Archives For August 2014

Quick update #2 02/09/2014



High pressure… getting the wobbles



Quick update #1 31/08/2014




Ex-hurricane Cristobal leaves trail of construction

Ex-hurricane Cristobal leaves trail of construction














Monday 25 Aug

Bank Holiday heavy rainfall wash out!


Nasty LOW








Into Monday morning: update:


Quick update here focusing on SE especially: please note this applies mainly to Reigate in Surrey and is an amateur analysis for educational purposes.  For updates through the storm please see @RGSweather on twitter for the Bertha story as it unfolds for us in Reigate and SE.  This is called NOWCASTING (as opposed to “forecasting”).

Ex-Bertha is turning out to be rather interesting meteorologically!  A convective potential has emerged today, which means there is more possibility of thunderstorms of some significance as the LOW passes across the UK, especially to the south of the system.  Convective gusts of 50-60mph could be possible and the odd tornado cannot be ruled out, though no need to panic because these are quite common and not usually powerful or damaging in the UK.  So it is still the case that the overall impact of this storm is still not likely to be extra-ordinary or wreak widespread havoc Daily Express style.  It is more likely to be underwhelming for most.  Nevertheless, rainfall totals in a short space of time for some places might be high and there could be some interesting weather phenomena associated with active fronts.

UPDATE Sunday 7:30am

Estofex and TORRO have issued severe convective weather warnings for the S UK. Estofex Level 2 storm warning is most unusual for the UK and TORRO do not issue tornado watches lightly.

(back to yesterdays update:) The UKMO fax chart below for Sunday midday shows a “triple point” of three fronts meeting near the SE (warm front, cold front and occlusion) Between the warm front and cold front the warmest humid air is wrapping into the centre of the LOW in the warm sector: this contains much of the moisture to fuel the storm as condensation releases latent heat driving up parcels of air.  On top of this a conveyor of cooler drier Polar air that flows over the cold front and warm sector and this increases lapse rates further encouraging lift throughout the system. The warm air eventually flows to the core of the storm as it occludes.



All the time the jetstream to the south is lifting air off the ground (by a process called divergence in the upper atmosphere) and lowering the central pressure causing air to converge into the centre of the LOW… this results in the surface wind rushing into the centre.   Converging air at the surface has nowhere to go except up.  Rising air, especially where tropical air meets polar air at the fronts, creates condensation, thick cloud and potentially plenty of rain.  The potential water available in this storm is large.  In addition, cloud top temps, with the influx of cold air aloft, are likely to be as low as -50C causing ice to form in turbulent air that can create charge up thunderstorms.  Such storms are only a risk and may not happen at all.2014-08-09_20-59-21

For Reigate and the SE it seems we can expect more rain during the morning than was previously the case in earlier models and forecasts.  Latest models suggest widepsread rain in the SE of up to 20mm and discrete patches of high totals possibly exceeding 50mm in the SE.  This is about a month of rain in one day, so local flooding could be a problem.

Rain will arrive tonight, after midnight, and persist throughout the morning.  Wind speeds, probably 30-40mph max gusts inland, possibly more gusty in any thunderstorms, will increase towards the middle of the day and potentially be highest as the cold front moves away which, on current models looks like early afternoon.  Strongest gusts will be associated with any thunderstorms.  The good news is that by pm the cloud should break rather rapidly, however, scattered showers could follow in the brisk westerly. This regime will continue for much of the early part of the week.

Needless to say, apart from the rain potential, Reigate is less at risk from tstorms than further N during this episode (Reigate storm shield!)

Even now much still remains uncertain about this storm and it is causing lots of interest and headaches for both professional and amateur meteorologists.  The nature of the fronts may produce some organised squall like features and some organised thunderstorms for places but predicting these is extremely difficult.  Any such storms can have the potential to deposit a lot of rain in a short space of time.

Late this afternoon Saturday Bertha split in two: one rain system moving north, the other pushing more ENE.  This was unexpected.  Currently the rain moving north across Ireland is the more significant but things can change.


Let’s finish with the UKMO forecast for Reigate.  It shows lots of rain, potential for thunderstorms and some unsually strong winds for the time of year.  As leaves remain on trees this might cause loose branches to fall and peak rainfall totals, if met, could cause some local flooding.  Certainly nothing to panic about but do look out for any interesting weather features and send them in to @RGSweather!  Sadly for @ridelondon the prospects are not terriibly nice in the morning.



