Archive | June, 2013

MJO – Influences In Forecasting

29 Jun

When it comes to weather forecasting there are a variety of weather models that are used to give insight as to the possible weather. Both higher resolution short term models help weather forecasts pin-point those more specific areas at risk of say heavy rainfall and thunderstorms, to then the ensemble forecast information from the likes of the GFS and ECMWF which out to 2 weeks or more help to gauge the possible weather trends within the medium and longer term. When it comes to longer term forecasts there are some other factors which can be taken into consideration and one of these is the MJO – Madden Julian Oscillation

This tropical feature which is characterised by an eastward movement of rainfall across the Indian Ocean and into the Western Pacific whilst it may not seem to be of that much importance to the UK at first glance, does have significant and widespread affects on weather patterns across the globe and including the UK. There are many studies on the MJO that prove this to be true and one particular analysis and seminar by Christophe Cassou in 2008 provides significant and detailed information that the MJO can and does affect the weather across the UK and the North Atlantic area in general.

What I will say at this stage as a priority however, is that the MJO is not the “be all and end all” when it comes to medium and longer term forecasting. Just like any part of weather forecasting a variety of pieces of information need to be looked at and acknowledged when forecasting the weather and the MJO can be overridden by other large scale, global weather events and features.

However, I want to use this blog as an experiment, a forecasting experiment to see whether, on this one particular occasion, the MJO can indeed be a guide as to the longer term weather trends across the UK and the North Atlantic in general. Reanalysis of past weather data has lead to a series of pieces of information that correlate to the expected phase of the MJO, time of year and how approximately 10 to 12 days ahead (Cassou 2008) it may then influence the weather across the North Atlantic. Just like weather charts that show weather forecasters how areas of low pressure and high pressure move, there are forecast charts that show how the MJO may progress as well – http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/CLIVAR/clivar_wh.shtml

So, using the last link and the forecast charts on that page you can see that the MJO Index is currently in Phase 8. For a June month, this correlates to the follow possible synoptic pattern within approximately 10 to 12 days;

JunePhase8500mb

What this is showing us is that the green areas are signalling areas where pressure is likely to be higher than average and the blue colours denoting lower than average pressure. It is interesting to note that on this chart there is a clear signal that for the UK and NW Europe pressure is likely to be high and the current expectations are that towards and beyond the opening week of July the UK will experience a progression towards high pressure leading to settled, summery and potentially very warm conditions. So this is one interesting correlation and example of how the MJO may well be highlighting/influencing this signal of better weather given the current phase of the MJO.

So lets look further ahead for a real challenge;

ALL_emean_phase_full

The latest forecast guidance regarding the progression of the MJO is that there is quite high confidence that by the time we move into early July (1st to the 3rd) the MJO will have progressed into Phase 1. Note on the above image how the forecast models (coloured lines) signal the progression of the MJO into the ‘1’ region. Now, using the reanalysis charts again as a possible guide to longer term trends, this is the following possible pattern for a July month given a Phase 1 of the MJO;

JulyPhase1500mb

Now, in this instance we have a totally different possible synoptic pattern across the UK. Remember that blue colours denote generally lower than average pressure and clearly there is a region of blue/lower than average pressure located across the UK.

So, using the above information and the latest forecast models regarding the track of the MJO, I’ll make a prediction that approximately 10 to 12 days after the opening few days of July the weather across the UK will become more unsettled as the high pressure and potentially very warm conditions from the 6th and 7th of July are replaced by cooler and more unsettled conditions as we progress towards mid-July (12th to the 15th approx).

I’ll come back to this blog and prediction around the middle of July to see whether it was accurate or totally wide of the mark, but it will be an interesting experiment to see, if on this one occasion, the MJO did have some influence on the weather longer term. If this prediction is also likely to be correct, it would seem that the possible settled and very warm conditions towards the 6th and 7th of July won’t be prolonged and lasting for less than a week.

Time will tell as ever!

Regard to all,

M.

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Atmospheric Soundings – Approaching Frontal System

21 Jun

In a recent blog I discussed the usefulness of atmospheric soundings in terms of aiding to predict thunderstorm and convective activity. Well, forecast soundings have many uses and one other primary one is using them to view an approaching frontal system or to see the atmosphere through an active frontal system. A great example of an approaching frontal system is in evidence today (Fri 21st June) across Ireland. So, lets take a look at some of the synoptics;

12Z synoptics18Z synoptics

The low pressure and associated frontal systems can be seen clearly to the west of Ireland on the two synoptic weather charts. At midday you’ll notice that across Ireland there is a minor ridge of high pressure in evidence, this is represented by the isobars ‘ridging’ northwards. At midday the atmosphere across Ireland is forecast to be relatively stable beneath this ridge of high pressure, but you can clearly see by 1800Z the frontal systems associated with the surface low pressure are across Ireland, the ridge has been removed and the weather by the end of the day is essentially wet and windy in association with the passage of an active set of frontal systems.

