The urban heat island is well observed in the climate records of metropolitan areas. If you reside in downtown Minneapolis/St. Paul you experienced a minimum temperature that was near or even above the thawing point of 32 degrees this morning. The International Airport recorded a low of 32 degrees, while Saint Anthony checked in with 34 degrees. All official temperatures are taken about five feet above ground level.
Meanwhile, it was a different story away from the urban region. Here is a sample of some other minimum temperatures recorded this morning: Chanhassen, Crystal, and Buffalo all at 27 degrees, Further away from downtown, it was a very nippy 23 degrees at Waseca, 25 at St. Cloud and one of the coldest readings of 21 degrees in Princeton. Temperatures in the middle twenties were observed in western Wisconsin as well, with a frosty 25 degrees in Eau Claire.
Obviously the the exact location of the thermometer has some bearing on the temperature, particularly when winds are calm and the sky is clear. But the reality is, we were all under the umbrella of the same air mass. When meterorologists predict lows from the middle 20s to the middle 30s, they are not hedging. They are bracketing the temperatures that can be different in a small radius.
Peter Synder of the University of Minnesota has embarked on additional research to capture the magnitude of the urban heat island in the Twin Cities. He is in particular need for observers within the Interstate 494/694 beltway. If you would be able to assist Peter and his work, he can be contacted at email@example.com. Here's the website for more information; http://www.islands.umn.edu/
I spoke with Greg Spoden at the Minnesota State Climate Office this afternoon and he related how some of his research has shown that with a slight north wind the urban heat island can expand to Farmington. Invaluable research here, especially when scientists are trying to resolve the human impact on global warming.
Greg also confirmed that the latest seasonal outlook for this winter, issued by NOAA yesterday, is using the new normals when defining regions that favor above or below normal temperatures.
Another nice day is in store for Saturday before the opportunity for moisture arrives on Saturday night. It felt quite comfortable in the sunshine this afternoon. We topped out at 60 degrees at the Twin Cities international Airport.
Often meteorologists refer to the prospect of showers as the "threat of rain". When you have been as dry as some locations in Minnesota, you call it an opportunity for rain.
Here's a look at how sparse rainfall has been since late July. As Greg noted, southern Minnesota is seeing moisture shortages that are extremely rare. Being in the zero percentile is not where you'd like to be if you're looking for soil moisture recharge for next year's growing season.
This afternoon's weather graphic from the National Weather Service in Chanhassen. Hopefully we can play out a win-win in the weather world, with precipitation falling overnight on Saturday.