By Nick Finnis
13th March 2023Spring Forecast Headlines
- Overall for the 3 spring months, temperatures are forecast to be close to the 1991-2020 long-term average. March below average, April around average, May above average.
- Average rainfall is expected across the season – March, April and May all around average
Continue below for the in depth forecast and explanation of the factors which are expected to influence the weather this Spring.
Spring Forecast Month By Month
March
The combination of tropical forcing from the MJO reaching the dateline (phase 8) at high amplitude and SSW reversal propagating down to the troposphere looks likely to promote an episode of high latitude blocking. High pressure near the UK in the first 5 days, bringing dry and settled conditions, looks to then drift northwest to become centred around Greenland. This means a greater likelihood of colder-than-average conditions with an arctic-sourced flow from the north or northeast developing across the UK and lasting for up to a week. Air cold enough for snow at times, causing some disruption in places, even to lower levels, more particularly in the north but perhaps elsewhere should Atlantic frontal systems bump into the cold air further south. Towards mid-month, milder but unsettled conditions look to push into the south and perhaps eventually all parts for a time.
The knock-on effects of the stratospheric warming may persist into the second half of March too. So, although we may see a spell of milder and unsettled weather mid-month, which may last into the last third of the month, there are signs high pressure may eventually return to the north while pressure remains lower to the south. This may mean return to colder conditions later in the month, though any snow mostly confined to higher ground in the north.
Probability for temperatures against 1991-2020 average: 50% chance of below average, 30% chance of near to, or 20% above average.
Probabilities for rainfall: 40% of chance for average, 30% chance of below average, 30% chance of above average.
April
The lingering effects of the SSW on weather patterns may persist into April, with a tendency for episodes of high latitude blocking, particularly over Scandinavia and lower pressure over mainland Europe. This increases the chances of colder-than-average conditions, particularly during the first half of the month. Rainfall most likely greatest in the south and west, but generally drier-than average first half.
As we enter the second half of the month, blocking to the northeast may begin to lose its grip allowing low pressure to move into close to the west or southwest, bringing a transition to more unsettled and milder conditions for the rest of the month, perhaps warm or very warm briefly late in the month. Rainfall amounts greater for all areas compared to the first half.
Probability for temperatures against 1991-2020 average: 40% chance of average, 30% chance of below average or 30% chance of above average.
Probabilities for rainfall: 40% of chance for average, 40% chance of below average, 20% chance of above average.
May
Blocking high pressure looks less likely than earlier this spring, with the jet stream back close to the UK or just to the north, with low pressure more likely to be control close to the north or west much of the time, but with occasional spells of higher pressure, especially later in the month.
Overall, it's looking like a warmer-than-average month, perhaps with a brief early hot spell later in the month. Rainfall likely to be around average overall, but perhaps quite wet in the northwest, drier in the southeast.
Probability for temperatures against 1991-2020 average: 40% chance of above average, 40% chance of average or 20% chance of below average.
Probabilities for rainfall: 40% of chance for average, 30% chance of above average, 30% chance of below average.
A look back at winter 2022-23 and how well we did with the winter forecast.
The main highlights of the Netweather 2022-23 winter forecast were:
- Overall for the 3 winter months, temperatures were forecast to be close to the 1991-2020 long-term average. December and January close to the average, February above average.
- Slightly below average rainfall is expected across the season - December and January below average, February average.
December
December was colder than average overall, the only below-average month in 2022, with a very cold and frosty spell until the 17th, then milder weather thereafter. Rainfall was limited during the cold spell, but from the 18th there was much more rain, bringing the total rainfall for the month close to the long-term monthly average. The UK mean temperature was 2.9 °C, which is 1.3 °C below the long-term average, making it the UK’s coldest December since 2010. Rainfall was slightly below average overall, but there were regional variations - western Scotland, Northern Ireland and central parts of England were drier than average, but parts of eastern Scotland and southern England wetter than average.
The Netweather winter forecast for December was for blocking high pressure near the UK early to mid-month with colder weather, then a more mobile unsettled spell with milder conditions later in the month. We went for a 40% chance of rainfall being close to average, 30% chance of above or 30% below average. A 40% of chance of rainfall below average, 40% chance of average, 20% chance of above average. The forecast was weighted towards average temperatures, in the end it was below. Rainfall was weighted equally average or below, when it was slightly below overall.
