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

Tropical Storm Bill

Tropical storm development
Tropical storm development shows how important atmosphere-ocean interactions are in climate and weather. Tropical storms introduce and transport enormous amounts of heat and moisture into the atmosphere.

The video below shows the formation of Tropical Storm Bill in June 15 through June 17, 2015  as it makes it way westward across the the Gulf of Mexico in the easterly trade winds (remember: easterly trade winds flow FROM the East carrying weather systems westward).  Note that this is not a hurricane. A hurricane would be rotating more quickly and have an "eye" in the middle as it intensifies. These systems can cause severe flooding because of the tremendous amount of water that evaporates from the warm ocean surface of ther Gulf of Mexico  in the atmosphere




Current satellite imagery
You will be able to see how this tropical system dies out as it makes it way northward and its moisture and cloud gets advected and dispersed (that is to say transported blown) eastward in the mid-latitude westerlies. A little guide to the imagery is provided below. Note that blue indicates cold cloud tops (the colder the higher) while red generally represents surface temperatures.   You should be able to see the cloud and precipitation due to the moisture of this system if you are in Southern New England.

Link to satellite imagery.