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What is the Wettest Month of the Year in the U.S.?

By: Christopher C. Burt, 7:06 PM GMT on October 10, 2014

What is the Wettest Month of the Year in the U.S.?

Brian Brettshneider of Borealis Scientific has done some impressive research concerning what the wettest calendar month of the year might be by employing data from 8,535 official NCDC sites from across the U.S. utilizing the latest 30 years of record (1981-2010). His conclusion is that June is, overall, most frequently the wettest month in the U.S. with 2,053 of the 8,535 sites reporting such. April, at the other end of the spectrum, reports only 76 sites of the 8,535 as their wettest month. This is a guest blog by Brian and below are the results of his research (both text and maps are his).

The Wettest Month in the U.S.

When is the wettest month of the year? The answer to that question depends greatly on where you live. If you live along the West Coast, the answer is probably one of the winter months. If you live in the Great Plains, the answer is likely one of the summer months. There are a myriad of reasons for this temporal and geographical disparity. They include: solar heating, jet stream position, sea surface temperatures, upper level patterns, and so on. In some instances, the distribution of annual precipitation is quite uniform and no straightforward seasonal pattern exists. In those cases, the variation between nearby stations can appear chaotic.

Methodology

To make the maps shown in the figures below, we used the NCDC 1981-2010 normal monthly precipitation values for 8,535 stations in the 50 states (this is the complete U.S. precipitation data set that NCDC has for the POR of 1981-2010). For each station, the month with the greatest normal precipitation was identified. There were 98 stations that had a tie for the wettest month and 1 station with a three-way tie for the wettest month. In the monthly maps that begin with Figure 4, stations that were tied are shown in any month where the tie exists.


Nationwide Wettest Months and Seasons

Before we begin with the map set, please take the opportunity to download a Google Earth file with all 8,535 stations that are color coded by wettest month. See this link.. Each station's monthly precipitation data can be viewed by clicking on the station's marker. EDITORS NOTE: This Google Earth file takes some time to download but is worth your patience! Zoom in to pick out the monthly precipitation data for all the sites used. Try and not let all the other Google Earth map ‘points of interest’ distract you!

The first 3 figures show the wettest climatological season (Figure 1) and the monthly peak precipitation values (Figures 2, 3).


Wettest Season

If we define the seasons by traditional climatological boundaries (Dec-Feb, Mar-May, Jun-Aug, and Sep-Nov) and add up the normal precipitation values for each of those time periods, we can easily identify which season is the wettest. Figure 1 shows the result of that analysis. Since there are only four categories, the seasonal boundaries are quite easy to discern. The West Coast has a winter precipitation peak and most of the rest of the country sees a summer peak – with the notable exception of an area bounded by Central Texas to the Ohio River Valley to the Deep South.



Figure 1. Dot map of wettest climatological season of the year. 8535 stations were used to make this map.


Wettest Month

Unlike the seasonal map, the combined monthly maps are much more difficult to visualize. That being said, it is important to distinguish between a January peak and a December peak (for example). Figures 2 and 3 show the station breakdown by calendar month – one as a dot map and the other as a color scale (choropleth) map. For example, the spring maximum noted in Figure 1 is shown in Figure 2 to be nearly entirely composed of stations whose wettest month is May. To the west of the May stations, a large area of June stations exist. While the seasonal map showed a significant break in the region, it is relatively minor when looking at the monthly data. However, a sharp transition to winter peak values exists to the east of the May stations. Again, having the monthly values is important for this type of assessment.




Figure 2. Dot map of wettest month of the year. 8535 stations were used to make this map. If a station had more than one wettest month, the month closest to its neighbors was chosen.




Figure 3. Continuous map of wettest month of the year. Map surface generated from points in Figure 2.

Individual Months

Instead of trying to decipher (often) complicated patterns, I though it useful to have an individual map for each month of the year. In the following 12 figures (Figures 4 through 15), each month of the year is pulled out individually. Only those stations with a peak precipitation value in that month are shown. If a station has a tie for the peak month, it is shown in all months for which a tie exists.



Figure 4. Stations where the wettest month of the year is January (n=277).




Figure 5. Stations where the wettest month of the year is February (n=332).




Figure 6. Stations where the wettest month of the year is March (n=263).




Figure 7. Stations where the wettest month of the year is April (n=76).




Figure 8. Stations where the wettest month of the year is May (n=1,881).




Figure 9. Stations where the wettest month of the year is June (n=2,053).




Figure 10. Stations where the wettest month of the year is July (n=1,073).




