The mysterious disappearing warm front

Courtesy UKMO

As you know I get quite agitated by some of the analyses that the UKMO come up with in their six hourly fax charts. At the moment they have a whole bunch of fronts of one sort or another queued up to the west of IONA. The first one of which was a warm front, which at midnight was aligned north-south from St Kilda* in the north to Valentia in the south. The front itself had been first identified at 06 UTC yesterday (17th), and hadn’t previously been marked as a weakening feature (frontolysis), nevertheless it had been completely omitted from their 06 UTC analysis this morning, which was quite surprising as it was responsible for a broad band of rain from Cape Wrath down to Anglesey in the weather radar. Some fronts in the UKMO analyses in my opinion don’t exist and the analysis in some respects becomes more of a nephanalysis, but on this occasion when they were 100% correct about the warm front at 00 UTC, which by 06 UTC had ceased to exist, and really should have been drawn where I’ve added the red line. All I can think is that they forgot to add it to the finished product and this analysis needs a correction. It will be interesting to see if they do that and what the 12 UTC analysis chart looks like.

It also seems to have got “Hello There” Louise Lear foxed in her forecast graphics this morning on the BBC which don’t seem to line up with the reality of the situation.

*Can I have a SIESAWS installed please?

4 thoughts on “The mysterious disappearing warm front”

  1. The warm front didn’t disappear – it occluded out. Note how narrow the warm sector was on the midnight analysis. The sequence looks fine IMHO.

    1. If that had happened how do you explain the overnight weather radar and the rain then?
      If it had occluded out it wouldn’t have simply shrunk back westward or disappeared it would have continued to move eastward in the upper flow.

      1. I don’t know the thinking behind the changes – perhaps the rain was just starting to shear forward from the surface feature at the time and wasn’t quite in a position to classify as an upper front.. What I do know is that it’s quite tricky representing developments in the upper air on a surface chart. Whether something is represented as a warm front or an occlusion (or even an upper occlusion) is pretty academic. I suppose that the forecasters make a decision based on the available evidence at the time and, to a certain extent, what they think is going to happen next (based on experience, NWP evolution, observation sequences, etc.). If that involves reclassifying a front then so be it. Equally, if a front is reclassified to an earlier representation then there is nothing wrong with that, as that reclassification will be based on available data/evidence. At the end of the day, the Norwegian frontal model doesn’t often model reality. Fronts as a representation of weather is the consistent theme, and is one that most people (of varying levels of meteorological knowledge) are reasonably familiar with. Whether it’s a warm front or an occlusion probably doesn’t matter to them. It may well matter to purists brought up on a diet of Norwegian frontal theory – but knowledge has been moving on from those days for at least the past 35 years.

        1. Fair enough, but I like continuity, and in that regard yesterday’s 06 UTC chart was a complete disaster.
          The charts since then have shown another separate warm front crossing western areas, before becoming quasi-stationary (something that’s every rarely done by the UKMO) and moving back westwards as a cold front across western Scotland at 06 UTC.

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