Keeping Perspective – Preliminary Vs. Final Tornado Reports

Posted by pmarshwx on 17 Apr 2011 | 6 comments

Over the last few days, the southern United States has endured a significant severe weather event that took the lives of a still increasing number of people. While many communities are still trying to sift through the wreckage, meteorologists, “meteorologists”, chasers, and other weather enthusiasts have taken to Facebook and Twitter to discuss what has happened. Unfortunately, a lot of misinformation is floating around. Here’s my quick attempt to clarify some of this information.

As of this writing, 248 preliminary tornadoes have been reported via the Storm Prediction Center’s Preliminary Storm Report webpage over the three days 14-16 April 2011. Much has been made about this number. Unfortunately this number contains many duplicate tornadoes, and potentially even some tornadoes that never were. Hence the label “Preliminary”. Over the next few weeks, National Weather Service Offices throughout the south will be conducting damage surveys to determine the number of actual tornadoes to the best of their ability. There is no doubt that this has been a significant three-day tornado outbreak. However, until the official numbers are released via the National Weather Service’s Storm Data publication, people should exercise extreme caution in trying to quantify where this week’s severe weather outbreak ranks in history.

It used to be the case that the preliminary number of tornadoes underestimated the number of actual tornadoes. However, near March of 2006 the pattern reversed itself with the number of preliminary tornadoes typically overestimating the number of actual tornadoes. For more information regarding preliminary vs. final tornado reports, please read this blog post on the topic by Harold Brooks, which can be found on the United States Severe Weather Blog.

In 2008, the final tornado count was roughly 75-80% of the preliminary count. Based on the number of spotters now reporting tornadoes, it’s not out of the question to assume this ratio is now closer to 70%, if not lower. Based on this an estimated guess to the number of final tornadoes might be closer to 198 (80%), 186 (75%), or even 173 (70%). Since I know people want to know below are the top three-day tornado counts on record using the final tornado numbers. The date given is the last day of the three days used in the count.

Also, a disclaimer to the above list: This is a pure count of tornadoes over a three day period. A four-day period with tornado counts of 10, 85, 80, and 0 would show up twice even though for many applications, it probably should only count once. If you wish to throwout duplicate days, please refer to the dataset linked above.

Drought and Tornadoes In 2006

Posted by pmarshwx on 3 Apr 2011 | 0 comments

As many already know, the southern plains is in the midst of a fairly pronounced drought. The drought reminds me of the drought we experienced in Norman during 2005-2006. During the drought, the rain stopped in late summer of 2005 and remained that way through November of 2006, when a very intense cyclone helped to alter the prevailing storm track.

As we head into the spring storm season, I’ve been getting asked more and more frequently how the lack of precipitation in the southern plains will affect storm season. The answer to this question is highly complex, and poorly understood at this time. However, I’ll offer one possible outcome.

The lack of southern plains precipitation, and more importantly, the dry soil, might allow the dryline to mix eastward faster than if the soil held more moisture. This, coupled with warm air aloft being advected over the southern plains from the higher elevations of New Mexico (which has also been extremely hot and dry of late), will help suppress thunderstorm and tornado development across much of western Oklahoma and the Texas Panhandle.

This is what happened in 2006, and you can see based on the graphic below, most of the tornadoes were to the north and east of western OK.

AOTW: Temperature Records for KOKC, KLIT & KMEM

Posted by pmarshwx on 7 Jan 2011 | 2 comments

This answers to this week’s “Question(s) of the Week” are:

1. When was the last time Little Rock (KLIT) or Memphis (KMEM) recorded a daily minimum temperature below 0F?
   • The last time either site recorded a daily minimum temperature below 0F was 23 December 1989. KLIT recorded a minimum temperature of -1F and KMEM recorded a minimum temperature of -3F.
2. When was the last time they recorded a daily maximum temperature less than 10F?
   • Since 1950, KLIT has not recorded a day where the the temperature failed to rise above 10F. During this time period, KLIT has recorded only 1 day where the maximum temperature was 10F, which was 24 December 1989. KMEM, however, has recorded two days since 1950 where the temperature failed to reach 10F: 10 January 1962 (6F) and 22 December 1989 (8F).
     
3. Since 1950, has either site recorded a day where the temperature failed to warm to greater than 0F?
   • Since 1950, neither KLIT nor KMEM has failed to warm to greater than 0F.

Congratulations to Tim for his correct answers!

Below are the data for these two sites. If you are interested in the full climatological data (that I have available), let me know and I’ll post it somewhere.

