Archives For October 2014

The end of October has continued to be warmer than average for the season and Friday 31 October could be the warmest Halloween on record (current record around 19C). Hi Res models show the highest temps most likely somewhere inland away from cooler seas and an incoming cold front from the west.  If the sun comes out, which is most likely in the SE or East Anglia and under such a dry southerly air flow, temperatures could rise to over 20C, or even 23C in places.  Whilst this unseasonably freaky warmth cannot really be described as a heatwave (there are technical requirements to classify as a heatwave) the headlines might be correct in suggesting a possible record breaking temperature maximum on Friday.

slide into normal

slide into normal

Thereafter, November temps take a tumble nearer seasonal average through next week and at times even below average. So whilst we can enjoy the treat of near summer like temps tomorrow by next week it will be some 10C cooler as Autumnal temperatures and more rain take a hold, though nothing extraordinary.  Compare this with the first winter storm warning of the year on the East Coast USA where temps are set to freeze and the first substantial snow in the mountains is likely to occur this weekend.

Along with temperature, pressure is set to fall through the weekend and into early next week as a depression hovers NW of the UK over Iceland and brings Reigate some breezy SW winds on fronts with rain reaching us in the SE on occasions over the weekend and into the early part of next week.  A cold front later on Sunday looks especially likely to be the one to usher in the distinctly cooler polar air overnight into Monday as a trough pushes out the high pressure which moves east taking any remaining mild air into the continent.

During the first part of next week the LOW moves SE over the UK and brings unsettled cooler and damper conditions to Reigate.  This LOW is due to then move further to the south / SE over the continent and deepen to become a feature called a “cut-off” low.  This is modelled to move over the continent to the Alps and N Italy where especially heavy rain is possible and then to parts of the Mediterranean which could cause unsettled conditions to arrive in the NW Med later in the week.

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There is some uncertainty as to how the end of next week and next weekend plays out for the UK and us in the SE once this cut-off feature moves off.  It seems likely that another depression with fronts could sweep across the N of the UK and bring more unsettled conditions towards the end of next week with fronts and rain for the NW.  But there are also signs of higher pressure building from the south again.  Either way, temperatures are going to be more averagely Autumnal and November-like, unlike the warm temperature tricks October has played.

A quick reminder that, unless November and December are markedly cooler (and the jury is out on this) then 2014 is STILL on target to become one of the warmest years on record for the UK and certainly one of the warmest.  Every month has been well above average temperature (see below, CET means Central England Temperature) except of course August which was the coldest for 21 years!  Despite this hiccup October looks to be warmer by over 1.5C and ending on a corker.  Nevertheless, perhaps celebration of such temperatures should be tempered by remembering that this kind of anomalous heat is exactly the kind climate change expected by IPCC predictions and, although anomalous warmth is pleasant for humans, it is just as stressful to wildlife and the environment as severe conditions such as storms and cold snaps.

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Finally, it’s worth noting recent press headlines regarding a possible COLD winter.  Such scare-mongering headlines in the Express and Star and similar papers are usually based on just one or two seasonal forecasts from a few meteorologists who might be described as on the fringes of mainstream weather forecasting.

Whilst RGSweather does not  write seasonal forecasts, it is worth sharing that some of the wider expert and reputable weather community, including both professionals and amateur, is pretty animated about the possibility of a colder than average 2014-2015 winter (that is of course Dec, Jan and Feb).  This is despite several of the standard weather models indicating a rather warm winter at this stage (including our own UK Met Office).  There is a lot of discussion about it.

Making a winter forecast is a complicated process because it is based on many interacting factors in the atmosphere, the cryosphere and the oceans which are combined together by expert forecasters to assess likely winter conditions.

Winter forecast indicators include factors such as the extent and build up of early Autumn snow cover over Siberia/Russia (encouraging HIGH pressure and cold easterlies), Arctic Ice cover (low ice cover warms Poles, increasing pressure, pushing cold air out to mid latitudes), the North Atlantic Oscillation and Arctic Oscillation (based on development of Azores High and Icelandic LOW) and associated amplification of the jetstream, how strong it is and how wiggly (meridional) or straight (zonal) it is, (a weak jetstream with -ve NAO allows polar air to leak out into mid latitudes), the strength of the polar vortex in the stratosphere as indicated by the an equatorial lower stratospheric wind pattern called the Quasi-biennial oscillation (QBO).  A westerly phased QBO is correlated with possible increase in likelihood of Sudden Stratospheric Warming episodes that have been linked to downwellling of cold air into mid latitudes some weeks after a SSW).  Phew, you can see there are loads of tricky indicators to watch.