Figure 11. Stations where the wettest month of the year is August (n=916).




Figure 12. Stations where the wettest month of the year is September (n=438).




Figure 13. Stations where the wettest month of the year is October (n=339).




Figure 14. Stations where the wettest month of the year is November (n=333).




Figure 15. Stations where the wettest month of the year is December (n=653).


Intra-Annual Variability

In many cases, there are substantial differences between wet and dry months. Some stations in California and Alaska receive 60% of their annual precipitation in a three-month window. On the flipside, many stations in the Northeast and mid-Atlantic have precipitation evenly distributed across all months.

The final map in this blog post (Figure 16) shows the month-to-month variability in precipitation values across the year. To make this map we calculated the difference between the NCDC normal precipitation for each month and compared it to the value that would occur if each month received 1/12th of the annual precipitation. This type of assessment is called a ‘goodness-of-fit’ test. In this case we used the Chi Square goodness-of-fit-test.

As you can see, some areas have low month-to-month variability and others have quite a bit. I initially assumed that all cold regions (e.g., Northern Plains, Alaska, New England) would have low winter precipitation values due to the moisture capacity of the air being greatly reduced. However, that is only the case in the Northern Great Plains and Alaska – not in New England. The other quite surprising finding is the low month-to-month variability in the Great Basin. Perhaps this is an artifact of multiple synoptic-scale parameters in other regions all converging in this location.

There are far too many patterns in this map to describe. It is worthy of its own blog post another day!



Figure 16. Intra-annual variability based on monthly totals. Stations with consistent precipitation values throughout the year are shown in green and stations with large month-to-month variation (i.e., distinct wet and dry seasons) are shown in red.


KUDOS: A big thanks to Brian Brettschneider of Borealis Scientific in Anchorage, Alaska for all of the above information and maps. Brian maintains a blog with other interesting climate analyses. The link to his site can be found here.

Christopher C. Burt
Weather Historian

NOTE: I will be on a leave of absence for the next twelve days so my next blog post will be on October 22nd or 23rd.



Extreme Weather Precipitation Records

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

Another excellent blog entry. Thanks so much. Good luck on your time away.
Thanks Chris! In addition to the Google Earth file referenced in the text, here is a YouTube video that I put together a few months ago showing the annual march of precipitation throughout the year based on 1981-2010 NCDC normals: https://www.youtube.com/watch?v=pz2DsQeF_UM.
Excellent! I thought Florida might have some stations with their wettest average month in October.

The patterns from month to month are very interesting!
I have a question. Has there ever been a day in the Continental U.S. without any precipitation? I'm guessing no, just because the U.S. is so large
I would guess no. In fact it's rare for there not to be precipitation occurring at at least one station in the CONUS during the previous hour.
A great question and something I've always been interested in, so I have always kept my eye out for such. However, I have yet to find a 100% dry day coast-to-coast. It would be an almost impossible task to find out if this has ever happened, one would have to go back and look at every single day's precipitation summary for every single site in the entire CONUS. Plus there is the problem of just what constitutes 'a day' with four different time zones to deal with.

Quoting 4. nwobilderburg:

I have a question. Has there ever been a day in the Continental U.S. without any precipitation? I'm guessing no, just because the U.S. is so large
I guess a 'day' could be defined as being according to GMT. Finding out whether it's happened for a full day would be very difficult--even if all hourly observations were entered in a file, there are more errors in the METARS than I'd like, with spurious precipitation observations. You'd then have to look at the maps.
Quoting 7. weatherhistorian:

A great question and something I've always been interested in, so I have always kept my eye out for such. However, I have yet to find a 100% dry day coast-to-coast. It would be an almost impossible task to find out if this has ever happened, one would have to go back and look at every single day's precipitation summary for every single site in the entire CONUS. Plus there is the problem of just what constitutes 'a day' with four different time zones to deal with.




Chris, I looked at all of the daily records for all airport stations with WBAN numbers (formerly called 1st-order stations and numbers about 2,000 today) and came up with a few dates with a nationwide average that came out to 0.00" in the 48 Contiguous states (no AK or HI and restricted to days with at least 200 stations reporting]. The dates were: October 25th, 1965; October 28, 1966; and February 9, 1991. Then, when I went to xmACIS2 and searched for those individual dates, there were a few Cooperative stations in Florida with measurable precipitation on each of those dates (maybe other states too but I stopped searching when I saw precip). So, to answer the question, there appears to not be any dates (since 1920) with no precipitation anywhere in the country.
I hate to be "that guy," but Alaska is part of the "continental" US. It's not part of the contiguous US. So, if you're looking for days without rain throughout the "continental" US, you'd need to also look in southeastern Alaska, where several towns are among the rainiest places in the US outside Hawaii.

