Author: jped

A Period of Quiet November Weather

Heavy rain and flooding has been a feature of the first half of November 2019.  The UK weather has turned a corner now to more settled conditions.  The reasons for this lie out in the Atlantic.

Some 300 nautical miles south east of Cape Farewell (southern tip of Greenland) there is an intense area of low pressure.  Analysed by the UKMO at 967 mb last night it is very intense.  The lowest recorded pressure ever was in 1979 when Typhoon Tip brought us down to 870 mb, but that was in the tropics; 967 mb is very low for the mid-latitudes.

191118UKMOsfcanal
UKMO Surface Analysis 180600Nov19

The marvellous website Earth.nullschool.net gives us the opportunity to investigate the winds around this low at different heights.  Current flows for the surface and top of the atmosphere are as follows.

191118ENS250mb
250 mb wind 18 Nov 19
191118ENSsfc
Surface Wind – 18 Nov 19

Often these patterns are different; different winds at different levels are what causes changes in weather patterns.  The fact the winds is uniformly circling the depression at all levels makes this a more static pattern.

The UK is somewhat to the West of this enormous system.  The graphics show an intense stream of air piling south over us at the top of the atmosphere.   Whole regions of air are being forced downwards causing high pressure and settled weather.

High pressure in winter can bring sunshine or settled gloom with.  Sunshine is accompanied by frosts at night.

Upper Air and Development

In general, the development of weather systems over middle latitudes will be driven by the upper winds which circulate around the earth.  The mathematics and dynamics of these processes is complicated but after much work development areas can be related to the pressure pattern at 30 000ft or so.

The study of the mathematics is very rewarding and brings together the conservation of momentum (and angular momentum), Newton’s Laws and the rotation of the Earth.

For the moment, the following images show the relationship of the 300mb contour chart with convergence and divergence.  The fact that air cannot be created or destroyed in the flow means that:

  •  if upper level air is in CONVERGENCE the air is forced downwards causing subsidence, a build up of pressure and good weather, and,
  • conversely, if upper level winds are in DIVERGENCE air is sucked upwards causing condensation, droping of pressure and bad weather.

Applying the rules in these images to the latest 250 mb flow (300mb not available, but 250 mb close) and surface flow form Earth.nullschool.net and then looking at the latest forecast sequence from the UK Meteorological Office (weathercharts.org) will give you an insight into current atmospheric dynamics and weather development.

The Convergence / Divergence Rules

St Martin’s Summer – not really on the cards in Kent for 2019

Folklore says St Martin’s Day, 11 November, brings with it a settled spell of weather known as St Martin’s Summer.

Disappointingly, the UKMO forecast chart for St Martin’s Day (weathercharts.org), 11 November 2019 leads to a text book forecast: blustery showers, some of which will merge to give longer outbreaks of rain.

UKMO forecast chart for St Matin's Day - 11/11/19
UKMO forecast chart for St Matin’s Day – 11/11/19

 

History (wikipedia) tells us that St Martin was a Roman cavalry soldier who resettled as a Bishop on leaving the army.  Leaving the Roman equivalent of brightly coloured trousers behind, his Christian career gained some meteorological notoriety in a snow storm.  Turning the tools of his military career to pastoral use he cut his cloak in two in order to share half of it with a beggar.

Höchster_Schloß_Tor_St_Martin.jpg
Statue of Saint Martin cutting his cloak in two above the Höchst Castle’s gate

By Eva K. / Eva K. – Eva K. / Eva K., CC BY-SA 2.5, Link

Mid-latitude weather can always be relied upon to give some settled spells and there is enough lore out there to expect such a respite on many dates.  In fact, Shakespeare connects St Martin’s Summer with a similar spell of calm weather, halcyon days, expected at the solstice.  Henry VI, Part 1, act 1, scene 2, line 131:

Expect Saint Martin’s summer, halcyon days.

The current forecast from the Met Office doesn’t really promise summer:

St Martin's Day Forecast

with heavy rain for the commute.  Tuesday and Wednesday seem better, but low pressure is not far away.

The time is always ripe for some continued Autumnal feasting as depicted in the featured image.

October 2019 UK Weather Headlines

Source: UKMO Climate Summaries and Wetterzentrale Archives

Summary

October 2019 was a typical Autumnal month of mobile UK-Atlantic weather with a succession of depressions, fronts and high-pressure systems bringing unsettled and changeable weather.  At the at the start of the month the ‘eyeballed’ mean low pressure track was to the North West, with depressions moving to the North East through the Iceland/Hebrides gaps trailing fronts across the country giving rain with some settled spells in between.  As the month progressed, this track gradually moved south, as befits the transition of the seasons.  Slower moving complex lows were centred over the UK on and around the 20th.  These were followed by some more intense areas of high pressure responsible for the hard frost in Scotland at the end of the month.

