A knife-edge winter 2019/2020

Winter is here... so here I will go again and try to find some direction to how the winter will progress amongst all the chaos. I will not be looking at analogues for this forecast as I do not believe they are useful anymore in a time which arctic is seeing unprecedented change.

First looking at the drivers which impact weather patterns in UK and afar.

The stratospheric vortex is a vital element in winter forecasting for the UK and Europe but limited use the models have beyond 2 weeks. The vortex naturally strengthens and expands over pole through late Autumn as polar night returns. Typically it reaches its apex strength around late December, it is therefore no surprise Christmas often is mild with no snow boo! The stratospheric polar vortex (SPV) is currently weaker than average but not significantly so. Today (02/12/19) the zonal mean zonal wind at 10hPa from GFS analysis is 15.2 m/s it should be around 30 m/s, it is forecast to remain weaker and weaken further, in a time which it should be at its most rapidly intensifying phase. A growing envelope in GEFS seem keen for a reversal in zonal wind to E'ly and therefore a Sudden Stratospheric Warming (SSW) approaching mid month. I however do not think this is likely as often with GFS we are chasing rainbows, at earliest I see this happening in late December add a few weeks and cold surface impacts?, would be quite a bit into January. With SSW approximately 2 in 3 result in extended cold weather in Northern Europe with nuance the UK is a little island. February 2018 was last recent example which spectacularly benefited W-Europe. If we do see a major perturbation which I'm inclined to believe we will, it is likely we will not see possible positive effects of this until midwinter, January or even into February.

Sea Surface anomalies in the North Atlantic as of now favour a slightly south of normal storm track indeed this has been observed through Autumn. With a shocking lack of gales in Scotland the jet stream has been further south than normal on an unusual axis, depressions which usually ride jet stream have been instead passing through central swathe of England. Sheffield has experienced its wettest Autumn on record, flooding has never been far from news. The current SST anomolies are not particularly promising for a relent in wet weather in these places as we go through December. However, very warm waters around Greenland's southern coasts may help to support ridging in area, indeed which we have seen of late in a colder spell of weather with Greenland blocking. This more hospitable region for blocking in conjunction with a jet stream which has been already more south solar? May allow for significant snowfalls in December and beyond but this is much more likely for northern UK.

Snow cover & ice extent

Snow cover is very high across Northern Hemisphere this October had 5th largest snow cover extent on record. it is believed a strong and extensive snowpack across Eurasia allows for a strong Siberian High, fostering disruption to vortex with a linked connection for a negative Arctic Oscillation (AO). This promoting a southerly tracking jet stream and very cold air bottled up over the high latitudes to spill into mid latitudes a greater risk of cold weather in Europe and US. The Siberian High can become more influential for UK later in winter as robust N Atlantic westerlies die down, I do see the second half of winter featuring coldest weather *spoiler*. While onto sea ice the Atlantic side only really relevant for UK and Europe has seen significant very much above average progress in the last month, something we have not seen in last 10 years. This will mean northerlies are likely to be a little colder than normal this winter and a higher chance of snow falling and accumulating to lower levels and further S.

Solar forcing

There is a problem equating a "cold winter" to solar minimum as a direct relationship but it is not truly understood some brilliant winter spells have been seen near to solar maximum such as February 1991 which London did not see equivalent winter weather till 2009. Although dissimilarly this was a solar minimum winter, the observational note in last 10 years has shown a cluster of blocked and colder winters at or following a solar minimum 2008-2013. Some literature suggests there is two year lag, with an uptick in sunspots in 2013 followed a cluster of much milder winters, but again careful pinning it on just one variable. Overall I'd say this winter solar minimum, increases the chances of blocking and therefore a colder spell of wintry weather but where it positions itself is key. A "bendy" jet has been present this autumn not typical flat W-E may already be having an influence with Autumn 2019 being the first cooler than average average to 61-90 average since 2012.

