Why Texas Is So Dry Part 2

It has been really dry lately in Texas. The reason it is dry is that we are in La Nina, which pushes the jet stream further north than usual. It causes storm systems to go further north, which means it allows warm air aloft above the surface to develop unchecked. The warm air aloft is from the deserts of Mexico. The warm air aloft is always there, but it is kept in check by storms systems that destroys them. Another reason why Texas is so dry is the upper level is drier than normal. It could be really humid on the ground, but go 5,000 feet (1,524 meters), it is much drier. The dry upper atmosphere is again from the dry and warm air of Mexican deserts. In wet years, the upper part of the atmosphere is wetter than normal. I am going to be comparing two springs from March to May. The wettest spring on record for Upper Texas Coast is 1997 and current spring, which is likely going to be one of the driest spring on record.

Spring 1997 Relative Humidity Anomaly

Spring 2011 Relative Humidity Anomaly

Notice anything? Spring 1997 was unusually humid at the surface and at the 5,000 feet (1,524 meters or 850 millibar) level. Spring 2011 humidity level is within average. However, at 5,000 feet, it is dry. The surface can be very humid, but when you go 5,000 feet (1,524 meters), it is much drier. The low relative humidity at the upper levels inhibit storms from forming. The low moisture level is what kills storms from forming over eastern part of Texas.

If you think Texas is in a drought and abnormally warm, we are not alone. Europe is also in midst of a severe drought and abnormal warmth for the same reason Texas is in, La Nina. Texas and Europe’s drought are one of the worst on record.

Spring 2011 Relative Humidity Anomaly In Europe

The level of moisture at the surface and 5,000 feet (1,524 meters) is abnormally dry.

Spring 2011 Air Temperature Anomaly In Europe

Europe is abnormally warm at the surface and 5,000 feet (1,524 meters).

Spring 2011 Geopotential Height Anomaly In Europe

There is a blocking feature over Europe at the 18,000 feet (5,487 meters). That upper level high pressure is blocking any storm systems that give Europe rain.

The same atmospheric setup in Europe is very similar to in Texas, low moisture, warmer temperature, and upper level block pattern.


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.

La Nina and Winter 2010-2011 Outlook

Winter is upon us. Last winter was one of the coldest winters on record for Southeast Texas and throughout the nation. This was due to El Nino we had. Right now, it is the opposite, La Nina. La Nina is when the equatorial Pacific cools down, unlike El Nino, when it warms up. La Nina and El Nino are part of the El Nino Southern Oscillation (ENSO) cycle.

What does La Nina winter mean for us and the nation? Typically, La Nina gives us a warmer and drier winter. The last time we had a La Nina winter was in 2008-2009. That winter was warm and dry. There were no big freeze that time in Southeast Texas. One difference with the current La Nina from 2008-2009 La Nina is that the current one is a strong La Nina.

Here is a scatter plot that shows a correlation of La Nina and El Nino winters in Upper Texas Coast in terms if temperature and rainfall. December to February is defined as winter season by the NOAA.

ENSO and Upper Texas Coast Precipitation

The scatter plot shows that the strong or warmer ENSO is, the more wetter the winter will be. La Nina for the Upper Texas Coast means a drier winter. However, sometimes there are heavy rain events in a La Nina winter. The winter of 1949-1950 had a strong La Nina and yet the Upper Texas Coast rainfall total from December 1949 to February 1950 was 14.89 inches. So, it could be possible to see heavy rain during a La Nina, but that is an outlier. In general, La Nina winters are going to be dry for the Upper Texas Coast. I expect this winter will be dry.

ENSO and Upper Texas Coast Temperature

The cooler the La Nina, the warmer the winter it is for Upper Texas Coast. Does this mean we are not going to have freeze and have no storms? No, in fact some of the biggest freeze have occurred in La Nina. The winter of 1988-1989 was a La Nina winter and there was this huge freeze that came in early February of 1989. It was below freezing for five days straight. I remember it because it was cold and there was frozen rain which iced up roads. There was even a few snow flurries during the freeze. Another La Nina winter was 1950-51 and there was a huge freeze that occurred from late January to February of 1951 and the duration remains the longest even to this very day for Southeast Texas.

How does La Nina affect the nation? I think the best analog for the 2010-2011 La Nina are:

Why did I choose those years? The current La Nina is a strong one and those winters had moderate/strong La Ninas.

Here are divisional maps I generated from US Climate Division Dataset Mapping Page.

Let’s start with precipitation.

La Ninas analog to the current one show that a large area of America is drier than average, especially in Texas, Gulf Coast, Florida, and Southwest. What does this winter bode for us? It is likely going to be drier than normal. There is a real possibility that we could see a serious drought situation by the time spring is around.

If you want to find a wetter than average area, you would have to go north. La Nina causes the jet stream to be further north than usual. The jet stream carries low pressure systems and cold fronts that give rain and cold weather. During a La Nina, the low pressure systems and cold fronts are further north, which makes most of the nation drier and warmer in the winter.

Let’s look with temperature.

La Ninas analog to the current one for America are warmer than average. In fact, most of the nation is warmer than average. We are less likely to freeze ourselves out in the middle of winter. That means less heating bills to be paid. If you want to find cooler than average in a La Nina winter, head out West. It is cooler than normal. Now, just because we are in a La Nina does not mean we are going to be free of Arctic blasts. As I have mentioned, Arctic blasts can occur in a La Nina winter. Next time I will look at other atmospheric and oceanic patterns besides El Nino and its impact on winter. They are North Atlantic Oscillation (NAO), Arctic Oscillation (AO), and Pacific-North American (PNA).