January
The first half of January was mild, unsettled and wet, the second half turned drier and colder - though temperatures were near or slightly above average in the final week. Mean temperatures ended up slightly above average, with a provisional UK mean temperature of 4.4 °C, 0.4 °C above the 1991-2020 average. Rainfall was generally close to average, with 103% for the UK overall, but this conceals the wet first half and dry second half; wettest areas relative to normal were Wales and western parts of England. We said January: “looks to start how December finishes off, with a mobile westerly +NAO regime of low pressure close to the north or northwest and high pressure over the European continent bringing unsettled and often mild conditions” and “after a wet start to the month, becoming drier and likely colder at times too, with an increasing risk of frost”. We went for a 40% chance of temperatures close to average, 30% chance of above or 30% below average. Rainfall 50% of chance for below average, 30% chance of average, 20% chance of above average. The winter forecast was close with temperatures, calling for average when they verified slightly above. For rainfall it predicted more likely drier-than average, when it was actually close to average.
February
We went with a mostly unsettled month, milder and wetter than December or January. But perhaps with some high latitude blocking towards Greenland bringing a west-based -NAO at some point, which could bring a risk of wintry precipitation in the north. The south would be milder-than-average, north close to average at times. A 60% chance of temperatures being above average, 20% chance of above or 20% below average. 40% of rainfall being average, 30% chance of above average, 30% chance of below average. We were right on it being a milder-than-average month, but wrong with being wetter than average, in fact rainfall has been well-below average, due to high pressure nearby or over the UK for much the month, and it looks like last month will be one of the driest Februarys on record.
Pointers for making the spring 2023 forecast
Stratosphere
After reaching near-record lows in early January, temperatures in the Arctic stratosphere have been rapidly rising since early February, with a technical sudden stratospheric warming (SSW) event occurring around the 17th February, with a reversal of stratospheric zonal (westerly winds). Following a brief recovery since the reversal mid-month, with winds at 10 hPa 60 degrees North rising above 0 m/s, a second warming and reversal of zonal winds at 10 hPa 60N occurred on Monday. This second warming looks to trigger a stronger reversal than the first warming, with the stronger easterly winds in the stratosphere at high latitudes looking to propagate down to the troposphere and influence weather patterns in the high to mid-latitudes of the northern hemisphere over the coming weeks.
The latest ECWMF weeklies show this reversal with zonal winds falling below 0 m/s this week, though with a recovery towards mid-month
Since the first warming and technical SSW on the 17th February, the stratospheric polar vortex has shifted into northern Europe and is forecast to drift further east across Russia. With the 2nd warming pushing the reversal down towards the troposphere, weather models have signalled a weakening of the recently strong tropospheric Polar vortex over Greenland shifting it over NE Europe and NW Russia, while height rises / ridging develop across Greenland and Iceland, kind of mirroring what has been going on in the stratosphere.
A disrupted polar vortex tends to have its strongest tropospheric impact over the North Atlantic, which increases the odds for colder conditions across northern Eurasia. The impacts of the SSW and early disruption of the polar vortex can persist for a few months, so may still impact weather in April too, though current forecasts do suggest a recovery of zonal winds in the stratosphere, but the polar vortex weaker than before.
However, the SSW recently will not be the only influence on our weather patterns out there. Other phenomena like La Nina, the Madden Julian Oscillation or the chaotic nature of the atmosphere can also affect how our atmosphere reacts to a disruption of the stratospheric polar vortex. Meaning, the best we can say now is a shift towards a higher likelihood—but no promises—of colder-than-average conditions somewhere in the mid-latitudes in coming weeks.
Madden Julian Oscillation (MJO)
Although the MJO is a wave of enhanced convection that circum-navigates the global tropics, it does impact the upper patterns at high latitudes of the northern hemisphere, so is a useful tool to use for predictions for 3-4 weeks ahead, but no more than that really.
