|HydroClim Minnesota - April 2002
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
- March precipitation was generally near normal
across much of Minnesota. In some west central and central Minnesota
counties, March precipitation totals finished above normal by about one
half inch. After a three month period lacking in significant snow events,
March compensated for the shortfall by bringing three substantial snow
storms to Minnesota. During the three day period, March 7 through March 9,
two major winter storms dropped heavy snow on central and northeastern
Minnesota, rain and damaging freezing rain on southeastern Minnesota, and
generated high winds nearing 50 mph in many areas. The third major winter
storm swept through Minnesota on March 14 and 15. Snowfall totals from
this event topped 15 inches along a 70 mile wide band from Canby to
Hinckley. Six or more inches of snow was reported over much of the
remainder of the southern two thirds of Minnesota. In some areas of south
central and southeastern Minnesota, the storm produced freezing rain that
coated surfaces with one quarter to one half inch of ice.
- in sharp contrast with a winter-long trend of unusually warm weather,
March temperatures finished significantly below normal. Temperatures
across the state were four to nine degrees lower than historical averages.
In a unique juxtaposition, the March mean temperature was colder than any
of the preceding winter months in many communities.
WHERE WE STAND NOW
- snow depths at the end of March were generally
less than four inches in all but east central and northeastern Minnesota.
Large sections of northwestern and southeastern Minnesota reported almost
no snow cover. The March 28 snow depth ranking map indicated that snow
depth values ranked near the historical median in a one hundred mile wide
band extending from southwestern to northeastern Minnesota. Elsewhere,
snow depths ranked below the 20th percentile for the date. Early April
snows, not depicted in the late March snow maps, exceeded six inches in
some east central Minnesota communities.
- as of March 26, the National Drought Mitigation Center designates much
of western and northern Minnesota as "Abnormally Dry". The
remainder of the state is free of any drought 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 March 30 Palmer Drought Severity Index (PDSI) from the Climate
Prediction Center places northwestern Minnesota counties in the
"Moderate Drought" category. Other areas are categorized as
"Near Normal". The Palmer Drought Severity Index is used for
assessing long-term meteorological conditions.
- the U.S. Geological Survey indicates that the majority of discharge
values for Minnesota streams (where winter reporting is possible) are in
the normal category for the date. Some streamflows in scattered areas
around the state are below normal.
- soil frost depth data gathered on April 1 by the U.S. Army Corps of
Engineers and the University of Minnesota indicate that frost depths, as
measured under sod-covered surfaces, were 6 to 16 inches in the south, 16
to 28 inches in the west and north. Frost depths across Minnesota are
highly variable depending on the amount of snow cover and soil type. For
some locations, frost is near maximum depth for the season. Soil frost
typically reaches maximum depth in late February and is gone by the first
week of April in the south, mid-April in the north. Soil frost will thaw
from both above and below, leaving a mid-profile ice lens to thaw last.
- the extraordinarily warm weather during December, January, and February
led to relatively thin and unconsolidated lake ice cover. It appeared
certain that lake ice-out would arrive early this spring. However, the
cold March temperatures delayed ice-out progress, and the outlook for
ice-out dates is unclear. Long-term average lake ice-out occurs during the
first week of April in the southern tier of Minnesota counties; near the
end of the second week of April in the Twin Cities metropolitan area;
towards the end of the third week of April for Brainerd, Alexandria,
Detroit Lakes area lakes; and during the final week of April in far
- the April precipitation outlook from the Climate
Prediction Center shows no significant tendencies away from climatological
probabilities. April precipitation normals range from one and one half
inches in northwestern Minnesota to around three inches in southeastern
- the April temperature outlook also shows no significant tendencies away
from climatological probabilities. Normal April high temperatures are in
the low to mid 40's early in the month, rising to near 60 by month's end.
Normal April lows are near 30 to start the month and climb to around 40 as
the month ends.
- the 90-day precipitation outlook for April through June shows no
significant tendencies away from climatological probabilities. The April
though June temperature outlook also shows no significant tendencies away
from climatological probabilities.
- the National Weather Service produces long-range probabilistic river
stage and discharge outlooks for the Red River, Minnesota River, and upper
Mississippi River basins. A hydrologic model is initialized using the
current conditions of stream flow and soil moisture across a basin. The
model is allowed to run into the future with multiple scenarios using more
than 30 years of climatological data. The climatological data are weighted
by the 90 day outlooks for temperature and precipitation trends. The model
output offers a complete range of probabilistic values of stream stage and
discharge for numerous forecast points. The product offers a risk
assessment tool which can be used in long-range planning decisions
involving flooding or low-flow concerns. These products are part of the
National Weather Service's Advanced Hydrologic Prediction Service (AHPS)
and will be produced near the middle of each month.
FROM THE AUTHOR
- heavy late November rainfall and snowfall brought
relief to areas affected by precipitation deficits established during the
later part of the 2001 growing season. Because the considerable November
precipitation fell before soil freeze-up, the soil moisture profile was
amply recharged in most areas of the southeastern three fourths of
Minnesota. Soil moisture changes relatively little while the soil is
frozen, therefore the early spring soil moisture situation should reflect
the situation found at soil freeze-up. The primary area of concern for dry
soils at this time is northwestern Minnesota. Many areas in the Red River
Valley were missed by the late autumn precipitation. Snowfall in these
areas was sparse throughout the winter, and wind erosion has been a
- in the coming months you may hear a great deal about the onset of an
"El Niņo". The El Niņo phenomenon is a semi-regular (every two
to seven years) warming of the sea surface in the eastern equatorial
Pacific. There are no known connections between El Niņo and Minnesota's
growing season weather. However, El Niņos are often associated with warm
and relatively snow free winters in Minnesota. I would like to emphasize
that El Niņo is not presently in place, but there are indications that it
may evolve later in the year.
NOTES FROM AROUND THE STATE
UPCOMING DATES OF NOTE
- April 18, Climate Prediction Center releases 30/90
day temperature and precipitation outlooks
- April 18-25, Probabilistic Flood Outlooks issued by National Weather
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
- U.S. Geological Survey, Minnesota
- U.S. Army Corps of Engineers, St. Paul District
http://www.crh.noaa.gov/ahps/index.html - National Weather Service - Central Region Headquarters
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