The release of convection is strongly dependent upon correct analysis and forecasting of boundary layer humidity and land surface characteristics. This can result in a mismatch, mainly in arid coastal regions, between the location and severity of forecasts of active convection and verifying observations. Showers may be forecast in the wrong location or not forecast at all.
Users should consider the possible effects of less or more moist air feeding into the boundary layer. The potential for moist marine air to spread further inland or for drier air to extend more widely than Ensemble Control Forecast (ex-HRES) forecasts suggest. Existing or previous rain in an area may increase boundary layer humidity more than IFS analyses. Users should consider the possibility of an influx of low level air that is dissimilar to forecast values - i.e. moist air across coastal areas that might allow release of convection, or the converse if an influx from drier areas occurs. Daytime heating in upland locations and/or upslope flow over the mountains can also cause destabilisation that may not be captured by the forecast models. Examples are given below.
Fig9.6.5-1: Large and vigorous convection over eastern Oman 6 July 2018 bringing heavy showers.
Fig9.6.5-2: Observed (black) and forecast (red) vertical profiles at approximately the same time as the satellite picture (Fig9.6.5.1) 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.
Locally heavy snowfall in Lebanon early on 24 February 2025 was not well forecast by IFS.
Cumulus and layers of stratocumulus lay trapped beneath an inversion in a cold northerly airflow. Only light snow showers might be expected to to form from the weak convection. IFS showed little if any precipitation. Deeper, active higher-based convection formed over the adjacent sea but precipitation would largely evaporate in the dry air beneath. The active convection was advected inland on the northwesterly flow above the pre-existing cumulus and stratocumulus. Precipitation fell through the underlying cloud and reached the surface. Generally as snow above 200m with some disruption but locally at low levels and in coastal parts.
The IFS does not advect maritime convection and associated convective precipitation inland and missed the significant precipitation. Users should allow for this when using IFS products.
Fig9.6.5-3: The vertical profile near the coast showed moisture confined within the lower atmosphere between 850hPa and 700hPa with cumulus and layers of stratocumulus. Any light snow showers from this would mostly evaporate in the underlying dry air. The steering flow for this convection was 15kn from the north-northwest. IFS showed little if any precipitation.
Fig9.6.5-4: Vertical profile with construction for maritime convection. Deeper convection over the nearby sea produced showery precipitation. The parcel curve, based on 18C sea surface temperature and slightly more moist maritime air, implies development of quite active convection between 800hPa and ~500hPa. The steering flow for this convection was 35kn from the northwest. The active maritime convection with high base drifting inland over pre-existing cloudy and weakly convective cold continental air allows active snow showers.
Fig9.6.5-5: 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 3°C 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.
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.
Fig9.6.5-6: 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.