NCEI versus PSR compared

For many years I have studied on and off the NOAA US divisional statistics of temperature and precipitation, because climate statistics — like all statistics — are always interesting to me if at times capable of making me very angry when runaway climate change is shown.

What is interesting is that NOAA has two sets of precipitation (and temperature) sets for each of the contiguous US states. One is published by the National Center for Environmental Information and the other by the Physical Sciences Laboratory. Over the period I have used them to study climate, I have assumed the two to be essentially completely interchangeable for the purpose of statistical tables, although for graphical purposes I can prefer one or the other (I prefer NCEI for precipitation maps and PSL for temperature maps) depending on the precise maps offered.

When I realised NCEI was offering comparisons for all months, I decided to try to look at each continental state (this excludes Hawaii) and tabulate its wettest and driest month since record began. However, when I tried to do this on the PSL site, I found that there appeared to be some differences between the two. To test this, I tabulated the wettest and driest months for all states east of the Continental Divide. For states west of the Divide there are generally sufficient months with zero or negligible precipitation that I decided only to tabulate each state’s wettest month. I have included both sets of units for convenience of readers not familiar with US units.

State	Wettest month						Driest month (omitted for states west of Divide)
	NCEI			PSL			NCEI			PSL
	Month	in	mm	Month	in	mm	Month	in	mm	Month	in	mm
Alabama	July 1916	17.59	446.8	July 1916	18.13	460.5	October 1963	0.03	0.8	October 1963	0.03	0.8
Alaska	September 1925	7.59	192.8				February 1979	0.72	18.3
Arizona	January 1993	5.22	132.6	January 1993	5.72	145.3
Arkansas	October 2009	14.13	358.9	October 2009	13.38	339.9	October 1963	0.13	3.3	October 1963	0.18	4.6
California	January 1995	12.50	317.5	January 1909	13.50	342.9
Colorado	April 1900	5.45	138.4	April 1900	5.13	130.3	November 1904	0.03	0.8	November 1904	0.03	0.8
Connecticut	October 2005	15.92	404.4	October 2005	16.02	406.9	March 1915	0.28	7.1	March 1915	0.27	6.9
Delaware	August 1967	12.58	319.5	September 1935	11.50	292.1	October 1963	0.12	3.0	October 1963	0.13	3.3
Florida	August 1898	13.49	342.6	September 2004	14.25	362.0	November 2016	0.21	5.3	October 1940	0.16	4.1
										November 2016
Georgia	July 1916	13.46	341.9	July 1916	14.06	357.1	October 1963	0.06	1.5	October 1963	0.04	1.0
Idaho	December 1964	8.52	216.4	December 1964	6.72	170.7
Illinois	September 1926	9.62	244.3	September 1926	10.18	258.6	October 1964	0.18	4.6	October 1964	0.15	3.8
Indiana	January 1950	10.11	256.8	January 1950	10.01	254.3	March 1910	0.21	5.3	March 1910	0.22	5.6
Iowa	July 1993	10.45	265.4	July 1993	10.75	273.1	October 1952	0.02	0.5	October 1952	0.02	0.5
Kansas	May 2019	10.51	267.0	May 2019	10.22	259.6	January 1986	0.00	0.0	January 1986	0.00	0.0
							November 1989	0.00	0.0	November 1989	0.00	0.0
Kentucky	January 1937	15.16	385.1	January 1937	15.49	393.4	October 1963	0.14	3.6	October 1963	0.14	3.6
Louisiana	May 1907	15.09	383.3	October 2009	16.41	416.8	October 1952	0.03	0.8	October 1952	0.03	0.8
Maine	October 2005	11.26	286.0	October 2005	12.62	320.5	March 1915	0.46	11.7	October 1947	0.37	9.4
Maryland	August 1955	11.85	301.0	July 1945	12.68	322.1	October 1963	0.15	3.8	October 2000	0.12	3.0
Massachusetts	October 2005	14.03	356.4	October 2005	13.99	355.3	March 1915	0.09	2.3	March 1915	0.11	2.8
Michigan	September 1986	8.72	221.5	June 1943	7.22	183.4	March 1910	0.26	6.6	March 1910	0.22	5.6
Minnesota	June 2014	8.07	205.0	June 2014	8.13	206.5	December 1913	0.07	1.8	December 1913	0.06	1.5
Mississippi	April 1991	14.36	364.7	December 1982	15.62	396.7	October 1963	0.07	1.8	October 1963	0.05	1.3
