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.

Year-to-Date Precipitation Update (Oct. 11)

How has aggregate precipitation looked so far in 2014?

Daily Average:

The chart below (Figure 1) shows the long-term daily average precipitation in the Contiguous U.S. along with the 2014 values (through October 5th) in red. In addition, the long-term 10th and 90th percentile lines are shown. In theory, each month should have three days exceed the 90th percentile and three days below the 10th percentile. If you add up the 2014 precipitation, it comes out to 25.25". The long-term average should be 25.52". Therefore, 2014 is just barely below average year-to-date. Nearly two months passed (mid-January to mid-March) without a single day exceeding the 90th percentile.

Perhaps more interesting are the normal and percentile curves. June is the wettest month of the year in the Contiguous U.S. but the winter months have the greatest variability. This is likely a result of summer precipitation occurring as thunderstorms on fewer days (with less spatial coverage) and winter precipitation being more stratiform (less intensity but greater spatial coverage).

The chart in Figure 2 shows the year-to-date daily precipitation averages for Alaska. Similar to Figure 1, the 90th and 10th long-term percentile lines are shown in Figure 2. A large number of days between June 1st and August 15th had precipitation values greater than the 90th percentile. Conversely, the period from mid-March through the end of May saw many days with low precipitation values.The summed daily precipitation values for 2014 are 24% higher than the long-term average. Therefore, we can confidently state that 2014 has been significantly wetter than average in Alaska through October 5th. A note of caution about the Alaska chart (Figure 2). It is not spatially weighted. Areas with a higher density of stations (e.g., Anchorage area, Fairbanks area, and all of Southeast) are disproportionately represented. 

Figure 1. Long-term average daily precipitation for the Contiguous U.S. with 2014 data (though October 5th) shown in red. 90th and 10th long-term percentiles are also shown.

Figure 2. Long-term average daily precipitation for the Alaska with 2014 data (though October 5th) shown in red. 90th and 10th long-term percentiles are also shown.

Year-to-Date:

The summary snapshot of the year-to-date precipitation is shown in Figure 3. Areas from Texas and Oklahoma westward to California and northward are running far below their annual normal precipitation through October 11th.Throughout the rest of the country, departures from normal are generally nondescript with the exception of portions of Alaska and Hawaii which are significantly wetter than normal. The nationwide leader in year-to-date precipitation departure among WBAN ("first order") stations is Molokai, Hawaii (203% of normal). In second place is Fairbanks, Alaska (179% of normal). The third place station is Twin Falls, Idaho (170% of normal). In last place is Imperial Beach, California (16% of normal). As noted in the earlier section, the nationwide average is only slightly below normal. The average grid cell value in Figure 2 is 98%.

Figure 3. Percentage or normal precipitation though October 11th.

The Driest Day in U.S. History

** Update: Here is a posting about the wettest day(s) in U.S. history **

Has there ever been a day when no precipitation was observed in the Contiguous U.S.? It would be pretty remarkable if no precipitation fell over three million square miles. Let's take a look. Before we answer this question, we need to set some parameters as to what exactly a dry day is. So, here are the rules:

Rules:

1) There is a fairly robust data set to use at the National Climate Data Center based on single calendar days. A very, very important caveat to this is the observation time for precipitation. Precipitation at Cooperative stations is usually observed between 7:00 a.m. and 8:00 a.m. This causes problems when precipitation falls between the observer's bedtime and the observation time the next morning. Which day does the precipitation get assigned to? Therefore, we will not use Cooperative stations in this analysis. Unlike Cooperative stations, WBAN stations always report data from midnight to midnight local standard time. In addition, they are almost always located at manned airport towers, so providing reliable data is especially important. Therefore, we will restrict the analysis to WBAN stations. Currently, there are about 1,200 WBAN stations across the U.S. (e.g., Chicago O'Hare Airport, Los Angeles Intl. Airport, etc.).

2) We are restricting the analysis to the Contiguous (Lower) 48 states. Stations in Alaska and Hawaii are often too sparse to capture significant patterns prior to 1950. In addition, there are locations in each state that receive precipitation nearly every day of the year. Therefore, Alaska and Hawaii are excluded from the analysis – although they are shown on the maps. (Note: None of the 139 stations in Alaska reported measurable precipitation on March 13, 1997 – the only time on record.).

