THEWEATHERMOGIL:: It’s all about source regions
This is an important factor in the weather forecasting world because winds from certain directions carry air from certain “source regions.” “Source regions” are large geographical areas that help certain temperature and moisture characteristics develop in the lower atmosphere.
Consider the Gulf of Mexico. It’s a large, relatively homogenous, region filled with “bathtub-like water.” Hence, heat and moisture from it’s waters can create a warm and humid air mass above it. Similarly, air that sits over an icy landscape becomes cold and dry (cold air cannot hold much moisture).
Once formed, these air masses (large geographic regions with similar temperature and moisture characteristics) can move. When they do, significant changes in local weather conditions, possibly far-removed from source region origins, can take place.
Today, a storm system is exiting the Great Lakes, allowing a little chilly air to move southward into the U.S. (Fig. 1a). But, west and southwesterly winds are already coming into play across the western Plains states. These will initially spread warm and dry air northward through the Nation’s mid-section. As the winds across the rest of the Plains states and Mississippi River Valley become southerly, warm and moist air from the Gulf of Mexico will be drawn northward. This will set the stage for yet another bout of severe weather as the week unfolds. In fact, NOAA’s Storm Prediction Center is indicating a large region of severe storm potential on Mar. 30 from Iowa southward to Louisiana (Fig. 2).
However, by the start of April, a very chilly air mass will charge southward from Canada, thanks to a significant shift in upper level winds (Fig. 3). By Apr. 2, cold air will cover the region from the northern and central Plains states to the Northeast and Middle-Atlantic (Fig. 1b).
Then, winds return to westerly across the U.S., until around Apr. 8, when another significant system is expected to race into the Northeast, bringing another brief period of cold temperatures into the Great Lakes and Northeast. Leading up to this Canadian air mass intrusion, another Gulf of Mexico air mass will help set the stage for severe storms across parts of the eastern half of the Nation.
© 2016 H. Michael Mogil
Originally posted 3/28/16