Archives For September 2013

Thursday pm and overnight to Friday looks like seeing heavy possibly thundery rain crossing the SE and Reigate.  As the Scandinavian HIGH slips south the LOW sitting to the west of the UK will sweep fronts across the country which could bring up to 10mm (max) of rain for Reigate.  The rain is likely to start Thursday pm, become heavy quickly early overnight and peter out to showers on Friday. Thursday will also be breezy, especially on the east coast. Saturday looks drier and brighter as a fresher westerly airflow arrives.

Perhaps a more interesting outlook is the appearance of some interesting winter forecasts for 2013-2014. Various models are illustrating a cold winter for the UK with northern blocking and cold easterlies, not dissimilar to the 2012-13, with an especially late cold spell in Spring.  Dig out those snow shovels, well, maybe you can wait a month or two yet!

For more details on the 2013-2014 winter forecast please visit our expert long-range weather friend Ollie at http://olliemillinweather.wordpress.com/long-range/

cfs december run

Chilly December… this is the anomaly 500hPa chart showing temperatures at approx 5000m (half way up through the atmosphere) and show how far up or down from the average they are predicted to be, they are NOT surface temps! It gives a good idea of a cool pool of air over Europe and the UK and warmer than usual blocked conditions over the Arctic (perfect for sending cold air our way).

cold march 2014This chart shows 2m surface anomalies for March and shows a colder than usual Europe with a really cold Russia.  Any easterly winds at this time of year would be bitterly cold for the UK.

Here’s a copy (below) of the IPCC AR5 report that has the juicy bits highlighted for ease of reading!

In essence the IPCC have toned down some of their projections because they recognise that there has been no global warming since 1997, something their models did not expect.  The IPCC believe the 1997 – present day heat is “hidden” in the oceans and will emerge at some point.  They also point out that warming will not be regular and that some of their previous models had “forcing inadequacies” (i.e. modelled temperature rise incorrectly?) and that in some models there was an “overestimate of the response to increasing greenhouse gases and other anthropogenic forcing”.

Read Simon Keeling’s @weatherschool musing on this. here http://www.weatherweb.net/wxwebtvsimonnew.php?ID=864

Read the summary report here…

WGIAR5-SPM_Approved27Sep2013

Slightly edgy stuff for the weekend for fans of airmass battles. Current weather is in a BLOCKING pattern with a HIGH NE of the UK over Scandinavia blockiing the passage of a LOW pressure cyclone which has been cut-off down off the west coast of Spain all week. It’s gradually filling and has been wafting a lot of nice warm upper air our way from the South, hence our mild temps for much of the past week and the stagnant weather… not much happens when the jetstream abandons the country!  This usually means the weather (good or bad) stays much the same for ages until the BLOCK moves or the JET returns.  

However, the weekend heralds a change, of sorts, for next week. The HIGH over Norway is building and will increase the pressure gradient as the LOW sneeks up the Channel. A brisk easterly wind generated by the pressure gradient will push the relatively drier continental air (from the HIGH) right into the warm, moist upper air arriving from Spain. The result will be lots of rain in the SW and for the Channel. Some of this will be heavy and possibly thundery on Saturday and any slight change in the location of this marriage of airmasses could bring rain anywhere to the south.
The fronts are not now expected to push much up from the coast or out of the SW.  However, whilst a dry mostly cloudy day is forecast for the weekend in the SE, with so much action nearby, we shouldn’t rule out the possibility of a shower. This is especially the case on Sunday afternoon when, despite a good general forecast, the fronts do wiggle closer up the Channel and might deliver some showers in the afternoon.  The SE and Reigate are on the dry side of the front but humid showery air is never far away.
Next week looks more interesting as a new LOW, invigorated by a refreshed jetstream, is due to deepen and stay out west due to the blocking high to our east.  It is set to deliver some waves of warm unstable air across the UK sometime midweek. On a stiff warm southerly wind this might generate heavy rain mid-week, some possibly thundery with plenty of interesting convection. A plume of so
rts.

