Reader Question- What's up with the Weather Changes?

Saturday was certainly the pick of the weekend.  Although it is not raining today temperatures are much colder this afternoon compared to yesterday.
Temperature comparison 
This prompted a question from a faithful reader of the blog.  He asks "What causes temp drops/increases on a day to day basis?  How is it that much colder today than yesterday?"

The simple answer to this question is a wind shift.  Here is the surface chart from yesterday afternoon.
WPC 18z Surface Analysis Saturday 2 PM
Remember that air flows clockwise around high pressure and counter clockwise around low pressure.  The front that brought rain showers Friday night didn't really have a significant air mass change as high pressure was not driving it like the second front.  Instead a surface low developed on the front and helped pull warm southern air into New England.  Now let's take a look at the 11 AM analysis today.
WPC 15 z Surface Analysis 11 AM Sunday
High pressure has taken control in the central US so the air source is from Canada, not the Southern US.  The wind this afternoon is combination of the incoming high and departing low in Northern New England.  Another way to look at this is at the upper levels where the steering of surface systems largely occurs.

2 PM Saturday
Saturday 18z NAM 500 mb vort Sat 2 PM (image NCEP)
Sunday 2 PM
Sun 12z NAM 500 mb vort Sun 2 PM (image NCEP)
It is generally the same story.  Saturday southerly flow with a source in the Atlantic/Gulf to Sunday north/westerly flow with a source in Canada.

Now those watching the Red Sox today see the sun peaking out from time to time.  The reason for all these clouds is very cold air in the mid levels of the atmosphere.  This cause the sun to warm the surface (it is Mid-May after all) and air parcels begin to rise.  The parcels rapidly cool with height so they condense into cloud droplets.  Think a cold drink on a hot day.  A dew will develop on outside of the container.  This is why some showers are possible this afternoon.
18z HRRR simulated radar 5 PM (image Weatherbell)

The rule this spring has been slow moving weather systems.  The same is true for this system as high pressure slowly works east.  Temperatures tonight will drop into the 30's but there are no frost/freeze headlines in Southern New England thanks to windy conditions overnight and a dry air mass.   The weather Monday is similar to today but probably more sun.  Temperatures right around 60.  We get back into the mild weather by the end of the week.

More on this week's weather tomorrow.

-Zack Green


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