Archives For February 2015

A system stirred up by a low pressure tracking out of Mexico into the Gulf of Mexico this week is on track to bring unsettled conditions for part of the weekend to the UK.  It’s nothing too severe for most but is an interesting feature that will bring some wind and rain everywhere.  Below is a satpic showing the development of this system as it interacted with a lively jetstreak on Saturday 28/02/2015.

development of LOW on jetstreak

development of LOW on jetstreak


This LOW illustrates nicely how extra-tropical systems can rattle clean across the Atlantic in a few days if they are picked up by an active jetstream.   This one does precisely that.  Spot the system leaving Florida on the chart below for today and the sat pic.  This system started as a low pressure crossing from Mexico into the Gulf of Mexico mid-week, so for weather systems it will be an aged fellow on arrival here in the UK.  Its’ longevity is partly due to the exceptional COLD over NE US which interacts with the warm tropical air and causes further deepening.

The Gulf low pressure is tracking quickly NE skirting the US east coast before being picked up and deepened further by an active jetstream.  The jetstream itself is particularly powerful at the moment due to intense cold spilling out of an exceptionally wintry NE USA meeting warm tropical air issuing from a strong subtropical Azores HIGH pressure converging with the moist Gulf airmass.  A result of the powerful jetstream is a positive North Atlantic Oscillation: the NAO is a measure that indicates the difference in pressure between Iceland and the Azores.  In positive NAO conditions the jetstream is often active, producing a strong westerly zonal flow keeping Europe mild and unsettled especially in winter, or early spring!

Our Gulf LOW is due to pass over Reigate fairly rapidly through Saturday pm and overnight into Sunday am and bring some moderately wet and windy weather, likely to go unnoticed because of the nocturnal transit.  Winds gusting in excess of 40mph are possible for Reigate into Sunday am in exposed places.  Notably, due to the TROPICAL origins of this airmass the temperature overnight Sat-Sun could climb to double figures in Reigate.  Tropical air crossing the Atlantic also picks up a tremendous amount of moisture so attendant fronts are likely to bring a lot of rain too, possibly exceeding 10mm overnight, which is a moderately wet night.

Here are the synoptic charts showing the Gulf low progress across the Atlantic, deepening and occluding into the North Sea.  Note the secondary wave which could bring additional rain later on Sunday to the South.

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The additional rain later Sunday afternoon / evening looks potentially heavy for the South and SE.  It’s a rapidly developing wave feature that needs attention as, on the northern edge, it looks to raise the possibility of snow across the Midlands.  Heavy rain is possible for the SE and #Reigate with a period of gales on the south coast.


The outlook for next week is for the Azores HIGH pressure to extend a ridge to the north and cause a NW then northerly airflow for the UK.  This will bring cooler temperatures to the UK.  Whilst it is likely to be mostly dry for Reigate and the SE with pressure rising, wintry showers especially on east facing coasts of the North Sea could be possible depending on how the HIGH develops.  Frost is likely with temperatures dipping below freezing at night from mid-week.


Atlantic ridge builds to the west and brings in northerlies

How long this Atlantic block persists, and the cooler weather, is uncertain.  The coldest scenario would depend on the HIGH moving north and east and building over Scandinavia to pull in easterlies from a cold continent.  This scenario is preferred by the ECM by later next week whilst the GEFS topples the high to the SE and brings back a zonal mild westerly flow from the persistent Icelandic LOW pressure that erodes the edges of the HIGH from the NW.  The charts below show the uncertainty as a wide spread of possible pressure and temperature towards the end of the first week in March.


