Climate Summary

Temperature and Precipitation by climate division
Departures from Normal for April 2010 - based on preliminary data.

April 2010 in North Carolina was hot and dry. Coming off the coldest winter in 30 years, the temperature patterns have swung in the opposite direction. The first week of April 2010 was generally the hottest for early April ever recorded. Many locations in NC experienced the earliest 90°F maximum temperature ever, and most locations observed less than half of their normal rainfall for the month. While milder temperatures were experienced in mid- and late-April, it was still on the warm side. For many locations in central and western NC, average maximum temperatures for April 2010 ranked as the top 5 warmest on record. Statewide, April 2010 ranked as the 12th warmest and the 9th driest since 1895.

Another Drought? Maybe.

With such hot and dry conditions in April, are we headed for another drought? Maybe. Maybe not. We had a very wet early winter, which brought recovery to all water resources. Similarly, there have been few impacts to agriculture since there is limited production during this time of year. However, fire activity has been a bit high this year. More importantly for the drought forecast is that winter and spring conditions are not a good predictor of summer climate. Based on analysis of past seasonal climate patterns, we find dry springs with wet summers and dry springs with dry summers. Summer climate is very challenging to predict, and the science isn't there…yet. There's lots of research looking at the prediction of summer conditions in the southeast, and so we hope to offer better summer season forecast within a few years. Right now, the outlook is for equal chances for above-normal, near-normal, and below-normal precipitation. That's a fancy way of saying "We don't know".

Precipitation for April 2010
Based on estimates from NWS Radar
Data courtesy NWS/NCEP

Precipitation for April 2010: Percent of Normal
Based on estimates from NWS Radar
Data courtesy NWS/NCEP

Impacts to Agriculture and Water Resources

While drier conditions were welcome to growers for a while, many are looking for rainfall again to ensure proper germination and early season growth.

The warm, dry weather hasn’t affected reservoir or groundwater levels, but streams, especially in eastern NC, are starting to drop down and have reached or are approaching drought levels. In late April, D0 (Abnormally Dry) was again introduced in parts of central and eastern NC. The NC Drought Management Advisory Council is again holding weekly technical meetings to review all drought indicators and provide recommendations to local agencies and the US Drought Monitor. The depiction below emphasizes the short-term dryness that especially central and eastern NC have experienced in April 2010.

Growing Degree Days

Growing degree days (GDD) are a measure of heat accumulation used as a guide for planting crops and for estimating crop maturity dates. Unless plants are stressed by other environmental factors such as drought, their development rate typically depends upon the accumulation of specific quantities of heat, as determined by the daily air temperature. GDDs are often used for assessing the suitability of a region for growing a specific crop, estimating the stage of growth for a particular crop, and estimating the heat stress on crops. GDDs are calculated by subtracting a given base temperature from the mean temperature (maximum plus minimum, divided by 2). While 10°C (50°F) is the most common base number used in GDD calculations, many utilize other base numbers that more closely represent the life cycle of their specific crop, such as 8°C for potatoes or 5.5°C for wheat.

Given that North Carolina and other southeastern states are favorable for growing a wide range of crops, the climate office has developed a map interface for displaying GDD accumulations across the entire southeast. Users can enter in a start date, end date, and base number for generating GDDs for all temperature-measuring stations, which are then depicted using Google Maps. Upon clicking on a given station, the user can view the GDD accumulation specific to that station, as well as some of the station’s basic metadata. Users interested in viewing GDD accumulations for all of the stations in a tabular format may click on the link located just below the map to generate an Excel spreadsheet containing this information.

The Growing Degree Day product can be accessed through the Agriculture section of the SCO website.

Cucurbit Downy Mildew Forecasting

Many tools are used to aid in the forecast of plant disease epidemics as they spread downstream from various sources. Cucurbit Downy Mildew (CDM) is a fungus that can affect vegetables such as squash, cucumbers, and pumpkins, often resulting in a decrease in yield, quality and harvest time. CDM is most common in humid climates with frequent rainfall, and can spread rapidly under favorable weather conditions. Preventing and treating diseases like CDM is also very time consuming and costly, leading growers to strive for a balance between protecting their crop and reducing costs by spraying only when necessary.

In conjunction with the Cucurbit Integrated Pest Management (ipmPIPE) team at NC State, the SCO has recently launched a new website to strike the balance between disease prevention and cost reduction by providing CDM forecast information to growers. Tools available to the public include national maps of downy mildew risk assessment and disease forecasting, as well as customizable email and text message alerts for growers who want to know when the disease is reported nearby.

The cucurbit downy mildew epidemic status map (right) allows users to see where the disease has been reported. Users can click on an affected county to see detailed information such as dates of disease symptoms, the type of plant affected, and the current status of the report. A smaller regional disease map is also shown on every page of the website, based on the users' internet location, allowing for quick link to this detailed information. New visual forecasts are also included to compliment the text forecast discussions provided by the ipmPIPE forecast team (below).

View these products and more at http://cdm.ipmpipe.org

Grant funding is provided by the USDA Risk Management Agency. Collaborators at NC State include Dr. Peter Ojiambo, Wendy Britton, and Thomas Keever in the Department of Plant Pathology.

Estimated Evapotranspiration and Open Water Evaporation

Of particular interest to agricultural and irrigation practices, evapotranspiration (ET) is a combination of evaporation and plant transpiration processes into a total moisture flux from the ground to the atmosphere. Since ET observations are only available for a limited number of locations across the southeastern United States, empirical models, such as the UN Food and Agriculture Organization paper number 56 (FAO56) Penman-Monteith equation, are typically used to estimate ET at local and regional scales.

The FAO56 Penman-Monteith method estimates reference crop ET for a well-watered surface based on physical atmospheric observations of solar radiation, temperature, wind speed, and relative humidity. The reference surface is a theoretical grass reference crop with a height of 0.12m, an albedo of 0.23, and a constant surface resistance of 70 s/m. While dependent on time of year and location, the equation is developed for the hypothetical grass reference crop and is thus independent of specific crop characteristics and soil factors.

Crop coefficients contain the effects of characteristics that differentiate a particular crop surface from the reference surface (crop height, albedo, canopy resistance, evaporation from soil) and can be applied to adjust the estimate for that specified crop under standard conditions (no disease, adequate soil water, efficient fertilization in large fields), known as crop ET. These characteristics differ based on the crop growth stage and thus, crop coefficients can vary during the growing season. The FAO56 paper provides guidance for estimating crop ET under non-standard conditions, such as soil water stress conditions.

The Penman-Monteith equation can also be adjusted to estimate open water evaporation by using open water and its associated characteristics as the reference surface. Specific characteristics include an albedo of 0.05, height of 2mm, and a canopy surface resistance of zero.

To obtain daily reference crop ET, crop ET, or open water evaporation, please visit our CRONOS interface. For more detailed information on the FAO56 Penman Monteith method, please refer to the UN FAO56 Penman-Monteith documentation.