Jacob's weather forecasts

 My current expectation for the next 2 weeks is a continuation of the weaker westerly flow, possibly interrupted by some anomalous high pressure, but nothing strong or long lasting enough to bring large changes in weather conditions. As a result, changeable weather conditions are expected for this period. Temperatures should oscillate near average, with drier conditions between fronts of rain, and potentially snow further north, on high ground The MJO is expected to move into phases 8-1-2, or with too little amplitude for those phases. These support a NAO+, westerly flow, but the lower amplitude of them means that the signal isn't as strong. Either way, they aren't supportive of a very blocked pattern at all Secondly, this MJO/ER analogue is very supportive of low-pressure anomalies over and to the north of the UK, and high-pressure anomalies to our south. This is a clear example of a NAO+, which brings westerlies to Euro-Atlantic Notice the lack of a very strong low-pressure a...

This weekend and into next week, high pressure over Scandinavia will draw in cold air into Europe, and also block and push the jet stream south, allowing this cold air to move over the UK, starting with the north. For the northeast of the UK, this easterly winds over the North Sea may stir up some snow showers under low temperatures. For the south, things look more uncertain, but temperatures will be below average, with risks of rain and snow Past this point, may we see further cold spells?  First of all, the EC46 suggests this, but at the same time, it does very often overestimate the risks of weather patterns that tend to bring cold weather to the UK. High pressure north of us blocks westerlies and forces the jet stream south, which can lead to cold air from the north, east or northeast filling in the gap Notice the high pressure to our north and low pressure to our south? That's a clear NAO- pattern here. Both the GEFS and CFS are on board with the NAO- being negative till mid-F...

Persistently high pressure over Scandinavia is blocking the jet stream, allowing notably cold air to rush in and fill the gap in continental Europe. This high pressure will likely move west, helping to reduce the dominance of the jet stream over the UK, and move it south (red arrow), which may cause the cold air over Europe to flood the UK (pink arrow). Setups like these are notoriously precarious, so I can't say that it is anywhere near guaranteed at the moment. If it does come off well, this cold air can pick up moisture from the North Sea to create snow, similar to lake-effect snow. While weather models haven't been supporting the risk too well recently, the chance of this setup (cold easterly) has increased last night and this morning. Here's some weather probabilities for the weather in 10 days: 31% - cold across the country, snowy, possibly rainy 29% - fairly cold, wet, possibly snowy in the north and east 28% - cool, wet, windy 12% - mild and dry in the south, cool a...

Not really sure how this stuff really, works, but I'll have a look at  AAM The drop in  AAM , at least for now, and it will likely remain weakly negative The January weakening of  AAM  seems to have led to the polar jet stream largely weakening, while the subtropical jet stream has weakened and shifted south, while the stalling in the decrease in  AAM  may have led to the polar jet stream shifting north, into the high latitudes (70 to 90N) My educated guess is that the weakening and shifting south of the subtropical jet stream will likely lead to less influence from the Azores high and southern Europe highs on our weather, so a lesser risk of them ruining the potential cold spell. Meanwhile, the polar jet stream, that's tends to be on the same latitude of us, weakens and is displaced north, causing a raise of pressure in the high latitudes. My guess is that both the jet streams moving away from each other would increase the amplitude of Rossby waves and the...

  Here's the EPS mean from day 10 to day 15 There's decent agreement towards higher pressure over the poles, pushing the negative Arctic Oscillation negative. This would normally help to inhibit a westerly for the Euro-Atlantic, but the  TPV  lobe over east Canada keeps pushing low pressure systems out, into the Atlantic, and with a lack of amplification, there's not much to stop the continued weak zonal flow, aside from higher pressure in northeast Europe, which is too far east to create a more blocked weather pattern over us without the jet stream taking more of a southeast route, which doesn't seem to be there yet, especially with the Azores high propping up the low pressure systems to keep sufficiently north. For those hoping for easterlies, the potential is there since the jet is weak enough and pressure over Scandinavia is anomalously high. One area for upgrades that is easy to overlook would be a weakening of the Azores high, to help the jet stream move southeast...