Gulf low for the weekend, then pressure builds

The cool start to March is shown below.  The overall outlook is for a persistent positive NAO and Arctic Oscillation to persist and this would suggest a brief cool epsiode without the formation of a persistent Scandinavian High.  Models have flirted with possible easterly winds by the end of next week but the outlook is for the positive NAO to persist and this rather suggests a quick return to milder zonal westerlies.  As the high builds in early in the week various troughs and fronts could even push some wintry precipitation as far as the SE on Tuesday (spot the pink on the rainfall chart below for Tues)

First week of March starts cool: MetOffice synoptic chart for mid-week shows Azores high briefly ridging north to block mild zonal westerlies and usher in a cold polar airmass, albeit briefly as this ridge looks to topple SE and by next weekend we could be in a pretty mild SW flow hitting mid-teens possibly.  So a cool, mostly dry middle part of the week for Reigate and much of S England but precipitation, some even wintry, pulling in on NW / N winds is not ruled out with a North Sea low possible.  As the Atlantic is likely to push westerlies back in later in the week we can expect more purposeful frontal rain pushing east across the whole country.



link to accuweather take on this system



HIGH pressure dominates but is it all calm?

High pressure is known for calm, clear conditions, with little wind, cold frosty and foggy nights especially when there is little cloud. Pretty unexciting weather.  However, HIGH pressure is not as unexciting as all that.  Anticyclones can sometimes be surprisingly windy especially round the edges.  We spend a lot of time learning about LOW pressure, with associated storms and gales and torrential rain but understanding the inner workings of HIGH pressure is important to get the full picture of mid-latitude weather.

So… buckle up for the ride and let’s get super-geostrophic!  Wind blows from HIGH pressure to LOW pressure.  The wind speed and direction is the result of two forces: the pressure gradient force (PGF) is the difference between high and low pressure and sets up the strength of the wind and the overall direction which is for winds to blow directly from HIGH to LOW pressure.  Coriolis force (or Coriolis Effect) is a result of the spin of the Earth and deflects resultant winds to the right of their intended path in the northern hemisphere.  Here are some video links to review these forces before proceeding with super and sub-geostrophic winds. Skip below these videos if you already know about PGF and Coriolis.


The winds do blow from high to low… but get pushed to the right by that Coriolis fellow!

The pressure difference between high and low pressure determines the speed of wind.  Winds do blow from high to low due to the pressure-gradient but are deflected to the right by another force called the Coriolis effect! Below is a chart showing upper winds at 850hPa (1500m) blowing round the same HIGH pressure shown on the synoptic chart at the top of the post.  Note the relatively high wind speeds circulating round the HIGH in the north of Scotland, the North Sea and across France and Biscay especially.  Winds obviously blow faster across the ocean but remember this is an upper wind chart so is above the boundary layer of most frictional forces upsetting the wind.  In any case, none of these locations is associated with a trough… it is all anticyclonic super-geostrophic wind.  So why is the wind blowing so strong when there is no LOW for miles?


Given the same isobar spacing the wind speed aloft round high pressure ridges is often greater than the wind flowing around troughs and low pressure. This is surprising because we associate gales and windy weather with “storms” and low pressure systems.  The chart above illustrates super-geostrophic winds circulating around the Azores high across Europe.  These look pretty strong at 850hPa (1500m), the level above frictional effects of the surface.  The chart also shows the trough of low pressure over the Mediterranean where, given some of the locations with similarly spaced and even tighter isobars, the wind strength is not especially any greater and perhaps even less than that circulating freely around the HIGH.


Wind is a result of pressure differences across the planet surface.  Wind wants to blow from high to low pressure.  This is called the pressure gradient force.  Due to the spin of the Earth winds in the northern hemisphere are deflected to the right of their intended path.  The two forces, pressure gradient and coriolis force, actually balance out to produce a theoretical wind that flows parallel to the isobars called the geostrophic wind, shown above. Unfortunately, isobars are almost always curved so the geostrophic wind hardly ever actually blows.


Assuming a constant isobar spacing.  Around troughs of LOW pressure the wind is sub-geostrophic. This means it blows less than the expected geostrophic wind.  In the chart above the wind is shown as a black arrow.  In addition to the coriolis force, the centrifugal force acts to “push” the wind away from the low centre and is acting in the same direction as the coriolis force.  Note that the resultant wind is pointing slightly away from the LOW towards the HIGH, which is of course not possible because the wind would be moving into and against increasing pressure.  As the pressure gradient force cannot change, the coriolis force must weaken to allow the wind to return parallel to the isobars.  This means that the wind flowing around troughs of LOW pressure has reduced force acting on them given the same isobar spacing of a similar HIGH. These winds therefore blow slower than geostrophic wind and are called SUB-GEOSTROPHIC.


