Archives For northerly winds

Heads up, Reigate!  Or rather… wrap up, stay in and keep warm: SE weather for the end of this week looks awful and, whilst nothing approaching the US Mid-West, it could be described as potentially severe for the SE.

Pick words from this list and repeat and then remind yourself it’s nearly June! “COLD, heavy persistent rain, windy, wet snow, sleet, hail, heavy thundery showers, freezing wind chill, 35 mph northerly gusts… at the end of May!

Thursday: bright start but with cold northerly winds +20mph and heavy showers building in the afternoon.  Feeling chilly at only 10ºC max.  Overnight temperatures fall to 3ºC and any showers could even fall as icy sleet or even wet snow on the Downs. 

Friday: things actually look worse than earlier this week: COLDER northerly winds will drag temperatures right down to 3 or 4ºC in heavy showers and a maximum of  8 – 10°C with temperatures falling lowest in showers.  The centre of a nasty deepening LOW pressure will track directly across the SE on Friday and we can expect persistent rain, some heavy at times and accompanied by really cold winds (for the time of year).  Later in the day heavy thundery showers have the potential to bring hail and sleet.  Significant instability is forecast (*CAPE >350 j/kg; before the Oklahoma tornado CAPE measured 5000 j/kg), currently the highest this year in the SE, which means there is a fair chance of severe weather events in the form of thunderstorms.  Watch out for mountainous cumulonimbus cloud formations extending to the height of Mount Everest and possible funnel or mamatus clouds under dark bases.
Freezing levels will be down to 600m and during any heavy showers evaporative cooling could potentially cause heavy rain to turn to sleet as snow forms above 1000m and heavy rain draws down colder air.  Evaporative cooling happens when enough of the heavy rain evaporates on the descent to take heat out of the air and freeze supercooled droplets or sleet into proper wet snow.  Strong winds gusting 35mph will make it feel truly nasty.  This all sounds ridiculous for May… but the potential is there..let’s hope it’s not quite this bad!

Friday night will be a chilly and wet experience outside.  Temperatures will fall to 4ºC but feel near freezing in northerly winds. Not good for camping.

The weekend recovers reasonably well with a weak high pressure ridge bringing bright and dry spell Saturday behind the Friday storm. The fine weather  could hang on through Sunday but showers could threaten as pressure falls away as a deep LOW approaches from the NW to threaten Monday with rain.  The rest of half term looks unsettled but a bit warmer and drier at the end and best in the NW of the UK where HIGH pressure is likely to ridge across.

*CAPE = convective available potential energy: how buoyant a parcel of air is and this determines speed of vertical extent of convective cloud formation.

In the end: Max 8.4C; Min 4.3C; Rainfall 8mm; gust 24mph NW; So, in the end, pretty foul and unpleasant but fortunately no observed wet snow or sleet anywhere.  Timings of light rain onset were good; heaviest rain pm and cold temps made it feel more like January. No thunderstorms heard but lightning was recorded in SE on ATD lightning detector; CAPES fell to just over 100j/kg in the last run of GFS in the morning, which is a little disappointing for extreme weather enthusiasts!

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.