end sept 30

HIGH cape and low LI indicate poss storms mid week

HIGH cape and low LI indicate poss storms mid week

A stubbornly slow moving “dishwasher” LOW spinning up west of Spain and just NE of the Azores has been abandoned by the jetstream. This LOW has sat there all week in a cool pool near the Azores. The tropical Azores have seen Tmax temps only a tad higher than Reigate this week as a cool polar pool gradually fills in that region. It is the position of the Atlantic dishwasher that controls our weekend weather.
Most models agree it will edge closer but this is a slow stuttering move without purpose or direction. Infact, it’s likely that this LOW will never move across the UK because of the stubborn ridge of HIGH pressure to the NE, causing the LOW to remain stuck out west sending occasional fronts and heavy showers our way as the pressure falls. The UK will remain in a relatively warm upper southerly flow with Tmax staying into the 20s through the weekend and possibly into next week, though things are uncertain at that range at the moment.
Expect windier conditions through the weekend, especially to the south coast.
Weekend weather could possibly see thundery showers edging over Reigate sometime Saturday through to Sunday but this is dependent on the location of the LOW. Current model runs suggest the heaviest rain will stay south in the Channel but check back as updates occur.
Between now and then… sluggish, overcast high pressure days with anticyclonic gloom for much of the week until the wind picks up.
Reigate was, again, the warmest place in the UK today… at 23.7C matching a rounded up figure for Charlwood, Gatwick at 24c.

Despite the gloomy stratocumulus which didn’t shift much this weekend, it has been warmer than usual.  Saturday night was especially balmy with Tmin of 16c and today saw Tmax a shade off 19c.  This week warm Tropical upper air will continue to waft up from the South being generated by a deepening TROUGH of LOW pressure spinning up just NE of the Azores. This will continue to pump a warmer than usual plume of upper air at +13c 850hPa (5000ft, the height air mass characteristics are usually measured) which flirts off and on with the south of the UK almost all week. The result is warmer than average September temps by some +4 ot 5c.  It will, however, gradually get edge away as the week progresses.

A weakening HIGH pressure over the UK will gradually melt away and drift over the continent and allow cooler air into Northern Britain. Reigate will remain in the warmer air most of the week by the looks of things.
Most models agree that the Azores LOW will fill gradually and drift NE over Spain and France during the latter half of the week and bring a nasty rash of showers to those areas. These might encroach on Southern England by the end of the week and weekend bringing potential for thundery outbreaks but this is too far off to be certain.
The distribution of rainfall is somewhat unusual, with a large accumulation of rainfall accumulating in a great spiral to the west of Spain associated with the Azores “Low” and lots of rain in Eastern Europe associated with the large Russian LOW. Note that the SE of England and Reigate in particular might just be one of the driest places in Europe by the end of next weekend as we seem to be sandwiched between these two great weather systems… well, models permitting! *one of driest places in Europe, probably! The GFS shows no rain at all for Reigate this week, but that’s quite a long shot! 

Another observation to match the LOW over the Azores is the negative forecast for the NAO: the North Atlantic Oscillation measure.  The NAO shows the pressure difference between Iceland and the Azores: when NAO is positive there is a big difference / pressure gradient and a strong jetstream and unsettled, fast moving weather across the Atlantic.  When negative, like this week, there is a meridional jetstream, a weak flow west to east and weather systems can get stuck… so not much change is expected.  This matches the forecast this week… not much change and no fast moving weather.

negative NAO sept

The ensemble model below shows that there will be a gradual decline in temperature during the course of next week and certainly beyond. More unsettled weather is also likely to arrive sometime next weekend but models still look rather uncertain so… keep checking back!

gefs 22 sept

Super Typhoon Usagi is blasting through the Luzon Straight and heading for Hong Kong. The Joint Typhoon Warning Center (JTWC) in Pearl Harbor expects it to make landfall in 24 hours, probably early Monday in HK. Wind speeds are over 100 knots and max wave height is 43 feet. Hong Kong sea port has been shut down and over 300 flights have been cancelled. All flights are expected to be cancelled from this evening (Sunday). Track Usagi with links below.

Updated location :usagi landfall

Typhoons in Pacific Sept 2013

Typhoons in Pacific Sept 2013

NOAA Usagi data

NOAA Usagi data

Warning Red Alert USAGI

Warning Red Alert USAGI

Hong Kong Observatory issued No8 storm warning signal (on a scale 0-10)  with the following alert as landfall occured 100km N of HK:

Hong Kong No8 signal


Meanwhile, in total contrast, the UK can expect the benign and balmy warm air feed from the SW to continue as a trough in the Atlantic deepens and brings warm air from Iberia. If the anticyclonic gloom shifts then temperatures will soon lift into the low 20s and maybe 23c is possible.  Models after about wednesday remain uncertain.  A general breakdown of the current HIGH over the UK with a return to zonal conditions with a strenghtening jetstream by next weekend is likely.  Some models bring on quite siginifcantly unsettled Autumnal weather next weekend, with heavy rain and gales, but this is too far off and models remain too wobbly with extra-tropical uncertainty still lingering in the N Atlantic.  The Atlantic hurricane season remains very quiet and no further incursions by ET storms are expected this week so Humberto maybe the first and last ETS affecting the UK this year … but the hurricane season has a while to go yet. ETS=Extra-Tropical Storm.

