Big Changes Ahead

Tuesday February 5, 2008 12:00 AM

It's been several days since a post to this section of the site. Thankfully, the weather over the last 3 to 4 days has been fairly quiet and rather mild and hasn't needed much elaboration from a forecasting standpoint. But, of course, this is New England and quiet stretches of weather don't last toooo long...

With that in mind...as we approach mid-week, we've got some very active weather ahead of us, complete with heavy rain, above normal temperatures followed by some snow and sleet and temperatures falling into the 20s and 30s. Throw in a possible rumble of thunder as well and maybe even some freezing rain.

The culprit: A big contrast in air masses and a corresponding series of low pressure centers riding northward from the Mississippi River Valley, up to the Ohio Valley and through New England. These storm centers are traversing a frontal boundary that's separating spring-like warmth over the southeastern part of the nation from wintertime cold over the nation's midsection. Along this boundary, heavy rains and severe storms have been common over the last 24 to 36 hours over parts of the Eastern U.S. In fact, this set-up has brought a major tornado outbreak to parts of the Mississippi River Valley. Locally, the first affects of this front were felt during the day on Tuesday with periods of light to moderate showers across the Cape.

Let's look at what we have in store:

1. Rain - the frontal boundary will sag southward across New England during the day on Wednesday, eventually reaching our latitude during the late afternoon hours. This boundary will serve as a conveyor belt of moisture, sending waves of rain across the region, especially later in the afternoon and into the evening hours when some pockets of heavy rain are possible. Model guidance cranks out nearly 1" of rain across the Cape through Wednesday night, and this could be underdone in a few spots should some downpours train across the area. The air mass is unusually warm at the onset of this event and rather moisture-loaded for this time of the year, which may allow for higher rainfall totals than computer guidance is forecasting and could even produce a rumble of thunder. However, on the flip side, radar would indicate that the heaviest axis of precipitation may head a bit north of the Cape. Either way, a healthy dose of rain is in store for the region with most of us seeing between 1/2 and 1 inch of rain..

2. A turn to colder weather and possibly some ice or snow? - The front will be pushed southward across New England by a cold high pressure area building in to our north over southern and eastern portions of Canada. As the front crosses the region it will shift our winds around to the north and northeast and begin to send much cooler air southward onto the Cape. You can use the Surface Analysis to track the colder as it sags southward - notice the very cold air well to the north. Timing the ill-defined front is a bit tricky, but it appears as though the boundary should drop across the region between 2 and 5PM.

Once the front is through the region, the temperature profile of the atmosphere will become quite complex, making for a very tough forecast Wednesday night into Thursday. The arrival of the cold air will happen first at the surface, as the cool dense air will easily undercut the mild air flowing in aloft. But the drainage of cold will be slow to occur aloft thanks to a persistent southwest wind in the mid levels of the atmosphere. Now, to our north and west, temperatures will cool more easily, enough so, in fact, to change rain to freezing rain, then sleet and eventually all snow during the course of the day Wednesday as the column of cold air thickens with time. Here on the Cape, the cold air will take the longest to establish itself, so the big question is when does the transition to frozen precipitation happen locally? It's very interesting to look at the forecast soundings from computer guidance, which show very marginal temperatures in the lowest thousand feet of the atmosphere by Wednesday evening, but very mild temperatures several thousand feet up. Freezing rain is not a common occurrence for us here on Cape Cod and this go around SHOULD be no different. The most likely scenario is for surface temperatures to remain above freezing by a few degrees for the majority of the event. Eventually, late Wednesday night, precipitation will go over to a mix of sleet and rain, followed by a sleet, rain and snow mix before flipping to all snow sometime Thursday. That said, it's going to be a very close call later tonight between just rain mixed with sleet and a far more complex mix of rain, freezing rain, sleet and snow. This will be highly depending on just how quickly and how strong the drainage of cold air is.

Updates through the day...