Hello everyone!

This evening’s update will focus on an impactful and messy winter storm forecast to move through the area Monday night into Tuesday. It will bring with it copious amounts of all precip types with significant snow in the far north, sleet and freezing rain for most, and heavy rain along the shorelines.

The event will technically begin tomorrow during the afternoon as shifting winds aloft bring some light moisture into the area. The greatest chance for precip will be across southern areas where some ocean moisture will become entrained in the NE winds and also in the mountains where some upsloping will aid in precip development. Precip will fall as sprinkles of the liquid variety along the southern coast, flurries in the mountains, and pockets of sleet/freezing rain across interior SW NH. No accumulation is expected but some slick spots are possible here or there.

Shown to the left is a series of maps that holds the key to this forecast. The model and time are kept the same with each map displaying a forecast for a different slice of the atmosphere. The top forecast is for about 25,000 feet followed by 10,000, 5,000, 2500, and 0 feet. The feature of interest is a back door cold front moving SW across the area tomorrow evening. Notice how it slopes backward over the cold airmass with the most forward progress of the cold wedge at the surface and the least progress aloft. This sets up a situation where you have a deep cold intrusion at the surface (and a high pressure system over Quebec to lock it in) and an environment only marginally cold aloft. This sets up a situation where warm air can easily flood in aloft but will have a hard time making headway at the surface. This is why we’re in for more freezing rain/sleet than snow for most of the area. The best chance for sizeable snowfall accumulation will be the far NW mountains. The poleward slope of the cold frontal surface illustrated above means that we’ll have an excellent isentropic overrunning surface as warm air is forced to glide up and over the low-level cold dome.

Here’s a cross section showing another view of the same phenomenon we discussed above. Because of the discrepancy between airmasses, this boundary acts similarly to a material surface along which air will rise. This sets up the arrival of precip Monday evening, initially as snow due to the deep cold air. You can see the warm nose moving in from the SW on the cross-section, it’s the little kink to the right in the purple (0C) line. That’s the warm air riding up and over our cold front. The image is valid 1 AM Monday morning. Also notice the low-level moisture on the cold side of the front. This is why we’ll see some light flurries/sprinkles/drizzle Sunday night and Monday morning.

Now that we’ve established that we’re looking at an elevated warm layer and a near surface cold layer, let’s look at a sounding (vertical profile of the atmosphere at a single point) to determine if we’re looking at inches of sleet or a damaging ice storm.

Thankfully for the power grid, the cold layer near the surface looks deep enough to allow for sleet to be the dominant precip type. While the deep warm layer will melt snowflakes, the deep cold layer near the surface will allow for those raindrops to refreeze into ice pellets rather than freeze on contact with the ground. That’s not to say that some folks won’t see ice accumulation but it likely won’t be heavy enough to cause power outage issues. Don’t worry though, we have winds for that job along the coast. For more on how warm/cold layers impact precip type, check out one of my recent UpPortland columns where I explain various forms hydrometeors make their way to the ground.

Here’s a look at the mid level winds which will be very strong out of the east/south east. The map to the left shows the anomaly in the u vector which is the east/west component of the wind. The highly negative u vector means the easterly component of the wind will be exceptionally strong. This will help not only to bring warm air into the region aloft, but it will also help introduce the threat for gusty winds out of the east along the midcoast early Tuesday morning.

Winds will be an issue along the midcoast and any other shoreline towns that happen to make it into the maritime airmass E of the coastal front. Wind gusts of 40-45mph are likely for a time Tuesday morning as low pressure approaches. On the western side of the coastal front, look for cold but lighter N/NE breezes to keep cold air locked in place at the surface resulting in slippery travel and messy precip types.

Here’s another view of the wind potential as shown by a vertical cross-section through time above Rockland. Notice the strong low-level jet just off the surface. Any winds below the yellow line are ‘eligible’ to be mixed down to the surface via momentum transfer and as a result, there is fairly high confidence in 40-45mph gusts along the midcoast Tuesday morning. For more on momentum transfer, check out one of my UpPortland columns from a while back where I explain it.

All those easterly winds over the Gulf of Maine will push water up along the coast. While thankfully we’re not experiencing astronomically high tides, some splashover is possible as shown by the black total water level forecast above the first red line which is mean higher high water level. The specifics of that value are complicated but you should know the little yellow line (storm surge) will be between 1 and 2 feet and will push water high enough for some minor coastal flooding though no major impacts are expected.

Here are my thoughts on precip type. Rain will be confined to the midcoast/shorelines and most of the area will see sleet for most of the event after an initial thump of snow. During that initial thump, 2-4″ can be expected for most. An additional few inches up in the northern mountains will likely bring totals to the 4-8″ range in the blue zone on the map above.

Precip moves out Tuesday evening and we’ll be left with a relatively mild airmass and dry weather for about 24 hours before another system approaches from the west. This one will be much weaker and is likely to bring just a brief period of snow/mix to the mountains with rain showers in the south. Little to no accumulation is forecast but some slick spots will once again be likely. Behind this system, colder air will begin working back into the region as winter’s comeback begins.

More tomorrow after I enjoy some turns up at Sunday River. As a result, tomorrow’s update may be a little later in the evening but will contain all the latest analysis.

-Jack