Blocking HIGH for the UK… opposite of Typhoon!

block high sept 22 2013

Some useful links on Usagi…

http://news.yahoo.com/super-typhoon-usagi-path-destruction-063833155.html

http://cimss.ssec.wisc.edu/goes/blog/archives/13969

http://www.wunderground.com/blog/JeffMasters/show.html

http://weather.noaa.gov/weather/current/VHHH.html

http://www.weather.gov.hk/wxinfo/currwx/tc1.htm

http://www.weather.gov.hk/wxinfo/currwx/tc_gis_e.htm

http://www.usno.navy.mil/JTWC/

http://www.bbc.co.uk/news/world-asia-24193201

A few wet days this week and then things looking warmer, brighter and more beautiful from the weekend!  Whilst not a heatwave it will be dry and pleasantly warm, even building to 23c possibly next week.

Reigate and SE is in for some heavy rain from lunchtime onwards Tuesday as a frontal wave forms under a brisk NW jetstream lying across the country. A wedge of warm tropical air will surge up from the SW and blast briskly up the Channel with gale force winds up to 50mph, and meet the Polar air blowing in from the NW lying over the country. The difference in the temperature of these air masses is pronounced: with places to the North of the front seeing Tmax 10c and those well to the south coast possibly hitting the high teens 17c, though in a brisk wind it won’t feel like that!  In between, the tropical air will lift, creating thick cloud and rain. Reigate can expect around 11-12mm of rain, possibly more, starting lunchtime and getting heaviest in the afternoon and getting lighter through the night.

After a drier Wednesday and another wet day on Thursday, things are still looking up for a pleasant weekend as a HIGH builds giving us a much drier run through maybe even to the end of the month.

 

Hurricane Humberto: a hurricane which left Cape Verde on 8 Sept and has been hovering in the Tropics ever since. Early models suggested a direct hit with the UK sometime next weekend which would have been wet and breezy. Latest charts show HUmberto gliding safely north of the UK. The GOOD thing about this is that Humberto will bring a great deal of Tropical warmth with him, especially on the south side. The heat embedded within the Humberto circulation will push warm upper winds from the Azores area north and build an upper ridge and surface HIGH pressure over the UK from the end of the week and hopefully over the weekend.  Warm air moving north aloft lifts air pressure through the whole warming column of the atmosphere and the result is increased surface pressure: an anticyclone (opposite of this weekend!).

Humberto is currently just a tropical depression having lost energy over a cooler part of the mid-Atlantic.  However, he is due to intensify when he meets the mid-latitude westerly circulation around the end of the week and entering the jetstream will deepen circulation further creating an extra-tropical mid latitude depression.  Charts say this will pass between the UK and Iceland… check back for updates.

humberto track into jet


So, it’s a way off to be sure, but the latest models seem to be pointing towards a nice weekend courtesy of Tropical Cyclone Humberto. Interestingly, Humberto left West Africa on 8 Sept and the first significant landfall he will make might well be Iceland next weekend!

 

The first proper Autumn storm of 2013 is spinning up in the N Atlantic today and bringing gale force winds to Scotland and the north of England. It is a product of a strong 200mph jetstream that has been blasting between Greenland and Iceland for the last few days and something meteorologists call convergence and divergence of air masses. The jet has been strengthening at the boundary between tropical and polar air: the temperature difference is very big this time of year (think of those tropical storms bringing hot topical air to meet the cold polar air over Greenland, for example).  This temp difference causes a steeper pressure difference between the air masses and this causes the jet to speed up, converging into a narrow tube of fast winds aloft called a jetstreak (#1). Where the jet spreads out, slows down or changes direction it causes air to diverge and this causes air on the surface to rise rapidly to replace the diverging and spreading air.  Surface pressure is lowered rapidly especially on any inside leg of a northward blowing limb of the jet where there is a positive spin on air circulation encouraging cyclonic winds: this is a development region for low pressure.  These combined processes have kick started the storm between Scotland and Iceland.  The diagrams try to explain more about what’s been happening to produce this explosive depression in the North Atlantic.