The approach and passage of a frontal system (warm front) has certain characteristics and events that take place within the atmosphere.  These include anything from increasing wind speeds at high altitudes, the backing of surface wind speeds at surface level to generally a south-westerly direction and also the introduction of cloud and moisture, initially at high altitudes and then progressively at lower altitudes. The approach of a frontal system is represented well on the following graphic;

warmfront

Now, taking a look at some forecast soundings through the course of the afternoon across Ireland;

sounding13.curr.1200lst.d2sounding13.curr.1300lst.d2sounding13.curr.1400lst.d2sounding13.curr.1500lst.d2sounding13.curr.1600lst.d2sounding13.curr.1700lst.d2sounding13.curr.1800lst.d2sounding13.curr.1900lst.d2

1200BST and 1300BST: On these two forecast soundings you’ll notice that the red line (temperature) and the blue line (dew point) are particularly separated, especially between 700mb and 500mb. On a forecast sounding this represents a particularly dry atmosphere and hence little cloud. Also notice that the wind direction is still westerly or west-southwesterly and the wind speeds haven’t really increased very much through the atmosphere. This profile is representative of being beneath a ridge of high pressure and this tallies in well with the weather chart earlier in the blog which highlights a ridge of high pressure across Ireland.

1400BST and 1500BST: As the afternoon progresses the primary importance and change here is that the space/distance between the temperature and dew point line is decreasing within the upper levels of the atmosphere in particular. This is indicative of cloud increasing within the upper levels of the atmosphere as the frontal zone approaches. If you refer back to the earlier image showing the profile of an approaching frontal system you will see that cirrus and cirrostratus clouds are first to be expected and that is what these forecast soundings are indicating, an increase in upper level frontal cloud. Also note how the wind direction at lower levels has backed significantly and is now south-westerly

1600BST onwards: From around 1600BST onwards you’ll notice that the temperature and dew point lines get progressively closer and closer and by 1900BST are practically the same around the 700mb level. This is a clear example of how the frontal system has continued to approach the region introducing thicker cloud at increasingly lower altitudes and by the end of the afternoon a large portion of the atmosphere is moist and cloud laden in association with the frontal systems. You’ll also note that the wind speeds have increased within the mid and upper levels of the atmosphere in association with the jet stream and low pressure system.

The difference between the first image at 1200BST compared with the image at 1900BST is significant. The atmosphere goes from being stable and particularly dry indeed around the middle of the atmosphere (500mb) to being almost completely saturated by 1900BST in association with extensive frontal cloud which will clearly be producing frontal rain at surface levels.

Regards to all,

M.

Acknowledgements:

http://www.southamptonweather.co.uk/fronts.php

http://www.avbrief.com

http://rasp.inn.leedsmet.ac.uk

‘Loaded Gun’ Atmospheric Sounding

7 Jun

The term ‘loaded gun’ may well seem quite a strange one, but is quite an appropriate one for this particular kind of sounding. Clearly a loaded gun can “go off”, just like the atmosphere can under the right conditions and of which the below illustrates;

sounding 1

Atmospheric soundings, or tephigrams as they are also known, have many uses, but one of the important uses during the summer is to gauge potential instability within the atmosphere and to discover whether there is a risk of some heavy and thundery conditions possible from large cumulonimbus clouds.  In this instance, I use a forecast sounding for Exeter and I’ve highlighted two sections of the sounding which are of primary importance;

A = This area of the sounding is indicating that this portion of the atmosphere from approximately 6,000ft up to nearly 30,000ft is unstable.  When I say unstable I simply mean this area of the atmosphere is cable of allow large clouds to develop and air to rise and given the height at which clouds could well rise to (30,000ft) then these would be very large cumulonimbus (thunder clouds). However, for the clouds to properly develop the whole of the sounding needs to become unstable and there is an important part of this particular sounding that is stopping convection and the large clouds from developing…

B = Whilst this region of the sounding looks quite insignificant, it is in fact very important. You’ll notice on the image that near 4,00ft to 5,000ft the temperature (red line) increases slightly after decreasing from the surface. This is known as an inversion and whilst it looks insignificant this is prohibiting convection from occurring. Think of being in a room with the central heating on and all doors and windows are closed.  The heat from the radiators rises and will continue to rise until it reaches the ceiling and then can’t go any further, the heat is essentially ‘capped’ at the level of the ceiling, this is the same principle in this case. The warmth and humidity at surface levels is attempting to rise but is unable to rise past that level near 4,000ft to 5,000ft due to the inversion.

So, how do we get past this problem and allow the atmosphere to become completely unstable and for those big thunder clouds to develop?

Well there are a number of ways, but one of the primary ways is from the surface and that is with further warming through the afternoon. Additional warming of the surface will increase the temperatures and will then essentially override that inversion and once that is the case, the entire atmosphere then becomes unstable, as is evident on this forecast sounding for an hour or so later:

sounding 2

Note how the dashed line is now visible from approximately 3,000ft all the way up to beyond 30,000ft.  This is a particularly impressive atmospheric sounding in terms of the instability. Temperatures at the surface and humidity have increased enough to allow complete instability and that ‘capping’ near 5,000ft has been removed from the equation and all the heat and humidity within the lower layers of the atmosphere, that has built up, is now free to rise rapidly and significantly, in essence the “loaded gun” has “gone off” and more often than not, it would usually go with a ” bang” in the form of heavy and likely thundery showers.  I’ve also highlighted, at the top of the image, one variable known as CAPE (J). CAPE stands for Convective Available Potential Energy and is a measure of the potential instability within the atmosphere.  In essence the larger the value the more unstable the atmosphere is and in this instance a value of 992 is evident.  Anything over 1000 from a UK’s perspective is pretty impressive and does signal some possible significant convective activity. Of interest CAPE is derived from the area between the dashed line and the solid red (temperature) line on a forecasting sounding.

So if you ever see soundings like the above images posted on twitter or from other sources then think convection, heavy showers, cumulonimbus clouds and possible thunderstorms. The next 3 months are the prime time of year to experience significant convection across the UK that more often than not leads to some very lively weather.

Regards to all,

M.