The 00 UTC models (the Canadian, GFS, and European) are in reasonably good agreement, showing a surface low developing in SE Texas late Friday, moving to north Alabama on Saturday, then into the Northeast U.S. as a powerful storm Saturday night and Sunday. If the models are correct, we could see some decent rain Saturday morning, then the rain could change to sleet or snow in Alabama Saturday afternoon and Saturday evening. According to the GFS, mid-level temperatures would become cold enough for sleet or snow by 3 pm Saturday, with surface temperatures dropping below freezing Saturday evening. IF THAT MODEL IS CORRECT, some spots could see a light accumulation of snow. But, most of the precipitation would fall before the cold air arrives. Why does this always seem to happen around here, and how might this turn out differently? Scroll down for a discussion.
GFS MSL pressure and 500 mb loop
Canadian GEM MSL pressure
I just finished teaching the vorticity and vertical motion part of a dynamics course at UAH…and the situation setting up over Alabama this weekend plays right into some of what we talked about in class. Upward motion is what causes precipitation in the atmosphere. Downward motion tends to suppress it. Two main processes cause upward motion on a large scale: increasing upper-level vorticity (spin, ie., an upper-level disturbance), and warm air advection (warm air overtaking cooler air).
Heavy precipitation events tend to occur when warm air is flowing in, or an upper-level disturbance approaches. Cold air flowing in, therefore, does not promote precipitation, it actually suppresses it. This is why snow in Alabama is so rare. To get heavy snow, you need the cold air to already be in place.
Sometimes, like the models predict to occur this weekend, we get warm air flowing in, it rains, then cold air pours in and begins to suppress precipitation just before it can change to snow. But, the upper-level disturbance is intense, which promotes precipitation. The two processes then compete. Trying to tell which one will win is difficult. See graphics below.
These graphics show upper-level winds and vorticity, and lower level temperature and winds, at 6 pm Saturday. At low-levels, cold air is flowing in quickly from the NW. But, two upper-level disturbances are still in the picture, promoting upward motion and precipitation ahead of them. One area of increasing vorticity is just leaving Alabama, with another approaching from the NW. According to the models, these upper-level disturbances will be strong enough to cancel out the effect of cold advection Saturday evening, allowing a brief changeover to sleet and snow in Alabama.
This will be a tough call, as it always is in Alabama! If the low moves farther north or is weaker, we may see no snow at all. If the low forms farther south and warm advection is limited, there could be more than an inch of snow. Hey, I’m interested. At least by Sunday, the high will be 30-50 degrees colder than today, and more like Christmas weather!