July 4th rains followed by very low dew points

July 4th rains for some:

The first week of July has seen near normal to cooler than normal temperatures with the only significant rainfall coming on July 4th. Many climate observers in the southern half of the state reported rainfalls of half an inch up to an inch. Some eastern and southeastern locations reported over an inch of rainfall, mostly from thunderstorms on in the morning and early afternoon of July 4trh. La Crescent in southeastern Minnesota reported 1.54 inches on July 4th, and an observer near Renville in central Minnesota reported 1.66 inches.

According to the U.S. Drought Monitor, areas of Moderate (57%) to Severe Drought (8%) expanded across Minnesota over the past week, despite the widely scattered thunderstorms on July 4th. The warmer than normal temperatures of June pushed rapid development of the corn and soybean crops, so that corn is near the tasseling stage and soybeans are in the flowering stage, both of which are important for yield potential of these crops. Continued more frequent rains and cooler temperatures would be helpful in this context.

Comment on Low July Dewpoints:

On Thursday (July 6) of this week about a dozen Minnesota climate stations reported a dewpoint in the 30s F. This is extremely low for the month of July, when average dew points are in the upper 50s to low 60s F. In fact, for the Twin Cities climate history, most of the record low dewpoints for the month of July range from 33°F to 43°F.

Dewpoints in the 30s F indicate very little water vapor in the atmosphere, and relative humidity readings in many places on Thursday (July 6) only ranged from 25 percent to 35 percent. About 2 pm in the afternoon on July 6, MSP reported a dewpoint of 39°F with an air temperature of 73°F and a relative humidity of 29 percent. It is interesting to note that since 1999 there have been only two years that July brought dewpoints in the 30s F to Minnesota. Those were in 2001 and 2009, both very brief in duration, lasting less than one day.

By July, our bodies are already acclimated to the heat of summer (especially this year because of the unusually hot June we experienced), so that a temperature in the low 70s F, with a dry dew point in the 30s F can indeed induce a chill in the human body, especially when getting out of a swimming pool or lake as rapid evaporation of water from our skin cools us down quickly.

What is the reason for such low water vapor content across Minnesota following July 4th? Probably two reasons: a drier and cooler Canadian high-pressure system passed over the state, and the geography surrounding Minnesota, is predominately in drought, including Wisconsin, Iowa, eastern South Dakota, eastern Ontario, and southern Manitoba. This means that regardless of which direction the wind blows across Minnesota, it is likely bringing dry air to the state. 

Weekly Weather Potpourri:

The Weather Underground summarized the global temperature anomalies for June of 2023 which was the warmest June in the global climate record. Much of the record-setting warmth globally was driven by very warm ocean temperatures, with many areas showing sea surface temperatures warmer than any June back to the mid-19th Century when record keeping began. Canada has sweltered through both a record hot May and a record hot June.

This week’s AGU-EOS Bulletin features an article about the declining extent of sea ice in the Arctic Ocean which has been measured since 1979. At the current rate of sea ice loss, the Artic Ocean may be ice-free within a quarter of a century. However some scientists suggest that the role of wind in the loss of sea ice is not fully understood nor is it factored into model calculations of sea ice loss.

As reported this week by the BBC, Eumetsat, the intergovernmental organization charged with managing Europe's meteorological assets in orbit released some images of thunderstorm lightning across portions of Europe and Central Africa. The new satellite-based sensors will provide much higher resolutions to help detect severe thunderstorms and their movement over landscape areas that do not have good radar coverage.

MPR listener question:

We were wondering with the extremely dry air this week, what is the record low July dewpoint in the Twin Cities climate record and what relative humidity was associated with it?

Answer:

Good question. The record lowest July dewpoint in the Twin Cities climate record for the month of July is 33°F on July 27, 1934. The relative humidity was just 19 percent. Even with an air temperature of 83°F, that translates to instant chill if you are coming back from a swim in the lake and there is a wind blowing.

Twin Cities Almanac for July 7th:

The average MSP high temperature for this date is 83 degrees F (plus or minus 8 degrees F standard deviation), while the average low is 65 degrees F (plus or minus 7 degrees F standard deviation).

MSP Local Records for July 7th:

MSP records for this date: highest daily maximum temperature of 101 degrees F in 1936; lowest daily maximum temperature of 65 degrees F in 1918; lowest daily minimum temperature of 44 degrees F in 1891; highest daily minimum temperature of 80 degrees F in 1936; record precipitation of 3.00 inches in 1955. No snowfall on this date.

Average dew point for July 7th is 58°F; the maximum dew point on this date is 76°F in 1955; and the minimum dew point on this date is 36 degrees F in 1965.

All-time state records for July 7th:

The state record high temperature for this date is 108 degrees F at Browns Valley (Traverse County) in 1988. The state record low temperature for this date is 24 degrees F at Tower (St Louis County) in 1997. The state record precipitation for this date is 5.00 inches at Elgin (Wabasha County) in 1990. No snowfall has been reported on this date.

Past Weather:

By far the two hottest July 7ths in state history were in 1936 and 1988. On July 7, 1936 every climate observer in the state, except for Grand Marais reported an afternoon temperature in the 90s F or over 100°F. In fact 53 climate stations reported a 100°F or greater. On July 7, 1988 over 40 climate stations reported a daily high temperature of 100°F or greater. Madison in Lac Qui Parle County reported a high of 101°F and an overnight low of 82°F (not a good night for sleeping).

Thunderstorms brought heavy rainfalls to portions of eastern Minnesota on July 7, 1990. Many observers reported 2 to 4 inches of rainfall, while Elgin, northeast of Rochester reported 5 inches.

A cold morning greeted campers in northern Minnesota on July 7, 1997, as morning lows ranged from 24°F to 32°F in portions St Louis and Lake Counties. The afternoon maximum temperature at Tower was only 55°F.

Outlook:

Temperatures will warm closer to normal over the weekend, with slight chances for rain in northern sections of the state. Temperatures will climb to near normal levels next week, and there will be chances for widely scattered showers or thunderstorms just about each day.