The MJO affects the Indian Monsoon, plays a role in the onset of ENSO events, has an impact on tropical cyclogenesis but also has more far-reaching impacts on northern hemisphere extratropical weather through Rossby Wave propagation. The Rossby Wave propagation depends on the longitude of where the enhanced convection associated with the MJO wave takes place. As the MJO has a significant impact on northern hemisphere weather patterns, including the North America, Atlantic and Europe, there are correlations that can be made between the 8 different phases of the MJO and the lagged impacts on the upper air patterns that might be expected over the North Atlantic and Europe based on composites of previous events. The impact of the MJO on North Atlantic and European weather patterns depends on the strength of amplitude of the wave.
The MJO since mid-February has been over the western Pacific (phases 6-7), the lag of these phases typically forces high pressure near the UK and indeed February closes with blocking high pressure close to the UK. The MJO wave is currently weak over the tropical western Pacific, but is forecast to strengthen and continue to move across the tropical Pacific towards the dateline through phases 7-8.
This strengthening of the MJO through 7-8 at high amplitude typically promotes high latitude blocking patterns in winter and early spring. ECMWF extended MJO forecast takes MJO to phase 1 too. Both phases 8 and 1 promote Greenland blocking. This in conjuction with the downwelling of easterly winds in the stratosphere to the troposphere from the recent SSW could re-inforce recent longer range forecasts, including the most recent ECMWF weeklies, for high latitude blocking and low heights over southern Europe to persist through much of March.
March MJO phase 8 amplitude >1
March MJO phase 1 amplitude >1
Looking further ahead into April and May, confidence decreases - with forcing on northern hemisphere patterns from the drivers such as the MJO and stratosphere becoming far less easy to predict at range. So, the forecast will lean more on analogs of previous years with similar ENSO state and these months in years following a SSW in February.
ENSO
La Niña is present a of 27th February, with equatorial sea surface temperatures (SSTs) below average across most of the Pacific Ocean, however, during the last 4 weeks, negative SST anomalies weakened across most of the equatorial Pacific Ocean, with positive SST anomalies emerging in the eastern Pacific. The NOAA's Climate Prediction Center (CPC) predicts La Niña will transition to ENSO-neutral this spring. Already we are seeing the weakening of La Niña allowing the Madden Julian Oscillation (MJO) to become more coherent as it moves across the Pacific, particularly in phase 8 as shown above. During La Niña the MJO tends to be more muted.
Direct connections of ENSO to imprint on the weather patterns months ahead are often (and usually) confounded by a myriad of other long-term and shorter-term drivers or teleconnections that can dominate through a season, which I will take into account.
One of the strongest drivers of weather patterns, at least over the next month or two, that looks to overwhelm the effects of ENSO, is the current stratospheric warming, as discussed above, which has displaced the polar vortex.
So, in conclusion, ENSO is not likely to play a big role through spring, particularly as it looks to go neutral.
Analogs
My top analog years combine years with La Nina trending neutral in spring and following a SSW in February, these years were 1963, 1979 and 2001.
March
April
May
Also, I have run composites for each month of years that featured a SSW in February, Those years were: 1963,1966, 1973, 1979, 1980, 1984, 1989, 1999, 2001, 2002, 2007, 2008, 2010, 2018.
March
April
May
Both composite sets above for March indicate blocking prevalent over Greenland and other areas in the arctic circle to the N and NW, with low heights over northern Europe. For April, again blocking at high latitudes over Greenland and also Scandinavia, low heights over northern Europe, indicative perhaps that the polar vortex never really recovers back over the pole. For May, lower heights to the northwest and over Europe, higher heights over Scandinavia and Azores.
NWP seasonal forecasts
The February update to the C3S suite of seasonal prediction systems, show a very mixed picture for March/April/May, so it's difficult to glean much from them, particularly as they are for all three months. But will consider ECWMF seasonal forecast for each spring month for temperature and rainfall (see link below). It was last updated on 1st February, it indicates March seeing average temperatures over the UK, but for rainfall wet in the south, dry in the north. April and May - both temperatures and rainfall around average.
Also, the UK Met Office contingency forecast is out. They are forecasting a higher-than-average likelihood of drier & warmer conditions across the UK from April onwards.
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