A Historical Perspective on U.S. Weather and Climate Extremes
WHAT HISTORY TELLS US ABOUT U.S. DAILY RAINFALL EXTREMES - METHODSBased on Hoerling et al. & Wolter et al. (2016), we utilize 987 meteorological stations in the contiguous US extracted from GHCN-D (Menne et al. 2012) with at least 100 years of non-missing daily observations during 1901–2014, as well as mostly complete data in 2015. The “RX1day” index (maximum 1-day precipitation), as defined by Sillmann et al. (2013), is computed at each station for all annual and bimonthly cases (base period 1901–80). We applied the generalized extreme value (GEV) distribution, known as the block or annual maxima approach for analysis of 20-yr precipitation events, using the Matlab NEVA package (Cheng et al. 2014). The lower confidence bounds (2.5th percentile) of the GEV-estimated return level for 20-yr events are applied in order to include all cases that might be considered of that intensity. We validated these results against the empirical estimates of the 20-yr events by ranking the annual and seasonal maxima at each station.
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for January-December 2018
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for November-December 2018
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for September-October 2018
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for July-August 2018
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for May-June 2018
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for March-April 2018
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for January-February 2018
January-February 2018 reported 19.7% of all 859 available stations with 20-yr daily extremes, 2nd only to 1997 (21.2%), but not associated with any significant trend. Regional counts were record-breaking in the Northeast with 51.0%, 5th highest on record in Ohio valley (33.3%), moderate in the Midwest (20.0% ties for 10th rank) and 21.7% in the South (12th). Only the South featured a significant trend through 2017 in this season (+4.4% per century; p=96%). There was one $1.1B disaster ( https://www.ncdc.noaa.gov/billions/events): A Nor'easter caused damage across many Northeastern states that clearly contributed to the record-breaking total of 51% stations with 20-yr extremes. This storm also resulted in 22 fatalities.
Annual daily extremes in excess of the lower estimate for 20-yr events for 2017
2017 kept 955 out of 987 stations sufficiently active (300 daily observations per year) to be included in this analysis. The final tally of stations with 20-yr daily precipitation extremes translated into a total of 17.5% for the year, narrowly beating out 2016, and second only to 2008 in the 117-year record. This strengthens a national upward trend that has kept the seven “wettest” years from 1990 onwards, and five of those during the last decade years, including and unprecedented THREE in a row since 2015 (2008, 2010, 2015, 2016, and 2017). The upward trend since 1901 is highly significant (p=99.99%) and amounts to an increase of 4.5% over a century (all trends computed through 2016). There were again large regional differences: the Ohio valley recorded a whopping 27.7% (second only to 2008), confirming a highly significant (p=99.99%) annual trend that amounts to about 6%/century; the Midwest came in 19.3% (10th), consistent with its upward trend of 6.2% per century (p=99.99%); the Southeast almost repeated its previous extreme year with 20.0% (10th), slowly increasing the likelihood that its upward trend of +3%/century is significant (p=88%); the South was extreme for the third year in a row (19.0%), continuing its upward trend of +5.6% per century (p=99.99%), and the Northern Rockies ended up with a 9th ranked 15.7% (upward trend of 2.2%/century has now reached p=95%). On the other hand, there was not a single region with exceptionally low counts.
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for November-December 2017
November-December 2017 recorded a national tally of “only” 7.5% of all 866 available stations with 20-yr daily extremes (for the first time dropping below 90% of the original set of 987), not nearly as high as during years during the last decade (only 2012 was lower since 2003). The national trend is still upward (+2.8%/century) through 2017 (p=97%). Only the Ohio Valley exceeded 10%, actually by quite a bit with 24.8%, ranked 11th highest since 1901. This confirms a highly significant regional upward trend (3.2% per century; p=99.8%). On the other hand, the neighboring Midwest had one the lowest counts on record with 0.9% (tied for 14th lowest), during a season with no significant trend. There was one Billion-Dollar Disaster ( https://www.ncdc.noaa.gov/billions/events) that picked up where October had left off: Southern California experienced extreme wildfire conditions in early December that culminated in the Thomas Fire complex ( https://en.wikipedia.org/wiki/Thomas_Fire). It burned 440 square miles to make it the largest wildfire in California’s recorded history, and killed two people while causing damages in excess of $2B.