3) A minimum number of stations must report observations on a day to be considered. If, for example, only 3 stations were used, many days per year would report no precipitation. So what should the minimum number be? We have arbitrarily decided that 100 stations is a minimum threshold for consideration. This ensures a wide enough network of stations to capture synoptic-scale weather event across the Contiguous U.S. The 100 station threshold means that every day since 1920 can be assessed (and some days back to 1915).

Let's run the numbers:

First, a word about data sources. My primary data source is the GHCN v. 3 database maintained by the National Climate Data Center. It is not a perfect database in that much station data are, unfortunately, not included. For example, there are approximately 40% more stations in the NOAA Regional Climate Center's xmACIS online application that report precipitation totals than in the GHCN database.

To the best of my knowledge, there is not a mechanism for programmatically accessing xmACIS so I am left with a systematic evaluation of GHCN data and then supplementing it with xmACIS data on a case-by-case basis.

After looking at all 18 million observations in the GHCN database and sorting them by date, we identified 10 calendar days when the combined total of all precipitation in the Contiguous U.S. was 0.10" or less. Table 1 shows those dates.

Table 1. List of lowest precipitation totals in the GHCN database and the number of stations reporting precipitation totals. Trace values are counted as 0.00" for mathematical purposes.

The "winner" is February 9, 1991. A combined 0.02" was reported for the entire Contiguous 48 states! Not only was it the winner, but the other stations that were in 2nd through 10th places all had fewer than 165 observing stations. This vastly increases the confidence that February 9, 1991, was indeed the driest day in modern U.S. history.

As stated earlier, the xmACIS application has a larger number of stations than the GHCN database that report precipitation so we manually looked at the xmACIS data on a state-by-state basis. A total of 714 WBAN stations in xmACIS reported precipitation on February 9, 1991. The grand total of all 714 stations in xmACIS was also 0.02". So there you have it.

Maps:

It turns out that 2 stations each reported 0.01" of precipitation on February 9, 1991, in the Contiguous U.S. Those stations were Lemoore Reeves NAS, California, and Russell Municipal Airport, Kansas. An additional 41 stations reported a Trace of precipitation – but those are mathematically treated as 0.00" for summation purposes. Figure 1 shows the location of all stations that reported a precipitation total (including 0.00"). Alaska and Hawaii are shown on the map but are not included in the analysis.

Figure 1. WBAN stations reporting precipitation in xmACIS on February 9, 1991 (n=714).

As we stated in the Rules section, Cooperative stations were not included. Figure 2 demonstrates why. A small number of stations along the southeastern coast reported up to 0.50" of precipitation on February 9, 1991. However, a closer inspection of the Cooperative observer forms and satellite imagery shows that precipitation fell before midnight on the 8th in several places but was not observed and recorded until the next day

Figure 2. All stations reporting precipitation in xmACIS on February 9, 1991 (n=8,055).

The satellite image in Figure 3 is from the GOES 7 infrared instrument and was taken at midnight Eastern Standard Time (EST) on February 8/9, 1991. A complex of storms is seen leaving the North and South Carolina coasts. These storms produced up to 0.50" of rain just before midnight but were recorded in the following day's precipitation total. An image from the same satellite at 3 p.m. EST is shown in Figure 4.

In contrast to the southeastern coast, the precipitation dots in Figure 2 in the Pacific Northwest appear to be valid based on the perspective shown in Figure 4. Still, the decision to only use WBAN stations appears reasonable given the uncertainty of Cooperative precipitation totals due to the time of observation issue. It is safe to assume that the time of observation issue does not vary across the length of the historical record; so therefore, any one date is just as likely to have rainfall amounts assigned to an adjacent day, of just plain missed, proportionate to the amount actually captured by the WBAN stations.

Figure 3. GOES 7 image of the Contiguous U.S. on February 9, 1991 at 6:01 UTC.

Figure 4. GOES 7 image of the Contiguous U.S. on February 9, 1991 at 21:01 UTC.

Conclusion:

I must admit to a slight disappointment to learn that there were no completely dry days to be found in the climate record. Oh well. That being said, is this methodology perfect? Not really. But it probably came to the correct conclusion. February 9th, 1991 was the driest day in modern history for the Contiguous U.S.