Monthly forecast

December

The first month of December is likely to be generally chilly and wet while the jet stream may remain strong it is in my opinion likely to be deflected south. Current modelling paints a likely cold snap/spell around mid month with cold northerly winds, and rather unsettled with it and therefore the risk of snow particularly in the north, significant snowfall(s) a distinct possibility there. I do not think significant cold is likely to feature in this month - but it is not necessary for snow to fall even in SE England, it would just not lay on ground for long. Looking towards pacific forcing the next ENSO cycle expected later this month looks to bring the MJO back into equation, with colder phase 7-8-1, possible this may allow for blocking to develop. So a drier final third I feel but potentially cold or very cold, rather than chilly which could prove good for festive period, with severe frost potential any significant snow I feel this is limited more likely into 2020. The month generally as a whole looks quite cold and seasonable.

January

This is difficult month I see there being two possibilities the first rests on major perturbation to vortex and of course downwelling, at earliest I can see this truly manifesting on ground level is middle of January. With possible much colder and drier weather, and enhanced risk of disruptive snow just about anywhere. Before this point it could be wet start to January and around average if milder temperature wise, somewhat what I feel like a fair swathe of December will be like. The jet stream remaining predisposed to south, I can see a wonderful season for Scottish mountains. If we fail to see a major disruption to vortex I firmly believe January could be the most stormy month of the winter, with a turbocharged +NAO pattern, this is what I believe seasonal models have gone with. No stratospheric impact for winter 19/20 which may prove wrong in my opinion & if is true wintry weather could be very limited to northern hills. I am placing my bets with first scenario.

February

It probably will be the driest month of the winter I will not give much detail here but as above in regards to January, this month may be the coldest of winter. With easterlies perhaps bringing exceptional cold from Siberia, in this case not like 2018 possibly much earlier in month, with the QBO transitioning to E by late winter. This aids in the polar vortex not being menacing and on top with a possible major perturbation to vortex and climatology support, northern latitude blocking may prove to be extensive and resilient a very strong -NAO signature this month might have. In contrast to my last winter forecast which I thought would bring an early start to Spring I do not believe this is particularly likely in 2020.

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So that is my winter forecast this may be seen as a daring forecast but it is a forecast which I see as being plausible and rooted in science. However, much emphasis on it leaning towards a colder winter rests heavily on a SSW happening and imprinting favourably. If this does not occur then the winter may be rather uninspiring and wet, if opposite is true which I think will be so, expect the winter to become increasingly drier as we progress but also colder. A winter we may not have seen in a number of years, feet firmly on ground but I am looking forward to what may come. I do not for the life of me foresee it being a snowless 2013/2014 winter low solar also to add, expecting for this winter colder than average departures to north and east and more around average for south and west. Thanks for reading. :-)

'Summer' slipping away for good as low pressure moves in?

Rather fittingly the weather has performed like clock work with the arrival of Autumn Equinox, tomorrow, heralding the start of astronomical autumn and for many marks end of summer. The rain and gloom is here but is it here to stay? After following weeks of high pressure influence and very little in way of rainfall - bringing particularly dry conditions to SE England which has seen a fifth (20%) of expected monthly total. This is set to change a breakdown as of now is occurring this did not prevent a warm day in east in which Norwich reached 26C in afternoon, in contrast London saw its high temp 23C late morning before cold front arrived.

From this point onwards through the new week the weather takes a distinctly more changeable flavour and potentially beyond....

However, I personally do not believe this unsettled period will carry much at all into October viewing the guidance. While there will be Atlantic weather systems giving some very wet weather to all in between some drier and sunnier spells are likely, additionally nights are likely to be mild, We see the remnants of ex hurricane Humberto move in on Tuesday and indeed this serves as a catalyst reinvigorating jet stream for unsettled weather to truly break through.

Monday sees outbreaks of rain and increasingly gusty winds spread across Wales, Ireland, SW England however elsewhere it will be a rather nice day, dry with sunny spells with temperatures lifting into the 20s in the London area.

Tuesday will be a windy and overcast day for most the showery rain seen early in the morning will clear all areas, however then an area of more persistent rain will push in across England and Wales and for southern counties including the SE the first substantial rain in quite some time. The Met Office currently have a yellow severe weather warning for rain covering areas to south/southeast of London extending all the way west to Pembrokeshire, Wales.

Midweek onwards ongoing theme of slow moving low pressure so remaining unsettled pluvial flooding is unfortunately likely and perhaps set to come too much at once. 12Z GFS is forecasting 60mm of rain come the start of October in London area this might be overboard but this does suggest some high rainfall totals may be seen in capital away from NW Scotland which often sees drenching rains a rain shadow is likely to be seen to east of London however still I'd expect some good rain for East Anglia but not as much.

