Kasim Awan
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Merged polar low & Atlantic low snow maker moving South at 126 hours. Original post: https://community.netweather.tv/topic/99760-model-output-discussion-colder-but-how-cold-and-for-how-long/?do=findComment&comment=5006022
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A significant number of GEFS members now produce a NW-SE runner which originates from energy over Iceland around the 120-150h mark. Some parse it into a front, and some keep it's circulation entact as a shortwave low. What is also of interest is the fact these GEFS members also struggle to process the potential big snow maker trough further North East, indicating that a more southerly track of the Azores low also increases the chance of this runner to develop well into the UK.
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Depends if we get the front north enough, if we're on the northern flank the easterly winds will produce a moderate leeward effect west of the Pennines with reduced snowfall rates as a result. Get it far enough North or get a front with lighter winds and the leeward effect will be minimal. Any short scale troughs would bring lower levels a risk as there would be no leeward effect in these instances.
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For a 25km model to show 30-40cm of snowfall indicates locally 60-70cm of snowfall is likely, say on hills or where streamers within the front align. This is due to the low resolution of the model missing the normal distribution associated with elevation and streamer activity. Combined with 40-50mph winds would likely result in very severe drifts.I can say the GFS18z would produce this with very high confidence. Objective analysis of a model run is not silly, it is factual analysis of data shown to us by the NWP.
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Lovely chart to look at, meteociel GFS algorithm looks a bit stingy, 20-25cm widely central areas with 40-50mph gusts. Some are adamant of a southwards trend, which I agree is possible. However, the governing factor that is mostly responsible for the latitude of this system is the Azores high. Given we usually see trends northwards in this feature, I wouldn't discount a trend north in this system over time as opposed to a trend south. Very 5050 stuff and we can't bank anything at this point.
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Ok so the ECM is not so good - it shows us the least we can probably expect from the upcoming pattern. These sorts of outputs are expected really when we're looking at a probabilistic chance of 70/30 of the cold staying in put after the initial slider 16th/17th/18th. We'll probably get the odd rouge run with those probabilities within the ENS/operationals. The main thing is ECM sticks to the plan broadly speaking within 192h so coldies should hope that it is just struggling with too much energy which it can do in these setups at this range. This does not discount the chance of a milder push around day 8 though which is still somewhat of a risk, personally I think GFS/GEM handling is more accurate.
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Not a fan of deep snow cover and severely cold temperatures? @Nick123 I think models are converging on the pattern now - and it's a very good pattern given the activity to our South West. Obviously details and potential warm sectors very uncertain but the trend that is gathering pace is highly supportive of eventful and cold UK weather.