It Means Weekly. Each year and each season has its "weather or climatic character" as a result of the runs and spells of weather that make it up. Often the spell is made of up weather events of a particular character falling on the same day of the week, week after week after week. Here is a case in point.
Jerome Namias (1966). A weekly periodicity in eastern U.S. precipitation and its relation to hemispheric circulation. Tellus XVIII:731-744.
No one wants it to rain on his or her parade. The phrase however is deeply entrenched in our culture
Once I mentioned all this to my best-secretary-ever of some years back (I have computerized her out of a job.) She told me that "Everybody knows that." I see her from time to time and she yells out her observations: "It's Tuesday this Spring. Every Tuesday!" She kept notes on her desk calendar and boldly predicts the weather a week in advance. Once you note the pattern it is very hard for the weather service and its numerical models to out-forecast you at the 7 days and beyond into the future, much less 14 and 21 days out! Just say: Next Friday will be like last Friday but more normal and you are in the know!
This weekly cycle is often present, especially in the winter half of the year. However, in any one year, it could fall on any of the 7 days that make up our weekly, work-a-day world. If we had 8 days in the week with each named so we could remember them, we would, perhaps, tend to see years with 8 day cycles. Actually, this is often the case during the summer half of the year. That is my observation based on my years work on coastal storms and beach wave climates along the Atlantic coast. Use and look for weather spells of 7 days return in winter and 8 days or more in summer. This quasi-cycle, sometimes called the synoptic cycle, records the time between crossings of weather systems across the span of North America.
In winter you can, in some years, see a 3.5 day cycle with every other one being stronger. Keeping a calendar is the best way to become aware of runs of weather like these. Without a calendar you have to rely on getting washed out of every Saturday afternoon football game or enjoying the Tuesday afternoon youth soccer league quagmire. A neat little paper on the subject paper on the subject is by Jerome Namias of Scripps. December 1, 1964 - February 14, 1965 (76 day period) 39 weather stations in the Southeast U. S.
In the data above, the first number in parentheses is the average 39 weather station total rainfall in inches for the period of record. The second number is the average total days with precipitation for the 39 weather stations and for the period of record.
In the example above, the day on which the rain tends to fall depends somewhat on geography. Mobil to the South and West of Richmond gets its rain a day earlier. Richmond got its rain on Sunday and Mobile on Saturday. Runs and spells such as these require that the waves in the jet stream march their way across the continent in an orderly manner. If their speed of propagation is fast, the synoptic cycle could shorten to less than 7 days or if the propagation is slowed then the cycle could be longer than 7 days. Of course the propagation speed (celarity in Rossby's frequency equation) can go to ZERO a period of time. Than the weather becomes persistent from day to day. The Flood of 93 sure fits this bill. People - guys who know some meteorological jargon - start mumbling about blocking patterns in the circulation of the atmosphere. The wave or trough in the jet stream, in this case, parked itself over the upper Mississippi Valley and stayed put. The same weather day after day. The divergence of mass aloft gave support to the growth of thunderstorms. This spell was made up of a run of days [a month and a half] with essentially the same weather. Iowa got rain. Virginia got hot. Here is how it works. The wet air for the thunderstorms came from the subtropical North Atlantic, the Caribbean and the Gulf of Mexico. Solar calories were invested in evaporating water not in heat the air (84 F isn't so bad). The wet air made its way up the Mississippi Valley. This is an every year thing so far. With the dip in the jet stream conditions were very supportive of thunderstorms. With the lifting of this wet air condensation and raindrop formations released the calories as sensible heat and the potential temperature of the air got higher and higher. (Potential temperature is the temperature it would be if brought back to about sea level.) Now this high potential air moved east and sank down into the Bermuda High, which resided over the east and southeast as persistently as the thunderstorms of Iowa. This high potential temperature air at the ground in Virginia assured temperatures in the upper 90s. It was hot, humid and hazy. Tropical heat for Virginia by way of Iowa.
Spells of Weather -- the continuance of some type, or repetitive sequence, of weather over several days or weeks. (H.H. Lamb. 1972. Climate: Present, Past and Future. V. 1 Fundamentals and Climate Now. Methuen & Co. LTD.
Spells of Climate -- the continuance of some type, or repetitive sequence of climate over several years, decades or centuries. I have added to Lamb's definition because spells, in the sense of Lamb, seem to occur on all time scales! After all, it is spells of that should really impact ecosystem dynamics. Ask your favorite farmer and he will tell you that weather and climate comes in runs and spells. A run is usually a short sequence of the same kind weather day after day. Spells more often involve a sequence of weather e.g., rain every Sunday and fair every Wednesday. We could have a run of years with above average rainfall or a spell of years with something like 2 to 3 wet years every 5 years.