The start of October is likely to see more settled conditions but it may do so unconvincingly away from Southern England, recent model output has backed off on much cooler conditions, courtesy of blocking area over Greenland dragging in cool northerly winds. American model (GFS) however remains keen on much cooler feel to end September and open October while European (ECM) lost scent. High lat blocking however is still likely to be seen north and northwest of British Isles, MJO (tropical forcing) orbiting phase 7-8-1 tends to give a rise of pressure in Greenland locale and as such higher risk of cool northerly airstreams. Some unusually cold and crisp mornings could be seen early October but it is not entirely clear yet.

Thanks for reading

Dan

Summer forecast 2019 'normal service returns'

It's that time of year again time never stands still; last summer seems a distant memory, forecasting this summer presents particular challenges. The teleconnections or signals atmospheric/oceanic which govern the weather we see, are not particularly strong and are conflicting with one another. In contrast to exceptional summer that was 2018 in which the blueprint for the summer began in May and continued well into August. As such all forecasts should be treated particularly cautiously, as possible scenarios, a lot can and does change in season after all this forecast is a breakdown and analysis of current state of play. Of systems which are fluid. This summer we are set to experience a weak Nino and neutral to positive PDO these parameters help support fine, warm weather nonetheless they are not particularly strong, which may weaken their positive influence. As such one should not expect a washout summer which is devoid of lovely weather like 2007 this does not present itself; however there is likely to be cooler and unsettled weather punctuated through summer. This will give rise to thundery outbreaks which 2018 disappointed in that department.

*Not a poor summer but not a great one either **

Summer 2019 forecast summary

Temperature: near average most likely to fall within 0.5 both -/+ of 1981-2010 average

Precipitation: normal to above 

'Middle loaded although extending into August'

June

June is rather likely to experience wetter than average conditions with persistent negative AO/NAO state (high pressure across pole, lower pressure in mid lats affecting UK and Ireland) probable pronounced by the current second lowest sea ice extent on record & low solar. Furthermore, the Atlantic SSTs are not particularly promising with it possibly reflecting in storm track perhaps being further south than usual and subsequently lows steered our way. The start of summer has seen suppressed convection in Pacific atypical and typical in La Niña state there are encouraging signs later in the month, we may see a reversal. Taking a more Nino signature which we should be seeing, this is good as 2007 and 2011 (washout summers) both featured strong La Niña which reinforced a very blocked pattern not conducive for longtime warmth. This expected uptick will have corresponding effect of ridges of HP being able to truly take a foothold so while I believe much of month will not be particularly warm or settled, there is a good chance of something much more along those lines late in month while also not being a fleeting affair.

Temperature: below average 

Precipitation: above average particularly in N/W

July

Confidence truly slips away at this period however if expected uptick in tropical forcing happens later in June there is broad expectation July will be largely anticyclonic and warm. Everywhere particularly in the south and east in which it may turn hot at times although nothing overly unusual in a typical summer. At least in the first half of the month widely, and I do believe it is more likely to be the warmest month of the summer. Overall through this month this warm weather may be punctuated by cooler Atlantic air, this will cause thundery outbreaks and rain, so indeed a better summer for those storm 'chasers' out there? It may be a month which has a pronounced E-W split with Atlantic trough backing west however perhaps not enough for Ireland in which I feel summer could be particularly wet and cool.

Temperature: warmer than average

Precipitation: near normal large regional variance

August

Confidence is very low and therefore not much detail can be provided however, it is probable warm summery weather will feature with a greater chance perhaps in first half. There is some signal for a more La Nina signature with a decline in +ve Nino state to neutral verging on La Nina later in the summer this would mean the summer is not likely to finish particularly well. There is also good expectation in conjunction sea ice may be in a truly disastrous state with lowest sea ice extent on record. This could give aid to give rise to return of extensive northern latitude blocking seen early summer, which is why we are currently locked in an unsettled pattern. With increased westerlies seen with seasonal change August could finish unsettled although much of month may be pleasant beforehand.

Temperature: above average generally

Precipitation: near normal

Thanks for reading I sense this summer will help alleviate drought fears a respectable summer it is looking.

Dan