A pleasant St. Patrick's Day but snow arrives Tuesday morning

7:53 AM, Mar 17, 2013   |    comments
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Happy St. Patrick's Day! If you celebrate the day like many people I know, you will probably be half in the bag before you read this article. I can live with that; it might actually make my forecast easier to take.

Today: Mostly sunny and cool for most of us. Temperatures struggle to break 35 F even over southern Maine and winds will gust up to 25 MPH from the northwest making it feel even chillier. I said mostly sunny for "most of us" because the mountains will quickly turn cloudy with an upper level disturbance moving through that region. In fact they could see some snow showers as well through the afternoon.

Tonight: Mostly clear and cold. Temperatures drop into the single digits in most spots with only low teens even in the usually warmer coastal plain.

Monday: High pressure will be directly over head so look for mostly sunny skies, light winds and dry conditions. Temperatures will continue to struggle after a chilly starting point, so highs will be in the 30 to 35 F range. Clouds will begin to stream into southern Maine by 3-4 PM in response to an approaching storm system.

About that approaching storm system: The computer models have converged a bit on the solution for this storm, but there are details yet to be ironed out.

Overall the trend has been to depict a colder storm system. The reason? The transfer of energy between the inland low and the coastal low is now occurring more quickly and more efficiently. In general the faster the coastal low takes over, the colder the storm plays out for Maine as we are quickly put on the west side of the low...the cold side (99/100 times at least). That being said the EURO is still bombing us with 14-15" totals while the GFS, NAM and our in house RPM model are more in the 8-9" range for most spots with less along the coastline.

Interestingly I'm discounting the EURO as too high NOT because of the rain/snow line, but because I think it's overdone on the amount of moisture this system can conjure. Looking at the charts for the mid levels of the atmosphere, I see strong signals for a classic "dry slot" to develop. These are well known (by nerds at least) for wrecking the structure Nor'easters and decreasing snowfall amounts. I'm particularly concerned with a dry slot for this storm because there is a lot of space between the coastal low and the "parent low" to the west, allowing dry air to work in.

Bottom line: My snowfall forecast is still pretty hefty, but I am accounting for some dry slot development and a little mixing with rain along the southern coastline. Instead of 15-16" I have most of the state in a 6-12" swath with 5-10" along the coastline due to that flirting with the rain/snow line.

Timing wise the snow develops over southern Maine late in the evening Monday and into Tuesday morning (3-4 AM) and then spreads northeast to the point where it reaches Bangor by 9 AM and then Machias by 11 AM-Noon. Snow is heaviest Tuesday evening and continues into Wednesday morning over the northern half of the state before tapering off Wednesday afternoon. (Southern Maine will basically be done by Wednesday morning)

Other considerations:

1) It's still over 2 days out so I'm sure adjustments to the snowfall map will need to be made. Nature of the beast.

2) The snow will be a heavy, wet variety along the coastline which will increase power outages there. Further inland it will be much less dense as air temperatures will be cooler, creating a lighter (although still not fluffy) snow.

3) Winds won't be too bad from this storm. The low isn't a true BOMB so we are probably looking at winds up to 35-40 MPH along the coastline in gusts. No higher.

4) It's time for me to go celebrate St. Patrick's Day. I suggest you do the same.

Twitter: @KeithCarsonWCSH





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