PREDICTION FOR APRIL AND MAY 2017: WARMER THAN USUAL BUT OFTEN WET IN THE NORTH

[Matthew.]

For April we've only received about 5mm of rain so far this month. The ground is starting to really dry out; comparable to October last year after a dryer period July-October. I can't see much in the way of rain forecast in the next week with the current set-up for here in the ENE England.

Edited April 12, 2017 by Summer 1976

[DAVID SNOW]

In my opinion Ians forecast should be judged at the end of May, and not picked to pieces before then.

[Interitus]

There is just far too much importance being applied to the QBO. The time-latitude chart below shows the zonal wind anomalies at the 30mb level from Jan 2014 to end of March 2017 -

The phases of the QBO can seen in the tropical anomalies on the left hand side, and the seasonal winter stratospheric vortex development in high latitudes on the right. There is no transfer of QBO anomalies towards the midlatitudes, with the anomalies effectively meridionally confined below 20°N. The reason is that meridional wind flow is very low, in the region of less than a couple of m/s at most.

The Holton-Tan relationship describes a link between a weaker winter time vortex with an easterly QBO but this is not because of a transfer of momentum, it is because of the effect of the critical line i.e. zero or easterly flow, on planetary waves generated from the troposphere. These are prevented from moving equatorwards and are directed more towards the pole so more effectively weakening the vortex. The effect of QBO phase can then be seen in stratospheric northern annular mode (NAM) and thence in the tropospheric AO/NAO.

This is restricted to wintertime however, as planetary wave propagation is greatly limited by summer polar easterly flow, requiring the presence of the vortex westerlies from autumn onwards. This also affects the major stratospheric meridional motion - the Brewer-Dobson circulation is generated by planetary waves during the winter half of the year, and does provide a northward flow which as it descends adiabatically warms the polar stratosphere significantly above what would be expected from winter radiative cooling. But it is largely above the 30mb level and the velocity is very, very low - tracers such as tropical ozone show that the age of polar stratospheric air may be 4 or 5 years.

Edited April 13, 2017 by Interitus

[Gael_Force]

1 hour ago, Interitus said:

There is just far too much importance being applied to the QBO.

I've been thinking the same thing but lacked the knowledge to coherently argue the matter. In a recent paper, the UKMO disregarded the QBO phases as a useful tool for following winter prediction ... other much better predictors. Thanks for the detailed post which clarifies the situation.

[weatherguru14]

Its funny how people do  a long range forecast after a warm spell or cold spell.. Well this weekend was 25c in parts of the UK. I will now do my long range forecast and strangely  its going to be a  warm spring.. since the weekend its been cold with a wind chill in a lot of the UK and its looking like remaining for the foreseeable future..  

 

 

[iapennell]

On 4/13/2017 at 12:00, Interitus said:

There is just far too much importance being applied to the QBO. The time-latitude chart below shows the zonal wind anomalies at the 30mb level from Jan 2014 to end of March 2017 -

The phases of the QBO can seen in the tropical anomalies on the left hand side, and the seasonal winter stratospheric vortex development in high latitudes on the right. There is no transfer of QBO anomalies towards the midlatitudes, with the anomalies effectively meridionally confined below 20°N. The reason is that meridional wind flow is very low, in the region of less than a couple of m/s at most.

The Holton-Tan relationship describes a link between a weaker winter time vortex with an easterly QBO but this is not because of a transfer of momentum, it is because of the effect of the critical line i.e. zero or easterly flow, on planetary waves generated from the troposphere. These are prevented from moving equatorwards and are directed more towards the pole so more effectively weakening the vortex. The effect of QBO phase can then be seen in stratospheric northern annular mode (NAM) and thence in the tropospheric AO/NAO.

This is restricted to wintertime however, as planetary wave propagation is greatly limited by summer polar easterly flow, requiring the presence of the vortex westerlies from autumn onwards. This also affects the major stratospheric meridional motion - the Brewer-Dobson circulation is generated by planetary waves during the winter half of the year, and does provide a northward flow which as it descends adiabatically warms the polar stratosphere significantly above what would be expected from winter radiative cooling. But it is largely above the 30mb level and the velocity is very, very low - tracers such as tropical ozone show that the age of polar stratospheric air may be 4 or 5 years.

