Saturday, October 3, 2015

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.

3 comments:

  1. Now, back to the cold air mass. October started off extremely hot and dry. We, at that time hit 96 degrees two days in a row, which was just two degrees off our then all-time record high of 98, set in 1950, but, as luck has it, the drought of the 2010's brought a 101 degree reading in October in 2013. Anyways, it was 70 degrees on the 7th, fell all day to 52 by midnight, meaning the high for the day happened at midnight. On the 8th, the high was 52 degrees, and fell to the mid 40's all afternoon, meaning the same thing happened on the 9th, with light drizzle. The official high temperature of just 47 degrees on October 9th would have been a record low as well, meaning that the highest we got on Oct. 9th, 2000, the lowest it has ever got in the then 114 years of record was warmer, not just here, but in MANY other areas ALL across South Texas. San Antonio's high temperature of 44 degrees on the 9th was 41 degrees below normal, and smashed the old record low-maximum for the date by a whopping 26 degrees! Laredo's high, normally the "hot spot" in South Texas was only 44 as well, with a normal high of 90 degrees, this was an incredible 46 degrees BELOW NORMAL. To have something happen like this even in the dead on winter is extremely unusual, I can think of just once, the February 1899 freeze, where some cities have not come within 4 degrees of their all-time record low temperature set since then, but for this to happen in early October, well before the start of winter? The high temperatures recorded on October 9th that day are the normal LOWS for January. Imagine Chicago having a high of 14 degrees in early October, or Minneapolis having a 7 degree high temperature in October? Can't find those types of temperatures until very late into November. So my question is, How in the world did such an event happen?

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    1. Unfortunately, a lot of the information from back in the day is not easily accessible. Area Forecast Discussions are archived since December 2000. The CPC 6-10 day forecasts are archived beginning in 2001. All I could find is a 1-month outlook for October 2000 and it called for above normal temperatures for all of Texas: http://www.cpc.ncep.noaa.gov/products/archives/long_lead/gifs/2000/200009month.gif

      I'll dig around some more and see what I can find.

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  2. Oops it forgot my first part. In September of 2000, just one month earlier, South Teas was experiencing the hottest temperatures even known to this area. Many all-time records were smashed. San Antonio broke their old record by 3 degrees, from 108 to 111, Houston by one degree, Corpus Christi by 2 degrees, set on the previous day!, Galveston by 3 degrees as well, set on the previous day. It has now reached 104 in Galveston, where before, like Miami, they all-time high was just 100 degrees. This would be just like Miami hitting 104 degrees, in fact, which will probably never happen. Since the year 2000, none of the new all-time record highs have been broken, but in August 2003, August 2005, and August 2011, every single city above would have broken their old all-time record high temperature, set before 2000. So, we have had no less than 4 heatwaves since 2000, including the Sept. event, where temperatures got higher than they ever did before 1999. Examples: Since 1886, San Antonio has recorded 5 of the top 10 hottest days in the past 15 years. This is going on all over South Texas as well.

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