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for September-October 2017
September-October 2017 reported the 7th highest national count of 20-yr daily extreme precipitation events for this season since 1901 (17.9% of 886 available stations). Combined with the 4th ranked total in 2016 (18.3%), this tally increased the overall trend from 3.5% per century through 2015 (p=99.9%) to 4.0% per century through 2017 (p=99.99%). The national outcome was anchored by diverse regional totals with 33.3% in the Southeast (4th highest), 25.0% in the Pacific Northwest (6th highest), and 24.0% in the Northeast (15th highest). Coming on the heels of 22.3% in 2016, the Southeast now shows a significant upward trend through 2017 (+5.2% per century; p=95%), compared to +4.3% per century through 2015 (p=87%). On the other hand, the Pacific Northwest is still not showing a significant upward trend, despite a record-breaking total of 45.1% in 2016 followed by this year's 25.0%. Finally, the Northeast confirms a strong upward trend that was already +8.9% per century through 2014 (p=99%), only to rise to +10.2% per century through 2017 ((p=99.8%), after recording 28.8% in 2016 as well. In contrast, the West only reported 2.9%, not nearly as bad as those 30 prior cases with only 0.0% (since 1901), but a far cry from 36.6% in 2016, and not associated with any significant trend. Other noteworthy regional totals include 19.7% in the Northern Rockies (11th highest) and 21.2% in the Midwest (9th highest). Both of these regions have not recorded a significant trend in this season (upward or downward). There were three "Billion-Dollar Disasters" ( https://www.ncdc.noaa.gov/billions/events) in this season, continuing an extremely damaging Atlantic hurricane season. Hurricane Irma hit the Southeast, in particular Florida, killing at least 97 people, and causing $51B in damages. The high regional fraction of station with 20-yr daily extremes in the Southeast (33.3%) is clearly associated with that storm in particular. Hurricane Maria devastated Puerto Rico with at least 65 fatalities as a direct consequence of the storm, and over $90B in damages, while its precipitation extremes are not part of the dataset used here. In contrast, the excessive rains of 2016-17 in Northern California were followed by severe drought conditions during the early fall that set the stage for extremely damaging wildfires in October and led to 4 of the 20 most destructive in the state’s history ( https://en.wikipedia.org/wiki/2017_California_wildfires), killing at least 40 people and causing damages in excess of $10B.
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for July-August 2017
July-August 2017 registered 13.5% of all 889 reporting stations with 20-yr daily extreme precipitation events (5th highest since 1901), cementing a significant upward trend for this region in the contiguous US through 2017 (+2.5% per century, p=99.99%). There were two regions with noteworthy total counts: the South (22.1%; 3rd highest, mostly associated with Hurricane Harvey) and the Ohio Valley (16.1%; tied for 10th highest). While the South has not shown a significant trend in July-August, the Ohio Valley had seen a significant upward trend through 2016 with +3.1% per century (p=99.6%). In contrast, the Pacific Northwest recorded not a single 20-yr daily extreme, tied for lowest rank with 13 other years, but not associated with any significant trend. There were two "Billion-Dollar Disaster" ( https://www.ncdc.noaa.gov/billions/events) during this season, but the first one was huge: Hurricane Harvey dropped record amounts of rain on Texas in late August, leading to at least 89 fatalities. Total damages in excess of $127B make this the biggest weather-related disaster in the US since Hurricane Katrina (2005), but also globally for 2017. The lack of rain in the northern plains since late spring arguably created the most severe drought since 1990 in that region, leading to losses in excess of $2B by the end of the year.
