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Fig9.4.1-1: An example of HRES forecast fog distribution on right hand chart compared with the observed visibility on left hand chart (visibility as in coloured scale).  Forecast run data time 00UTC 23 January 2017 T+30 verifying at 06UTC 24 January 2017.  Thick fog, less than 100m visibility in places, caused problems over southern England.  Many flights were cancelled and others delayed.  In the early morning hours the fog was more widespread over England and Benelux countries.  HRES forecast visibility was too poor in some areas but provided good guidance to forecasters regarding areal extent of the fog risk. 

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Fig9.4.1-2: Fog probability in the early medium-range from an earlier forecast run, data time 00UTC 21 January 2017 T+78 verifying at 06UTC 24 January 2017.  The ensemble also highlighted England and Benelux countries as areas where fog was likely.

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Errors in forecast near-surface data associated with cases of thick fog.

Fig9.4.1-83: An example of incorrect temperature and dew point forecasts in a case of predicted fog.  Imperfectly modelled mixing processes near the surface induce errors in 2m temperature and humidity.

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IFS forecasts of reduced visibility can be difficult when fog is patchy or shallow.

Fig9.4.1-64:  Forecast visibility at 30hr lead time (left) and 6hr lead time (right) with verifying observations, in a freezing fog situation.  Although there is a good deal of consistency between the forecast fog distribution, there is slightly more fog forecast on the T+6 forecast chart than on the T+30 chart.  However, consistency should not necessarily be taken as a good guide to the true extent of fog.  The large area of fog over north France is captured quite well,  but over Britain observations of fog do not in general match either of the forecast locations at T+30 or T+6 (e.g. East Anglia).   However, fog is often quite patchy and may not be captured by the observations (observations are reports of fog at an individual location, not an area).    Additionally it should be noted that forecast visibility charts show poor visibility where cloud covers hills.

Fig9.4.1-75:  Forecast probability <200m visibility at 30hr lead time (left) and 6hr lead time (right) with verifying observations for the same freezing fog case as in Fig9.4.1-64.  In this example the areas of low/moderate probability capture the observed 200m visibility fairly well over England and Wales, and the probability distribution sharpened up as lead time reduced (e.g. over East Anglia), though not clearly in a manner that verified better.  A chart of probability of fog with visibility <200m (or other low thresholds) should only be taken to be very roughly indicative of those areas most at risk.

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Fog banks formed as a weak westerly flow brought relatively moist surface layer air across anomalously cold sea surface temperatures near the south coast of Sicily.  HRES captured the development and eastward advance of the fog banks particularly well.   Previous HRESforecasts showed a strong signal for fog banks in the general southern Sicily area, even at four days lead-time.  These gave a useful warning to coastal craft and recreational sailing, especially given the unexpected nature of the event.  Little if any fog was predicted for any other parts of the Mediterranean.

Fig9.4.1-3 6: Hour-by-hour visibility HRES predictions around Sicily 16 June 2022 00 to 24 UTC, DT 16 June 2022 T+0).  The colour scale shows the visibility in m.  Note the eastward movement of the fog bank with some detail shown of the changes in area.  Some areas with visibility <100 m (purple) are evident.

Fig9.4.1-47: Mediterranean sea surface temperatures around Sicily HRES at 00 UTC 16 June 2022.  The colour scale shows the temperatures in ºC

Fig9.4.1-58: Example of visibility meteogram showing forecast visibility in cumulative frequency form.  Colours represent forecast visibility ranges (see scale). 

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