This year, the October Pattern Index (OPI) is gaining attention as a possibly reliable predictor of winter outcomes.  Basically, the OPI is a clever measure of upper atmospheric conditions in October and particularly how amplified (or wiggly) the jetstream is looking.  Scientists have found that atmospheric conditions in October correlate well with winter outcomes.  A negative OPI, for example,  correlates amazingly well with a negative NAO (north atlantic oscillation) occurring later in the winter.  A negative NAO indicates a weak jetstream, itself possibly caused by low Arctic Sea Ice and high Siberian snow cover.  A negative NAO can, given the right synoptic pressure patterns (placing of HIGH and LOW pressure), be conducive to incursions of COLD air from the Poles reaching mid latitudes like the UK.  All this is experimental but extremely interesting and rather important given the rapid climate change going on in the Arctic.

Update!

Records were smashed today… warmest end to October on record at 24C

 

Reigate weather for next week to 10 days is overall set to calm down somewhat and, for us in the south, be generally drier and warmer than average for the time of year, most of the time. Temperatures are unseasonably warm over the next few days with night times barely falling below teens and daytime reaching 21C.  This is due to the southerly winds bringing warm air from Spain.

2014-10-17_22-57-53

A slack static cold front divides the really warm air over the SE from the cooler air to the NW. This cold front will bring cloud and rain at times across the SE, especially to the south coast, as it drifts SE over the weekend.  Winds on the coast will also be more noticeable over the weekend with 20mph+ possible, but staying mild over the weekend. The temps are likely to slip very slightly as the weak cold front edges SE on Sunday bringing some rain through the SE and more wind to the south coast.

Unfortunately, there is a hiccup to this generally benign warm weather. A gradual rise in pressure is set to be sharply upset temporarily by the remains of Hurricane Gonzalo, now battering Bermuda as a Cat 3 storm and due to arrive UK early next week late Mon /Tues.

Gonzalo hiccup

Gonzalo hiccup

Gonzalo is modelled to arrive early/mid next week.  Forecasts suggest Gonzalo will merge with the persistent Atlantic low near iceland and ride the jetstream across the Atlantic arriving late Monday as a 980mb low (not especially low) , bringing gales initially to the west and NW coasts and then gales to through the North Sea later the same day.  Some heavy rain is likely but the SE and Reigate looks currently likely to miss the worst.

After its passage across the north of the UK, the centre of Gonzalo appears to move SE down into the North Sea and behind a potentially vigorous cold front usher in significantly cooler NW winds of some potency, albeit briefly.  These appear to peg down temps a while, especially on NE coasts. Thereafter, indications are for a gradual improvement in the south as high pressure builds over the continent, albeit with some fronts reaching across to the south at times before a HIGH pressure seems to be suggested for the last week in October.

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If this comes off a dry and settled spell can be expected for half term.  The location of the HIGH starts in the south where mild conditions can be expected but the anticyclone could slip north and topple east.  if this happens then temps will fall as a cool easterly flow are brought in from a cooler continent creating a more foggy autumnal feel.  The charts below show the possible scenario for the last week in October and very start of November with things cooling off.

So in summary the weather for half term is generally settled, warmer and drier than average for the time of year but with a significant hiccup as Gonzalo arrive early next week bringing a plunge of cool polar maritime air down across the UK and the north sea behind this sharp active system.  Thereafter, a gradual improvement to the last week of October when an anticyclone is set to build across the UK bringing autumnal foggy end to October as temperatures drop to November.  Note that extra-tropical storms upset models so keep an eye on any changes to this forecast.

forecasters: SAC and Chris M

 

 

 

Update: confirmed tornadoes today (see foot!)

Potentially interesting, albeit tricky, weather tomorrow for UK, Wednesday. A deep surface LOW sits to the SW of Ireland and is dragging through complex series of fronts associated with various airmasses.  Occluded fronts are tightly wrapped around the LOW, which is due to migrate NE away from the UK during the weekend and pressure to rise.  For the SE tomorrow it’s not so much the fronts but an unstable mass of warm southerly surface air that will be the main cause of any heavy showers tomorrow and some potentially thundery weather, especially when this warm air is forced up by anything… coast, hills or fronts.

