ABOARD A GULFSTREAM IV, over the Pacific>> The winter storms pounding California this month often have come into being thousands of miles to the west, in the moist air above the Pacific.

That has given a group of scientists and technicians a few days before one blows ashore to examine satellite readings, run computer models and plan crews and equipment, all of which culminates in a uniquely full-contact effort to understand these storms’ inner workings: by dropping sensors into them from the sky.

This past week, aboard a Gulfstream IV jet that was streaking toward Alaska as a giant storm seethed miles below, an engineer gave a quick countdown: “Sonde’s out in 5, 4, 3, 2 ... ”

A hatch in the plane’s belly sprang open. The sonde, a tube of instruments about the size of a model rocket, was sucked out into the frigid air and began plummeting toward the clouds, where it would investigate the storm’s innards and transmit its findings to the world.

Atmospheric rivers have caused weeks of flooding, power outages and evacuations up and down California and killed at least 18 people. But the devastation almost certainly would be even greater were it not for weather forecasts that roll in before each storm. Emergency responders, dam operators and farmers now have piles of rapidly updated information at their fingertips about where these storms are headed, how soon they might arrive and how much rain and snow they could bring when they get there.

Assembling those predictions still begins, however, by getting close to the action. The West Coast’s atmospheric rivers spend their early days over huge, empty expanses of the Pacific. Clouds can obstruct satellite measurements, and drifting buoys mostly gauge conditions near the ocean’s surface.

A program called Atmospheric River Reconnaissance, or AR Recon, is trying to plug this data void. It is led by scientists at the Center for Western Weather and Water Extremes, part of the Scripps Institution of Oceanography at the University of California-San Diego.

AR Recon is partly a research initiative, but since 2019 it also has been part of the federal government’s winter meteorological operations, with support from the Air Force and the National Oceanic and Atmospheric Administration.

This winter, AR Recon started flying missions earlier than ever, in November, to sample more of the early-season storms that have caused devastating flooding on the Pacific Coast in recent years.

The data these planes gather is part of a series of advancements in weather forecasting and the computer models behind it.

Each AR Recon flight begins with scientists and officials convening virtually and in a San Diego conference room to draw up a plan of attack. It was Wednesday. Another river of moisture was barreling toward California. A big one.

With only three aircraft at their disposal — the NOAA Gulfstream and two Air Force C-130s — mission planners have to be strategic. They conduct analyses to determine where extra data from inside an atmospheric river might be most useful for improving forecasts. Then they chart flight paths to hit those spots economically.

With the forecasts before any storm, “very small errors have the potential to grow to make a precipitation forecast really off,” said Anna M. Wilson, a scientist at Scripps and AR Recon’s mission director for the past week.

Early the next morning, a small team set off in the Gulfstream from Honolulu.

The mission: fly about 1,500 miles almost straight north, toward the Aleutian Islands in Alaska, before doubling back.

This would allow the plane to cross two sections of the atmospheric river’s moisture-laden core as it swept east.

And flying at an altitude of 41,000 feet to 45,000 feet would let the plane sample the storm itself and the jet stream, whose powerful winds help shape the system’s course.

Total flight time: about eight hours.

The NOAA Gulfstream, nicknamed “Gonzo,” is mostly kitted out for science, with a few concessions to the needs of crews spending long hours in close quarters.

There are jugs for water and coffee, plus an icebox. Snacks aren’t provided. Squeezed into its rear is a lavatory.

Heavy racks of gear have been installed throughout the cabin with artful efficiency.

A tail-mounted Doppler radar estimates how quickly the moisture is moving below. A GPS receiver measures how much satellite signals are refracted in the air to estimate the properties of the atmosphere around the sides of the plane.

Then there are the sondes, which cost NOAA about $800 each.

The crew dropped more than 30 of them during Thursday’s flight, spacing them out to sample a wide section of the atmospheric river.

The data is transmitted to a global repository that feeds weather forecasts around the globe. But the first to see it are the onboard meteorologists, whose screens dance with colorful squiggles representing wind speed and direction, temperature and humidity — each sonde’s chaotic 15-minute dive toward the sea.

In some cases, scientists have found improvements in forecasts by the sondes of up to 25%.