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In reality the showers take time to grow while also being driven downwind. The snow that falls from them also drifts downwind as it falls. However, neither "drift" mechanism is represented in the IFS and the net effect is that the snow in reality is spread across a much larger distance downwind than in the raw model output .
Fig9.6.1-5A6: The diagram shows an area of NW Scandinavia with snow accumulation indicated by colours (large accumulations blues, small amounts, green). Topographically the area is complex, but the key feature are strong upslopes near the exposed NW coast of Norway, with a line of mountains reaching about 2000m but interspersed with lower lying gaps.
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Fig9.6.1-67: Forecast CAPE Extreme Forecast Index (EFI) in northern Italy and western Austria for 00UTC 12 to 00UTC 13 Dec 17, T+24 to 48 from data time 00UTC 11 Dec 17. The darker orange area over far North Italy and Tyrol (roughly shown by the pin) has EFI>0.8 denoting high probability of an out-of-the-ordinary significant event. EFIs are shown by colours - Yellow >0.5%, Dark Yellow >0.6%, Orange >0.7%, Dark Orange >0.8%, Red >0.9%.
Fig9.6.1-78: Forecast CAPE-shear Extreme Forecast Index (EFI) in northern Italy and western Austria for 00UTC 12 to 00UTC 13 Dec 17, T+24 to 48 from data time 00UTC 11 Dec 17. The red area over far North Italy and Tyrol (roughly shown by the pin) with EFI>0.9 and is rather larger in extent than CAPE EFI owing to the influence of the bulk shear in the lower troposphere. EFIs are shown by colours - Yellow >0.5%, Dark Yellow >0.6%, Orange >0.7%, Dark Orange >0.8%, Red >0.9%.
Fig9.6.1-89: Forecast probability of precipitation (>5mm/12hr) in northern Italy and western Austria for 12hr period ending 00UTC 13 Dec 17, T+36 to 48 from data time 00UTC 11 June 17. Higher probability of precipitation over Tyrol. Probabilities shown by colours - Light blue >5%, Blue >35%, Dark blue >65%, Purple >95%.
Fig9.6.1-910: Forecast CAPE-shear EFI superimposed upon forecast probability of precipitation (>5mm/12hr) in northern Italy and western Austria ending 00UTC 13 Dec 17, T+48 from data time 00UTC 11 June 17 as figures shown above. High CAPE or high CAPE-shear alone show only the potential for active convection - if it can be released. It is also necessary to to identify where the atmospheric model is actually producing precipitation, and if this overlaps with high CAPE or CAPE-shear then severe convection may be forecast. In this example the area over far North Italy and Tyrol (roughly shown by the pin) has high CAPE-shear and high probability of precipitation and hence severe weather may be forecast in the area.
Fig9.6.1-1011: Multi-parameter EFI CHART (a clickable chart) for 00UTC 12 to 00UTC 13 Dec 17 (T+24 to T+48) from data time 00UTC 11 Dec 17. The dark green area over far North Italy and Tyrol (arrowed) indicates a high EFI for rainfall.
Fig9.6.1-1112: Cumulative Density Function (CDF) for far North Italy (derived from the clickable EFI chart in Fig9.6.1-67) for period 00UTC 12 Dec 17 to 00UTC 13 Dec 17. Note the steep slope of rainfall and temperature CDFs which implies high confidence (low spread among recent ENS members). Latest precipitation forecast has not as many high rainfall totals as previous ENS members but retains a high EFI value.
In such a winter case the values of CAPE and CAPE-shear are unlikely (inland) to be dramatically large, particularly when compared to summertime values. But nevertheless, the EFI values for CAPE (Fig9.6.1-67) and especially CAPE-shear (Fig9.6.1-7) are high, indicating the forecast values are towards the high end of the M-climate distributions for these parameters, and are therefore worthy of further investigation. In particular, there is an enhanced likelihood that the convection scheme's totals may not be so representative and that much more extreme local totals are possible.
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Land surface characteristics (soil moisture, leaf area index) have an impact upon land and temperature forecasts. Changes in land characteristics is especially important where there is a sharp discontinuity in ground type or vegetation cover. This can produce significant difference in temperatures or moisture content of the lower atmosphere over a short distance, and hence to air temperature and/or the development of convection. Users should inspect model information on land surface, soil moisture and leaf area index to identify areas where significant changes in precipitation or other weather phenomena over short distances may occur.
Fig9.6.1-1213: Illustration of the impact of differing land cover and type in the vicinity of Flagstaff, Arizona. Showers broke out over the vegetated west part of the area but not over the rocky region to the east. The central diagram shows the ensemble 98th percentile of "point rainfall", with tephigrams DT 00UTC 18 July 18 T+24 VT 00UTC July 19. The parcel curves have very different CAPE values - greater in the west and hence greater risk of very wet weather, but lesser in the east even though temperatures were higher over the bare surface. This illustrates high sensitivity to humidity mixing ratios and altitude. Humidity mixing ratios can reflect land surface processes related to evapotranspiration which control the moisture exchange with the lower troposphere. And in turn these relate to the soil moisture which controls moisture availability. Also of critical importance on the soundings are the light winds with shear. Here the land surface characteristics changed rapidly across a short distance (forest to rock), which is in fact reflected on the deep (1m) soil moisture plots from the IFS, and also in the leaf area index (LAI), which is a multiplying factor for evaporation.
