Persistent Warm Up in Store
Up on the Roof:
During the infancy of the National Weather Service in the late 19th century, observational networks were established and maintained by the U.S. Army Signal Service. Guidelines were quite stringent for locating instruments in suitable exposures, maintaining and calibrating
instruments and filing reports. Failure to perform duties using these guidelines was not tolerated. Sergeants and corporals who failed to adhere to these guidelines were reduced in rank to first-class privates, while first-class privates were reduced to second-class for similar violations.
One of the exposure guidelines advocated for decades was to place the instruments (primarily thermometers and rain gages) on the rooftop of a tall building. This was indeed the case for the Twin Cities climate record as it shows rooftop placement of instruments on the U.S. Court House Building in Minneapolis (Marquette and 3rd St) from November of 1890 to April of 1938. This location essentially measured the climate 105 ft above street level. In addition, even when the Weather Service moved to the MSP airport location, rooftop readings were made from the 1930s until nearly 1960 at elevations that ranged from 30 to 40 ft above ground level.
So what are the consequences of these rooftop measurements? A number of studies have shown that temperature records kept for such locations are consistently higher than those kept near the
ground. Some studies show average differences of 1 or 2 degrees F, while other studies show differences of several degrees, especially where overnight inversions are quite common, or where the radiative properties of the building produce a great deal of heat storage and re-radiation which affects the air temperature. In all cases, rooftop readings are warmer than those taken in standard exposures 5 ft above the ground. This means that in climate change detection studies and forecast verification studies researchers must be careful to correct for observations that are based on rooftop measurements. It is interesting to note that in the private sector and various school systems around the country there is a marked increase in the number of rooftop measurements, likely producing a positive temperature bias in their reports.
Weekly Weather Potpourri:
Typhoon Mawar in the Western Pacific Ocean was producing winds over 130 mph and sea waves from 40 to 50 feet this week. It knocked out much of the power grid on the island of Guam and was expected to move westward towards Taiwan through the weekend. The Joint Typhoon Warning Center was closing following this storm.
The BBC features an interesting article this week about the rising cost of damages from extreme weather events globally, but simultaneously the relative number of deaths caused by extreme weather has been declining. According to World Meteorological Organization studies “better early warning systems and disaster management in developing countries mean fewer people are dying.”
MPR listener question: Has it ever snowed on Memorial Day?
Answer: Observed on May 30th from 1868 to 1970, and then as the last Monday of May since 1971 Memorial Day (formerly known as Decoration Day), is somewhat difficult to track when it comes to weather history. Nevertheless, the Minnesota records show that the Twin Cities has never recorded snowfall on this holiday. On a statewide basis, Decoration Day of 1897 (May 30th) brought snowfall to some northern Minnesota Counties, from a trace amount to 0.1 inches at Bemidji, where the temperature hovered in the 30s and 40s F most of the day. Then, in the more modern record, Memorial Day of 1992 (Monday, May 25th) brought some snow to southern Minnesota locations, from trace amounts at Alexandria, Wadena, Owatonna, and Gaylord, to as much as 1.3 inches at New Ulm, where temperatures hovered in the 30s and 40s F. Whew!
Twin Cities Almanac for May 26th: The average MSP high temperature for this date is 73 degrees F (plus or minus 10 degrees F standard deviation), while the average low is 54 degrees F (plus or minus 8 degrees F standard deviation).
MSP Local Records for May 26th:
MSP records for this date: highest daily maximum temperature of 96 degrees F in 1978; lowest daily maximum temperature of 45 degrees F in 1906; lowest daily minimum temperature of 34 degrees F in 1992; highest daily minimum temperature of 72 degrees F in 1911; record precipitation of 1.60 inches in 1873. No snowfall on this date.
Average dew point for May 26th is 47°F; the maximum dew point on this date is 69°F in 1959; and the minimum dew point on this date is 0 degrees F in 2017.
All-time state records for May 26th:
The state record high temperature for this date is 103 degrees F at Tracy (Lyon County) in 1914. The state record low temperature for this date is 20 degrees F at Cook (St Louis County) in 1961. The state record precipitation for this date is 3.48 inches at Grand Meadow (Mower County) in 1978. Record snowfall is 2.0 inches at Kelliher (Beltrami County) in 1970.
May 26, 1914 brought a mid-summer Heat Wave to Minnesota. Climate observers in 27 counties reported afternoon high temperatures of 90°F or greater. After a morning low of 59°F, Tracy (Lyon County) reached an afternoon high of 103°F.
Some newly emerged Minnesota crops suffered frost damage on May 26, 1961 as morning temperatures dipped into the 20s and low 30s F. Caledonia in Houston County dropped to 28°F damaging some newly emerged corn fields. In many areas the afternoon high temperatures were in the 50s F.
A strong low-pressure system with an associated cold front brought a mixture of precipitation to Minnesota over May 26-27, 1970. Many areas reported over an inch of rainfall and Rochester reported 2.08 inches. In northern areas of the state both sleet and snow were reported. Climate observers in St Louis and Beltrami Counties reported measurable snowfalls up to 2.0 inches.
Mostly sunny and warm over Memorial Weekend, becoming breezing on Sunday and Monday. There will be increasing cloudiness with a chance of showers in northern counties on Monday, then a more widespread chance for showers across the state on Tuesday. Warmer than normal temperatures will prevail much of next week.