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- At shorter lead-times the ensemble solutions will usually be fairly similar. The probability of the types of precipitation are likely to be high. Charts will show a lot of detail.
- As lead-time increases the spread of ensemble solutions will be greater. The development, timing and location of precipitation events will become less certain. Charts will show larger areas of grey (<50% probability of precipitation).
Fig8.1.10-6: 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.
However, IFS models is able to predict freezing rain several days in advance despite the finely balanced vertical thermodynamics structure required.
Fig8.1.10-7: Sequence of precipitation types DT:00UTC 04 March 2023. VTs:00UTC 04 Mar (T+00) to 12UTC 06 Mar (T+60). Precipitation type shown by colours.
Interpretation of colouring on charts
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- 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.
Fig8.1.10-78: Opencharts Probabilities. Most probable precipitation type DT 00UTC 31 June 2023, VT 12UTC 01 July 2023. 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 the precipitation (between 10-30 or 30-50%) is shown by the shades of grey. The histogram location is for the SE of Iceland and shown by the arrow.
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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 accumulation.
Fig8.1.10-8(left)9A: HRES forecasts 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 are not the same as accumulation of glaze or glazed ice although some proportional accretion must be expected.
Fig8.1.10-8(right)9B: The ensemble forecasts freezing rain probability (>1mm over 12hr). DT 00UTC 23 February 2018, T+72 VT 00UTC 26 February 2018. Colour scale: Yellow 5-35%, Green 35-65%, Darker Green 65-95%, Blue >95%. The pin marks the location of Quebec City with the forecast probability of >1mm 12hr total of freezing rain given in the probe information frame.
Fig:8.1.10-910: 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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The following plots illustrate how the handling of precipitation type by ensemble forecasts in a marginal rain-snow situation can make physical sense; and specifically how user-relevant subtle changes are predicted with some precision.
Fig8.1.10-1011: Illustration of the probabilities of different instantaneous precipitation types. The flow is mainly easterly over England and illustrates the impact of different length sea tracks over the relatively warm sea, and the impact of topography. Consider an approximate low level trajectory from Belgium to Wales (dashed arrow). Colour is used wherever the probability of some precipitation falling is 50% (from the ensemble). The colour itself illustrates the most likely type, whilst the darkness of the shading indicates how likely that type is. Dry snow over Belgium, wet snow and sleet over the relatively warm southern North Sea, wet snow over colder southeastern England, dry snow further away from the sea (most likely as the air re-cools partly via evaporation), then over Welsh mountains the probability of dry snow is very high (because the high probability of temperatures being below zero and of high probability of precipitation falling due to orographically forced ascent).
Fig8.1.10-1112: Typical histograms of probabilities of instantaneous precipitation type for locations within the airstream. Colours in Fig8.1.10-10 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.
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Freezing drizzle can be as much of a hazard as freezing rain. The "Probability of Freezing Rain" option on ecCharts can show 0% when the default threshold is used. Freezing drizzle can be seen if a very low threshold (say >0.01mm in previous 6hr) is applied.
Fig:8.1.10-12:13 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.
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