Tropical surge, heat and humidity build this weekend

Let's face it,  we've led charmed weather lives when it comes to heat and humidity so far this summer.

Our near continuous supply of free AC is brought to you courtesy of the Canadian Board of Tourism. I've talked with many of you who are happy with our windows-open air masses so far this summer, and the good sleeping weather nights.

Some numbers:

• 62 percent summer 2014 AC costs vs. last summer to date in Minnesota (we're saving about 38 percent overall)

• 80.1 percent vs. average on cooling costs in Minnesota vs. average so far this summer (we're saving about 20 percent overall)

• 255 cooling degree days (CDD) since June 1st at Minneapolis-St. Paul International Airport

• 318 average CDD at MSP since June 1

• 411 CDD last year at MSP to date

Bottom line? We're paying about 38 percent less than last summer and 20% less than average for AC costs in the Twin Cities so far this summer.

So just what the heck is a "cooling degree day" anyway?

Think of it as adding the number of degrees for any day where the average temp is over 65 degrees. If the daily temperature average is 75, it takes 10 "cooling degree days" to get your home back to what many consider ideal comfort.

Here's a more detailed description of degree days from the National Oceanic and Atmospheric Administration's Western Regional Climate Center.

Degree days are computed from each day's mean temperature (max+min/2). Each degree that a day's mean temperature is below or above a reference temperature is counted as one degree-day. The amount of fuel necessary for home or industrial heating is indicated by the mean temperature for that day. Estimates are that most people use their furnace when the mean daily temperature drops below 65F.

Heating degree-days are determined by subtracting the mean temperature for the day from the reference temperature. Thus, if the mean temperature for a day is 50F and the reference temperature is 65F, there would be 15(65-50) heating degree-days on this day. On days when the mean temperature is above the reference temperature, there are no heating degree-days. Therefore, the lower the average daily temperature, the more heating degree-days and the greater the consumption of fuel.

Cooling degree-days are used during warm weather to estimate the energy needed to cool indoor air to a comfortable temperature. Mean daily temperature is converted to cooling degree-days by subtracting the reference temperature from the mean. For example, a day with a mean temperature of 80F and a reference temperature of 65F would correspond to (80-65), or 15 cooling degree-days.

Higher values indicate warm weather and result in a high power production for cooling. Knowledge of the number of cooling degree-days in an area in the summer gives power companies a way of predicting the energy demand during peak energy periods. A summary of heating and cooling degree-days can give a practical indication of the energy needed over the year.

In a previous weather life, I used to work operational meteorology for a company called Weather Command in Chicago that has a large number of gas and electric utility clients.

We did very specific hour by hour temperature forecasts to help companies like Commonwealth Edison and Northern Illinois Gas estimate the daily gas or electric load for their customer base. Then we applied various "correction factors" for cloud cover, wind and dew point to get an even better estimate of energy demand.

Accurate daily load forecasts save gas and electric companies millions each year. If they are off on customer demand, and caught short of supply they may have to buy expensive gas or electricity on the spot market, and that's pricey.

Next time you turn on your AC unit or flip the light switch, know there is a meteorologist on the other end of the line helping your utility company to plan for the extra load you are demanding. That meteorologist is also saving you money by making your utility company more efficient and keeping your energy costs down.

Risk of summer: heat and humidity build this weekend

It may not get any better than this for those who covet a warm, increasingly humid summer weekend in Minnesota.

Temps rise through the 80s and make a run at 90 by Sunday afternoon. Dew points climb into the 60s to near a tropical 70 degrees by Sunday. You'll feel the humidity creeping up this weekend. A weak front may bring a few attempted isolated thundershowers Saturday afternoon.

Here's a more detailed temp and dew point break down this weekend from the European Centre for Medium-Range Weather Forecasts Model.

Heat Builds: 90s by Monday

A massive bubble of high pressure over the southwest U.S. will nudge into Minnesota by Monday.

That should be enough to punch temps to the 90 degree mark in the metro for the 2nd, and possibly third times this summer Monday and Tuesday.

More free AC arrives with a cool front later Tuesday into Wednesday.

Thankfully for many the heat won't last, as cooler breezes should return by midweek.

Enjoy the summery forecast.