Photo mosaic of the squall line that passed over Reigate on 10 Aug afternoon: quite a feature!

and tornado reports of damage from various locations including Hull, plus other news here:














Update: Fri pm



Reigate and UK weather looks increasingly unsettled through the next week or so as the jetstream goes up a gear and becomes much more active and autumnal, possibly culminating early next week with the delivery of our first extra-tropical storm of the season, Bertha, on a fast jet from across the Atlantic. More on her later.


This week a couple of significant secondary LOW pressure systems are forecast to spin up rapidly out of the SW bringing wet breezy weather in the South.  Wednesday early morning looks especially wet but with a possible rapid clearance behind the active front.  Rain totals could be significant and warrant warnings nearer the time, watch the UKMO for these.

Friday looks a similar performance, even wetter for some, with another secondary depression developing rapidly from a parent LOW over the mid-Atlantic near to S Iceland.  This secondary low emerges quickly from the south bringing potentially wet weather but it is further off so things could change…

Hurricane season in the Tropical Atlantic basin starts in June and ends in November.  The National Hurricane center in Miami keeps an expert eye on every development in the tropical Atlantic but forecasters in the UK also take a great interest in them too.  This is because hurricanes can impact the UK as extra-tropical storms.

Several tropical disturbances look like they could potentially affect UK weather in the next few weeks, Bertha being the first.  Of course, the UK has never seen a “true tropical hurricane” (i.e. with an eye, wind sustained no fronts etc etc) but we certainly get the remains of tropical storms (called extra-tropical storms when they leave the tropics and arrive in the mid-latitudes).  Many / most extra-tropical storms never make it to our shores (e.g. Humberto), many also die a death or veer off long before they have any real impact on the UK.  Some arrive as late summer or autumn depressions often characterised by wet and breezy weather: this could be Bertha next week.

potential Bertha tracks

potential Bertha tracks

Whether or not extra-tropical storms arrive on UK shores they can still inject a lot of heat and moisture and energy into the late summer and autumn mid Atlantic at mid-latitudes.  This can perk up UK weather significantly and spoil the dog days of August. This can cause trouble for forecasters as models become unreliable.  Bertha is significant because models seem to be agreeing, at this stage anyhow,  that she will arrive near the UK sometime late next weekend or Monday.  Some models show her deepening into a significant summer storm (GEM), others show her moving south into Biscay and producing a Spanish plume of warm/not thundery weather.  In any case, expect more rain this week and next and do keep an eye on forecasts as the weather gets interesting, I would not be surprised to see a flurry / slurry of weather warnings for heavy rain at times.

Finally, the overall outlook remains rather unsettled for us as the charts above show: slightly below average temps with no heat wave on the cards yet (although heat cannot be ruled out at times as a plume event is possible) and plenty of rain spikes with over 150% more rain than normal shown on the NCEP charts.  As a rough guide, daily rain totals in Reigate exceeding around 10mm would be considered a really wet day… so there are several potential candidates for some wet days in the next week or so.

Some places in Japan have recently seen more than 250mm IN ONE DAY in tropical storm Nakri, so our worst weather is still nothing compared with some global extremes. In addition, think of Japan as Super-Typhoon Halong bares down on the southern islands this week.


Unusually Bertha appears on Atlantic UKMO chart as a tropical storm status and is downgraded about 12 hours later to ex-TS Bertha.  The charts for late Sunday / Monday look astonishingly autumnal with a perky jetstream invigorating the storm under a trough. Nothing certain yet, as exact track and intensity is variable still but this is a turn up for the books.  Last year no ET-hurricanes made it to the UK, Humberto drifted off harmlessly in mid September and actually built us a nice HIGH pressure as it made “landfall” over Iceland.  Perhaps Bertha might do the same thing?



July summary for Reigate, Surrey

Tmax 30.2C on 23 July

Tmin 8.4C on & July

Total rainfall 31mm, with half of that falling during storms on 8 July.

Sunshine 201 hours

Max wind gust 28 mph on 26 July

July 2014 was notable for significant thunderstorms and a particularly spectacular meso-scale convective system (MCS) that drifted up from France and across the Channel on 18 July and then again on the 28 July with attendant shelf clouds and torrential rain and amazing lightning displays across much of the SE.  Whilst Reigate saw some thundery activity neither storm impacted significantly in Reigate, skirting further east, but did cause some spectacular lightning.  Flooding and hail was notable along the south coast, especially Brighton.  RGSweather was on holiday (typical!) so missed the action directly but will collect some links to add below. Any local photos available gratefully received.