Tonight and into tomorrow, a warm front will pass over the UK, bringing widespread rain, turning to snow over areas where cold enough air still lingers As a result of the milder weather taking over, a thaw is expected soon after, wherever snow accumulates. Here's a snow forecast:  

In all honestly, a cold easterly still seems to be highly an outside risk. The risk was never a major one imo, but I'm not going to assume that it won't happen My reasoning is firstly that the CFS, even after several days of watching, has not predicted a  NAO - a single time during mid or late January. The CFS managed to pick up on the current cold spell, and the one in mid-late November a few weeks before they happened Secondly, these  MJO  and Rossby Wave analogs, which picked up on the potential for a Scandi high in late February, and the retrogression in early January, also weeks before they happened does show some potential for an easterly, but it leans more towards the weather pattern being too far east to bring proper cold to us. Here's the link to it, since imo it's an underrated long range forecast tool and it seems impossible to find without the specific link:  n = 10 Thirdly, and most importantly, there's a lack of support from ensembles. I understand tha...

Context: the ECMWF 12z weather model on this date brought a cold easterly to the UK What made this possible is the arctic high that's left over from the Greenland high migrating over to Scandinavia, contributing to the Greenland high. While this isn't guaranteed, this step isn't an unlikely scenario, especially due to the upcoming zonal flow being weak at most. It essentially acts as a block to the jet stream, so cold air from the eastern lobe of the TPV shifts into northern and eastern Europe The next part is much more precarious, which is keeping Atlantic low-pressure systems just weak enough to allow the jet stream to move south, so the cold air over Scandinavia is lead our way. Subtle changes in the strength of lows pressure systems and their trajectory can have massive ramifications for the UK, which will be demonstrated below. Here's the GFS chart at 102h As you can see, the slight ridge into west Europe is stronger, so therefore the jet stream is further north o...

According to the ICON-EU  ENS , the median outcome would be an M4 north snow event The 25th percentile looks supportive of snow everywhere but the south coast And the 75th percentile restricts it to Northern England  

Significant potential for snow this week, especially on Friday, where an area of low pressure can sweep to the south of the UK, bringing snow The position of the snowfall is unclear, anywhere from France to the Pennies, but it’s very possible that even a few miles north/south could make the difference between  torrential rain and blizzard conditions Here’s one possible scenario, with historic levels of snowfall for the southeast, but it’s important to consider that this is a best case scenario for the London area. Even if they are favourably positioned just north of the snow line (which it most likely won’t be), very minor changes in temperature, depth of the low pressure, dew points etc could just turn it into some sleet that barely leaves any snow on the ground

 

It's notable that the upcoming Atlantic spell is more of a weak zonal flow, rather than something much stronger As a result, pressure in the Arctic isn't looking all that low, especially in Barents Sea area. The low-pressure systems that push through the Atlantic may manage to pull some warmer air over that area to generate a weak Arctic High when combined with the leftovers of the current diminishing Greenland high. While that by itself isn't all too helpful to cause cold spells, if that signal on the chart above were to trend south, which isn't very unlikely, considering that it's at day 10, brutally cold air could build to our east, while the jet stream undercuts it, allowing this frigid air to make its way over us. Take this  GEFS  member for example, considering that this is very cherry picked, and it isn't a trend at all in the ensembles In addition, teleconnections are supportive of a Scandinavian high in late January

 

 

Minor shifts west of the trough, like what happened on the ICON 12z significantly increases snow shower potential, even far inland. Using London as an example again. The LCL height is only at roughly 300m high on the ICON 12z on the 5th of January This then allows clouds to be closer to the surface, and grow thicker. In this specific case, the cloud seems to get to 2km thick based on RH, which is supportive of little flurries, snow showers, and possibly even bursts of heavy snow with the lapse rate at 7.0. I doubt that these would be possible to forecast unless a polar low develops, which ends up However, it's important to consider that sounding at this range aren't reliable at all, which I forgot to mention in my morning post. This just serves as an example of what is possible with westward shifts. The sea level pressure at this point is 1013, the 850hPa temperature was -6 to -8, and the 2m temp was roughly around 1C. If this were to verify, and precipitation were to develop, ...

Terminology, in bold is defined below There's a trend towards further upgrades the longevity of the cold spell by having a more distinct shortwave (see the curves on the sea level pressure isobars) This flattens the eastern side of the Atlantic ridge as it glides over it, towards the UK, where it may create a snow event. This shortwave then combines with the trough , allowing it to push more strongly against the Atlantic ridge . The combination of the flattening of the east side of the ridge and it getting more of a pushback by the trough keeps it further west, where increased amplification forces it northwards, creating an area of weaker high pressure in the high latitudes Arctic highs , even if they're weak often cause the jet stream to push south and weaken to the east of them, so cold air still manages to cling on to the north of the UK, and likely the south as well. While this doesn't create an easy path back into very blocked weather, it can allow the cold spell to...