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Fig8.1.8.2: Ensemble forecasts instantaneous type of precipitation chart, DT 00UTC 20 February 2018, T+36 VT 12UTC 21 February 2018. A type of precipitation is shown wherever the forecast rate of precipitation is greater than 0.1mm/hr. The charts show the most probable type of precipitation based on the proportion of types of precipitations forecast by the ensemble members. These are represented by colours:- Green-Rain, Red-Freezing Rain, Blue-Snow, Dark Blue-Wet Snow, Cyan-Sleet, Orange-Ice Pellets - darker shades indicate greater probability. Where the total precipitation probability is less than 50% the type of precipitation is not shown, but the total probability of any precipitation (between 10%-30% or 30%-50%) is shown by the shades of grey. The probability of each type of precipitation (taken as the proportion of ensemble members showing this type) is also given in the "probe" information (top of diagram) according to a code - in this case for Quebec City, 3 represents >70% probability of rain at this time. It should be remembered that an indication of freezing precipitation gives no information of likely accumulation of glazed ice although there must be a serious risk, and heavier precipitation rates suggest a potential for greater accumulation.
Fig8.1.8.2A: Ensemble forecast of instantaneous type of precipitation chart, DT 12UTC 19 February 2018, T+45 VT 09UTC 21 February 2018 (run time 12hours earlier than in Fig8.1.8.2). The histogram shows the distribution among ensemble members of types of precipitation at Quebec City (pin shows location) at 09UTC 21 February 2018. The ensemble chart suggests that the area of freezing rain lies to the northeast of Quebec at this time and the most probable type of precipitation at Quebec is rain. However, the histogram for Quebec shows the probability of freezing rain overnight was very high but that the risk has greatly reduced by 09UTC. However, the probability of freezing rain at 09UTC is still about 21% - heavy (dark red) 8%, moderate (red) 10% – so a risk of occurrence of a potentially dangerous phenomenon persists. It is important to inspect the histogram of precipitation types and not rely only on the probability of precipitation charts alone. It should be remembered that an indication of freezing precipitation gives no information of likely accumulation of glazed ice although there must be a serious risk, and heavier precipitation rates suggest a potential for greater accumulation.
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Fig8.1.8.3: Ensemble forecast of instantaneous type of precipitation chart, DT 00UTC 20 February 2018, T+36 VT 12UTC 21 February 2018 (the same time as Fig8.1.8.2). A type of precipitation is shown wherever the forecast rate of precipitation is greater than 0.1mm/hr. The types of precipitation are represented by colours:- Green-Rain, Red-Freezing Rain, Blue-Snow, Dark Blue-Wet Snow, Cyan-Sleet, Orange-Ice Pellets. The type of precipitation is also given in the "probe" information (top of diagram) according to a code - in this case for Quebec City (pin shows location), 3 represents freezing rain at this time. Note this is different to ensemble guidance for this time (Fig8.1.8.2) where the greatest probability is for rain at this time at Quebec at 12UTC 21 February 2018 (but there may be a lower probability of freezing rain). An indication of freezing precipitation gives no information of likely accumulation of glazed ice although there must be a serious risk, and heavier precipitation rates suggest a potential for greater accumulation.
Fig8.1.8.4: An example of the most probable instantaneous type of precipitation product displayed over a larger area. Type of precipitation is shown by the colour scale. To help users to have a better understanding of the situation, where the total precipitation probability is less than 50% the type of precipitation is not shown, but the total probability of any precipitation (between 10-30 or 30-50%) is shown by the shades of grey.
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Fig8.1.8.6: Method to display histograms of probability of types of precipitation from ecCharts. The location of the histogram may be selected using the probe tool on the chart.
Fig8.1.8.7: ecChart forecast chart of the most probable type of precipitation DT 00UTC 20 January 2018, T+60 VT 12UTC 22 January 2018 (Same ecChart as Fig8.1.8.1). The resolution of the orography is 9km. The most probable type of precipitation is shown by the colours but gives no information on any alternative types of precipitation. The histogram, however, shows probabilities of each type of precipitation as measured by the proportion among the ensemble members of each type of precipitation forecast at the selected location. The ensemble grid point altitude is displayed at the top of the meteogram.
