for example, the Red River flood at Grand Forks, North Dakota, in April 1997 or flooding from Ohio to New England during the January thaw of 1996. Floods can create health risks to humans and the local ecology due to the biological and chemical constituents in flood drainage that otherwise would not be present.

Because these storms are large, typically 1000 km in diameter, they are rather well observed over land. Those that cause flooding and landslides along the West Coast, as in January to February 2005, are almost always centered offshore. Occasionally, however, a long plume of moisture in southwesterly flow will cause soaking orographic rains along the California coastline without the presence of a well-defined cyclonic circulation. In either case, more in-situ observations are needed within a few hundred kilometers of the coast, especially of temperature, wind, and moisture below 600 mb, to supplement satellite observations. In-situ observations inside of cloud systems within a day or so of reaching the West Coast would also be very helpful.

Mesoscale features within the storm circulation often mark the difference between merely soaking rains (say, 0.20 inches per hour) and serious flooding (>0.50 inches per hour, prolonged). In many parts of the country, tropospheric wind observations, especially within cloudy areas, are too far apart to resolve these details. Moisture observations, especially below 600 mb, where most atmospheric moisture is concentrated, are sparse. For these mesoscale features, lower tropospheric soundings at ∆x=50 km, ∆z=200 m, and ∆t=3 h resolution are appropriate. ∆x refers to horizontal spacing, ∆z to vertical spacing, and ∆t to temporal frequency.

For longer forecasts than those considered here, the Winter Storm Reconnaissance Program in the North Pacific Ocean provides targeted aircraft observations. These benefit the entire country but especially the West.


Definition: A Nor’easter is a large cyclonic storm occurring from late fall through spring that moves northeastward along the U.S. Atlantic coast or a few hundred kilometers offshore. Sometimes intensifying rapidly, Nor’easters bring strong onshore winds, often from the northeast (hence the name), storm tides, flooding, and heavy precipitation. For practical purposes, a Nor’easter may be considered an Atlantic coastal storm accompanied by an onshore component of the wind of at least 40 mph for at least 12 hours.

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