After a nice little storm today, we turn sunnier and cooler tomorrow as low pressure moves east. Another low is already developing in the Gulf of Mexico and will be moving offshore well to our SE Monday. Normally, this would be a slam dunk miss but a strong upper low will be driving SE and there’s a lot of energy on the leading edge. Surface low pressure will develop over the Great Lakes in response to the upper low and a surface trough will develop between the Great Lakes low and the offshore low. This sets up a tricky forecast for Monday as this trough could take on some Norlun traits leading to narrow and possibly heavy snow bands.
Looking at the 500mb setup, there are a lot of things going for a Norlun event. A strong upper low is moving in from the NW and on the leading edge, a strong peice of energy is moving right over us. Also, being ahead of the upper low, the flow aloft is divergent. To put this in simpler terms, the air at 20,000 feet is spreading out. This leaves a void that air from below must rise to fill. Think of what happens when you pull your hands apart on the surface of a lake or bath. Water rises to replace the water you pushed to the side. This rising motion helps generate precip. This is how everyone will see a coating to an inch or two of snow tomorrow night into Monday morning. The real fun is along the southern coast with a Norlun trough.
At the surface a storm will blast by well SE tomorrow. Another weak wave will develop over the great lakes. The link between the two will manifest itself in the form of a Norlun trough extending NW-SE across the area. Everyone sees light snow from synoptic (large) scale divergence aloft but any moderate snow amounts will be dominated by a mesoscale (small) norlun trough. These troughs generate narrow, heavy snow bands (think lake effect type) that can drop moderate to heavy accumulations in one town while the next town over sees some flurries and maybe a coating. My point here is that uncertainty is quite high.
Another thing to watch out for with Norlun type events is the threat for a mesolow to develop. Mesolows are caused by rising air along and near the trough and also westerly winds accelerating and veering as they move from a high friction environment (over land) into a low friction environment (over water). Imagine the right front tire of your car running into a patch of ice. As that tire moves from a high friction environment (pavement) into a low friction environment (ice) it accelerates while the left front tire is stuck on the high friction pavement and can’t move as fast. The result is your car spinning out. The same thing happens with air moving from over land to over water. Above I’ve drawn up a quick graphic showing how west winds and a cold airmass emerging over water can generate a low. The divergence aloft and strong upper energy aids in this process. This doesn’t happen behind every cold front because often the air is and sinking. The air monday will be rising due to the approaching upper low/divergence which will help with the development of the low.
The 12Z CMC shows this potential well. Should this occur, most of the coast could see a moderate snowfall and amounts would be slightly less dependant on the fickle Norlun. While most models don’t show this, based on warm waters (still over 40 degrees!), rising air ahead of the upper low, the strong energy aloft, and the movement of the surface airmass, I think there is a good chance some sort of mesolow develops and strengthens tomorrow afternoon.
Here are my thoughts for snow. This is tentative as Norlun troughs are notoriously fickle. I’ll have another update tomorrow morning.