Seasonable weather will return to the U.S. east of the Mississippi River through the rest of the week before cooling reaches the upper Great Plains, according to forecasters.Outlooks for April 17 to April 26 call for a high probability of cooler-than-normal weather across the northern U.S. stretching east as far as the Great Lakes, according to Commodity Weather Group LLC and MDA EarthSat Weather. The Southwest will probably be warmer than normal through April 26, both forecasters say.Bethesda, Maryland-based CWG says the trend will spread to the South starting about April 22, while MDA narrows that to the southern Atlantic coast.Natural gas traders monitor weather forecasts to determine whether temperatures may boost heating or cooling demand. Predictions of below-normal or above-normal temperatures may prompt traders to buy or sell gas futures. Commodity traders watch precipitation forecasts for the potential impact on planting and crop yields.New York's average temperature in mid-to-late April is about 54 degrees Fahrenheit (12 degrees Celsius), according to MDA in Gaithersburg, Maryland. The average temperature in St. Louis is about 58 degrees; in Boston, 50; in Philadelphia, 55; in Atlanta, 62; and in Dallas, 66, according to MDA.