@Interitus; You might be interested in this little article about the effects of the Quasi Biennial Oscillation on the Westerlies at higher latitudes:

https://books.google.co.uk/books?id=iOfqCAAAQBAJ&pg=PA46&lpg=PA46&dq=QBO+AND+MID-LATITUDE+WESTERLIES,+ANGULAR+MOMENTUM+INFLUENCES&source=bl&ots=T9HNYMMpJv&sig=GBllwYfPTMcpoGoD4lxteqLk6JA&hl=en&sa=X&ved=0ahUKEwi966-p-aPTAhUsLcAKHZnWAyUQ6AEISzAE

There is, nonetheless, a link between the QBO phase and the strength of the Circumpolar Vortex. It is, as you rightly point out, rather less in late spring/summer when there are easterlies in the stratosphere in all latitudes and the tropic to polar circulation in the troposphere is weaker. Nevertheless, you have a large area of the tropical stratosphere between about the 20 mb and 80 mb levels where mean winds are from the west at speeds exceeding 10 mph (locally 35 mph where strongest): See here (note of the large blob of blue in the middle of the chart):

The Earth-atmosphere system is a closed system as regards angular momentum (save for small impacts from tidal effects of the Sun and Moon and momentum lost through atmospheric out-gassing to space and - periodically- the influence of Solar Flares);  such a large area of such Westerly AAM over the tropics is (however it was originally caused) inevitably going to have an impact on the global circulation elsewhere as these unusually strong tropical stratospheric Westerlies work their way down through the stratosphere. Such a large area of strong westerlies will inevitably have an effect through wind-shear and mixing on surrounding parts of the tropical stratosphere and the upper troposphere-some of this westerly momentum will find it's way into the upper part of the tropical Hadley Circulation through which the poleward transport of westerly AAM normally occurs in all seasons. Over the coming months it will carry a bit more of this Westerly momentum than normal despite the seasonal easterlies in the stratosphere above.

Now, I would normally agree, that the QBO has little impact on the weather we get in Britain in late spring/summer but the QBO has been breaking records in terms of the strength of the westerly winds and their extent. Westerlies also extend to the Equator at around 100 mb (the top of the troposphere) with just a small  scarecely-easterly blip around 70 mb and that is (of course) at the top equatorial end of the Northern Hemisphere Hadley-Circulation. It would be unwise to discount this completely in a monthly-season-based forecast. Here is one of the conclusions in the paper I have referred to above:

The main influence on our spring/summer weather will , of course, be the likely positioning of the ridges and troughs in the Circumpolar Vortex in higher latitudes. This will, in turn be influenced by the patterns of anomalous cold and warm water across the North Atlantic and temperature/snow-cover over North America and Eurasia and (of course) the effect of topographical locking due to the Rocky Mountains. You will be aware that there is a region of the North Atlantic that is locally 2C or more colder than normal- to the south of Greenland. This will encourage an upper trough in the upper Westerlies a little way downwind of it (i.e. to the east). The wettest weather associated with depressions tends to be on their south-east quadrant- which would tend to put Britain (certainly the north-west) in the firing line. The extent and strength of the Westerlies high over the Equator would, in that context, underscore and emphasise that the wetter conditions are more likely. Some of the forecast charts for a fortnights' time show signs of bearing this out.

Aside from that disappointing picture I do have confidence, however, that during the second half of May that the retreating extent of Arctic pack-ice (which is already at near-record low extent for mid-April) combined with the unusual warmth of sub-arctic waters will result in the Polar Vortex shrinking closer to the Arctic itself. With the northwards extent of the subtropical high-pressure belt that means central and southern England at least can look forward to a good deal of fine and very warm weather by then- of that I am certain.  

Edited April 14, 2017 by iapennell
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[iapennell]

18th April: The air temperature fell to -4C overnight at just over 400 metres' elevation in the North Pennines following a chill day with north-east winds yesterday (vicious hail-shower about 7.pm last night). We have the predicted mid-April cold snap!

Looking cold and showery for many by Friday (22nd):

[draztik]

Ian, as always, thank you for your insights and general forecast. May I ask, as for back end of this month and seemingly start of next month... what seems to have upset your projections in terms of rainfall, esp for the North of the country. 

[iapennell]

On 4/30/2017 at 14:14, draztik said:

Ian, as always, thank you for your insights and general forecast. May I ask, as for back end of this month and seemingly start of next month... what seems to have upset your projections in terms of rainfall, esp for the North of the country. 

@draztik; As regards temperature I predicted a mean of 9.5C for England and Wales in April 2017, which is not far off the actual CET average of 8.9C which has just been logged: I don't think the actual figures deviate far from what I predicted on that score and there was a cold spell that brought widespread night frosts and some wintry showers, although this occurred later in the month than I predicted it to. For lowland Scotland, I predicted a mean of 7C- and this seems to be close to what the average has actually been. Drier clearer conditions however, seem to have resulted in more frosts at night in some areas although I would not put the overall incidence above average.