Daily Minimums of 0F or less for KLIT:
1951-02-02: -5 (2.0)
1962-01-10: -1 (5.0)
1962-01-12: -4 (3.0)
1963-12-24: -1 (5.0)
1983-12-25: 0 (0.0)
1985-01-20: -2 (0.0)
1989-12-23: -1 (0.0)

[The format is Date: Minimum (Observed Snow Depth)]


Daily Maximums of 10F or less for KLIT:
1983-12-24: 10 (0.0)

[The format is Date: Maximum (Observed Snow Depth)]


Daily Minimums of 0F or less for KMEM:
1940-01-19: 0 (0.0)
1940-01-25: -2 (2.0)
1940-01-26: -2 (1.0)
1940-01-27: -1 (1.0)
1940-01-28: -1 (0.0)
1951-02-02: -11 (5.0)
1962-01-10: -1 (3.0)
1962-01-11: -1 (3.0)
1962-01-12: -4 (2.0)
1963-01-24: 0 (1.0)
1963-12-23: -8 (13.0)
1963-12-24: -13 (12.0)
1982-01-17: 0 (1.0)
1983-12-25: 0 (0.0)
1985-01-20: -4 (2.0)
1989-12-22: -4 (0.0)
1989-12-23: -3 (0.0)

[The format is Date: Minimum (Observed Snow Depth)]


Daily Maximums of 10F or less for KMEM:
1962-01-10: 6 (3.0)
1989-12-22: 8 (0.0)

[The format is Date: Maximum (Observed Snow Depth)]


QOTW: Temperature Records for KOKC, KLIT & KMEM

Posted by pmarshwx on 3 Jan 2011 | 1 comment

In meteorology we tend to deal in extremes and anomalies. After all, this is what causes the biggest impact on society. Days in which the temperature and precipitation are average, or nearly average don’t result in significant impacts to travel, construction, etc. It’s the extreme or anomalous events that get people’s attention and negatively impact society. With this said, consider the current forecast for the upcoming weekend into much of next week. Below is the Climate Prediction Center’s (CPC) 6-10 Day Temperature Outlook. Notice how the entire contiguous United States is expected to have below normal temperatures!

Both the ECMWF and GFS are signaling a major cold air outbreak of the likes we haven’t seen in years. Forecast temperatures from the GFS keep the central and southern plains below 0C (32F) for well over a week. Additionally, the GFS forecast from Sunday evening (00 UTC Monday) for the Martin Luther King, Jr holiday weekend has daily minimum temperatures of -20C (-4F) or below as far south as northern Texas and northern Louisiana. (Although, admittedly, the 12 UTC run was slightly warmer.) Also, as the next sequence of images shows, portions of Arkansas are not forecast to have daily maximum temperatures above -4F!

Surface (2-meter) temperatures valid Monday, 17 January 2011, at 6 AM CST

Surface (2-meter) temperatures valid Monday, 17 January 2011, at 6 PM CST

Surface (2-meter) temperatures valid Tuesday, 18 January 2011, at 6 AM CST

After seeing the 00 UTC GFS, I was curious about the frequency of such cold periods. I examined the daily temperature and precipitation records for KOKC dating back to 1 January 1948, to determine the number of times KOKC has recorded a minimum temperature of 0F or less. I then repeated this exercise to determine the times in which the daily maximum temperature was less than or equal to 10F. Since 1948, OKC has recorded a minimum temperature of 0F or less 31 times and a maximum temperature of 10F or less 7 times.

Broken down by decades:
1950s, 3, 0
1960s, 5, 0
1970s, 8, 1
1980s, 13, 5
1990s, 2, 1
2000s, 0, 0

[The format is: Decade, # of mins, # of maxes]
(Actual dates/values can be found at the end of this post)


As you can see, the 1990s and 2000s had fewer of each than did the 70s and 80s. This leads me to a point that Harold Brooks brought up earlier today. The National Weather Service (NWS) updates its 30-year averages (normals) every 10 years, in contrast to World Meteorological Organization (WMO) standards of updating them every 30 years. This means that 2011 is the first year that NWS, and specifically the CPC, should be using the “new normals”. Thus, if CPC is using the “new normals”, what impact does this have on the Temperature Outlook above? Simply looking at the extremes listed above, replacing the 1970s with the 2000s in any 30-year average calculation would result in a warmer average. Thus, it would be (theoretically) easier to have a Temperature Outlook where the entire CONUS has a 33% or greater chance of below normal temperatures this year than last year. (Note, the next 30-year average update for WMO will be in either 2020 or 2021).

A word of caution here, I do not know what the actual time range of the 30-year averages are in the CPC forecast above. However, assuming that a contiguous 30-year period is used in the averaging, and we’ve replaced at least one of the 1960s, 1970s, 1980s, with the 1990s or 2000s, then the argument I’ve laid forth is still valid, regardless of the actual years used.

So, readers, my questions to you:

1. When was the last time Little Rock (KLIT) or Memphis (KMEM) recorded a daily minimum temperature below 0F?
2. When was the last time they recorded a daily maximum temperature less than 10F?
3. Since 1950, has either site recorded a day where the temperature failed to warm to greater than 0F?

Leave your answers in the comments! The answers will be posted on Friday!

And as promised, below are the data for the OKC Minimum/Maximum statistics mentioned in the article.

