The Most Anomalous Cold Spell in Recent History

Was the October 2000 cold snap in South Texas the most extreme temperature event in U.S. climate history when looking at high temperatures? Perhaps. Let's take a look. But first, some background.

NCDC Normals

There are certain times during the year, usually winter, where large temperature swings are not uncommon. In fact, portions of the Great Plains can expect 24-hour temperature changes of 50°F at least once per year. The variability in daily temperatures is measured statistically by calculating the standard deviation for a 30-year period and smoothing it over a 15-day moving filter. For the 1981-2010 time period, the National Climate Data Center (NCDC) has published daily normal temperatures and daily standard deviations for over 6,000 stations nationwide. These values are used to assess how anomalous daily temperature reading are in comparison to the current time period. Unfortunately, this data does not extend back to earlier 30-year normal periods. The NCDC changed their computational methodology for the 1981-2010 climate normal period and reconstructing previous epochs is nearly impossible.

Comparing a reading from January 1, 1950 (for example), to the 1981-2010 climate normal period is not entirely appropriate. The 30-year baseline was developed by the World Meteorological Organization to account for slow changes in the climate of a location. In the case of the 1981-2010 climate normals, they are the comparison standard for data collected between 2011 and 2020. That being said, we can still look at the January 1, 1950, data and state how far above or below it was in comparison to the 1981-2010 normal with the understanding that it may be somewhat of an apples to oranges comparison.

September/October 2000

Early September produced all-time record high temperatures (since broken) for areas between Central Texas and Mississippi. This blog's author spent an entire week outside in Northeast Louisiana in 106°F+ temperatures conducting fieldwork during the record streak and can attest to the severity of the heat. By mid-September the temperatures had returned to seasonal normals and some record lows were even set in the Deep South. However, this was just a taste of things to come.

As early as October 1st, forecasters were calling for a major buckle in the jest stream to drive cold air down through the center of the U.S. Figure 1 shows a 48-hour forecast map issued on October 1, 2000, The leading edge of the cold front is forecasted to drive through North Dakota and Minnesota at that time. The text portion of the forecast calls, underlined in the figure, calls for the cold air to make it all the way to the Deep South.

Figure 1. NCEP 48-hour forecast discussion issued on October 1st, 2000. Source: http://archive.atmos.colostate.edu/data/misc/QHUQ00/0010/00100118QHUQ00.png 

By October 5th, the Arctic cold had already swept from Montana through the Front Range and began to enter the Texas Panhandle. Temperatures were 10°F-15°F below normal north of Lubbock. On the 6th, the front pushed through Central Texas and on the 7th it made it down to deep South Texas (see Figure 2). Not only were temperatures substantially below normal, but they were historically below normal.

Figure 2. Surface weather map from October 7, 2000.

The chart depicted in Figure 3 shows the hourly temperature observations at Houston's Bush Intercontinental Airport between October 6th and October 11th, 2000. The cold front arrived at the airport during the evening of October 6th and temperatures dropped all day through October 7th. There was a 37-hour period where the temperature did not exceed 50°F.

Figure 3. Hourly temperatures at George Bush Intercontinental Airport from October 6-11, 2000. The average hourly temperature is shown as a dashed line. Note: year 2000 hourly observations were not included in the average hourly calculations.

Nationwide Analysis

When querying the Global Historical Climate Network (GHCN) database for high temperature anomalies as compared to current (1981-2010) normals, a single event stands out. Figure 4 shows the sixteen greatest daily high temperature departures from current normals in the GHCN database at 1st-Order stations. Cooperative stations were excluded from the analysis due to time of day observations issues.

Figure 4. Greatest high temperature departures from 1981-2010 climate normals for all airport stations in the GHCN v.3 database. All entries have passed an initial quality control check. Hourly observations from Lihue in October 1965 appear to validate their cool October day in 1965.

Twelve of the fifteen most anomalous events were recorded in South Texas between October 8 and October 10, 2000. The Port Isabel high temperature of 51°F on 10/9/2000 is one of only two days in the GHCN database that if the observation were to occur today would be more than eight standard deviations below normal. If you are not familiar with what a standard deviation is, here is a brief description. If temperature data are normally distributed, which they generally are, we expect 68% of daily observation to fall within 1 standard deviation of the daily normal; 95% to fall within 2 standard deviations of the daily normal; and 99.7% to fall within 3 standard deviations of the daily normal. Considering that approximately 18 million daily climate records exist in the GHCN database, and only two are more than 8 standard deviations from the daily normal, this was truly a historic event. 

The next two maps show the maximum high temperature departure from the 1981-2010 normal (Figure 5) and the maximum standard deviation from the 1981-2010 normal (Figure 6).

Figure 5. Largest daily temperature departure from normal for entire U.S. between October 8-10, 2000.

Figure 6. Largest daily standard deviation departure from normal for entire U.S. between October 8-10, 2000.

Texas Maps

While large portions of the U.S. were significantly below normal, the core of anomalously low temperatures was south of Interstate 10 in Texas. The following three figures show just how unusual the high temperatures were in Texas. Each figure is color shaded and primary stations are marked and labelled. The first figure (Figure 7) shows the coldest high temperature during the three-day period. The next figure (Figure 8) shows the greatest high temperature departure from normal during those same three days and finally, Figure 9 shows the number of standard deviation below normal for the coldest day during that period.

Figure 7. Lowest high temperature for Texas and vicinity between October 8-10, 2000.

Figure 8. Largest daily temperature departure from normal for Texas and vicinity between October 8-10, 2000.

Figure 9. Largest daily standard deviation departure from normal for Texas and vicinity between October 8-10, 2000.

Why so Cold?

So why was it so anomalously cold in Texas? There are several answers. First, the airmass was especially cold. Figure 10 shows the upper air conditions from the Corpus Christi, TX, RAOB sounding on 10/9/00 0Z. The temperature at 850 mb is only 3.7°C. This is, by far, the lowest for so early in the season.

Figure 10. Upper air sounding for Corpus Christi, TX, at 0Z on October 9, 2000.

Just as importantly, clouds and light rain were present. If the skies were clear and the 850 mb temp were low, that would drive record low minimum temperatures. However, this post is about record low maximum temperature. An overrunning situation took shape with moist air lifting over the shallow cold airmass (isentropic upglide). The cloud cover (see Figure 11) prevented solar energy from warming the surface, and the precipitation falling through cold airmass acted to suppress temperatures due to latent heat processes.

Figure 11. GOES-8 infrared satellite image on October 8, 2000.

Conclusion

If you live in Montana, a temperature of 35°F below normal is nothing to write home about. But that same discrepancy in Texas, in October, is one for the record books. If you live in South Texas, go outside on October 8-10 of this year and imagine what temperatures in the 40s feel like.