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Poorly forecast heavy showers in Oman
Fig9.6.1-1314: Large and vigorous convection over eastern Oman 6 July 2018 bringing heavy showers.
Fig9.6.1-1415: Observed (black) and forecast (red) vertical profiles at approximately the same time as the satellite picture (Fig9.6.1-1314) for a radiosonde location (Seeb, WMO:41256) just northwest of Muscat. The lowest layers were observed to be quite moist while the forecast vertical profile indicated much drier conditions. The low level winds are shown as drifting air from the nearby sea on both observed and forecast profiles. Higher moisture at low levels would allow deep and active convection to be released with sufficient energy input to overcome the convective inhibition (CIN), either by surface heating or by uplift over the mountains. Heavy showers did develop but were not well forecast, if at all.
Poorly forecast heavy showers in SE England
Fig9.6.1-1516: An example of the effect of under-representation of low-level temperature and dew point by the IFS near an urban heat island. Forecast values of near-surface temperature and dew point are about 3C cooler than observed. Modifying the forecast vertical profile using observed values results in significantly greater CAPE than forecast which, together with the forecast shear, would indicate much more active convection. Flash flooding and a tornado were observed near the urban heat island.
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Where medium level instability is forecast above a dry lower atmosphere, users should use forecast lightning charts and forecast vertical profiles to extend and improve precipitation forecasts. Where medium level instability is forecast (even with only moderate CAPE), some additional showers should be forecast within the areas of forecast lightning. Owing to resolution issues, forecast intensity of lightning strikes gives only a rough idea of regions where there is more active medium level instability but it does not reliably indicate that showers will penetrate to the surface, nor their intensity if they do so. However, probability of precipitation should be increased.
Fig9.6.1-1617: Forecast IFS data for central and northwest Australia 17 Jan 2019. Local time is about 10hrs ahead of European time zones. The circled triangle locates Alice Springs.
- Fig9.6.1-1617(a): Total 6hr precipitation from 9km resolution model DT 12UTC 16 Jan 2019, T+21 VT 09UTC 17 Jan 2019.
- Fig9.6.1-1617(b): Lightning density in 6hr (flashes 100km-2hr-1) DT 12UTC 16 Jan 2019, T+21 VT 09UTC 17 Jan 2019.
- Fig9.6.1-1617(c): ENS probability of total precipitation >1mm: DT 12UTC 16 Jan 2019, T+12 VT 00UTC 17 Jan 19 to T+36 00UTC 18 Jan 2019.
- Fig9.6.1-1617(d): Total 6hr precipitation from 9km resolution model DT 12UTC 16 Jan 2019, T+21 VT 09UTC 17 Jan 2019, and Observed lightning flashes VT 09UTC 17 Jan 2019.
- Fig9.6.1-1617(e): Forecast vertical profile at Alice Springs DT 12UTC 16 Jan 19, T+18 VT 06UTC 17 Jan 2019.
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In this example, medium level thunderstorms developed and extended well into central parts of Australia (Fig9.6.1-1617(b), with observed lightning) but no underlying surface rainfall is forecast (Figs9.6.1-1617(a) & 9.6.1-1617(d), nor any probability of rain (Fig9.6.1-1617(c)). Forecast lightning flashes (Fig9.6.1-1617(b)) is overly extensive in northwest Australia but although there is some indication in central parts it is under-indicated (compare with Fig9.6.1-1617(d)).
The model boundary layer was generally dry in central Australia but observations showed much higher dew points where showers have occurred. Near Alice Springs the model T2m dewpoint was 4.2C lower than the observed dew point, and at a location to the northwest the error was 11.8ºC. Both discrepancies were probably due to storms that the model didn't represent.
The forecast vertical profile for Alice Springs (Fig9.6.1-1617(e)) shows possible (surface-based) medium level instability with just moderate CAPE (e.g. cyan line construction). Note that some ensemble members have higher low-level dew points which means a lower CIN to initiate medium level convection with greater CAPE (e.g. red dashed line construction). Further, if the boundary layer is moistened after any medium level showers penetrate to the surface then there is a higher likelihood of more energetic convection being released afterwards with much greater CAPE (e.g. black dashed line construction).
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Large values of CAPE lie in a zone across the Aegean Sea and parts of mid-Greece coincident with a belt of strong vertical wind shear resulting in very high values of CAPE-SHEAR (Figs 9.6.1-18(a) & 9.6.1-18(b)). In particular, high forecast values of CAPE and CAPE-SHEAR are indicated at Pilio while much lower forecast values are shown at Kavala. This might suggest at first sight that any instability that is released in the region of Pilio would be very active with the possibility of severe storms and rainfall. At the same time much less showery activity might be expected at Kavala on the northern flank of the CAPE and CAPE-SHEAR zone. Such a snap assessment would be incorrect.
Fig9.6.1-1718(a): Forecast CAPE (Blue high, Red low).
Fig9.6.1-1718(b): Bulk Wind Shear (Orange high, Yellow low). T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.
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