Rainfall Feast Or Famine: Part 1
I recently was asked a great question about rainfall from a reader asking "does it seem like every time it rains around here anymore, it's a 2-3 inch type deal every single time out?"
Well, even though the forecasts for this weekend busted badly (thank you computer guidance, weaker low pressure, and stronger high pressure for the forecast bust!), it still is an interesting question. It seems that we have seen some significant "feast or famine" cycles in rainfall where we go from getting little or no rainfall to getting significant, ark-building rains for weeks at a time in short order. Over the next couple of days, I'm going to take a look at Philadelphia's last 30 years and see how often they have been hit by excessive rainfall. I have to throw major caution out there that this is a single data point assessment going back 30 years and that with summer rainfall some locations can be "victimized" by more significant rainfall than others, so any of the information that's posted in here needs to be taken for what it is worth -- just one data point out of tens of thousands of climate sites in the country.
Going back to 1978, Philadelphia has seen one inch or more of rain fall on 318 calendar dates, including three so far in 2008. That averages out to 10.5 one inch rainfall dates per year. The chart below shows the year-by-year progression (in bars with the left axis line being your guide) of the one inch rainfall days over the past 30 years. The years with the most one-inch rainfall dates are 1983 (21 such dates), 1996 (17 such dates), and 1979 (16 such dates). The years with the fewest were 1997 (4 such dates), 1990 (4 dates), and 1998 (5 dates). 2006 and 2007 both had 12 one-inch rainfall dates in Philadelphia, just slightly above the 30 year average.
You probably see the second line going across the graph. That symbolizes the number of days per year with 2" of rain. That line corresponds to the axis on the right hand side. There have been 46 of those events going back to 1978, about 1.5 per year. Some years we see no two inch rainfall events at all. This most recently occurred in Philadelphia in 2003. However, the "feast" concept really dates back to the recent uptick in excessive rainfall we saw in 2004, 2005 (two dates per year with 2" of rain or more), and 2006 where we saw five two inch rainfall events, including two just four weeks apart in June (where we saw significant flooding along the Schuylkill and Delaware basins).
The thought of "feast or famine" also has to do with how far apart these rainfalls occur. For instance, 2007 had 12 one inch rainfall dates. Seven of them occurred before May 1st, with six of them occurring in March and April! In 2006, four one inch events occurred in a ten day span between August 27th and September 5th with another three one inch events occurring between October 11th and November 8th. Those of you who remember the 1999 flooding with Hurricane Floyd might not remember that there were eight one inch rainfall events between August 14th and October 4th, with four occurring in a two week span in September (including Floyd's rainfall on September 16th). When we get into patterns in the atmosphere that are conducive for moist storms bringing copious rainfall we typically see a repetitive nature to that pattern. These typically occur when the Bermuda High sets up shop and prevents troughs from moving over the top of us in the atmosphere. When we are stuck between the ridge to our east and south and the trough to our west, a southwest wind flow in the atmosphere can transport copious amounts of moisture up from the Tropics...which leads to our rains.
Part two of this post will focus on the cyclic nature in more detail, showing how excessive rainfall (just like the lack of rain in general) can sometimes be a tough pattern to break.
Well, even though the forecasts for this weekend busted badly (thank you computer guidance, weaker low pressure, and stronger high pressure for the forecast bust!), it still is an interesting question. It seems that we have seen some significant "feast or famine" cycles in rainfall where we go from getting little or no rainfall to getting significant, ark-building rains for weeks at a time in short order. Over the next couple of days, I'm going to take a look at Philadelphia's last 30 years and see how often they have been hit by excessive rainfall. I have to throw major caution out there that this is a single data point assessment going back 30 years and that with summer rainfall some locations can be "victimized" by more significant rainfall than others, so any of the information that's posted in here needs to be taken for what it is worth -- just one data point out of tens of thousands of climate sites in the country.
Going back to 1978, Philadelphia has seen one inch or more of rain fall on 318 calendar dates, including three so far in 2008. That averages out to 10.5 one inch rainfall dates per year. The chart below shows the year-by-year progression (in bars with the left axis line being your guide) of the one inch rainfall days over the past 30 years. The years with the most one-inch rainfall dates are 1983 (21 such dates), 1996 (17 such dates), and 1979 (16 such dates). The years with the fewest were 1997 (4 such dates), 1990 (4 dates), and 1998 (5 dates). 2006 and 2007 both had 12 one-inch rainfall dates in Philadelphia, just slightly above the 30 year average.
You probably see the second line going across the graph. That symbolizes the number of days per year with 2" of rain. That line corresponds to the axis on the right hand side. There have been 46 of those events going back to 1978, about 1.5 per year. Some years we see no two inch rainfall events at all. This most recently occurred in Philadelphia in 2003. However, the "feast" concept really dates back to the recent uptick in excessive rainfall we saw in 2004, 2005 (two dates per year with 2" of rain or more), and 2006 where we saw five two inch rainfall events, including two just four weeks apart in June (where we saw significant flooding along the Schuylkill and Delaware basins). The thought of "feast or famine" also has to do with how far apart these rainfalls occur. For instance, 2007 had 12 one inch rainfall dates. Seven of them occurred before May 1st, with six of them occurring in March and April! In 2006, four one inch events occurred in a ten day span between August 27th and September 5th with another three one inch events occurring between October 11th and November 8th. Those of you who remember the 1999 flooding with Hurricane Floyd might not remember that there were eight one inch rainfall events between August 14th and October 4th, with four occurring in a two week span in September (including Floyd's rainfall on September 16th). When we get into patterns in the atmosphere that are conducive for moist storms bringing copious rainfall we typically see a repetitive nature to that pattern. These typically occur when the Bermuda High sets up shop and prevents troughs from moving over the top of us in the atmosphere. When we are stuck between the ridge to our east and south and the trough to our west, a southwest wind flow in the atmosphere can transport copious amounts of moisture up from the Tropics...which leads to our rains.
Part two of this post will focus on the cyclic nature in more detail, showing how excessive rainfall (just like the lack of rain in general) can sometimes be a tough pattern to break.
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