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for May-June 2017
May-June 2017 managed a national count of 9.6% of all 885 available stations, well below the median of more than 13% during the preceding decade, which was a result of a highly significant upward trend of 3.0% per century since 1901 (p=99.99%). Highest regional totals were found in the Ohio Valley (17.3%; 19th highest) and the Northeast (14.3%; 27th highest). The Ohio Valley tally is supported by a strong upward trend of 6% per century (p>99.9%), while the upward trend in the Northeast fails to reach even the 90% significance level. In contrast, the Southwest confirmed its marginally significant DOWNWARD trend (p=94%) with a total of 2.9% (23rd lowest). Two other regions came in under 5%: the Northern Rockies (3.5%, tied for 7th lowest, in fact, the lowest since 1958) and the Pacific Northwest (3.8%, 32nd lowest). The dry spring in this region set the stage for the subsequent Northern Plains drought and severe fire season. Five "Billion-Dollar Disasters" ( https://www.ncdc.noaa.gov/billions/events) include an extreme hailstorm in Denver (May 10th) that caused record-breaking damages of $2.2B (a record for Colorado), luckily without fatalities, Midwestern severe weather (three events in June, also no fatalities), and flooding in Missouri and Arkansas that took 20 lives without leaving much of a footprint in regional extreme tallies.
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for March-April 2017
March-April 2017 reported the 3rd highest tally of 20-yr daily precipitation events on record for this season (16.6% of 889 available stations). This came on the heels of the March-April 2016 record of 20.0%, consolidating a highly significant upward trend of +3% per century (p=99.7% through 2016 and p=99.9% through 2017). The national result was anchored by high regional totals in the South (26.8%; 4th highest), Ohio Valley (25.7%; 6th highest), and Pacific Northwest (19.6%; 11th highest). Two extreme March-April seasons in a row in the South (2016 was highest on record with 41.3%) anchor a statistically significant upward trend through 2017 (p=98%), while the upward trend through 2015 was not significant (p=85%). Both the Ohio Valley and the Pacific Northwest have not shown significant changes in their extreme tallies for this season since 1901. On the other hand, an outcome of 0% in the West ties for last place with 23 other years, but not associated with any significant trend. Four Billion-Dollar Disasters (https://www.ncdc.noaa.gov/billions/events) focused on the South and Ohio Valley, causing 8 fatalities. Three of these events were associated with severe weather, while the fourth was due to a hard freeze after unusually warm weather preceding it (in the Southeast).
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for January-February 2017
January-February 2017 had the 21st highest national count of 20-yr daily precipitation events for this season since 1901 (12.9% of 884 reporting stations which in turn are 90% of the original set of 987 stations). This national outcome was anchored by three regional totals at or above 20%: 25.5% in the Pacific Northwest (4th highest), 23.4% in the South (9th highest), and 20.0% in the Southwest (tied for 11th highest). Only the South showed a marginally significant upward trend through 2016 (p=93%), while the other two regions show no significant trends in this season. There were two Billion-Dollar Disasters ( https://www.ncdc.noaa.gov/billions/events) during this season. The first one was a tornado outbreak in the South that killed 24 people, in a region that also showed the aforementioned high total of 20-yr daily extremes. The second one was due to flooding rains in California, especially in February when the dam at Lake Oroville was almost breached and the city of San Jose suffered extensive flooding that also took 5 lives. While the West overall ended up with 16.7% coverage of 20-yr daily extremes, this was "only" tied for 20th highest place, nevertheless a change from a long-term (if insignificant) downward trend.
