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Extension > Mark Seeley's WeatherTalk > Random pattern of thunderstorms prevails this month

Friday, July 20, 2012

Random pattern of thunderstorms prevails this month

Random pattern of thunderstorms prevails this month

Much of the state has been missed by significant rainfalls this month, leading to more Minnesota counties placed in drought status by the U.S. Drought Monitor. Portions of Rock and Nobles, as well as Clay, Norman, Polk, Mahnomen, Pennington, Red Lake, Marshall, and Beltrami Counties were place in the "severe drought" category this week. Much of the rest of northwestern and southwestern Minnesota remains in moderate drought. Elsewhere portions of southern Wisconsin, southern Illinois, Indiana and northern Missouri were placed in the "extreme drought" category.

Despite this, some areas have seen significant, even record-setting amounts of rainfall. On Friday, July 13th International Falls received a record 2.68 inches while Spring Grove received a record 2.95 inches. On July 18th (Wednesday this week) Bethel, Isanti, and Rice received over 2 inches of rainfall, while Milaca reported 1.35 inches and Spring Valley received 1.51 inches. For the month of July so far, the random pattern of thunderstorm activity has produced normal or above normal rainfall amounts for a handful of observers in widely dispersed sections of the state including:
Brainerd (4.17"), International Falls (4.02"), Spring Grove (3.91"), Stillwater (3.73"), Milaca (3.43"), Kabetogama (3.40"), Moose Lake (3.38"), Mora (3.31"), Ottertail (3.11"), and Hallock (2.28").

New Seasonal Climate Outlooks

The NOAA Climate Prediction Center issued new seasonal climate outlooks on Thursday (July 19) covering the period from August through October. For Minnesota the outlooks favor warmer and drier conditions to prevail in August. For the August through October period continuation of the above normal temperature trend of the past year is expected, while precipitation is expected to be below normal in the southeastern part of the state.

Hot weather continues

Temperatures across the region remained well above normal again this week. According to Greg Spoden of the Minnesota State Climatology Office the temperature pattern so far this month is on pace to give us one of the warmest Julys in history. Many places reported Heat Index Values of 100 F or greater again this week, and there were some record temperatures set. On July 16th Milan reported a record warm overnight low of 76 degrees F after a daytime high of 100 degrees F. Then on July 17th Madison (Lac Qui Parle County) reported a record high of 100 degrees F and La Crosse, WI also reported a record high of 100 degrees F. Fortunately cloud cover and somewhat cooler air prevailed on July 18 and 19 holding temperatures down in the 70s and 80s F many places. But a return of the heat is expected this weekend.

25th Anniversary of the Twin Cities greatest rainfall

Between 7:00 pm and 1:00 am the night of July 23 (a Thursday), 1987 a severe thunderstorm complex brought ten inches of rain to MSP International Airport, by far the most ever measured in a single day there. For three consecutive hours, 7-10 pm, the rainfall intensity exceeded 2 inches per hour (a once in 25 year occurrence), and for two hours, 8-10 pm, the intensity exceeded 2.50 inches per hour (a once in 100 year occurrence). The maximum rainfall rate brought 1.55 inches in only 30 minutes. Earlier that week the Twin Cities had received over 4 inches of rainfall and soils were saturated. Needless, to say most of the Metro Area landscape experienced flash flooding that night.

Dew points remained in the low to mid 70s F during the storm, while air temperatures fell from the low 90s into the low 70s F. Flood waters closed many roads and much of the Interstate Highway system around the Twin Cities. Thousands of basements were flooded, and some roofs collapsed. Storm sewers spouted like geysers, leaving manhole covers scattered on roads and sidewalks. This storm combined with other thunderstorm events during that month produced the wettest month in history for most climate observers in the Twin Cities Metro area. July 1987 record total rainfall included.

MSP International Airport 17.90" Edina 17.91" Richfield 17.44"
Fridley 15.33" Chaska 14.67" Brooklyn Park 14.60"
Stillwater 14.04" Mounds View 12.66" Rosemount 12.08"

In fact for the Twin Cities the 1987 total annual precipitation was about 32 inches.....nearly 56 percent of that fell in one month (July) and over 30 percent of that annual total fell in six hours.

Weekly Weather potpourri

Because much of Indiana is in extreme drought, the first ever mandatory water restrictions were put into place by the Indianapolis Metro Area. Stream and river flows are extremely low around the state of Indiana as well.

Earlier this month NASA scientists analyzed imagery data from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor on board the Terra satellite and found that it accurately depicted the drought which has encompassed much of the USA. Minnesota looks relatively lush compared to much of the rest of the nation. You can view the image and read the discussion here.