Here is the HIGH pressure situation.  This time the centrifugal force is acting with the pressure gradient force to push the wind into low pressure.  As the pressure gradient cannot change the coriolis force must INCREASE to pull the wind back parallel to the isobars.  This means that the wind flowing around ridges of HIGH pressure has GREATER forces acting upon them than winds flowing round lows with equivalent isobar spacing.  These winds therefore blow faster than geostrophic wind and are called SUPER-GEOSTROPHIC.

Usually, of course, low pressure cyclones and depressions exhibit tighter isobar spacing than HIGH pressure and so resulting wind speeds round LOWS are most frequently higher than the HIGH pressure feeding them.  Nevertheless, assuming the same pressure-gradient force, winds exiting anticyclones can produce higher wind speeds than those entering depressions.

useful reference


January 2015 summary for Reigate, Surrey

Average temp 4.5C (UK av 3.7C, CET 1C above long term average)

Tmax 14.7C (12.2C 2014)

Tmin -4.9C (-1.5C 2014)

Total precipitation 69mm  (183mm 2014)

Max wind gust 42mph (52mph Jan 2014)

snowfall was recorded on 3 days, snow lying on 2 mornings

Total sunshine 96 hours (98hrs 2015)


January was mostly under the influence of an occasionally strong westerly and NW airstream with some stormy weather for the NW of the UK but, predictably, Reigate and the SE was sheltered from most of the weather action and our rainfall total was about on the long term average for the South East at 69mm. This was, of course, considerably less than the 183mm rainfall last January 2014!


Overall, January was just about average temperature in the UK. This average hides the variation though… the start of January was extremely mild with some very warm Tmax temperatures early in the month reaching nearly 15C in Reigate.  The end of January was considerably colder than average although nothing extreme.

The end of January cold snap ran into the first week of February.  During this period Reigate saw temperatures drop modestly below average and we experienced 3 spells of modest wet snowfall, albeit lying snow from night time falls rapidly melted by early morning and the lowest temperatures we a mere -4C.  Reigate experienced one notable but brief “thundersnow” event on the afternoon of 29 Jan. This caused some local traffic disruption.

The cause of our swings in weather a neatly summarised in the mean monthly sea level pressure and 500hPa anomaly chart from the JMA.  Here you can see the building high pressure over the Azores towards Iceland and the low pressure to the North and NE nudging towards Scandinavia.  It is this configuration of building ridge in the Atlantic and LOW over Scandinavia that eventually brought our modified “Arctic blast” through the last week of January.  As is normal for an Arctic airmass the SE of England away from the North Sea coast of Kent rarely sees any prolonged snowfall and this was the case.  An interesting feature developed on the 29/01/2015 within the Arctic airmass: a polar low may have spun up and moved South through the Irish Sea.  This was controversial and not accepted as a true polar low by everyone but it seemed to have many of the characteristics.  A post on this polar low can be found here.


The evolution of the January 2015 cold snap was interesting because it was initiated by an unusually cold and unstable NW airstream on the back of a deep low that crossed Scotland.  The NW airstream was unstable enough to bring the thundersnow event to Reigate. Oddly the original NW “blast” from Greenland pushed through so quickly that a lot of polar maritime returning and tropical maritime air was secluded in the low core over Scandinavia.  It was this secluded / occluded warmer airmass that a) probably contributed to the polar low feature and b) modified the Arctic blast and , at least initially, made it much less cold than is usual for such an airmass direction.  It took several days for any truly cold air to reach the SE of England and, even then, 850hPa temps never fell below -6 or -7C.

The CET for January was nearly 1C above average for the long term average, quite a lot more than the MetOffice.  As can be seen from the chart below this January was not exceptionally warm, being moderated by the cold snap at the end of the month.  Only 6 of the last 20 years have come out colder than the long term CET average.


Globally January was +0.35C above the long term (30 year) average temperature.  The northern hemisphere was +0.55C above the long term average.



Is this it for winter? Read on…




arctic airmass





