Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 02:53 PM GMT am 21. März 2012
Summer in March continued for the eighth day yesterday, toppling dozens of records for hottest March day in both the U.S. and Canada. Nearly every major airport in Michigan's Lower Peninsula tied or set records for their hottest March temperature, including Detroit (82°), Flint (84°F), Saginaw (83°F), Grand Rapids (83°), Muskegon (82°), Lansing (83°), Alpena (84°), Gaylord (80°), Pellston (84°), Traverse City (86°), and Houghton Lake (81°). Most of these records will be broken again today or tomorrow. Detroit's current string of six days over 70° is unprecedented so early in the year. One has to go back over 125 years to find an early-season warm spell that compares, and even that streak occurred in April, a full month later (April 16 - 24, 1886.)
Pellston, Michigan in the Northern Lower Peninsula is called "Michigan's Icebox", since it frequently records the coldest temperatures in the state, and in the entire nation. But the past four days, Pellston has topped out at 80° - 84°F, the first 80°F March days in their history. Yesterday's 84° reading broke the previous record for the date (55° in 1976 and 1948) by an unbelievable 29°, and was 44°F above average. Nearby Traverse City hit 86°F yesterday, which was 45°F above the average high for the date, and was the fourth consecutive day with a hottest March temperature on record.
Figure 1. Summer-like temperatures this March in the Midwest have heated up Lake Michigan to record warm levels for this time of year. The average temperature of the lake is characteristic of what occurs in June. Image credit: NOAA. Thanks to wunderground member Neapolitan for posting this image in my blog comments.
Hot times in Lake Michigan
The NWS in Chicago reported yesterday that the Windy City's high of 85°F that day boosted the average temperature for the month to levels that would make March the 7th warmest April in the city's 140-year record. The unprecedented March warmth in the states surrounding Lake Michigan have heated the lake to temperatures never seen before this early in the year. Water temperatures at the South Lake Michigan buoy were 46 - 47°F yesterday (8°C), which is about 10°F above average for this time of year, and typical of early June temperatures.
Record March warmth spreads into New England
Temperatures across much of Vermont, New Hampshire, and Maine yesterday were the warmest on record for so early in the year. Burlington, Vermont's 80°F was 39°F above the average high for the date, and the earliest 80° reading in recorded history. Concord, New Hampshire (81°) and Bangor, Maine (78°), also had their warmest temperatures for so early in the year. The 73°F recorded in Caribou, Maine tied for that city's highest March temperature on record, and broke the record for the date by a remarkable 23°F.
Record warmth continues in Canada
Numerous all-time warmest March temperatures were recorded in Ontario, Canada yesterday, including Windsor at 27°C (previous record, 26.6°C), Sarnia (26°C, previous record 25.6°C), and London (25°C, previous record 24.8°C). High temperatures for Wednesday and Thursday across Ontario, Quebec, New Brunswick, and Nova Scotia are likely to break records for hottest March day for most of the major cities in these provinces, including Toronto, Ottawa, Montreal, St. Johns, and Halifax.
Figure 2. The jet stream pattern features a large, southwards dipping bulge over the Western U.S., creating a trough of low pressure with cold and snow, and a large, northwards looping bulge over the Central U.S., creating a record-strength ridge of high pressure. The Western U.S. trough has cut off into a "cut-off low" that will slowly drift eastwards during the remainder of the week.
Summer in March ends by Friday
The unprecedented Summer in March conditions are due to a loop in the jet stream that has created a large upper-level ridge of high pressure that is stuck in place over the Eastern U.S. Since the jet stream acts as the boundary between cold air to the north and warm air to the south, and the large loop in the jet places its axis far to the north of the eastern U.S., summer-like warmth has developed over the eastern half of the U.S. Conversely, colder than average temperatures have developed over the western third of the U.S. behind the southwards-dipping loop of the jet stream. This jet stream pattern is too extreme to be stable, and the big loop over the Western U.S. has broken off to form a giant eddy. The resulting area of low pressure is known as a "cut-off low", because it is cut off from the jet stream. The cut-off low will drift slowly eastwards during the week, gradually bringing an end to "Summer in March" over the Eastern half of the U.S. by Friday.
How rare is this Summer in March heat event?