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It is important to note that the colour scales on type of precipitation charts and the accompanying histograms indicate different things - they are not interchangeable. The colours in the chart show the most probable type of precipitation; the colours in the histogram show the probability of each intensity of precipitation at the chosen probe location.
Fig8.1.8.8: An example type of precipitation chart. Ensemble forecast charts, DT 00UTC 23 February 2018, T+83 VT 21UTC 26 February 2018. It is important to note that the colour scales on chart and histogram indicate different things - they are not interchangeable. The colours in the chart show the most probable type of precipitation represented by colours:- Green-Rain, Red-Freezing Rain, Blue-Snow, Dark Blue-Wet Snow, Cyan-Sleet, Orange-Ice Pellets - darker shades indicate greater probability (not the intensity of the precipitation). The colours in the histogram show the probability of each intensity of precipitation at the location of the pin (the colours are given in the scale above the histogram - darker shades indicate greater intensity). In this case at the location of the pin in the English Channel the dark blue area implies >70% pobability of wet snow. This agrees with the histogram where, at 21UTC 26 February 2018, there is ~82% probability of wet snow composed of ~37% light intensity (pale blue), ~41% moderate intensity (mid-blue), ~4% heavy inensity (dark blue).
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HRES (and CTRL) gives no information on the probability of a type of precipitation and should not be used on its own; reference should always be made to ENS output, particularly by use of the histogram, to assess the probability of other types of precipitation. It should be remembered that an indication of freezing precipitation gives no information of likely accumulation of glazed ice although there must be a serious risk, and heavier precipitation rates suggest a potential for greater accumulation.
Fig8.1.8.10A (left): Type of precipitation forecast chart from HRES. DT 00UTC 31 December 2017, T+84 VT 12UTC 03 January 2018. The HRES forecast type of precipitation for a location in central Slovakia (shown by pin) is sleet (grey).
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Fig8.1.8.12: Typical histograms of probabilities of instantaneous precipitation type for locations within the airstream. Colours in Fig8.1.8.11 represent the most likely (i.e. produced by the greatest percentage of ensemble members) but don't show other types, even if they are marginally less likely. The use of histograms gives a better overview of precipitation types and enables a more confident forecast to be made by the user. Considering 12UTC 31 Jan 2019: In Belgium there is high confidence of dry snow with very small probability of any alternative precipitation type.
Sometimes the probabilities of two or more precipitation types may be very similar. For example, the the difference in probability between problem-causing wet snow and less hazardous sleet or rain may be small and it may be may be may be difficult to decide between them. The The charts of most probable precipitation type only show the precipitation type that has the greatest probability, no matter how small the probability is greater than the next most probable precipitation type.
histograms of precipitation type in conjunction with charts with charts of most probable precipitation type to assess the most likely precipitation type and the probability of alternatives. The histograms are readily available on ecCharts.
Probability of freezing precipitation charts modified to show drizzle???
Freezing drizzle
Fig:8.1.8.13: ecCharts presentation of ensemble forecast of probability of freezing rain accumulations, threshold 0.01 mm in previous 6 hr, DT 12UTC 24 January 2023, T+21 VT 09UTC 25 January 2023. The vertical profile at Reading (located by the pin) shows a saturated layer of stratus at about 950 hPa with temperature above 0°C. Very slight precipitation produced from the stratus by the model falls through an underlying sub-zero layer near the surface. The model gave above 65% probability of very light freezing precipitation. Freezing drizzle was observed at Reading during the period.
Fig:8.1.8.14: ecCharts presentation of HRES forecast freezing rain accumulations over the previous 6hr, DT 00UTC 16 February 2023, T+42 VT 18UTC 17 February 2023. The vertical profile at Bangor, Maine, USA shows the classic freezing rain structure of a very moist and precipitating frontal zone overlying the sub-zero layer near the surface.
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