Autumn is Upon Us; NM braces for Unusually Cold Winter
Guest Writer Seth Price
While the summer temperatures have only cooled slightly, and some days still don’t feel like Fall, autumn is upon us. Meteorologically speaking, the pattern has begun to shift into a ridge and trough pattern for the mid-latitudes.
Last Thursday (9/25), a backdoor cold front was observed as moving into northeastern New Mexico and afternoon showers can be expected as a result. Another cold front is expected on Monday (10/1). Winds are characteristic of a backdoor cold front, blowing across the temperature gradient at the 850 mb level in the eastern part of the state. In the east, this pressure level represents a level a little ways above ground, and here in Socorro it represents almost ground level. The image shows winds coming from the northeast pushing against the temperature gradient. Meteorologists call this “Cold Air Advection”, which, if at a sharp enough boundary, is called a cold front.
In New Mexico, one of the common Fall patterns is the “backdoor cold front.” In the continental United States, weather systems typically approach from the west and push east. However, in a backdoor cold front, cold air pushes into the state from the northeast, and the frontal boundary moves to the southwest.
Backdoor cold fronts indicate that colder, denser, and drier air has moved south from Canada into the Great Plains. Because the Great Plains are relatively flat, the colder, denser air from the north (sometimes called Continental Polar, or denoted CP), can move unobstructed all the way into Texas.
As this air mass tracks south, it meets warm, moist air to the east, forming storms through the Ozarks and into the Deep South. This is why these areas are called “Dixie Alley” and experience a fall storm season, with tornadoes possible in the fall months.
To the west, the Rocky Mountains block the movement of the cold, dry air. The eastern half of the state sees colder, drier weather during the normal backdoor cold front. But what about Socorro and the Albuquerque metro area?
At the beginning of fall, however, the monsoonal pattern that dominated the late summer weeks is declining; the monsoonal rains have likely trickled off. The daily thermal lows (due to strong daytime heating) have weakened a bit, and moisture is not being pulled into the region as strongly. However, the air is still moderately humid, as shown by the afternoon and evening clouds present most evenings.
As these backdoor cold fronts sweep west, if they are strong enough, they will be forced into and over the mountains. With this orthographic lift, the warmer, humid air is lifted, forming clouds, showers and a few non-severe thunderstorms.
Eventually, the left-over monsoonal moisture will mix out and move out. Backdoor cold fronts will no longer bring showers and thunderstorms, just colder temperature. This is why winters are often dry and rain-free.
Source: https://weather.cod.edu/ (College of DuPage – Next Generation Weather Lab)