Evapotranspiration Outlook
For the North American Great Plains
INTRODUCTION
This is a subjective assessment of potential evapotranspiration (ET) for the great plains of North America. The intended users for this outlook are storm chasers and operational meteorologists. To better appreciate the discussion in the outlook, it is highly recommended that readers review the "ET BACKGROUND" link above.
Traditionally, the Great Plains is divided into three regions: the Northern Plains, Central Plains and Southern Plains. A review of many references reveals that there is no clearly defined delineation for the Great Plains nor its three regions. However, based on a composite of these sources, the following map will be used to define these areas for the purpose of this outlook:
The "high plains" of the three regions is shaded in pale yellow. It roughly represents the dry, higher elevations of the Great Plains whose native vegetation is dominated by either short or steppe grasses.
IMPACT OF CROP-BASED ET
Crop-based ET will impact moisture advecting through a region. As a rough guide:
Very low ET (white areas) - will normally see no affect on afternoon surface dewpoints
Low ET (green areas) - only subtle enhancement to advected dew points (1 to 2C)
Moderate (yellow areas) - modest enhancement to advected dew points (3 to 4C)
High (red areas) - notable enhancement of advected dewpoints (5+C)
Crops produce the most ET when they are actively growing on sunny days, especially when the humidity is low. However, sunny low humidity days will mean that the dew points will struggle to climb as the boundary layer mixing will be relatively deep. For chasers/forecasters, these days could mean slightly stronger storms than expected. The dewpoint rise will normally be more noticeable on sunny days with modest humidity. These days can catch chasers/forecasters off guard as storm potential may initially only appear modest. The moisture boost in the afternoon can increase BL moisture to more significant levels.
On sunny high humidity days, the crops will require less moisture as stress due to evaporation will be limited. For chasers/forecasters, these days already have ample moisture and ET contributions are much less critical. Finally, cool and cloudy days will mean low ET production because there will be less evaporation and and a reduction in crop activity.
SOIL MOISTURE ASSESSMENT
In the U.S., deep soil moisture (NOAA, Climate Prediction Center) remains generally near to above normal over much of the central and northern Plains, while much of the Southern Plains continue to suffer with generally dry conditions. There is a narrow swath of dry conditions from NE Nebraska through NW Iowa and most of southern Minnesota. Western MT is somewhat dry and this area extends into the western Canadian Prairies. There, drought-like conditions cover much of Alberta and western Saskatchewan. Soil moisture conditions into eastern SK while much of Manitoba is generally above normal.
CROP CONDITIONS
Canadian Prairies - AB crops are a bit behind schedule due to cool weather and very dry conditions. Significant rainfall is needed in many areas. In SK, cool conditions have hindered crop growth and late spring frost has damaged some crops. Concern about dry soil in the west remains. Over MB, due to cool weather and some earlier flooding, crop growth ranges from on-schedule to about 3 weeks behind schedule. Some reseeding has been required due to frost.
MT - Near normal moisture. Winter wheat progressing and spring wheat has emerged. Overall conditions are near normal.
ND - planting complete and most crops have emerged. Progress still about 2 weeks behind schedule.
SD - spring crops have emerged though a bit behind schedule. Winter wheat has progressed well with areas commencing the MAX ET phase.
MN - Crops are progressing. Most are near (south) to behind (north) schedule. The southern half of the state is very dry.
WY - Winter wheat doing well and and is entering its MAX ET phase. Spring wheat planting is nearly complete and are emerging/growing, though a bit behind schedule.
NE - Corn is growing and is generally on schedule though a long way from notable ET production. Winter wheat will be ripening over the next few weeks and ET is beginning to slowly decline.
IA - Corn has emerged and is on schedule. Soybeans are back on schedule and have mostly emerged. Notable ET production will begin in July.
CO - Winter wheat ET production has peaked and is slowly starting to decline. Spring wheat progressing.
KS - Winter wheat ET is declining rapidly and is no longer a factor. Corn and Soybeans have emerged though ET won’t be a factor for many weeks.
MO - corn and soybean planting complete and crops are or have emerged, though about a week behind schedule. Notable ET production expected in July.
OK - ET production no longer a factor.
TX - ET production no longer a factor.
ET OUTLOOK
The primary areas for ET for the middle and latter part of june will be in parts of WY, NE and SD with winter wheat its max ET phase. Upslope ET-enhanced moisture into eastern WY/MT could be notable in this period. After that, winter wheat crops in these areas will gradually start to ripen.
The longer term outlook shows everything shifting northward with decent ET potential for later in June and early July in the northern half of the U.S. Plains. Soil moisture conditions are generally better than the past few years. However, continuing cool weather could push the ET peak back a bit into July over the eastern Northern Plains. Upslope ET-enhanced moisture into MT could quite evident during late June and July this year. Corn and Soybean potential over the eastern Plains also looks good for July.
The Canadian Prairies currently have the best ET potential over eastern areas with the peak likely in mid-late July. The Western Prairies remain dry this year. However, May and June tend to be the wettest months, so all of that could easily change.
This Outlook compiled using June 7-12, 2009 data
Valid: June 13 - June 27, 2009
SOURCES:
