Page History

...

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 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.

...

Fig9.6.5-6: 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.

(FUG Associated associated with Cy50r1)