Think of the jetstream as the owner of an unruly unfit DOG.  The Jet owner travels faster than the dog and pulls it along, controlling what it does, lifting it up, pulling it along, making it sit and stopping it moving by different tugs on the lead and a firm hand.  This is pretty much, in a nutshell, what the jetstream is doing to surface weather patterns.  So, come on Fido, heal boy! Diverging (spreading out) of air aloft is when air is removed from an area more quickly than it arrives; there is therefore a “deficit” of air and this will drag air off the surface to redress the imbalance which therefore causes LOWER air pressure to form at the surface, as today. Air will converge into this low pressure and rise to replace diverging air aloft. Converging air aloft causes the opposite to happen: air piles up like lorries going up a hill (air arrives quicker than it leaves) so this means air can only descend to escape… causing a high on the surface. Convergence tends to occur where the jetstream is turning SOUTH from the Poles (where cooler air has a tendency to sink).  Divergence aloft tends to favour areas where the jet it turning NORTH towards the poles and carrying comparatively warmer air which has a tendency to rise.  All these complex processes combine to create LOW and HIGH pressure on the surface.

jetstreams cause convergence and divergence

Today, gale force winds will travel down the country and reach #Reigate this afternoon. A band of heavy rain will arrive shortly after the highest winds have eased a little but they will veer to the North and temperatures will drop further: the arrival of the polar air proper. The rain is fairly short in duration overnight and is not expected to amount to more than a few mm in total.
Down in the sheltered SE there are no warnings of damaging winds but peak gusts to 40mph in exposed places are possible and this is a “near gale” Force 7 on the Beaufort Scale: impeding progress, whole trees in sway and twigs breaking off trees, that sort of thing. Also, umbrellas difficult to use (I like that one :-)!

Think of Iceland, though… winds over the ice sheets will be touching 90mph overnight tonight with temps of -10c.  Waves in the North Atlantic will likely exceed 35 feet high during this period.

The central pressure of the LOW has fallen to 964mb which is low for a mid-latitude depression. In comparison, Hurricane Ingrid today bearing down on Mexico is “only” 986mb at the moment, though peak gusts in that system are expected to top 100mph.

low forms
It will remain windy through the first part of this week, with rain, heavy at times now and then as the LOW sits and fills in the North Sea. Tmax temps in the low teens, perhaps 16c on some days but cool mornings. Temps are expected to climb through to the end of the week when, if models come right, a HIGH pressure looks to make next weekend rather nice: perhaps low 20’s next Saturday with some good weather! Hurricane Humberto looks to whizz north past the UK, perhaps brushing N Scotland on the latest charts. Things will change so check back for autumn updates.

GALE WARNING: Reigate could see 40mph gusts in exposed places building through Sunday peaking in the afternoon and early evening.

Sunday will see the first Autumnal storm bringing rain and strong winds and a cool plunge of air from the Poles through Sunday and influencing our weather for much of next week.

Rain is due to arrive Sunday pm, and winds will build through the day to reach peak gusts possibly close to moderate gale force in exposed places 40mph into Sunday evening. Rainfall totals overnight into Monday could amount to 6-10mm, less than the rain yesterday and overnight, but heavy at times.
The cool air will be a feature as it follows the cold front with a Polar airmass fresh from Iceland. Expect temps to struggle into low double figures and feel especially cool in the breeze.
A gradual recovery through the week but the LOW will never be far away, lingering in the North Sea / Atlantic between Scotland and Norway: bringing northerly winds much of the week.
The end of the week might see ex-hurricane Humberto knock on the door. At least he will bring tropical air instead of Polar! Autumn is winding up!

Formation of our first Autumnal storm… 14 Sept

surface low formationThe chart above shows an unusually fast 200mph jetstream at 30,000 feet blasting across the North Atlantic from S Greenland to to S Iceland.  This jetstream then loops south and changes speed.  Any alterations in the speed and direction of an active jetstream are potential development areas for mid-latitude depressions.  The one South of Iceland, developing as we speak, is being created by the sudden decelaration of the polar jetstream and a steep temperature gradient between each side: warm to the south, cold to the north of the jet.  This steep temperature and pressure gradient across the jetstream axis encourages rising air (drags it off the ground), a fall in air surface pressure and convergence of airmasses at the surface: a cyclone.