|HydroClim Minnesota - September, 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:
- rainfall was generally near to below normal for most of Minnesota during the month of August. Some northwestern and north central Minnesota counties were exceptions to this general pattern and reported August precipitation totals an inch or more above the norm. Additionally, isolated heavy August thunderstorms brought larger amounts of rain to some small areas of southern Minnesota. Rochester set an all-time record for total rainfall for the months of May through August. The May through August total of 30.70 inches breaks the old record for the same period set in 1990 of 27.38 inches.
(see: http://climate.umn.edu/cawap/monsum/monsum.asp , http://climate.umn.edu/doc/weekmap.asp)
- a significant rainfall event occurred after the end of August, but before the posting of this note. Therefore, it will be mentioned here in the August summary. During the evening of September 2, very heavy thunderstorms formed just to the west of the Twin Cities metropolitan area and dropped three to four inches of rain in less than three hours across portions of Ramsey County. A six inch rainfall total was reported in northern Ramsey County. During a one hour period, 2.60 inches of rain fell on the University of Minnesota - St. Paul campus. A one hour total of 2.60 inches is approximately a "50 year event" for the Twin Cities (see "FROM THE AUTHOR" below).
- August temperatures were near the historical average across Minnesota. The state experienced a number of sultry days, with dew point temperatures climbing into the 70's on six occasions.
(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 August 7. A destructive tornado struck in and near the city of Lakefield (Jackson county), causing significant damage. In another tragic weather-related item, a man was killed in Cass county on August 14 when thunderstorm winds caused a tree branch to fall upon his tent.
WHERE WE STAND NOW:
- in an area of central and east central Minnesota, extending from roughly St. Cloud to the northern suburbs of the Twin Cities, growing season (April 1 - present) rainfall totals are short of normal by more than 30%. Unlike much of the rest of the state, this area has never fully rebounded from the precipitation deficits accrued during the fall, winter, and early spring. For example, total precipitation for Santiago (northern Sherburne county) is approximately nine inches below normal for the last 12 months. (See note below from Professor Bob Weisman of St. Cloud State University concerning central Minnesota dryness.) Pockets of dryness also exist along the north shore of Lake Superior, especially Cook county. In contrast, late summer rainfalls have pushed growing season precipitation totals in much of northwestern Minnesota to 25% or more above the historical average. This is a continuation of the wet regime this area has seen over much of the last 10 years. As noted earlier about Rochester, growing season precipitation in south central and southeastern Minnesota remains well above historical averages, with some locations falling at or above the 98th percentile for April 1 through September 5 rainfall.
- as of their August 31 release, the National Drought Mitigation Center - "U.S. Drought Monitor" does not classify any Minnesota region in a drought category. The NDMC index is a blend of science and subjectivity where intensity categories are based on six key indicators and numerous supplementary indicators. The NDMC depiction attempts to describe drought conditions on a fairly large regional scale, and has difficulty capturing geographically isolated precipitation deficits such as those currently found in portions of central and east central Minnesota (see discussion in Palmer Drought Severity Index section below).
- the September 2nd Palmer Drought Severity Index (PDSI) depicts most of Minnesota in the "Near Normal" category. Counties in south central and southeastern Minnesota are categorized as experiencing an "Unusual Moist Spell". The Palmer Drought Severity Index is used for assessing long-term meteorological conditions. One of the inherent weaknesses of the PDSI is the size of the geographic regions ("climate districts") covered by the analysis. The persistent dryness found in portions of central and east central Minnesota is "washed out" by relative wetness in other counties within the same climate district. Thus in this case, the PDSI fails to illustrate areas of Minnesota experiencing significant precipitation deficits. For this reason, it is always useful to cross-check the PDSI depictions with the State Climatology Office seasonal precipitation maps.
