Dog days of summer

Introduction

The main weekend weather story for Vermont is the hot, humid, and unstable air mass over Vermont. These conditions are typical of the dog-days of summer in New England. This weather is driven by a high pressure system to the south of New England transporting in warm , humid air from the Gulf of Mexico via the central U.S.   The high pressure keeps skies sunny, but the hot, humid air is unstable and can spawn strong  thundershowers, especially in the afternoon when temperatures are highest. We focus on Barre-Montpelier because this is in central Vermont and  an airport station with a long climate record and provides continuous data reliably.

Weather analysis

Figure 1 Daily maximum and minimum temperature temperatures for August 2015 compared to climate normals at Barre-Montpelier, VT, USA. Retrieved from  NOWData website of National Weather Service.    

Figure 1 shows the daily  temperature trends  at Barre-Montpelier for August 2015. Over all, temperatures in August so far have been close to normal on the whole. For the last five days we see a warming trend in day time maximum temperature (top of blue bar).   70 F to just above 80 F for the weekend.  This is higher than normal. However, night-time minimum temperatures have dipped to about  to about 50 F from temperatures closer to 60 F early in the week and were lower than normal Friday morning. This indicates a clearing trend, with cloud free skies contributing to more sun during the day and more cooling during the night as heat radiates into outer space through the clear skies. 

Figure 2 Surface analysis showing weather systems, station data, and precipitation from weather data. retrieved from AMS-OWS Datastream website. 

Figure 2  shows the USA weather system map at 12Z (8 AM EDT) and identifies a large high pressure system over Virginia that extends over all of the eastern US and into Canada. This is associated with a large zone of warm morning temperature around the 70 F and high dewpoints ranging from the mid 70's in Florida to mid 60's in New England. Associated with this are scattered showers associated with unstable conditions.
Figure 3 represents a 3 day animation between Wed. 12 Aug. and Sat. 15 Aug 2015 that illustrates the progression of the high pressure system. It moves very slowly from the midwestern U.S. to it's present position over Virginia. The cursor moves in the direction of the anticyclonic (clockwise) circulation, indicating the flow of warm air into New England.

Conclusion

The warm, humid, and unstable conditions over New England this weekend are associated primarily with a large  slow-moving high pressure system that has set up a weak but warm southerly flow into the region. What happens to this warm-weather system  will likely affect the weather over the entire Eastern half of the continent for the next