One challenge is nowcasting convective initiation when it occurs within environments which are very dry, and/or when the convection is substantially elevated from surface-based heating, leading to convective clouds which fail to produce echoes of 35 dBZ intensity at ground level. In these instances, a CI nowcasting algorithm that identifies this convective development technically fails as the traditional radar-based CI definition of a 35 dBZ echo (whether at ground level or at the altitude of the -10 C isotherm) is never met. Yet, convective clouds do in fact grow to produce rainfall. This leads to the notion that "you know it when you see it" in terms of CI, but in these situations a hard radar-based definition is less easy to follow.
In this example, the SATCAST (proxy Official AWG CI Algorithm) flagged a cluster of cumulus clouds at 1845 UTC, which are seen in 1 km GOES-13 visible satellite data at 1932 UTC. The first radar echo was seen near 1924 UTC with a maximum dBZ of <20 dBZ. This small cell developed over the Plains, and propagated north-northeatward (to the northeast of Trinidad, CO), and maintained only 10-20 dBZ rainfall. Radar data are shown through 2000 UTC. Also shown is the RUC sounding for near this location which confirm (a) the storm occurred in a dry easterly flow, to the north of a warm from, (b) most unstable CAPE values were near 471 Jkg^-1, and (c) cloud bases were near 680 hPa, with storm tops perhaps reaching ~300 hPa (i.e. it was elevated convection).
This example highlights the sensitivity of the SATCAST algorithm for identifying CI in less than obvious situations (when CAPE values are not high, and when cumulus updraft widths do not necessarily fill a 4 km^2 GOES pixel).
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