I know that "CONUS" is sometimes understood to mean "lower 48," but the original question in Comment 4 was about the "continental" US.

(I don't even live in Alaska any more, but when I did it used to drive us nuts when we'd see ads for things that said "valid only in the continental US" and when you tried to take advantage of the offer they'd tell you "that excludes Alaska and Hawaii." )

Sorry to quibble about this.

Quoting 7. weatherhistorian:

A great question and something I've always been interested in, so I have always kept my eye out for such. However, I have yet to find a 100% dry day coast-to-coast. It would be an almost impossible task to find out if this has ever happened, one would have to go back and look at every single day's precipitation summary for every single site in the entire CONUS. Plus there is the problem of just what constitutes 'a day' with four different time zones to deal with.



Quoting 10. EdwardinAlaska:

I hate to be "that guy," but Alaska is part of the "continental" US. It's not part of the contiguous US. So, if you're looking for days without rain throughout the "continental" US, you'd need to also look in southeastern Alaska, where several towns are among the rainiest places in the US outside Hawaii.

I know that "CONUS" is sometimes understood to mean "lower 48," but the original question in Comment 4 was about the "continental" US.


EdwardinAlaska, you are absolutely correct about the distinction between Continental U.S. and Contiguous U.S.I would guess that the question in Comment #4 was probably intended as a question regarding the Lower 48.As an Alaska resident myself, I am well aware of the difference and you will notice that I used the term Contiguous in Comment #9. Many friends of mine have PhDs in Geography and have never thought about the distinction.

Speaking of Alaska, they have only one day with no measurable precipitation with a minimum of 50 reporting stations. That date was March 13, 1997 (122 stations). Of course we could look at the Continental U.S. precipitation days by excluding Hawaii and all of the Southeast Alaska Islands; e.g.,Revillagigedo Island (Ketchikan), Baranof Island (Little Port Walter), etc.

Speaking of Hawaii, there might have been some Hawaii stations bundled into my earlier analysis. I'll try to re-run it later today without them to get a true Contiguous U.S. reading.
Great maps! :)

I am curious though as to why you changed the color scheme for the monthly legends between the month to month station maps and the month to month continuous maps.

In doing so things are indicated on the continuous maps that don't match up with any station on the month to month.

For example in the deep South, the area on the continuous map shows regions where march is the peak month bordered by places where April is the peak month.

Yet on the station map there are no stations that indicate April as the peak month. It goes from March to May or March to February for the most part with NO April stations in between.
Quoting 12. JohnnyMorales:

Great maps! :)

I am curious though as to why you changed the color scheme for the monthly legends between the month to month station maps and the month to month continuous maps.

In doing so things are indicated on the continuous maps that don't match up with any station on the month to month.

For example in the deep South, the area on the continuous map shows regions where march is the peak month bordered by places where April is the peak month.

Yet on the station map there are no stations that indicate April as the peak month. It goes from March to May or March to February for the most part with NO April stations in between.



Johnny, to answer your first question, one of the perennial problems in cartography is figuring out a color scheme. In this case, using the dot colors for the continuous map just looked horrible – so I changed it up to be more aesthetically pleasing. It was as simple as that.

For your second question, the continuous map is actually a surface that looks at the values of the 12 closest stations and weights them according to the distance from the target grid cell. This is necessary to assign values to cells that do not have any stations underneath them. Also, it prevents a salt-and-pepper effect.

As for Deep South Texas, there are no stations with a March or April wettest month on any of the maps Figures 2, 3, 6, & 7).
Here is a write-up on quite possibly the driest day in modern U.S. History: The Driest Day in U.S. History.
Other readers have argued over the meaning of 'day' and 'continental'. I would like to question the meaning of 'wet'.

The maps here are based on recorded monthly rainfall. I live in an area (North Carolina) with low monthly variation. The main cause of variation in perception of wetness is seasonal. The map says we get the most rainfall in summer. Summer rain falls mostly from thunderstorms. It can be torrential, but then the hot sun comes out, and soon the ground is dry again. Plants and trees are growing at their maximum rate, so they transpire a lot of water. We frequently get drought in summer. The grass turns brown. Lakes and streams dwindle. The ground is hard. Cracks appear in the clay.

