The Cool Summer of 2004
An Analysis of Three Cold Outbreaks
The summer of 2004 will be remembered as a chilly one for those living in much of the central and eastern portions of the United States. In fact, surface temperature anomolies are revealed to be strongly negative overall for the period June 1st - August 31st, 2004 (the meteorological summer). But was the entire summer cool, or did a few powerful outbreaks of chilly air skew the anomolies toward colder temperatures?
Figure 0: Surface Temperature Anomolies for the United States, 6/1/04 - 8/31/04
A careful analysis of the meteorological data reveals that three powerful cold snaps, one in each month of the summer, were primarily responsible for causing this summer to be remembered as cool overall. The first period was June 18th to June 24th, the second July 26th to July 30th, and the third August 10th - August 16th.
This study will analyze these three national outbreaks of chilly air, and sample their impact on Pennsylvania weather.
Figures 1a and 1b: 500 mb and 850 mb Height Anomolies for US, 6/18/04 - 6/24/04
The June outbreak was associated with an upper trough located over Manitoba and the Upper Midwest. The coolest temperatures from this outbreak tended to be in the midwestern states.
Here is an example of the surface temperature DFN on June 20th. Parts of the South Dakota and Nebraska are 15 degrees below normal!
Figure 1c: US Surface Temperature Anomolies for June 20th, 2004
Figures 2a and 2b: 500 mb and 850 mb Height Anomolies for US, 7/26/04 - 7/30/04
The July outbreak featured an upper trough that extended further southward than the June event, centered over southern Canada and the Dakotas, but reaching through the Central Plains. The coolest temperatures from this outbreak reached all the way down to Texas.
Here is an example of the surface temperature DFN on July 27th. There is an area in both Texas/Kansas and southern Illinois/Kansas of 10 degrees below normal air temperatues.
Figure 2c: US Surface Temperature Anomolies for July 27th, 2004
Figures 3a and 3b: 500 mb and 850 mb Height Anomolies for US, 8/10/04 - 8/16/04
The August outbreak revealed a strongly negative upper atmosphere temperature anomoly centered over Wisconin and Illinois, which was further east than the previous two outbreaks. Cool temperature anomolies reached southward to the Gulf Coast, and eastward to the Atlantic Ocean.
Here is an example of the surface temperature DFN on August 15th. There is a broad area of 10+ deg below normal temperatures stretching from Texas east to Ohio. Especially extreme conditions occured in the Deep South, where six consecutive record minimums were established in Meridian, MS and Mobile, AL. This event was the longest cold spell in their observed weather history, which contains records dating into the mid 1800’s!
Figure 3c: US Surface Temperature Anomolies for August 15th, 2004
When comparing the three events, the 500mb cold pool (the strongest negative height anomolies) gradually drips southeast out of Canada, progressing further each outbreak. In June, it is in Manitoba. In July, the Dakotas. And in August, Illinois. The most impressive daily surface temperature anomolies similarly progress south and east.
Evidence of these three cold snaps are evident in the temperature departures from normal for specific Pennsylvania locations.
Here are the summer DFNs for the largest cities in Pennsylvania, Philadelphia and Pittsburgh. The cold snaps are circled in black:
Figure 4a: Surface Temperature Departures from Normal for Philadelphia, Summer 2004
Figure 4b: Surface Temperature Departures from Normal for Pittsburgh, Summer 2004
From the strength of these three anomolies, it is apparent that they played a significant role in making the Summer of 2004 a cool one overall.
- Greybush, Knight 12/04