|HydroClim Minnesota - August, 2000
A monthly electronic newsletter summarizing Minnesota climate conditions and the resulting impact on water resources.
Distributed on the Wednesday following the first Monday of each month.
State Climatology Office - DNR Waters
WHAT HAS HAPPENED:
- precipitation patterns varied widely across Minnesota during the month of July. The northern one third of Minnesota reported near to below normal precipitation for the month, whereas the southern two thirds of the state reported near to much above normal monthly precipitation. For some southern Minnesota counties, the first 10 days of July brought a continuation of the heavy rainfalls which commenced in May and June. During the second half of the month, nearly all of Minnesota reported below normal precipitation.
(see: http://climate.umn.edu/cawap/monsum/monsum.asp , http://climate.umn.edu/doc/weekmap.asp)
- a number of heavy precipitation events occurred during the second week of July in Minnesota. Two of these events led to significant urban flooding. Torrential rains fell upon the Twin Cities metropolitan area during the weekend of July 7 - 10. The spate of severe weather began during the morning of Friday, July 7 when severe storms rumbled through the east metro. The south metro, specifically the city of Eagan, received an extraordinary amount of precipitation later on the 7th, and during the early morning hours of July 8. Over eight inches of rain fell in a three to five hour span across a small area of northern Dakota county. The climatological probability of receiving eight inches of rain in a five hour period for a given location in the metropolitan area, in a given year, is far less than 1%. An additional two to three inches of rain fell over many of the same areas during the evening of the 8th and morning of July 9th. On July 9th and 10th, south central and southeastern Minnesota was also doused by very heavy rains. Three to five inches of rain fell upon soils already saturated from late spring and early summer storms. During the afternoon of July 10, the Cedar River at Austin (Mower county) crested at a record level of 23.4 feet, 1.5 feet above the previous flood of record. Numerous homes, businesses, bridges, and city streets in Austin were affected.
- July temperatures were generally near historical averages across Minnesota. Nonetheless, Minnesota experienced seasonally cold weather for a brief period. Record cold maximum temperatures were reported in many areas on July 18, with daytime temperatures remaining in the 50's. Minimum temperatures dropped below freezing the next morning in some northeastern Minnesota communities.
(see: http://climate.umn.edu/cawap/monsum/monsum.asp , http://climate.umn.edu/doc/whatsnew.htm)
- while not directly tied to hydrology, a notable atmospheric phenomenon occurred on July 25. A destructive and deadly tornado struck the city of Granite Falls (Yellow Medicine county) on July 25, 2000. One person was killed, over a dozen injured, and millions of dollars of damage was done to residences, businesses, and public facilities.
WHERE WE STAND NOW:
- precipitation totals for the growing season (April 1 to present) are 25% or more above historical averages for much of southern central and southeastern Minnesota, and for a small portion of northwestern Minnesota. In an area of east central and central Minnesota, extending from the northern suburbs of the Twin Cities to St. Cloud, growing season rainfall values are short of normal by more than 25%. Pockets of dryness also exist in northwestern and north central Minnesota, and along the north shore of Lake Superior.
- as of their August 1 release, the National Drought Mitigation Center does not classify any Minnesota region in a drought category. Extreme southwestern Minnesota edges into the "abnormally dry" designation. The NDMC index is a blend of science and subjectivity where intensity categories are based on six key indicators and numerous supplementary indicators.
- the August 5th Palmer Drought Index depicts most of Minnesota in the "Near Normal" category. Northeastern Minnesota has made a relatively sudden jump into the "Moderate Drought" designation. It should be noted that real-time data available to the Palmer Drought Index are sparse. In the case of northeastern Minnesota, data from three stations are used. Two of these three stations are located on the shore of Lake Superior where State Climatology Office depictions show precipitation deficits (see above). South central Minnesota is categorized as experiencing an "Unusual Moist Spell". The Palmer Drought Index is used for assessing long-term meteorological conditions.
