Drought Misery Index

With the ongoing drought, I came up with the Drought Misery Index (DMI), which is similar to the Misery Index for the economy. Here is the formula for Drought Misery Index.

Palmer Drought Severity Index (PDSI) + Precipitation (PCP) = Drought Misery Index

Example:
-8.00 + 1 = -7.00

The more negative the DMI, the worse the drought is. Here is the current DMI, based on September outcome.

America
-2.78
-4.78 (PDSI) + 2.00 (PCP)
Lowest: -6.01 August 1934
Highest: +9.91 May 1983

Texas
-2.49
-3.78 (PDSI) + 1.29 (PCP)
Lowest: -7.22 September 1956
Highest: +15.51 October 1919

Upper Texas Coast
0.38
-2.05 (PDSI) + 2.43 (PCP)
Lowest: -4.64 July 1925
Highest: +20.98 October 1949

All data is from the Division Data and it can change as more data is put in and averaged out.

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How Bad Is This Drought?

It has not rained much in Southeast Texas. In fact, we have not seen over 1 inches of rain since January and it has been 4 months in a row that the Upper Texas Coast has seen less than 1 inch per a month. In fact, October to May rainfall total is the driest on record as shown on the list of top 10 driest October to May rainfall total in the Upper Texas Coast.

1.) 2010-2011 14.17
2.) 1924-1925 15.20
3.) 1955-1956 16.60
4.) 1908-1909 16.74
5.) 1950-1951 17.36
6.) 1916-1917 17.90
7.) 1917-1918 17.92
8.) 1901-1902 18.10
9.) 1939-1940 18.67
10.) 1962-1963 19.26

However, when it comes to Palmer Drought Severity Index (PDSI), it is not the worst. PDSI uses uses temperature and rainfall information in a formula to determine dryness. 0 is considered normal, while negative value is drier, while positive value is wetter. Here is the list of lowest or worst PDSI for Upper Texas Coast.

1.) 1917-1918 -4.86
2.) 1924-1925 -3.85
3.) 1955-1956 -3.68
4.) 1999-2000 -3.57
5.) 1954-1955 -3.37
6.) 1916-1917 -3.30
7.) 1950-1951 -2.98
8.) 2005-2006 -2.93
9.) 1951-1952 -2.90
10.) 1915-1916 -2.87

The drought of 1917-1918 is the worst even to this very day. It is legendary in its own right. The 1950s was the longest drought in Texas as it lasted for 7 years. October 2005 to May 2006 has a lower PDSI than right now. The winter of 2005-2006 was abnormally warm, which would have a more negative PDSI. The PDSI for October 2010 to May 2011 in the Upper Texas Coast is -1.54, which is serious. To compound it, there are more people living in Texas today than in the late 1910s and 1950s. More people use water for drinking, farming, and watering their yards. It is going to worsen the drought as water supplies are becoming more strained.

If we do not see rain any time soon this summer, the drought will only get worse and easily be in the top 10 worst droughts for Upper Texas Coast in terms of PDSI. Also, if it is dry, it will get hotter as drier grounds is more favorable for heat. Moisture moderates air temperature. It is no surprise that wetter years are cooler than compared to drier years, which are warmer.

Here is a chart of Upper Texas Coast October to May PDSI from 1895 to 2011.

Here is a chart of Upper Texas Coast October to May rainfall total from 1895 to now.

It shows that droughts come and go. It is cyclical in nature. Here is the top 10 highest October to May PDSI for Upper Texas Coast.

1.) 1919-1920 4.35
2.) 1991-1992 3.94
3.) 1941-1942 3.60
4.) 1973-1974 3.47
5.) 1992-1993 3.47
6.) 1913-1914 3.35
7.) 1979-1980 2.96
8.) 1960-1961 2.85
9.) 2006-2007 2.71
10.) 1923-1924 2.41

Top 10 wettest October to May rainfall for Upper Texas Coast.

1.) 1991-1992 46.97
2.) 1940-1941 45.29
3.) 1913-1914 44.01
4.) 1994-1995 43.40
5.) 1925-1926 42.79
6.) 1943-1944 42.41
7.) 1949-1950 42.41
8.) 1992-1993 42.26
9.) 2006-2007 42.23
10.) 1990-1991 41.82

Some of the highest PDSI and rainfall came after droughts, so there is hope in the end. I do remember the early 1990s being very wet.

Here is a NCDC of May droughts for all of Texas.

The current drought for all of Texas is one of the worst droughts in history, along side with 1918 and 1956. If Texas does not get rain, it will be a serious problem for everyone.

As for why we have a drought, it is because of La Nina cycle of El Nino Southern Oscilliation (ENSO). This past October to May was La Nina. La Nina is the opposite of El Nino, in which the Pacific Ocean water off the coast of South America is cooler than normal. The cooler water pushes the jet stream further north, which causes storm systems to go further north than usual. That means all the energy is further away from Texas. In an El Nino, the jet stream is further south, which means storm travel further south than normal, which means a wetter year. Some of the wettest years happened in El Nino years. The correlation between La Nina and rainfall and PDSI is strong.


Pearson Correlation
r = 0.41
p-value = < 0.01


Pearson Correlation
r = 0.42
p-value = < 0.01

The correlation value and p-value are strong, so there is a significant relationship between ENSO and PDSI and rainfall.

Data Source
NOAA-Divisional Data

El Nino/La Nina Data
FSU-Japan Meteorological Agency ENSO SST

October To April Rainfall

This past October to April has been very dry, easily one the driest on record. The culprit is La Nina, which causes the jet stream to go further north, carrying storms further away. I did a correlation between October to April rainfall total and May rainfall total and only April and May rainfall totals in the Upper Texas Coast. It is from the NOAA Divisional Data.

Two-tailed p value: 0.198
Pearson’s R statistic: 0.121
Degrees of Freedom (df): 113

Based on analysis, there is a positive correlation for October to April rainfall total and May rainfall total. It is not significant since the p-value is above 0.05.

Two-tailed p value: 0.443
Pearson’s R statistic: 0.072
Degrees of Freedom (df): 113

April and May rainfall total does not correlate much at all. It could go either way, a dry or wet April may give way to a dry or wet May. Since the p-value is above 0.05, it is not significant.

What is the verdict? October to April and only April rainfall does not predict how May rainfall will be like. So, there could be a chance that May could be either be wet or dry.

Why Texas Is So Dry

It has been very dry in Texas, while other parts of the nation are very wet and stormy, like the massive tornado outbreak in the Carolinas which claimed at least 45 lives. Wondering why it is so dry in Texas? Well, there is La Nina, but it is dying and maybe heading into a neutral phase, which is neither cool or warm. Typically, La Nina causes the jet stream to be further north than usual. During El Nino, the jet stream is typically further south than normal, which gives Texas a wet winter and spring, which means cooler. Also, there is a persistent area of high pressure over Texas and Southwest, which pushes the jet stream further north.

Here is a 500 millibar (18,000 feet) Geopotential Height Anomaly Map from March 1 to April 15, 2011. Source is from NOAA Daily Climate Composite

The area of high pressure at the upper level is pretty much over Texas.

Compare this from March 1 to April 15, 1997. Spring of 1997 was one of the wettest ever for Southeast Texas as the strong El Nino of 1997-1998 was developing. Source is from NOAA Divisional Data

The area of high pressure is further west and north. The jet stream is also further south, which allows low pressure to travel further south.

The other culprit is a persistent warm air aloft blowing from the southwest in the upper atmosphere at the 850 millibar or 5,000 feet level. The dry and warm air is blowing from the deserts of Mexico and goes unabated if there are no low pressure systems to keep them in check. The ground level is very humid, especially in the Houston area. Don’t let that fool you because at the upper level it is drier, which serves as an inversion and cap. Here is a map of temperature anomaly from March 1 to April 15, 2011.

It is warmer than normal at 5,000 feet. It is the same one that deprives us rain and keeps us dry.

Since, the jet stream is further north, which pushes low pressures further away from Texas. This means the energy for storms to form is further away and less chance for rain and storms to form. Why? Lower pressure allows lift in the atmosphere. That is why stormy weather often occurs from a low pressure system. Also, the persistent area of high pressure produces winds in the upper level that blows in dry and warm air from the deserts of Mexico. For the warm air aloft to crack, the low pressure system and jet stream has to be further south or the upper wind patterns has to come from the east, which occurs in the summer. These easterlies give us rain and sometimes allow tropical cyclones to make landfall on Texas.

Feast or Famine

We are in a drought in Texas. Texas seems to go through cycles of drier and wetter patterns. Some years are wetter than others, while others are drier than usual. Upper Texas Coast is no stranger to that as I can attest living there. So, I made a chart of annual rainfall differences to see if there is a cycle. It is based on annual rainfall minus average rainfall. The data is from the Divisional Weather Set.

Average Rainfall For Upper Texas Coast (1895-2010)
47.51

Standard Deviation (1895-2010)
10.97

Abnormally Dry
36.54 or below

Abnormally Wet
58.48 or above

Here is graph that shows the dry and wet cycle for Upper Texas Coast.

Dry Cycle
1895-1899
1909-1912
1915-1918
1924-1925
1927-1928
1930-1933
1936-1940
1950-1957
1962-1965
1988-1990
1999-2000
2008-?

Wet Cycle
1902-1903
1907-1908
1913-1914
1919-1920
1922-1923
1934-1935
1941-1942
1944-1946
1959-1961
1972-1974
1985-1987
1991-1995
1997-1998
2001-2002
2006-2007

Dry and wet cycles are about equally common, but dry cycles tend to last longer than wet cycles. Currently, we are in dry cycle, which started in 2008 and is currently going on. Dry or wet spells are not free of wet or dry months. Even in the driest cycle, there are heavy rain and flood events, while in the wettest cycle there are really dry months. It is all about how much rain fall in a whole year or years. It is feast or famine for us Texans. In many ways, it is a constant tug-of-war of rain or drought. We are always one notch away from drought.