A change in weather patterns is making it more difficult for forecasters to determine how cold the U.S. East will be as March ends and April begins.Commodity Weather Group LLC today predicted near-normal temperatures in the East with the possibility of above-average warming in Maine and the Canadian Maritimes from April 3 to 7, said the group’s president, Matt Rogers.MDA Federal Inc.’s Information Systems has a similar forecast for the period. However, MDA’s Robert Haas said he has low confidence because of unsettled weather erupting as the patterns change next week.“A faster progression could allow for a much cooler looking map,” Haas wrote in a note to clients.An alteration to patterns affecting weather across the continent is adding a layer of difficulty to forecasting, according to Rogers. The current cold in the East is being brought on by a negative phase of the North Atlantic Oscillation, a change in pressures near Greenland that can determine U.S. weather.The negative phase goes away near the end of the 6- to 10- day forecast time frame and will be replaced by a pattern that will bring cold southward in the 11- to 15-day period, Rogers said.‘Struggling’ Computer Models“The models are definitely struggling with pattern transition issues in the 11-15 day,” Rogers said in a note to clients today.Average temperatures for the end of March and early April in New York are about 47 degrees Fahrenheit (8.3 Celsius), according to MDA in Gaithersburg, Maryland. In St. Louis, the average is about 52; in Boston, 43; Chicago, 44; Atlanta, 58; and in Houston, 65.Haas said a storm pattern across the South March 29 to April 2 has caused problems for computer models.Yesterday, the U.S. Climate Prediction Center, in Camp Springs, Maryland, forecast an above-average chance of cooler weather spreading across the eastern half of the country until at least April 6.In the 6- to 10-day outlook, the climate center and Rogers agree an area from Nebraska to the Atlantic has an above-average chance of below-normal temperatures. Haas predicts the cold will be more concentrated in the Ohio Valley, mid-Atlantic and Northeast.Haas and Rogers diverge from the U.S. forecast in the south.The area from eastern Texas to Florida is included in the climate center’s forecast for possibly colder-than-normal weather.