The forecast for the British Open Golf Tournament at Royal Lytham this weekend is for somewhat unsettled weather, with occasional showers and perhaps gusty winds at times. Conditions may be especially challenging for the world's best golfers.

The NOAA Storm Prediction Center has received only three reports of tornadoes far this month, unusually low for July. However with drought so widespread across the mid-section of the nation this might be expected.

MPR Listener Question

Can you please explain the difference between relative humidity and dewpoint one more time?

Answer: Sure. Relative humidity, expressed as a percentage, is a measure of the relative content of water vapor in the air compared to the maximum amount it could hold at a given air temperature. During the summer months daytime relative humidity commonly varies from 35 to 60 percent. Humans can tolerate a wide range in relative humidity as long as the temperature ranges between 35 and 75 degrees F. Dew point, measured in degrees F, is the temperature at which the water vapor content of the air is at saturation (100 percent relative humidity) and the vapor would condense to droplets. Generally when dewpoints are above 65 degrees F in the summer we start to feel uncomfortable, and when they are above 70 degrees F everyone is uncomfortable regardless of the air temperature and relative humidity. More often than not in the summertime when dewpoints are above 70 degrees F the National Weather Service has to issue a Heat Advisory because the high water vapor content of the air makes a temperature of 85-95 degrees F feel several degrees higher (translated to a Heat Index Value) and effects our own thermoregulation, not letting us easily dissipate our own body heat. The fact is in the summertime dewpoint relates more to our comfort and health, than relative humidity does. At an air temperature of 95 degrees F, with a dewpoint of 78 degrees F, the relative humidity is only 58 percent (not bad for human comfort), but the Heat Index is 111 degrees F (really bad for human comfort and health).

Twin Cities Almanac for July 20th

The average MSP high temperature for this date is 83 degrees F (plus or minus 7 degrees F standard deviation), while the average low is 63 degrees F (plus or minus 6 degrees F standard deviation).

MSP Local Records for July 20th

MSP weather records for this date include: highest daily maximum temperature of 102 degrees F in 1901; lowest daily maximum temperature of 62 degrees F in 1912; lowest daily minimum temperature of 51 F in 1950; highest daily minimum temperature of 80 F in 2011; and record precipitation of 2.75 inches in 1987.
 
Average dew point for July 20th is 62 degrees F, with a maximum of 80 degrees F in 2002 and a minimum of 42 degrees F in 1947.

All-time state records for July 20th

The state record high temperature for this date is 110 degrees F at New London (Kandiyohi County) in 1901. The state record low temperature for this date is 30 degrees F at Fort Ripley (Crow Wing County) in 1871. State record precipitation for this date is 10.75 inches at Beaulieu (Mahnomen County) in 1909; and no snowfall has been reported on this date.

Past Weather Features:

Widespread frost visited central Minnesota over July 20-21, 1871. The observer at Fort Ripley reported consecutive morning low temperatures of 30 degrees F and 34 degrees F.
July 20, 1901 was the middle of a 7 day Heat Wave with temperatures in the 90s F all the way into the Iron Range. Over 30 Minnesota communities reported afternoon highs of 100 degrees F or higher. The overnight low at St Cloud never fell below 80 F. The Heat Wave broke on July 26th as temperatures fell into the 70s and 80s F.

One of the heaviest thunderstorms to cross northern Minnesota occurred over July 19-20, 1909. It brought over 11 inches to Bagley and Beaulieu, and nearly 9 inches to Fosston. Walker reported nearly 6 inches, while Park Rapids had 4.33 inches. Some farm fields were underwater for days.
A 5 day Heat Wave gripped Minnesota over July 17-21, 1932. Over a dozen communities reported temperatures of 100 degrees F or higher. Crops wilted and dry soil below around with the wind. Milan reported an overnight low of 81 degrees F on the 20th.

Last year, July 17-20, 2011, brought a Heat Wave to Minnesota with Heat Index values ranging from 105 F to 130 F. These were driven by high dewpoints as the Twin Cities recorded a temperature of 96 degrees F on the 20th with a dewpoint of 76 degrees F, producing a Heat Index of 110 F. The Heat Wave broke as temperatures fell off into the 80s on July 21st. But, July of 2011 was the 5th warmest in Minnesota history.

Outlook

Continued warmer than normal temperatures over the weekend and into next week. Slight chances for showers and thunderstorms Saturday and Sunday, but widely scattered. Another chance for showers by Wednesday next week.

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