Annual daily extremes in excess of the lower estimate for 20-yr events for 2016
2016 kept 974 out of 987 stations sufficiently active (300 daily observations per year) to be included in this analysis. The final tally of stations with 20-yr daily precipitation extremes translated into a total of 17.4% for the year, second only 2008 in the 116-year record. This continues a national upward trend that has resulted in the six “wettest” years from 1990 onwards, with four of those during the last nine years (2008, 2010, 2015, and now 2016). The upward trend since 1901 is highly significant (p=99.99%) and amounts to an increase of 4.1% over a century (all trends computed through 2015). There were large regional differences across the map: the Midwest tied its record of 2010 (23.5%), riding an upward trend (+6% over the last century, with p=99.99%); the South was second only to 1998 with 23.5% as well and a similar trend (+5.2%/century, p=99.99%); the Southeast was in the same ballpark with 23.7% (but “only” ranked 8th; no significant upward trend), and the Northern Rockies ended with 15.7% (9th; upward trend of 2%/century at p=93%). On the other hand, the Southwest reported only 4.5% of its station with a 20-yr daily extreme, ranking the 20th lowest, consistent with a downward trend of 3%/century at p=93%).
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for November-December 2016
November-December 2016 came in at a modest 9.8% nationally (of 900 reporting stations), higher than the long-term average, but low compared to recent years that have been more supportive of a significant upward trend of +3.0% through 2015 (p=97%). Noteworthy regional tallies include the Southwest with 20.0% (17th highest) and the Northeast with 0.0% (one of 25 such cases since 1901). While the latter region has shown a significant upward trend since 1901 (+6.2% per century; p=96%), the former has a non-significant negative trend. Therefore, both outcomes were unexpected. A particularly noteworthy Billion-Dollar Disaster ( https://www.ncdc.noaa.gov/billions/events) were the firestorms that hit Tennessee near the Great Smoky National Park in November 2016, killing at least 14, thus becoming the deadliest wildfires in the eastern US since 1947 ( https://en.wikipedia.org/wiki/2016_Great_Smoky_Mountains_wildfires). The Ohio Valley region that includes Tennessee reported 6.1% coverage of 20-yr daily extreme rainfall during this season, below-average but not extreme, and also bucking a highly significant upward trend of +7.4%/century (p=99.6%).
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for September-October 2016
September-October 2016 registered the 4th highest national count of 20-yr daily precipitation events for this season since 1901 (18.3% of 911 stations). This confirms a highly significant (p=99.9%) upward trend of 3.5% per century, just like in July-August. The national count was anchored by the following regional totals: Pacific Northwest (45.1%, a new record), West (36.6%, 4th highest), Midwest (28.3%, 5th highest), and Northeast (28.8%, 9th highest). Only the Northeast had shown significant upward trends in this season prior to 2016 (p=99%). Hurricane Matthew skirted the southeastern seaboard around the 10th of October, leading to flooding rains that caused 49 fatalities and $10.6B in damages to tie with the flooding rains of Louisiana for the most damaging event of 2016 ( https://www.ncdc.noaa.gov/billions/events). The 20-yr daily extreme coverage for the Southeast was 22.3% (12th highest since 1901), confirming an upward trend of 3.9% per century that nevertheless only reached 86% significance through 2015.
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for July-August 2016
July-August 2016 had the 6th highest count of 20-yr events for this season since 1901 (13.1% of all 913 available stations), confirming a highly significant (p=99.9%) upward trend of +3.5% per century. Regional totals were highest in the Midwest where a record 27.4% of the stations were flagged in this manner. The Ohio valley was not far behind with 19.3% of its stations, 3rd highest on record. These two regions are also the ones with the most significant (both p=99%) upward trends in this season (4.6% and 2.7% per century, respectively, through 2015). In the South, flooding rains around August 12-15 caused 13 deaths in Louisiana along with estimated damages of $10.6B ( https://www.ncdc.noaa.gov/billions/events). This event ties with Hurricane Matthew (October 2016) for the highest weather-related damages of the year. Severe weather from the Northern Rockies eastward into the Northeast caused $1.6B in damages, but no fatalities nor any noteworthy 20-yr event totals.
