glazed frost Glazed frost in the Netherlands

[Vorticity0123]

A very interesting weather situation will be unfolding in the Netherlands tonight. Basically any type of precipitation has the chance of falling. To keep it short, I will briefly describe the synoptics, accompanied by a sounding illustrating the potential of various kinds of precipitation.

 

Synoptics

 

Tonight, an occluding front will move over the Netherlands. The frontal system is not yet completely occluded when it reaches the country, giving rise to a very small warm sector during frontal passage. Furthermore, the air at the lower part of the atmosphere (near the surface) is still below 0*C. This cold layer is forecast to persist during the passage of the warm front, only giving away after the cold front. An illustration of the fronts by midnight is given below:

 

Analysis of the current weather chart, accompanied by frontal analysis as of 00 UTC.

 

Note that just to the west of the Netherlands, a frontal system is visible. There is still some room between the warm and the cold front, which will be becoming smaller as the front nears Holland.

 

Sounding

 

As an illustration, take a look at the GFS sounding of 04 UTC over the eastern part of the Netherlands:

 

GFS sounding of eastern part of the Netherlands (04 UTC)

 

The sounding can be made clearer by clicking on it.

 

To avoid things from getting too complicated, only follow the 0*C isotherm (which is slightly skewed) and the red line (which is the temperature line). The y-axis is altitude. What one needs to focus on is the temperature line close to the surface.

 

As can be seen, there is a large area of above-zero temperatures present between about 500 and 1000 meters. This means that precipitation falling from above that layer will melt into rain. However, below 500 m, temperatures drop below 0*C again as a result of the cold layer sticking at the surface. This might induce re-freezing of the rain droplets, probably generating widespread glazed frost. The Dutch equivalent to the MetOffice (KNMI) has issued the second-highest warning level (code orange) due to this phenomenon.

 

The intriguing part is that one degree colder or warmer in the upper air or at the surface can yield a major difference in the precipitation type being experienced. For example, if the above 0*C layer will be smaller than expected, the precipitation may well fall as snow.

 

Tomorrow I'll come back on this situation if possible, to see how this finely balanced situation unfolded and possibly to evaluate the models

 

Sources:

http://www.meteociel.fr/modeles/sondage_gfs.php

http://www.knmi.nl/waarschuwingen_en_verwachtingen/extra/guidance_modelbeoordeling.html

http://www.weer.nl/weer-in-het-nieuws/weernieuws/ch/0eca4be5d673ef35eb45503ac6543761/article/op_grote_schaal_ijzel_komende_nacht.html

http://webservice-nl-nl.weeronline.nl/vakman_hirlam

Edited January 23, 2015 by Vorticity0123

[Vorticity0123]

Ultimately, the forecasts for the situation verified quite well. However, the amount of glazed frost was higher than forecasted, literally 'derailing' the train traffic in the country.

Basically, the weather situation observed was a start of snowfall (warm front), being replaced by rain (warm sector passage at high altitude), then glazed frost and finally returning to snowfall (cold front).

 

The snow was nicely visible on satellite imagery:

 

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Satellite image of the Netherlands at 13:45 local time (12:45 UTC). Courtesy: Buienradar.

 

Note the snow cover is clearly visible over the eastern part of the Netherlands.

 

Source:

http://buienradar.nl/

Edited January 24, 2015 by Vorticity0123