Prediction of heavy precipitation requires knowledge of the spatial distribution of water vapor. This distribution can often include mesoscale variations that are crucial in determining the amounts and location of precipitation, and yet such spatial variations are often difficult to observe with the traditional observing network. New satellite-based remote sensors offer some promise in determining the distributions of integrated precipitable water vapor (IPW), which can be invaluable over oceanic regions where moisture observations are extremely limited.

In an effort to characterize the moisture fields in extratropical cyclones approaching the west coast of the United States, the California Land-falling Jets experiment (CALJET) focused on making mesoscale observations of the low-level jet (LLJ), a region in these storms where the largest moisture content is typically found. In addition to direct aircraft observations of latent heat fluxes arising from air-sea interaction in the high winds associated with the LLJ, one of the goals of CALJET was to compare satellite-derived moisture measurements with more precise in situ measurements from aircraft and dropsondes.

Observations from 30 dropsondes deployed by a NOAA P-3 aircraft in an open polar frontal wave 500-1000 km west of the California during CALJET are used to describe the water vapor distribution. This is then compared with satellite observations from a GOES satellite and from the SSMI instrument on a polar orbiting satellite. The dropsonde observations reveal a narrow plume of greatly enhanced IPW (over 3 cm in a swath <100 km wide) that lies mostly beneath a layer of high clouds and between the surface positions of two parallel surface cold fronts separated by 250 km. The high clouds hide the core of the plume from the GOES IPW retrievals, which see at most 2 cm near this region. Comparison with the SSMI measurements reveals a narrow moisture plume hitting the coast early the next day, with a near-coast maximum >4cm that is located near where the greatest coastal precipitation was observed (4-6 inches in 24 h). Implications of these results for operational forecasting and for the design of the PACJET experiment are described.