Tuesday, October 14, 2014

The Wettest Day in U.S. History

A few days ago we identified February 9, 1991, as the driest day in the Contiguous U.S. in modern history. The natural question to ask now is what the wettest day was. The previous post outlined several of the decision criteria in great detail for deciding what counts and what does not. Very briefly ...

Here are the rules:

1) Only WBAN ("first order") stations are evaluated.
2) Only the Contiguous ("Lower") 48 states are used.
3) A minimum of 100 stations must have submitted data for a day to be considered.

Results:

Right off the bast we encounter a major dilemma. What constitutes the wettest day? Is it the number of stations reporting precipitation or is it the amount of rain. When looking at the driest days, the dilemma does not exist. Instead of answering this question definitively, we will answer both questions.

Greatest Average Precipitation:

On September 16, 1999, the average WBAN station in the Contiguous U.S. reported 0.465" of precipitation. That works out to 24,243,346,168,800 gallons of water – or 80% of the volume of Lake Erie! This date barely edged out October 1, 1927 for first place. Given the low number of station observations back in 1927, our confidence level in their amount is fairly low. Table 1 shows the 10 largest daily average precipitation amounts on record.


Table 1. The ten largest nationwide average precipitation amounts on record for WBAN stations in the Contiguous U.S.

A map of the precipitation for September 16, 1999, is shown in Figure 2. As you can see, the spatial coverage of precipitation was not very large. In fact, only 1/4 of the reporting stations in the U.S. had any measurable precipitation at all. However, where it did rain, it rained a lot! Savvy readers of this blog will recognize the date in the title as corresponding to the landfall of Hurricane Floyd. Figure 2 shows the surface weather chart for September 16, 1999, from the NOAA map library.

Figure 1, Observed precipitation on September 16, 1999.


Figure 2. Surface weather map for September 16, 1999, from the NOAA map library.

Greatest Spatial Coverage:

On February 4, 1975, a remarkable 71.8% of WBAN station in the Contiguous U.S. reported measurable precipitation. This exceeds any other daily total on record. Table 2 shows the list of days with the greatest spatial coverage of precipitation in the Contiguous U.S. Not surprisingly, all of the days occur in the cool/cold months of the year. Colder temperatures generate more widespread, stratiform precipitation events than summer days.

The precipitation on February 4, 1975, fell as snow in many places. For example, Washington Dulles reported 6.0" of snow, Roanoke reported 6.3", Baltimore reported 4.4", and Philadelphia reported 2.0".

A map of the precipitation totals is shown in Figure 3. While a core of heavy precipitation is indicated in the Southeast, large areas of the country reported over 0.25" liquid (equivalent) precipitation. The surface and upper air charts (Figures 4 & 5) show that a large portion of the country was influenced by the low pressure near Nova Scotia and a developing subtropical jet stream.



Table 2. The thirteen most widespread precipitation events on record for WBAN stations in the Contiguous U.S.

Figure 3, Observed precipitation on February 4, 1975. 


Figure 5. Surface weather map for February 4, 1975, from the NOAA map library.


Figure 5. 500 millibar weather chart for February 4, 1975, from the NOAA map library.

Conclusion:

So which date was wetter? You be the judge. Both days top the list in their respective categories. However, many people lost their lives due to Hurricane Floyd and many billions of dollars of damage were incurred.


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