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Fig9.6.1-12: 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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Fig9.6.1-20(b): Precipitation extreme forecast index (EFI).  Red shading represents EFI>0.8, Dark red >0.9 EFI. T+24 VT00UTC 11 July 2019, DT00UTC 10 July 2019.

Forecast vertical profiles are very helpful in assessing the potential for severe events.  The forecast vertical profile at Pilio shows large CAPE but with relatively dry convection, possibly released by high surface daytime temperatures.  Very little moisture is indicated and precipitation looks very unlikely.  However some moisture is available locally over mid-Greece (Fig 9.6.1-20(b)) mainly at medium levels producing possible local showery outbreaks given some form of dynamic uplift.  The relevant wind shear to consider for this is probably between medium and upper tropospheric levels rather than between lower and medium levels (the bulk shear).  Inspection of the hodograph suggests the upper tropospheric shear is not great, so shower organisation/activity would lack this element of support.  Note, however, that heavy medium level showers can penetrate downwards through underlying dry layers more than IFS forecasts tend to suggest, even reaching down to the surface.  Ensemble control/HRES show a very humid boundary layer at Pilio, but it would require large energy input at the surface (2m temperatures above about 35°C) to overcome the large CIN and to lift the low level moisture to release moist convective cells.

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