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- total snowfall probability,
- total snowfall rate probability,
- probability of combined events of wind gust and total snowfall.
Instantaneous, single member CONTROL-10
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(HRES)
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Within ecCharts a layer is availableon ecCharts to show CONTROL-10 type type of precipitation. This is for precipitation rates greater than 0.1mm/hr (shown by colorationcolouration). However, CONTROL-10 type these types of precipitation charts give no information on probability of the type of precipitation shown, or on any alternative types. For this, it It is important to assess the temperature structure of the lower layers of the atmosphere to enable a tentative probability, or at least a risk, of potential severe weather types.
Fig8.1.8.1: ecChart CONTROL-10forecast instananeous forecast chart of instantaneous type of precipitation chart with CONTROL-10 orography DT 00UTC 20 January 2018, T T+60 VT 12UTC VT 12UTC 22 January 2018. The resolution of the orography is 9km. 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 location of
At Straubing, Lower Bavaria is indicated Lower Bavaria (shown by the pin where, as the precipitation area moves east, CONTROL-10 indicates ) the initial snow will turn to freezing rain followed by rain . as the precipitation area moves eastwards. However, CONTROL-10 this type of precipitation charts chart give no information on probability of the type of precipitation shown or on any alternative types of precipitation.
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Instantaneous, ensemble
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Charts
A more informative ecChart display of instantaneous type of precipitation is based upon ensemble output and shows the most probable type of precipitation. The indicated type of precipitation, however, gives no information on any hazardous precipitation having a lower probability. Such information can be obtained by reference to the instantaneous type of precipitation meteogram/histogram product.
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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 (location is indicated by the pin). 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 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.
CONTROL-10 is the unperturbed The unperturbed member of the ensemble and may or may not be the same as similar to all or any of the ensemble member solutions. CONTROL-10 It 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 ensemble output to assess the probability of some other type of precipitation.
Fig8.1.8.3: CONTROL-10 forecast instananeous type Ensemble forecast of instantaneous type of precipitation chart, DT 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 location of Quebec City is indicated by the pin. 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.
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The most probable type of precipitation product clearly shows the areas at greater risk for precipitation and also the areas where there is greater uncertainty. However, the uncertainty also varies with forecast lead-time. At shorter lead-times the ensemble solutions will usually be fairly similar and probability of the types of precipitation are likely to be high and charts will show a lot of detail. However, as lead-time increases the spread of ensemble solutions will be greater and the development, timing and location of precipitation events will become less certain with time and charts will show larger areas of grey (<50% probability).
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Fig8.1.8.5: Multiplots of ensemble probability of type of precipitation forecasts all verifying at 12UTC 21 February 2018 from a series of ensemble forecast runs at 24hr intervals. As lead-time increases the more hazardous, less common types of precipitation are less prominent or do not appear, and greys (total probability <50%) are more prominent.
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Histograms
More information regarding the probability of alternative types of precipitation may gained from inspection of the results from all ensemble members. Histograms of the ensemble types of precipitation that show this are available by use of a option on ecCharts.
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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: (Same ecChart as Fig8.1.8.1). DT 7: ecChart forecast chart of the most probable type of precipitation DT 00UTC 20 January 2018, T+60 VT 12UTC 22 January 20182018 (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 ENS ensemble members of each type of precipitation forecast at the selected location. The ENS ensemble grid point altitude is displayed at the top of the meteogram.
Fig:8.1.8.7 shows that at Straubing at 12UTC 22 January 2018 several possibilities of types of precipitation are forecast by the ENS ensemble members: ~26% rain (of which ~4% light, ~10% moderate, ~12% heavy intensity), ~2% sleet, ~2% wet snow, ~14% Snow (varying intensities), ~18% ice pellets (varying intensities), ~34% freezing rain (of which 4% light, ~20% moderate, 10% heavy intensity), ~6% no precipitation. Thus freezing rain has the greatest probability and would be shown as red areas in the probability of type of precipitation product in ecCharts. Nevertheless, the chances of rain falling at this particular time are only slightly lower which in this particular instance happens to match the forecast precipitation type provided by CONTROL-10, as shown on the chart. Without viewing the histograms the user would not know of this significant alternative. Freezing rain is such a significant hazard that even a small probability (as at 03UTC 24 January 2018 - see histogram) might be worth a warning to customers sensitive to this type of precipitation even though rain might be more probable and shown as such on the charts. It is very important that the histograms for a location of interest are consulted to capture occasions when hazardous precipitation is forecast with a lower probability, as often occurs at longer lead-times. 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.
Interpretation of
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colouring on
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charts
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.
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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).
Further
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examples related to
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types of
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precipitation
Freezing rain is a very hazardous type of precipitation which brings a significant risk to aviation, transport, communication and indeed to life. There are outputs to highlight the probability and amounts of freezing rain. 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 for greater accumulation.
Fig8.1.8.9A (left): CONTROL-10 forecast freezing rain (total over 12hr). DT 00UTC 23 February 2018, T+72 VT 00UTC 26 February 2018. Colour scale: Light Blue 0.0-0.2mm, Mid Blue 0.2-0.5mm, Dark Blue 0.5-1.0mm, Dark Red 1.0-2.0mm, Red 2.0-5.0mm, Pink 5.0-10.0mm, Orange >10.0mm. The pin marks the location of Quebec City with the forecast 12hr total of freezing rain given in the probe information frame. Freezing rain total values do not the same as accumulation of glaze or glazed ice although some proportional accretion must be expected.
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