Christmas Week will start and end dry, but the latter half of the week looks rather stormy.
We start the week off with high pressure trying to build in while low pressure continues to meander around near Atlantic Canada. The result is dry weather through Wednesday as the high will be the dominant feature, but the gradient between the high and the offshore low will result in breezy conditions today and Tuesday. Temperatures will be fairly close to where they should be in the latter half of December.
The end of the week is where things get rather complicated. Most of the models are trying to spin up a very strong storm and send it towards the Great Lakes. As a result, you’ve no doubt seen plenty of forecasts out there for a wind-swept mild rain on Friday. The people making these forecasts are taking the lazy way out, believing everything the models are saying, and not looking too deeply at the situation. That’s not how we do things here. Yes, the precipitation will be mostly likely be in the form of rain with this storm, and temperatures will probably be milder than normal. But a wind-swept mild rain? Far from a lock. In fact, we’re not sure that will materialize for a good chunk of the region. Allow us to explain:
First, the upper-level pieces of energy that will create the storm are still over water, in areas where there is little data, so the models are just guessing at this point. That will change in the next 24 hours or so as those upper-level disturbances move into Alaska and California respectively, so we’ll start to get a better idea of what will happen. Second, this storm may be in two parts, one late Thursday into early Friday and the second late Friday and Friday night, with a bit of a lull possible during the day on Friday. The first piece of energy, with the southern jet stream, likely stays weak and scoots along ahead of the main system. This may help to kick off a weak area of low pressure near the East Coast on Thursday, which could bring in some rain, but it may start a snow or a wintry mix across the interior before changing over to rain. As the northern stream energy moves into the Plains States, it will carve out a rather deep trough of low pressure at the upper-levels, which in turn will generate a rather potent storm system in the nation’s mid-section, which likely heads toward the Great Lakes. This is where our uncertainty starts to increase. Right now, the models keep those two streams separate. What happens if the southern stream storm is a little slower than the models forecast, and they do merge (or “phase”, as we usually call it)? Well, that could result in a stronger storm in the Midwest, possibly even a little farther west than the models show now. It also would lessen the likelihood of that rain and/or mix moving in here later Thursday. What about if that trough in the Plains States isn’t as sharp as the models predict? Well, then the storm ends up farther east, which has different implications here. When storms end up as far west the models currently show, it’s easy to look at the maps and say “it’ll be windy and warm east of the storm,” and assume that is the case all the way to the coast, but that’s the lazy way out.
In reality, when storms head up across the central and western Great Lakes, what usually happens is a weak wave of low pressure forms ahead of the main storm somewhere near the Mid-Atlantic states. What that does, is it pinches off the warm air at the surface. Yes, we’ll still have strong southerly winds pumping in lots of moisture and warmer air aloft, but at the surface, you end up with east or northeast winds, keeping the cooler air locked in under what we call an inversion. Normally, temperatures decrease with height as you head up into the atmosphere. When it’s warmer as you go higher, the temperature profile is “inverted”. This also prevents those strong winds aloft from mixing down to the surface. So, you may get windy and mild conditions near the South Coast, but places like the Merrimack Valley and southern New Hampshire could stay stuck in the upper 30s and lower 40s with little wind, but some rain, drizzle, and fog. We’ve seen this happen countless times, and wouldn’t be shocked if that’s what ends up happening around here on Friday. Eventually, as the storm moves into southeastern Canada, it will send a strong cold front in. Some rain, possibly heavy is likely ahead of that front, it the gusty winds associated with the front will eventually mix out that inversion, so temperatures could briefly spike well into the 50s or even lower 60s as the milder air moves in for a brief period ahead of, or even behind the front.
For Christmas Eve and Christmas Day we’ll have high pressure trying to build in with clearing, breezy, and colder conditions.
Monday: Sunshine and some afternoon clouds, breezy. High 34-41.
Monday night: Clear skies. Low 22-29.
Tuesday: Plenty of sunshine. High 33-40.
Tuesday night: Clear to partly cloudy. Low 18-25.
Wednesday: A mix of sun and clouds. High 33-40.
Thursday: Mostly cloudy, rain developing late in the day, possibly starting as some snow or freezing rain across the interior. Rain may be heavy at night. High 39-46.
Friday: Cloudy, breezy along the coast, rain likely, possibly heavy at times, especially early and again late in the day. Rain may mix with or change to wet snow before ending at night. High 51-58 during the evening, but temperatures may stay in the upper 30s and 40s for much of the afternoon across the interior.
Christmas Eve: Becoming partly sunny and breezy with a few flurries possible, especially early. High 30-37 early, temperatures drop a little during the afternoon.
Christmas Day: Partly to mostly sunny, breezy, and chilly. High 25-32.
Storm HQ 