The New England coastal storm we were anticipating is now affecting the area from Maine to Connecticut, but for Providence and Boston, the primary precipitation type has been plain rain. Snow is common from Worcester County, MA through the interior of New Hampshire (the Dover-Rochester area on north). If we trace the origin of the low level flow, we can one reason for this. The flow in the rain areas is strictly coming from the ocean. At Nantucket, it was 44 degrees at 8AM.
The map below shows the isobars. Notice how over Maine the isobars are associated with a high pressure area to the northeast. The lines come in from the ocean, then curve northward around the west side of the high. (The analysis off the Maine coast suffers somewhat from the lack of data over the ocean. I suspect that if pressure data was available out there, we would see a clearer split between the air associated with the the high pressure area and the ocean flow farther south.)
The actual wind blows from high toward low pressure at a 20-30 degree angle from the isobaric orientation. This means the air at ground level is coming southwestward over land through Maine. In contrast, the isobars farther south are affiliated more with the low pressure center. At the boundary between the flows, there really isn't much of a difference between the orientation of the lines, but the origin of the flow farther south is definitely from the ocean. The ocean is cold at this time of year, but not as cold as the land where the flow around the high pressure area is originating. At Bangor Maine the temperature was 28 degrees at 8AM.
In a case like we have this morning, that subtle difference in air mass origin in the lowest layers determines the snow-rain line. As the low pressure area moves northeastward, the flow along the coast should become more northerly, and so the cold air will advance. This will favor a changeover from rain to snow at more places, but the question then is how much precipitation is left before the storm slowly departs.
Now, a video:
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Here is one snapshot from this morning's NAM run, depicting the precipitation it predicts for Saturday between 1 and 7 p.m. ET. The rain/snow line should be close to the Pennsylvania-Maryland border.
it appears a storm will form the Southeast and trek toward eastern New England this weekend. Current models suggest this will turn into a mostly rain event in the I-95 corridor...
This map shows a draft of our starting time lines and expected accumulation from tomorrow's quick-moving East Coast storm.
A storm that has brought hardship and danger to parts of Texas and Arkansas with an assortment of ice and snow will send a swath of snow northeastward today and tonight. Here is a map showing our overall estimates as of 10 a.m. ET:
That could lead to tough travel at the end of the weekend. This map for Sunday at 7 p.m. ET shows where those troubles could be (north of the line with the label "snow rain line.")
This table shows the ensemble means for the next two weeks at Philadelphia: It suggests that whereas it does turn cold, any snowfall looks quite limited.