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for May-June 2016
May-June 2016 recorded the lowest national tally (6.9% of 900 reporting stations) for this season since 1988, despite a highly significant upward trend (p=99.99%) that amounts to 3.2% per century through 2015. Only one of the regional totals exceeded 10% (13.4% in the Southeast, not even in the top 20 rankings, also with no significant trend to support it). On the other hand, the Southwest tally of 1.6% is the 14th lowest, fitting a long-term downward trend of almost -3%/century that is not yet statistically significant, even including 2016. Three Billion-Dollar Disasters ( https://www.ncdc.noaa.gov/billions/events) associated with severe weather and tornadoes ravaged the Great Plains and Ohio Valley, causing a total of 25 fatalities along with $4B in damages, despite comparatively low 20-yr precipitation extreme counts.
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for March-April 2016
March-April 2016 tallied the highest national count on record for this season’s 20-yr events (20.0% of all participating 909 stations). Even without including 2016, the upward trend of +2.7% per century was already significant (p=99%). The national result was anchored by record totals in the Northern Rockies (33.6%; mainly in Nebraska) and South (41.3%). These two regions show significant upward trends in this season (just over 4% per century each), although the upward trend in the South did not even reach the 90% significance threshold before 2015. On the other hand, the Northeast ended up with not a single station reporting a 20-yr event, equaling a 0% total for only the 17th time in March-April in 116 years. This bucks a highly significant (99.99%) upward trend of 8.8% per century through 2015. Texas and Louisiana suffered through multiple flooding events, most notably in the Sabine River basin on March 12th (5 fatalities; $2.3B) and around Houston on April 17-18 ($2.7; 8 fatalities). Among the total of six Billion-Dollar Disasters ( https://www.ncdc.noaa.gov/billions/events), two Texas hailstorms with combined damages of $5.9B are noteworthy as well (no fatalities).
Bimonthly daily extremes in excess of the lower estimate for 20-yr events forJanuary-February 2016
January-February 2016 had one Billion-Dollar Disaster (tornado outbreak in the eastern US that killed 10 people; https://www.ncdc.noaa.gov/billions/events). The tornado outbreak occurred during an unusual time of year, leading also to high daily extreme precipitation counts in the Northeast (33.3%; 6th highest since 1901), while the rest of the country had less than 15% for regional 20-yr event counts, not reaching either the top-10 or bottom-10 rankings since 1901. The national tally of 10.7% is based on 912 reporting stations. This season has shown no significant national or regional trends through 2015. However, the high number of daily extremes in the NE region in January-February 2016 anchored an upward trend that reached 94% significance for the 1901-2016 period of record.
Bimonthly daily extremes in excess of the lower estimate for 20-yr events for November-December 2015
November-December 2015 saw more daily 20-yr events in this season (29.0% of 913 reporting stations) than in any other season of any year on record. This was not expected, since the national trend had been a modest 2.8% per century through 2014 (p=92%). Most of the extreme daily totals were associated with a powerful storm system that swept northeastward from Texas through the Upper Midwest in late December, causing at least 50 deaths and $2.1B in damages (Source: https://www.ncdc.noaa.gov/billions/events). Regional records for the season were set in the Midwest (50.4%), Ohio Valley (39.9%), and South (36.6%). Other regions with notable totals were the Pacific Northwest with 21.3% (9th highest) and the Southeast with 25.3% (4th highest). None of these regional or national extremes were preceded by significant upward trends over the last century.
U.S. Climate Regions
Click on a region to display data for that region, or here for Continental U.S.
Continental U.S. 20yr Precipitation ExtremesExtreme Weather and Climate datasets are compiled for the US Climate Regions and the continental US. Raw data can be accessed here.
Totals by time period:
Annual Totals by period
Data Table Values in the table are the percent of available stations that exceeded the 20yr Precipitation for each time period.
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- Wolter, K., M. Hoerling, J.K. Eischeid, and L. Cheng, 2016: What History Tells us about 2015 US Daily Rainfall Extremes (in "Explaining Extremes of 2015 from a Climate Perspective"). Bull. Amer. Meteor. Soc., 97, S9-S13. doi:10.1175/BAMS-D-16-0166.1 PDF
- Explaining extreme events in 2015 from a climate perspective - Press Conference
NOAA's Martin Hoerling explains the context for precipitation extremes