Averages

Mean Temperature 9.0 °C (-0.5 °C compared to average)
Rainfall 109% of average
Sunshine 95% of average

Extremes

Max Temperature 21.3 °C Trawsgoed (Dyfed) 1/10/19
Min Temperature -6.0 °C Altnaharra (Sutherland) on the 31/10/19
Max Rainfall 101.0 mm  Lebanus (Powys) on 26/10/19
Max Wind Gust 66 kt (76 mph) Needles (Isle of Wight) on 18/10/19

UK Sea Level Predictions to 2300

Source: UKMO UK Sea Level Projections

Beach hut owners on the North Kent Coast acquired their plots in the full knowledge that the once if fifty year storm would reduce their hut to broken planks. In fact the last weather disaster in the folk memory was the great flood of 1953.  In fact, the University of Brighton’s Screen Archives South East supplies some photographic evidence of the aftermath:

Hampton - Herne Bay 1953
Hampton – Herne Bay 1953, Screen Archive South East University of Brighton

The Met Office’s UK Sea Level Projections to 2300 reports that “at some locations the 1 in 10,000-year flood event of today could be expected more than once per year by 2300”. This seems to be bad news for beach hut owners; the once in fifty year storm might be expected every week!

These new sea level projections developed by the Met Office Hadley Centre have been published in a report by the Environment Agency. They show continued rise beyond 2100 under all climate change scenarios.

Sea Level Projections Sheerness (UKMO)
extended sea level projections London (Sheerness) – UKMO

Important news for all; not just beach huts at stake.

September 2019 UK Weather Headlines

Source: UKMO Climate Summaries and Wetterzentrale Archives

Summary

September 2019 started and ended with an unsettled westerly flow of fronts and depressions but enjoyed a more settled, high pressure dominated, spell in the middle. Slightly warmer than the long term (1981-2010) average, with a little more rain and sunshine than normal.

Averages

Mean Temperature 13.1 °C (0.5 °C above average)
Rainfall 127% of average
Sunshine 115% of average

Extremes

Max Temperature 27.7 °C Weybourne (Norfolk) on the 22nd
Min Temperature -1.6 °C Aboyne (Aberdeenshire) on the 8th
Max Rainfall 59.4 mm Fontmell Magna (Dorset) on 24th
Max Wind Gust 61 kt (70mph) Baltasound (Shetland) on the 15th

Towering Cumulus over the London Array

Towering cumulus over the London Array, outer Thames Estuary.  Complex low pressure lies over the North Sea countries.  Watnall balloon suggest unstable to sea surface temperatures with drying up at about 15,ooo ft.  AVHRR image, 1420Z, picks out this development at the end of a cumulus boundary running through the south east.

Atlantic Tropical Storm and Hurricane Forecast

UKMO North Atlantic and Tropical Storm forecast , 1 June 2017, predicts slightly enhanced activity when compared to long term averages. The first storm of the year, Arlene, happened in April.

2017 forecast is:

Tropical Storms (> 39mph wind): 13

Hurricanes (> 74 mph wind): 8

ACE index : 145

The Accumulated Cyclone Energy (ACE) index. A measure of the collective intensity and duration of all named tropical storms and hurricanes during the season.

The first storm of the year, Arlene, happened in April.

Atlantic Hurricane season starts on Wednesday, first of June, what’s in store?

The Atlantic Hurricane seasons starts this week on Wednesday the first of June.

Tropical storm naming is well under way already.  ‘Bonnie’ (featured image) has just been downgraded from ‘Tropical Storm’ to ‘Tropical Depression’ and is the first system to make landfall on the USA this year.

D: Tropical Depression – wind speed less than 39 MPH
S: Tropical Storm – wind speed between 39 MPH and 73 MPH
H: Hurricane – wind speed between 74 MPH and 110 MPH
M: Major Hurricane – wind speed greater than 110 MPH

The question arises as to the expected intensity of the 2016 season.  Theories link the El Nino (ENSO) to the intensity of the hurricane season.  It is thought that higher than average sea surface temperatures in the tropical Atlantic follow an El Nino and sea surface temperatures feed energy into tropical storm systems.  This has been an El Nino year.  Other observations report unusually cold regions of the Atlantic Ocean further north which may mitigate any warming later in the season.

Reports are a little mixed:

Reuters reports that US meteorologists at NOAA predict greater than average numbers this year.  NOAA’s website itself is sticking to a ‘near normal’ pattern of tropical storms.  Other reports abound.

Always thought that weather forecasting was a tricky business; it makes weather watching interesting though….