Missouri	September 1993	11.31	287.3	June 1928	11.69	296.9	January 1986	0.10	2.5	January 1986	0.21	5.3
Montana	June 1944	6.38	162.1	June 1944	6.40	162.6	October 1987	0.11	2.8	October 1987	0.12	3.0
Nebraska	June 1967	8.20	208.3	June 1967	8.64	219.5	November 1914	0.01	0.3	November 1914	0.01	0.3
										November 1939	0.01	0.3
Nevada	January 1916	3.65	92.7	January 1916	3.63	92.2
New Hampshire	October 2005	13.65	346.7	October 2005	13.23	336.0	March 1915	0.19	4.8	March 1915	0.22	5.6
New Jersey	August 2011	15.67	398.0	August 2011	15.79	401.1	October 1963	0.25	6.4	October 1963	0.25	6.4
New Mexico	September 1941	5.48	139.2	July 1911	4.60	116.8
New York	June 1972	8.58	217.9	October 2005	9.38	238.3	October 1963	0.34	8.6	October 1963	0.41	10.4
North Carolina	September 1999	13.22	335.8	August 1901	13.00	330.2	October 2000	0.08	2.0	October 2000	0.06	1.5
North Dakota	July 1993	7.97	202.4	July 1993	8.11	206.0	November 1939	0.03	0.8	November 1939	0.03	0.8
Ohio	January 1937	9.69	246.1	January 1937	9.42	239.3	October 1963	0.23	5.8	October 1963	0.24	6.1
Oklahoma	May 2015	14.44	366.8	May 2015	14.40	365.8	January 1986	0.03	0.8	January 1986	0.04	1.0
Oregon	December 1964	13.05	331.5	December 1964	16.33	414.8
Pennsylvania	June 1972	11.22	285.0	June 1972	11.63	295.4	October 1963	0.16	4.1	October 1963	0.22	5.6
Rhode Island	March 2010	14.75	374.7	March 2010	14.76	374.9	March 1915	0.14	3.6	March 1915	0.14	3.6
South Carolina	July 1916	14.41	366.0	August 1901	23.60	599.4	October 2000	0.02	0.5	October 2000	0.07	1.8
South Dakota	May 1962	6.78	172.2	May 1962	6.88	174.8	November 1939	0.01	0.3	November 1939	0.01	0.3
Tennessee	January 1937	14.53	369.1	January 1937	13.41	340.6	October 1963	0.03	0.8	October 1963	0.03	0.8
Texas	May 2015	9.05	229.9	May 2015	11.15	283.2	October 1952	0.03	0.8	October 1952	0.02	0.5
Utah	October 1972	3.69	93.7	October 2004	4.10	104.1
Vermont	November 1927	10.61	269.5	November 1927	10.48	266.2	March 1915	0.32	8.1	March 1915	0.35	8.9
Virginia	September 1999	13.84	351.5	September 1999	10.37	263.4	October 2000	0.09	2.3	October 2000	0.03	0.8
Washington	December 1933	16.61	421.9	December 1933	24.36	618.7
West Virginia	November 1985	9.53	242.1	September 2018	9.48	240.8	October 1963	0.24	6.1	October 1963	0.23	5.8
Wisconsin	September 1986	8.38	212.9	September 1986	8.43	214.1	February 1958	0.11	2.8	February 1969	0.10	2.5
							February 1969	0.11	2.8
Wyoming	May 1908	5.09	129.3	May 1908	5.83	148.1	November 1939	0.02	0.5	November 1939	0.02	0.5

If we look closely, we see that there are a few cases where NCEI and PSR do differ substantially (shaded in red). Almost all of these are for states’ wettest months — the driest months vary less between the two sets and there are many fewer months listed even comparing only the states east of the Divide.

In the cases of Washington state and Idaho, whilst the identity of the state’s wettest-ever month is definite, the estimated actual amount of precipitation varies by as much as a quarter.

In the cases of California and West Virginia, the precipitation figures for the state’s two wettest months are very nearly equal and which is the wettest may not be perfectly clear.

The reason for these discrepancies if not perfectly clear. In the late 2000s, state precipitation measurements were significantly updated to take into account much higher precipitation at high altitudes (similar updates occurred for Tasmania in Australia, and for the United Kingdom). Differences in this update for mountainous regions may be important, but a variety of other factors like:

1. difficulties measuring tropical cyclone rainfall in the South Atlantic States and New York
2. difficulties measuring lake-effect precipitation in Michigan

may also be important explaining the observed differences. I have not yet researched possible causes of the differences between NCEI and PSL, but still find it interesting to note that they exist.