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
compiled 8/9/00
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.
(see: http://climate.umn.edu/doc/journal/flash_floods/ff000710.htm)
- 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.
(see: http://climate.umn.edu/doc/weekmap.asp)
- 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.
(see http://enso.unl.edu/monitor/monitor.html)
- 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.
(see http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/regional_monitoring/palmer.gif)
- 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
,
http://swroc.coafes.umn.edu/Weather/Charts/Soil/00_soil_water.html)
- 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.
(see: http://www.dnr.state.mn.us/forestry/fire/)
FUTURE PROSPECTS:
- 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.
(see: http://www.cpc.ncep.noaa.gov/products/predictions/multi_season/13_seasonal_outlooks/color/seasonal_forecast.html)
- 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.
(see http://www.cpc.ncep.noaa.gov/products/predictions/multi_season/13_seasonal_outlooks/color/seasonal_forecast.html)
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
ACKNOWLEDGEMENTS:
Dana Dostert, DNR Waters Hydrologist - St. Paul
Dave Ruschy, Department of Soil, Water and Climate - U. of M.
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