Daily Minimums of 0F or less:
01 Feb 1951: -1 (1.0)
04 Jan 1959: -4 (2.0)
05 Jan 1959: -2 (1.0)
11 Jan 1962: -1 (0.0)
12 Jan 1963: -3 (0.0)
13 Jan 1963: -1 (0.0)
23 Jan 1963: -1 (0.0)
24 Jan 1963: -1 (0.0)
09 Jan 1973: 0 (5.0)
12 Jan 1973: -1 (5.0)
09 Jan 1977: -2 (3.0)
10 Jan 1977: -3 (3.0)
18 Feb 1978: -1 (3.0)
08 Jan 1979: 0 (2.0)
31 Jan 1979: -1 (2.0)
09 Feb 1979: -3 (5.0)
11 Feb 1981: 0 (0.0)
10 Jan 1982: 0 (0.0)
11 Jan 1982: 0 (0.0)
21 Dec 1983: -2 (1.0)
22 Dec 1983: -3 (1.0)
24 Dec 1983: 0 (1.0)
25 Dec 1983: -1 (0.0)
19 Jan 1984: -3 (2.0)
08 Jan 1988: -4 (12.0)
03 Feb 1989: 0 (0.0)
21 Dec 1989: 0 (0.0)
22 Dec 1989: -4 (0.0)
23 Dec 1989: -8 (0.0)
03 Feb 1996: 0 (0.0)
04 Feb 1996: -3 (0.0)

[The format is Date: Minimum (Observed Snow Depth)]


Daily Maximums of 10F or less:
09 Jan 1977: 9 (3.0)
19 Dec 1983: 9 (0.0)
22 Dec 1983: 9 (1.0)
23 Dec 1983: 10 (1.0)
24 Dec 1983: 3 (1.0)
22 Dec 1989: 4 (0.0)
22 Dec 1990: 6 (2.0)

[The format is Date: Maximum (Observed Snow Depth)]


Day 336: Definitions of “Winter”

Posted by pmarshwx on 2 Dec 2010 | 3 comments

Yesterday (1 December 2010) began what most meteorologists refer to as “meteorological winter“, which lasts until the end of Februrary. This differs from the astronomical definition of winter, which begins on the winter solstice, which occurs each year on either the 21st or 22nd of December, and lasts until the Vernal, or Spring, Equinox.

This morning, I saw yet another definition of winter offered by the Iowa Environmental Mesonet (IEM). This definition is a slightly different take on meteorological winter; in this new definition, winter is defined as the 91-day period in which the coldest mean temperature is observed. The 91-day period is used because this is almost exactly 1/4 of a calendar year. The image above was produced by IEM and depicts the start of this modified meteorological winter. It’s fascinating to see that the onset of winter is generally later the farther east one goes.

For those who tire of my daily dose of meteorology, the IEM offers their own daily dose, although it is typically Iowa centric. You can check out the IEM Daily Feature at this top of IEM website homepage.

Day 294: Official NWS Winter Outlook

Posted by pmarsh on 21 Oct 2010 | 0 comments

Today the National Oceanic and Atmospheric Administration’s (NOAA) Climate Prediction Center (CPC) issued their seasonal outlook for the upcoming winter. In my opinion it contained absolutely no surprises. The graphics depict what I would consider typical winter conditions for La Nina.

You can read the whole announcement here.

Day 256: Oklahoma City Temperature Climatology (2000s)

Posted by pmarsh on 13 Sep 2010 | 0 comments

If you’ve been following along the past two days, you will know that I’m in the midst of a three-day series of posts on the temperature climatology of Oklahoma City during the 30 year period from 1980 – 2009. Two nights ago I focused on temperatures in the 1980s, yesterday focused on the 1990s, and this post focuses on the 2000s.

Above is a graphic depicting the daily temperature in Oklahoma City, OK from 1 January 2000 through 31 December 2009. In red, the daily maximum is plotted with the “normal” daily maximum temperature in black underneath. In blue, the daily minimum temperature is plotted with the “normal” daily minimum temperature in black underneath. As you can see, we are rarely ever near the “normal” temperature on a given day…

I should also note that the 100F temperature is denoted in a red horizontal dashed line, the 32F temperature is denoted as a blue horizontal dashed line, and the 0F temperature is denoted as a purple horizontal dashed line.

Below are some extremes from the 2000s:

• Lowest Daily Average: 12.0 (2003-02-24)
• Normal Lowest Daily Average: 36.0
• Highest Daily Average: 94.0 (2006-08-10)
• Normal Highest Daily Average: 83.0
• Lowest Minimum: 6 (2002-03-03, 2005-12-08, 2005-12-09, and 2009-01-28)
• Normal Lowest Minimum: 26
• Highest Minimum: 81 (2006-08-02 and 2006-08-10)
• Normal Highest Minimum: 72
• Lowest Maximum: 15 (2003-02-24)
• Normal Lowest Maximum: 46
• Highest Maximum: 108 (2000-09-02 and 2000-09-03)
• Normal Highest Maximum: 94

This figure (below) depicts the daily anomaly from the normal maximum temperature of the day.

This figure (below) depicts the daily anomaly from the normal minimum temperature of the day.

This figure (below) depicts the daily anomaly from the normal mean temperature of the day.