When the summer nor’easter last weekend tracked far enough south to miss us, it opened the door for high pressure to build and start a wonderful stretch of weather.  

This high will keep control of the weather for several days until we come under the influence of a cold front to the west late this week and over the weekend.

Summer high pressure systems, like this week’s, often bring fairly predictable weather.  Confidence in forecasts dominated by high pressure is often greater than those with low pressure.  

The forecast for the rest of the week is partly to mostly sunny with a few afternoon or evening storms and highs in the upper 70s at the midcoast to mid- and even upper 80s over interior southern Maine.

The reason the weather is somewhat easier to forecast with highs is because of their structure. The air generally sinks and stays precipitation-free or is limited to a few showers.

 There can still be showers or storms, as was the case Monday.  Any weak system moving through the flow can ignite these storms, but much of the time it remains sunny or partly sunny and dry.

The wind flow around a high is also predictable. The right side of a high has a northerly flow. This usually brings comfortable cool weather and low humidity during the summer.  (If this was winter, this is when we would be seeing an arctic blast of air.)  

We’ve been on the right side of high pressure the past few days and that’s why it’s been relatively pleasant in terms of humidity.


Winds flow clockwise around high pressure. NOAA


As the high moves east and New England sits on the left or western side of the high, the weather turns even warmer and more humid.  This is due to the southerly flow of air bringing high temperatures from the south.  

Additionally, the air mass itself will warm simply due to the heating of the sun each day.  This is called air mass modification and even if a high just sits in one spot, the temperature will warm at least a degree or two day-to-day.

The backside of a high pressure area brings warmer air and more humidity. NOAA

Because the humidity goes up it doesn’t take much to create a shower or storm.  This is why from Tuesday and into the weekend the forecast holds a chance of a scattered thunderstorm in the afternoon.  

This is very typical early August weather.  The days are warm and humid and there’s a risk of a shower, but there’s a lot of sunshine as well. It’s basically great beach weather.

Eventually, highs move away, often pushed out by a cold front.  This is the case later this weekend.  

As the cold front approaches it creates a squeeze play between the exiting high and itself to bring even more humidity up from the south.

This also increases the risk of showers and storms, which peaks within 100 or so miles of the front.  

Saturday brings the greatest risk of widespread showers this week along with higher levels of moisture in the air.


A cold front approaches from the west on Saturday. NOAA

As the cold front goes by, a new area of high pressure and a new air mass replace the old one.  Then it’s typically back to the cooler and drier air for a few days.  

During some summers, the highs from Canada don’t make it this far south very often and we have heat waves, or the cold fronts get stalled and we have several days of humidity and showers.

This weekend, as the cold front approaches we’ll have to watch closely to see how fast it’s able to pass.  If the front moves slowly, it could keep showers in the forecast beyond Saturday, but for now Sunday looks great.

You can follow Dave Epstein on Twitter @growingwisdom

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