One measure of how record-breaking this "Summer in March" heat wave has been is the impact it had on NOAA's National Climatic Data Center web site. The extremes section of the their web site has been down since last Friday, since their software has been unable to handle both the huge number of records being set and the huge demand from people wanting to see these records. The web site came back on-line this morning with software re-engineered to handle the load, but only with data through Sunday.
We can also quantify how rare a meteorological event is by looking at statistics of past years. By averaging together at least 30 years of data to take a representative snapshot of the climate, we can generate a mean and a standard deviation of the data. The standard deviation gives a measure of how much the data fluctuates around the mean.
In comparing deviations from normal across wide regions, it helps to normalize the deviations. A temperature deviation of 3 degrees C may be not that unusual in one region, but may be very significant in another. The solution is to use climatological anomalies (which we often refer to by the Greek letter, sigma.) Calculating the climatological anomaly is a two step process. First, we calculate the difference between a quantity (i.e., temperature) and it's 30-year average value. Then we normalize the difference by dividing it with the 30-year standard deviation. From statistical theory, we know how unusual climatological anomalies are by value:
Odds of a deviation > 1 climatological anomaly=31.7%
Odds of a deviation > 2 climatological anomalies=4.5%
Odds of a deviation > 3 climatological anomalies=0.27%
Odds of a deviation > 4 climatological anomalies=6.34/1000%
Odds of a deviation > 5 climatological anomalies=5.7/100000%
Odds of a deviation > 6 climatological anomalies=1.9/1000000%
So, if we have a 30-year history of high temperatures for a particular date, we'd expect 20 of those years to be 1-sigma years, when the temperature is plus or minus 34% of average (ten colder years, and ten warmer years.) Rare 2-sigma events occur 4.5% of the time, so we should have about 16 of these per year. Even rarer 3-sigma events occur just 0.27% of the time, or just one day per year, on average. Truly extreme 4-sigma events should only occur once every 43 years. Much of Michigan, Wisconsin, Illinois, Minnesota, Northeast Iowa, and the eastern Dakota have experienced multiple 4-sigma days over the past week.
Wunderground is computing 30-year means of the weather for each day of the year using data from the Climate Forecast System Reanalysis (CFSR) (NOMADS data repository). Here is a description how a reanalysis works. CFSR is notable because it is the first reanalysis to use a coupled atmosphere-ocean model. As a result, CFSR has physically consistent estimates of the conditions of the atmosphere, ocean, and land. CFSR has data from 1979 (When polar-orbiting satellites became able to estimate vertical profiles of temperature) to 2010. We can take the mean and standard deviation for each calendar day using this history, and compare it to the current forecast from the GFS model. The result is an image showing how far from average the temperatures are. Yesterday, the analysis showed that Michigan experienced temperatures that were 4 - 5 climatological anomalies warmer than average (4-sigma to 5-sigma), the type of extreme that occurs between once every 43 years and once every 4779 years. Of course, using 30 years of data to estimate extreme events with a return period of centuries is a sketchy proposition. However, keep in mind that had we used a century-long climatology instead of using the past 30 years, yesterday's warmth would have been classified as much more extreme, since the climate has warmed considerably in the past 30 years. It is highly unlikely the warmth of the current "Summer in March" heat wave could have occurred unless the climate was warming.
Figure 3. Climatological anomalies for March 20, 2012. Michigan experienced temperatures that were 4 - 5 climatological anomalies warmer than average (4-sigma to 5-sigma), the type of extreme that occurs between once every 43 years and once every 4779 years. Wunderground plans to make these plots available in real time on our web site later this year.
Heavy rains create flash flood concerns in Texas, Louisiana, and Oklahoma
Widespread rainfall amounts of 3 to 5 inches have fallen over the past two days in Eastern Texas, Eastern Oklahoma, Western Arkansas, and Louisiana over the past two days, creating a serious flash flood hazard. So far, no major river flooding has been reported, and it appears the damage from this flood event will be limited. A few rainfall amounts from the event, from 7 pm CDT Sunday - 3 am CDT Wednesday, taken from the latest NWS Storm Summary:
FORT HOOD AAF/KILLEEN 5.43
DALLAS LOVE FIELD 4.49
NWS FORT WORTH 4.13
OKLAHOMA CITY 3.18
FORT POLK 6.14
LAKE CHARLES 2.11
FORT SMITH 3.49
LITTLE ROCK 2.22
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