As regards rainfall, it has however, ended up much drier than my original prediction for 90mm rainfall for England and Wales with total rainfall amounts of just one inch (25mm or less) over a wide area of the South and Midlands. the unsettled wet spell I expected to set in, over northern Britain in particular, did not occur over most of the UK.  There was, alas, plenty of cold wet weather but it was rather further north- and east- than I expected it to be: The far north of Scotland- i.e. Wick, Caithness, Tongue and the Shetland Isles did have a wet month in April 2017 with rainfall well above normal (locally over 100% more than usual) and (of course) it was cold with some unusual late snowfalls across parts of mainland northern Europe and over Scandinavia. The upper trough, which I expected to occur near the United Kingdom during the second half of April formed further east and brought the chill wet conditions to Scandinavia, Germany and across the Baltic- indeed they have had some shocking conditions for the time of year.

In the North Atlantic, there is a patch of colder-than-usual water and I anticipate that this would cause the upper Westerlies to re-curve south-eastwards and lead to an upper trough downwind- i.e towards the United Kingdom. During April, this unusually cold patch of the North Atlantic has extended further east, and there has been a patch of colder-than-usual water forming in the Norwegian Sea- to the north of Britain. Sea-ice extent around the east Greenland Coast has (if anything) been slightly above normal extent but is well below normal extent for the time of year in the Barents Sea to the north-east: Combined with the Polar Stratospheric Westerlies ending (and being replaced by easterlies) a couple of weeks' earlier than usual and with a Stratospheric High over Greenland this seems to have encouraged higher pressure to the west of Britain as well as over Greenland- leading to the spell of Arctic winds over Britain late in April whilst the passage of sub-arctic depressions was (consequently) well to the north and north-east (over Scandinavia). I also factored into my forecast that there has been a record Westerly QBO (the wind-pattern high above the Equator in the Stratosphere), which has been blowing at record speeds from the West over recent months: I am a little surprised that this has not led to stronger Westerlies coming across the North Atlantic as this excess Westerly momentum is transferred into the general global circulation even though one would expect much less of an influence from the QBO in the Northern Hemisphere late spring/summer.

What I will say however, is that (at least where I live in the North Pennines) the weather in April 2017 may have been characterised by low rainfall (32 mm as recorded at my weather-station), but we still had a large number of days with trace or very small rainfall totals (i.e. 0.8mm). The middle and latter parts of April were not characterised by warm sunny conditions overall where we live (though there were some bright fine days) but there was often cloud, cold winds (often from a north or north-west) and light showers- sometimes with hail or snow. On 26th April Cross Fell was white as were mountains in the Lake District. I would suggest that conditions across much of Scotland-  along with other parts of northern England have had broadly similar conditions to those that I have recorded, if not as chilly (I live at just over 400 metres above sea-level). Frontal influences did affect the northern half of Britain during mid?late April but they were much-weakened by proximity of high-pressure further south and west.

[Interitus]

20 hours ago, iapennell said:

I also factored into my forecast that there has been a record Westerly QBO (the wind-pattern high above the Equator in the Stratosphere), which has been blowing at record speeds from the West over recent months: I am a little surprised that this has not led to stronger Westerlies coming across the North Atlantic as this excess Westerly momentum is transferred into the general global circulation even though one would expect much less of an influence from the QBO in the Northern Hemisphere late spring/summer.

Well it was suggested that this was given too much prominence. The total angular momentum is a combination of the motion relative to the earth, in this case the QBO, but more importantly the motion caused by the earth which is a function of latitude. Theoretically, a parcel of air will gain around 134 m/s eastward velocity just by moving from the equator to 30°N, which is an order of magnitude greater than the relative motion of the QBO. It doesn't achieve this velocity because of turbulent mixing and looking at the region of the subtropical jet, the average April zonal wind speed at 30°N/250mb is 27.9 m/s (NCEP reanalysis). Also, momentum is calculated for a unit of mass i.e. proportional to density, and air at 30mb has a little over a tenth of the density of that at 300mb for example so the momentum is further diluted.

Using the March QBO figures from 1956 onwards, the 20 strongest (westerly) QBO average 8.91 m/s, 20 weakest (easterly) average -13.34 m/s. The corresponding April 30°N/250mb wind speeds are 27.6 and 27.4 m/s respectively; the correlation is an insignificant 0.18. Interestingly, for the 500mb wind over London, there's a tendency towards anticorrelation (-0.24) with the 20 strongest QBO have weaker 500mb wind 4.99 m/s vs 7.38 m/s for the easterly QBO. These average zonal winds at 50°N/500mb are not very strong and clearly, the Coriolis deflection at this latitude would dwarf any QBO effect.

Edited May 2, 2017 by Interitus