Today will feature more sun than clouds along with chilly temps and continued NW breezes. Highs will range from around 20 in the north to around 30 in the south. NW breezes will gust to 25mph at times as cold air continues to move into the area from Canada. No precip is expected but a few upslope flurries can’t be ruled out. Today’s cold will be setting the stage for what appears to be a fairly impactful winter weather event on Monday.
Snow will move into the region late tomorrow night and will continue into Monday. Everyone will start out as snow with a few inches expected by daybreak Monday. After that, the forecast becomes trickier for southern and coastal areas as warm air threatens to intrude enough to cause some mixing with sleet or rain. Any mixing will be limited to the coast, southern York County, SE NH, and the midcoast.
The snowfall forecast for the coast will hinge on the westward progress of the coastal front which is the boundary between warm marine air and cold continental air. Most models currently bring it into the I-95 corridor but guidance has a notorious warm bias with these types of events. The immediate coast will likely mix regardless but I think the front will set up closer to rt 1/the beaches. Exactly where the coastal front sets up will determine who sees 4-6″ of snow and who barely gets 2″. Those who have lived in this area for a while know that the difference between the two can sometimes be a mile or two.
Snow will be light to moderate Sunday night into early Monday morning but a period of moderate to heavy snow is likely Monday morning into the early afternoon. This will be driven by intense lift as a result of a sharp temperature gradient between very mild air to the south and the remains of the very cold air we’re currently enjoying. The NAM highlights this gradient though I think it mishandles the northern extent of the warm air which will likely be sliding east faster than moving north due to another important feature.
The intense lifting associated with this gradient is evident in the mid level vertical velocity field on the NAM model. This map shows how much air is rising and how fast it’s rising. Notice the reds and oranges over ME/NH late Monday morning. This isn’t the bright pinks of an intensifying Nor’easter but nonetheless it points to a period of moderate to fairly heavy snow on Monday morning that will be sure to cause a mess on the roads.
Those who have read my discussions for a while know that I often focus on the behavior of troughs and ridges at 500mb (~25,000 feet) to determine how storms at the surface will behave. Monday’s storm has weak upper level support at both the 500mb and 300mb levels. The positively tilted 500mb trough means that there isn’t enough energy for a storm to strengthen. Weaker storms have a harder time uprooting well entrenched cold air like that sitting over New England now.
The 300mb jet stream shows the same thing. The jet across all of North America is blowing from west to east in more or less a straight line. This is known as zonal flow and favors weak, fast moving storms like the one coming Monday. As I said before, weaker storms have a hard time uprooting cold air and the upper levels are indicating the storm Monday will be of the fairly weak variety.
All that to say that the storm coming Monday will bring a burst of snow Monday morning right around commute time that will possibly mix with rain along the coast. The storm will have a hard time uprooting the cold air now in place across the area because the upper level pattern indicates the storm will be fairly weak. How about accumulation?
Here’s my forecast for accumulation. The foothills and mountains will be the jackpot and there will likely be a few places in the Mt Washington region that see 6-8″ but most folks are likely to see 3-6″ with lower amounts along the coast where some rain is possible and up near the Canadian border which will be farther removed from the heaviest snow.
I’ll try to have another update tomorrow morning.