Here’s likely scenario for us in Reigate, Surrey SE: Rain is likely, possibly exceeding 10mm, which is fairly wet for SE: higher is possible.  Most of this is likely to be convective rainfall due to unstable and moist air moving in from the south overnight.  Showers, possibly heavy and thundery, are most likely in the morning as the warm surface air moves into our area, causing lapse rates to increase moderately and this encourages lift and cumuliform clouds.  If the sun comes out then surface heating could spark heavy showers and thunderstorms as warm air rises freely through the atmosphere, encouraged by the jetstream overhead that effectively drags air off the ground.  One of the ingredients for thunderstorms, LIFT, is therefore partly in place tomorrow, although it will depend on sunshine for greatest effect.  If it stays overcast, which is possible, then little exciting weather action beyond just rain is likely.

The above charts also show that the southerly / SW airstream is humid because, as it converges on the coast, the model shows rainfall increasing significantly. This increased rainfall on coasts is often caused by convergence which is due to air arriving onto the coast quicker than it is leaving (check the lower wind speeds inland) so the wind effectively PILES UP on the coast and is forced to rise as it has nowhere else to go except UP!  This is called convergence. It is clear from the charts above that any HILLS also encourage lift as South Wales and even the South and North Downs appear to be pushing rainfall totals up locally: this is orographic or relief rainfall.  The charts below shows another feature of the weather tomorrow: the winds are shown to be VEERING with height (left diag) which allows WARM air to advect (move into) into our region (right diag).  Read on for more about how veering winds and WARM AIR ADVECTION can encourage stormy weather.

 

Another ingredient for potentially unstable weather is that winds are VEERING tomorrow, albeit not dramatically, which means they are rotating clockwise directionally with height thus allowing warmer air from the south to move into a location: it is like opening the door to warm air: winds move through a southerly direction and therefore allow warm air to “advect” into our area.  A moderate wind veer is taking place overnight and into tomorrow morning.  Warmer air at the surface is overrun by cooler westerlies aloft that increases lapse rates: steepens the temperature difference between surface and air at altitude.  The air at altitude tends to stay the same temperature and is associated more with direction and origin of airmass than it is with surface heating or advection of warmer air at the surface. An increase in lapse rates adds to instability which encourages air parcels to LIFT off the ground, should surface heating occur if the sun comes out.  The chart above right shows how WARM air is ADVECTED into the South of the UK and migrates NORTH during the day.  This has nothing to do with solar heating… it is an 850hPa chart (1500m) and shows the airmass temperature which is largely independent of surface influences.  It’s a good example of WARM AIR ADVECTION with a moist air stream increasing instability causing showers and possible thunderstorms.

Finally, the warm relatively unstable airmass is being overridden by a NE turning jetstream that will encourages lift and wind shear.  Wind shear is the vertical change of direction and/or speed with height: rotation.  Shear is moderate tomorrow which might also add a twist to rising air that could even produce the odd tornado.  Nice 🙂  After writing this Estofex issued a tornado warning Level 1.

Update 8 Oct: confirmed tornados from today 8 October 2014

http://www.bbc.co.uk/news/uk-england-derbyshire-29542740

 

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September was a warm and unusually dry month.  Central England temperature (CET) came out officially as 15.1C which is 1.5C above the 30 year average (Hadcet) and made September the 4th warmest since 1910. In Reigate our average September temp was 16.5C, not unexpected as we are in the warmer SE of the country.  In 2013 the average for September was 14.4C but the range of temperature was greater: 2013 Tmax 30.9C and Tmin 4.6C were both more “extreme” than the respective Tmax 26.7C and Tmin 5.2C of 2014.  Whilst the remainder of Autumn looks to be cooling down markedly now, 2014 remains on target to be an exceptionally warm year overall.  Despite a cool August all the other months have been above average.

The UK average rainfall for September 2014 was 19.4mm for the month, the driest since 1910.  In Reigate we had 22mm of rain recorded (CoCoRahs) compared to over 47mm in 2013.  The rainfall we got in Reigate this September was restricted to a few heavy thundery showers: many of these moved over London and missed us completely.  However, a notable thunderstorm overnight on 19 Sept delivered some cracking thunder and lightning at about 1am and brought 10mm of rain in less than 20 minutes, and woke quite a few people up!

Overall, however, the month was dry: 16 consecutive days of no rainfall at all came early in the month and mostly dry days with just odd showers characterised the rest of the month.  Sunshine hours totalled 120 hours. Pictures below are a medley of photos from an “extreme” month due to the exceptionally dry conditions, the driest in 100 years in some places.  September in Reigate.