Archives For zonal flow

Post-Tropical storm Andrea arrives in the Atlantic this week and sets the scene for a significant switch back to a more Atlantic dominated weather pattern (called “zonal”: where a stronger west to east flowing jet stream brings in Atlantic weather from the west). Andrea is the first tropical storm of the 2013 Hurricane season and soaked the east coast of the USA from Florida northwards last week.  Her arrival in the Mid-Atlantic coincides and perhaps helps to nudge our weather back to a more “zonal” flow and a positive North Atlantic Oscillation (HIGH over the Azores and LOW over Iceland = “normal”!). Andrea is ageing and much-weakened since thrashing the US east coast.  Whilst she is unlikely to storm ashore by herself, she is set to merge forces and deepen a slack LOW in the Atlantic and gather strength ready to sweep fronts across the UK, especially the NW which will get most of the rain this week.  The “Andrea” system will arrive in SE England on Wednesday with windy conditions as isobars tighten.  Reigate and SE is likely to escape the worst of the rain, at least early in the week, as HIGH pressure builds, at last, over Europe (giving them a drier week after record breaking floods and rain in Germany, Austria and Czech Republic). update Mon: some rain arr Tues with fronts. Most rain Wednesday.

The plus side of this switch back to a westerly weather pattern is that the east will see the back of the “spoiler” cool NE wind which brought some gloomy cloud cover in the mornings and colder than expected “warm dry days” this weekend and last week where promises of 25°C never happened in the SE.  With a warmer flow swinging around from South and SW temperatures should climb away this week to a real 20°C and actually feel properly warm as humid air is fed over the SE from the balmy South West rather than the cold North Sea.  Update Monday: temps will fall away to mid-teens Tmax as rain spreads more quickly east than first models predicted. For Reigate and the SE the heaviest rain is likely to be at the end of the week as heat builds over Europe and possibly sees convective storms associated with a front over France bringing in heavy rain for Friday / Saturday.   This warm feed of air has the potential to be a Spanish Plume event causing significant thundery activity in the SE.  Check back later for updates on this exciting potential! (looks less likely on latest runs; but wetter weather mid-week more likely now!)

uncertain forecast

Thereafter, things look to be set in a cooler unsettled cyclonic pattern with LOW pressure systems storming off the Atlantic bringing wetter conditions for the rest of June but still much uncertainty on this one. 

In summary: this week the wind will clock round each day from a cool easterly (from North Sea) through Southerly to humid SW and weather is likely to become more mobile but with dry weather hanging on longest in SE and feeling warmer too!  What’s not to like?  It’s Summer, enjoy!

links Andrea on NOAA http://www.nhc.noaa.gov/refresh/graphics_at1+shtml/143437.shtml?gm_track#contents

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!