SATCAST CI Nowcasting: Translation & Evolution

One substantial improvement to increase the utility of the SATCAST CI algorithm (the GOES-R Proxy CI Algorithm) will be including details on where a given object, forecasted to become a new convective storm, is moving. For a forecaster, a present limitation of this CI nowcast product is that there remains uncertainty on where a nowcasted cumulus cloud object will in fact be located when it produces ~35 dBZ intensity rainfall (or perhaps lightning, if the environment is conducive). The example shown illustrates how adding propagation information will help in the CI nowcasting process, especially since the cloud object evolves substantially as CI occurs and thunderstorm development proceeds.

Shown is an example between 1432 and 1645 UTC 17 May 2011. The synoptic setting was a well-defined upper low, with strong cyclonic rotation and cold air aloft, leading to scattered generally low-topped, shallow convective storms. RUC surface-based CAPE values were 500-700 Jkg^-1. Convective clouds were propagating northwestward from central North Carolina to south-central Virginia. Circled is a northwest-southeast oriented cumulus cloud line in the vicinity of Raleigh, NC at 1432 UTC (Fig. A), with an added propagation vector included. Figures B (1445 UTC) and C (1602 UTC) show where the developing cumulus clouds and new storms, respectively, had moved since 1432 UTC. CI occurred by 1515 UTC; the 1606 UTC radar image is also shown. The cumulonimbus cloud with an obvious anvil is seen extending into far south-central Virginia at 1645 UTC (circled in Fig. D). In Fig. D, the full translation vector is shown beginning where the first object in Fig. A was seen. Only a few lightning flashes were observed by 1645 UTC associated with this storm. In this example, the translation vector points in an unusual direction, with storm propagation toward the northwest. This translation vector will also help forecasters maintain identification of a developing cumulus cloud, which will change substantially (in terms of size and shape) as it evolves, leading to less uncertainty in a CI nowcast.

Future enhancements to SATCAST will include translation vectors per nowcasted objects designed to help forecasters more precisely predict the location where heavy (~35 or greater dBZ) rainfall will occur. In many cases, the distance (time) between a highlighted CI object can be several 10's of kilometers (30-60 min) from where the rainfall/thunderstorm is actually observed to occur.