- the Minnesota Agricultural Statistics Service reports that topsoil moisture conditions across the state were rated 7% very short, 26% short, 66% adequate, and 1% surplus as of Friday, August 4. They state that row crops are showing signs of moisture stress in sandy soils. Plant available soil moisture measured at the U. of M. facility in Waseca (Waseca county) on August 1 was at 69% of field capacity. Plant available moisture measured at Lamberton (Redwood county) on August 1 was at approximately 35% of field capacity. At the time of measurement, moisture in the top two feet of the soil profile was low at both locations. Measurements are taken at plots planted in corn, soybeans, or a corn/soybean rotation.
(see: http://www.nass.usda.gov/mn/cwmn.htm , http://climate.umn.edu/img/soil_moisture/wassm12.gif ,
- current discharge values for most Minnesota streams rank between the 25th and 75th percentile for this time of year. According to U.S. Geological Survey reports, flows rank above the 75th percentile in some southeastern and northwestern streams.
(see http://water.usgs.gov/cgi-bin/daily_flow?mn , http://www.dnr.state.mn.us/waters/programs/surwat_section/stream_hydro/productsf.html)
- the wildfire danger potential is rated as "moderate" for portions of northwestern, north central, and northeastern Minnesota. In far northeastern Minnesota, the "blow down" zones are rated in the "high" fire danger category.
- the 30-day outlook from the Climate Prediction Center calls for near normal August precipitation statewide. Normal August precipitation ranges from near three inches in the west, to near four inches in the east. The August temperature outlook is for near normal conditions statewide. Normal August high temperatures are in the low 80's in early August (except near Lake Superior), dropping to the mid 70's by month's end. Normal August lows drop from near 60 early in the month to the mid 50's by late August.
- the 90-day outlook for August through October indicates near normal precipitation statewide. The August through October temperature outlook tilts towards above normal conditions throughout Minnesota.
FROM THE AUTHOR:
- one of the more confusing phrases used in meteorology and hydrology is "100-year storm". The phrase implies that an intense rain storm dubbed as a 100-year event brings rainfall totals heretofore unseen for 100 years, and not to be experienced again for another century. This is a logical, but incorrect conclusion to draw from the phrase. A "100-year storm" drops rainfall totals having a one percent probability of occurring at that location that year. Encountering a 100-year storm on one day does nothing to change the odds of seeing the same amount of precipitation the very next day.
Intense rainfalls are typically geographically isolated. Therefore, increased population density and improved precipitation monitoring networks have increased the likelihood of capturing (measuring) heavy rain events. Also, improved communication allows faster and more complete transfer of weather information. When the neighboring county is walloped by a "100-year storm", we hear about it quickly. Invariably we will vicariously "experience" the event and wonder why "100-year storms" seem to be occurring every month! July's flash flood event in northern Dakota county occurred in an area struck by a storm of similar magnitude in July of 1987. In both downpours, the precipitation totals far exceeded the "100 year storm" design threshold of six inches for the area. The communities affected by these storms tragically beat the odds twice in 13 years.
NOTES FROM THE FIELD:
- none this month
UPCOMING DATES OF NOTE:
- August 17, Climate Prediction Center releases 30/90 day outlooks
WEB SITES FEATURED IN THIS EDITION:
http://climate.umn.edu - Minnesota Climatology Working Group
http://enso.unl.edu/ndmc/ - National Drought Mitigation Center
http://www.cpc.ncep.noaa.gov - Climate Prediction Center
http://www.nass.usda.gov/mn/ - Minnesota Agricultural Statistics Service
http://swroc.coafes.umn.edu - University of Minnesota Southwest Research and Outreach Center
http://water.usgs.gov/cgi-bin/daily_flow?mn - U.S. Geological Survey, Minnesota
http://www.dnr.state.mn.us/waters/ - DNR Waters
http://www.dnr.state.mn.us/forestry/ - DNR Wildfire Information Center
Dana Dostert, DNR Waters Hydrologist - St. Paul
Dave Ruschy, Department of Soil, Water and Climate - U. of M.
Contributions of information and suggestions are welcome!