The U.S. Signal Service of the 19th Century used two specific storm signals (which for all I know may the earliest recursors of the old National Weather Service storm signals).
The two storm signals were:
a square red flag with a black square in the center, the signal for "approaching storm." Subsequently this was the flag for gale force winds, and two flying one above the other for hurricane-force winds.
a square white flag with a black square in the center, flying above flag 1, the "cautionary offshore signal" or, at Great Lake Ports, the signal for high winds out of the north or west.
Coastal stations of the Signal Service monitored flag signals from offshore vessels and, on request, forwarded those messages by War Department land-line telegraph. A signal station with such telegraphic capability that was not concurrently a life-saving station was identified by a square white flag with a red square in the center below the U.S. national flag.
image by Joe McMillan, 30 June 2001
Joe McMillan, 30 June 2001 Source: "Official Danger, Distress and Storm Signal Codes for Signal Service Sea-Coast Stations and Mariners (Washington: Government Printing Office, 1883) (a small book issued by the War Department's Chief Signal Officer)