in the wind. Night time temperatures could fall as low as 4ºC and any wind will make it feel distinctly cool.  A weak ridge over Sat and Sun may bring pleasantly warmer and drier weather but some models are showing a return to LOW pressure in a storm arriving from the NW on Monday bank 

Meanwhile, sincere sympathy and thoughts to those caught by the terrible EF5 tornado which caused such terrible damage in Moore, Oklahoma yesterday.  Following events on Twitter and news reports on destroyed schools was very sad and upsetting indeed.  The tension between following exciting weather and the potential for witnessing terrible disasters unfold in front of their eyes was palpable for the storm chasers and met-enthusiasts involved.  Unfortunately, the weather in MidWest continues to threaten areas with tornadic conditions: lately in New York state too.  Take care out there.  Our UK weather is usually mercifully benign in comparison.  

Last week our total rainfall in Reigate was 14mm, some models are predicting 40mm+ this week! Much of this will fall in heavy frontal rain mid-week and in thundery showers Weds/Thurs thereafter.  The jetstream is developing a meridional flow at the moment, which means it is meandering wildly and developing slow moving weather systems which sit around for long periods; the more usual zonal flow is when the jetstream flows west to east and drags weather systems briskly across the UK and keeps weather moving. Meridional flow gives rise to interesting weather: LOW pressure with heavy showers is the basic scenario this week: great for growing awesome cumulus clouds!

LOW pressure sits right over the UK all this week bringing sunshine and showers but with an added twist mid-week when a deepening short-wave trough is predicted to cross southern regions bringing potentially heavy frontal rain across the whole of the south, including Reigate. Thereafter, on Wednesday heavy thundery showers with hail could develop as winds slacken off.  Watch out for tall cumulonimbus clouds and rainbows! Any showers could build into single cell thunderstorms which will be slow moving in the slack winds and persistent and cause localised flooding. Around Reigate these showers will probably be quite isolated and some places will miss them completely but if you get caught in one you’ll know it! Temperatures remain uninspiring at 15C, although climbing slightly through the week. The end of the week remains very uncertain but could develop more heavy rain for us in the SE as the cut-off LOW over Europe feeds a miserably cool NE wind our way.
As always, this is early days and the end of the week looks especially variable and interesting!
Please check @RGSweather for updates!