Hello everyone!

With today’s Norlun trough fizzling out over the Midcoast and cold air pouring in as forecast, attention can now turn to a mid-week mess due to arrive Tuesday across the area as low pressure passes over the region. Because the low will be passing over us and not to our east, this will be a mix event with a medley of precipitation types including but not limited to rain, snow, sleet, and freezing rain. I’ve covered tonight and tomorrow’s bitter cold several times in the last few days so refer to last night’s blog post for the most recent info on that. Nothing has changed since then. With that said, let’s dig into the Tuesday storm.

The Arctic high pressure containing the bitter cold we feel tonight and tomorrow moves offshore Monday bringing behind it warm SW winds. Temps will rise on Monday but keep in mind, they have a lot to rise as temps Sunday remain largely below 10 degrees. The airmass ahead of the storm is also extremely dry so there will be room for evaporational cooling at the onset of precip. I think we get a good burst of snow at the start of the storm across the area before warm air intrudes from south to north (this looks like a solid front end thump event for those familiar with New England weather terminology). How far north that warm air makes it and how fast it makes it there will determine precip type and thus what accumulations will end up being.

How do we go from 20 below zero to rain in a matter of days? The answer lies in the placement of the Arctic high pressure. Notice how the high has departed to the east on Monday and behind it, warm southerly winds are already eroding the cold in place long before the storm gets to us. This is why I’m not super bullish on cold air remaining locked in. That being said, I’ve learned over my time forecasting in Maine that cold air has tremendous sticking power and is not easily moved. This is why I think the mountains stay mainly snow while the rest of the area only briefly sees rain with the exception of the coast where there will be more rain.

In terms of what sort of precip will fall from the sky, the short answer is too early to tell exactly. However, I do feel comfortable roughly outlining my thoughts for general precip type. I think the mountains is mostly snow as the low passes over the coastal plain. They look to remain on the cool side of the low and even if the low passes over the mountains, I think you’d be hard pressed to get enough warm air that far north-west to allow for an extended period of rain. I do think though that the mountains mix with some sort of sleet/freezing rain at some point. The coast is the other higher confidence forecast (relative to the storm in general, confidence is medium at best with any aspect of the storm). The low would have to track significantly farther south than forecast for cold air to remain in place throughout the storm. For coastal areas, look for snow changing to a mix then changing to rain with briefly heavy rain possible. For inland areas, expect some sort of a mix with snow at the onset and rain to finish. What happens in between is anyone’s guess. Confidence will increase in the coming days so stay updated for the latest info.

I’ll try to have an initial guess at accumulations tomorrow evening in my next update on the storm. I’ll have tomorrow’s forecast as usual tomorrow morning.

-Jack