In winter rain falls from stratiform clouds. Rainfall rates are generally light to moderate, but a storm may last all day. Puddles form. The ground gets soaked and squishy. Sometimes it freezes, or snow covers the grass, so that drying is impossible until it thaws. Plants and trees are dead or dormant, so they are drawing no water from the soil. Under the feeble winter sun drying is slow.. Seems to me that this is the definition of 'wet'. Put on your galoshes!
Quoting 15. Bogon:

Other readers have argued over the meaning of 'day' and 'continental'. I would like to question the meaning of 'wet'.


That is an excellent point Bogon. Unfortunately there is probably not a satisfactory answer. For example, which is a wetter city, New York or Seattle? Probably 9 out of 10 people would say Seattle. In reality, New York receives about 5" more precipitation annually. However, Seattle has more days with precipitation annually. Does that make it a wetter city? Over long periods of time – climatological lengths of time – raw precipitation totals will almost certainly be the main drivers of fluvial processes and ecological diversity. But there is certainly room for debate on the subject.
In North Carolina summer rain adds to the humidity, which aggravates the heat. Humidity is typically high throughout the summer here. The weather pattern is subtropical. The wind originates over the ocean. The air is wet, even if the soil is dry.

The situation reverses in winter. The north wind blows across arctic snowfields, where all of the moisture is frozen. The air is dry (and, aside from being cold, much more comfortable), even while the ground is squishy.

In western Washington, on the Olympic peninsula not far from Seattle, there is a rain forest. Eastern Washington is a desert. New York doesn't reach as far north as Washington (It's about the same latitude as Oregon.), but Buffalo is one of the snowiest places in the nation.

Let's call it 'nuance', or maybe just natural complexity. The world is not constructed for us to understand easily. Rainfall is something that we can easily measure. Thanks to you, we can look at the data. Understanding begins there.
.
A new record all-time hottest temperature in Brazil's capitol city, Brasilia, of 36.0 C / 96.8 F on October 15 is being reported.

Link
I recall some mention of a possible all time record high being set in Novosibirsk this past July, but haven't been able to find out if it was confirmed. Max Herrera doesn't have a listing for Novosibirsk on his page, but he can't list every reporting station.
A new record all-time hottest temperature in South America's most populous city, São Paulo, Brazil, of 37.8 C / 100.0 F on October 17, 2014 is being reported.

Link
Quoting 22. DCSwithunderscores:

A new record all-time hottest temperature in South America's most populous city, São Paulo, Brazil, of 37.8 C / 100.0 F on October 17, 2014 is being reported.

Link


Water Crisis Seen Worsening as Sao Paulo Nears ‘Collapse’

Sao Paulo residents were warned by a top government regulator today to brace for more severe water shortages as President Dilma Rousseff makes the crisis a key campaign issue ahead of this weekend’s runoff vote.

“If the drought continues, residents will face more dramatic water shortages in the short term,” Vicente Andreu, president of Brazil’s National Water Agency and a member of Rousseff’s Workers’ Party, told reporters in Sao Paulo. “If it doesn’t rain, we run the risk that the region will have a collapse like we’ve never seen before,” he later told state lawmakers.


By Vanessa Dezem Oct 21, 2014 12:46 PM CT
Hello Weatherhistorian,

I find it very interesting that May or June are the wettest months. I have always figured that the wettest months would be during the winter.

Dave
Quoting 25. ColoradoBob1:



Water Crisis Seen Worsening as Sao Paulo Nears %u2018Collapse%u2019

Sao Paulo residents were warned by a top government regulator today to brace for more severe water shortages as President Dilma Rousseff makes the crisis a key campaign issue ahead of this weekend%u2019s runoff vote.

%u201CIf the drought continues, residents will face more dramatic water shortages in the short term,%u201D Vicente Andreu, president of Brazil%u2019s National Water Agency and a member of Rousseff%u2019s Workers%u2019 Party, told reporters in Sao Paulo. %u201CIf it doesn%u2019t rain, we run the risk that the region will have a collapse like we%u2019ve never seen before,%u201D he later told state lawmakers.


By Vanessa Dezem Oct 21, 2014 12:46 PM CT


The president of So Paulo's water utility company said "According to the country's waterworks technology foundation FCTH, this type of weather condition will not occur for another 3,338 years".
weatherhistorian has created a new entry.
I looked for a while and couldn't find a state that has at least one location with its average wettest month covering every month of the year. There are a couple with 11 months but none with 12 unless Hawaii or Alaska have some that are hidden on smaller islands. I appreciate your research and this very interesting topic!