Weather news and views from Brian Gaze.
Published 18th March, 11:12
The unsettled period of weather looks set to continue through the rest of March. Therefore, all regions will have showers or longer spells of rain, and after a mild start temperatures are expected to dip. Will it be cold enough for the risk of snow to become more widespread?
The chart below is for Birmingham. It shows forecast air temperatures and precipitation from all of the runs in the GEFS ensemble model. The horizontal axis plots time with the chart reaching out to April 3rd.
To begin with, air temperature forecasts from all of the runs are close to or above the thick black line which represents the 30 year average. However, there is a distinct downwards trend from March 26th and a majority of the runs show a colder period developing.
The forecasts are for about 1500m above sea level which is high enough not to be subject to significant day to night (diurnal) variation. The thick purple line, the ensemble mean, falls to about -5C by March 28th.
The snow row at the very bottom shows the number of runs which are forecasting snow on a given day. It reaches a peak of 15 out of a possible 33, which suggests there is a significant risk of snow falling. Nonetheless, it's important to remember that it could just be a few flakes of snow mixed into a shower because the snow row doesn't attempt to forecast accumulations. If you're interested in the possibility of accumulations check out the Birmingham snow depth chart or other locations which can be selected from that page.
Will there be much precipitation around, regardless of whether it falls as rain, sleet or snow? The lower half of the plot suggests the answer is yes. Each spike indicates precipitation (mostly rain) being forecast by one of the runs in the model at that time. Of course there will be day to day variability and dry interludes, but on the whole it looks like quite a wet picture.
The air temperature forecasts above suggest it turn colder, but for a variety of reasons the relationship between them and what happens at the ground level is not always straightforward. Therefore, the data table below showing forecast 2m temperatures (basically those at the ground level) for Birmingham is useful. It categorises all of the runs in the ECM ensemble model and goes out April 1st.
In the short term the columns are dominated by light green and yellow shading. Those colours show runs forecasting temperatures of between 6C and 15C. Therefore, most of them forecasting daytime temperatures to be close to or above the average.
However, from March 24th dark green shading starts to appear and it shows runs forecasting temperatures of between 1C and 5C. Values like those are distinctly cold for the time of the year! The amount of dark green peaks at 29% on March 31st and there's even a tiny amount of blue which represents a run going for very low temperatures.
Comparable data table for locations around the UK point to the risk of colder conditions being greater the further north you go. That is to be expected because the colder air will be moving down from the Arctic and if it doesn't reach the north of the UK it can't make it to the south! The air mass also warms up a little the further south it flows.
Unsettled weather is expected to continue through the rest of March. Current computer model data is suggesting that after a mild start the trend will be towards lower temperatures, particularly in the north. Although it doesn't look set to become very cold, the risk of snow will probably increase. At this time of the year accumulations are a lot more likely over high ground in the north, and at low levels they would be short lived. The colder conditions also lead to an increased possibility of frost.
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