(see http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/regional_monitoring/palmer.gif ,
- the Minnesota Agricultural Statistics Service reports that northwestern Minnesota has surplus topsoil moisture, while the southern half of the state has areas of very short topsoil moisture. Wet soils in northwestern Minnesota have hindered fall harvesting operations whereas crops in moisture stressed fields of southern Minnesota are not ripening normally. Topsoil moisture conditions across the state were rated 14% very short, 30% short, 50% adequate, and 6% surplus as of Friday, September 1.
(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 in the "normal flow" range for this time of year. However, for some portions of central and east central Minnesota, and in one basin in southwestern Minnesota, stream flows are in the "low flow" and "protected flow" categories. Conversely, stream discharge ranks in the "high flow" category in some northwestern and southeastern watersheds.
(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 a ring of counties just north of the metropolitan area, and in Cook county. The "blow down" zones of the BWCA are rated in the "high" fire danger category.
- the 30-day outlook from the Climate Prediction Center calls for near normal September precipitation statewide. Normal September precipitation ranges from near two inches in far western Minnesota, to over three inches in the east. The September temperature outlook tilts towards above normal conditions statewide. Normal September high temperatures fall off rapidly from the low 70's early in the month to the low 60's by month's end. Normal September lows drop from the low to mid 50's early in the month to the upper 30's and low 40's by late September. The average date of the first occurrence of 32 degrees or lower ("frost date") falls within the second week of September in some low-lying areas of far north central and northeastern Minnesota, the third week of the month for the remainder of the northern third of the state, and the last week of September for the northern two thirds of Minnesota. The average frost date for the southern one third of Minnesota occurs during the first week of October.
(see: http://www.cpc.ncep.noaa.gov/products/predictions/multi_season/13_seasonal_outlooks/color/seasonal_forecast.html ,
- the 90-day outlook for September through November indicates near normal precipitation statewide. The September through November temperature also calls for near normal conditions throughout Minnesota.
FROM THE AUTHOR:
(repeated from last month)
- 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 AROUND THE STATE:
- from Bob Weisman, St. Cloud State University:
(excerpted from "St. Cloud August, 2000 and Summer, 2000 weather summaries")
The dryness, which has plagued a small piece of central Minnesota from eastern Stearns, southeastern Morrison, and Benton Counties to the northern Twin Cities Metro, intensified in August, according to the monthly statistics from the Saint Cloud Airport. August's rainfall was only 1.21 inches, 2.75 inches below normal. The low rainfall made August 2000 the driest August since August 1981 (no measurable rain recorded) and the 10th driest August of the 109 years on record. Almost all of the monthly rainfall occurred in a cool storm on the 16th when 1.00 inch fell. Occasional high heat, especially during the first portion of the month, put additional strain on water resources and forced restrictions on outdoor water use in Sauk Rapids, Sartell, and other communities. The low rainfall contributed to the low summer (1 Jun-31 Aug) total of 7.56 inches, the 16th lowest total in St. Cloud records. While both 1992 (7.49 inches) and 1996 (6.74 inches) had lower summer rainfalls, the 2000 rainfall was lower than in the drought years of 1988 (8.14 inches) and 1999 (8.29 inches). This brought the growing season rainfall (1 Apr-31 Aug) to only 11.53 inches, more than 5 1/2 inches below normal.
- from Mike Mueller, DNR Waters Area Hydrologist - Cambridge
Total precip for August at Cambridge was 1.24". From July 12 to the current, we've received about 2" total.
UPCOMING DATES OF NOTE:
- September 14, 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/fire/ - DNR Wildfire Information Center
Bob Weisman, Meteorology Professor, Earth Sciences Department - St. Cloud State University
Mike Mueller, DNR Waters Hydrologist - Cambridge
Dave Ford, DNR Waters Surface Water Engineer - St. Paul
Dana Dostert, DNR Waters Hydrologist - St. Paul
Mark Seeley, Agricultural Meteorologist - U. of M. Extension Service
Contributions of information and suggestions are welcome!