This evening's weather map features a strong high pressure dome moving over New England while a strengthening storm center is pulling east across the Lower Mississippi River Valley and prepared to eject northeast across the Tennessee and Ohio Valleys tonight into Friday before moving through New England Friday night while continuing to strengthen.
The dome of high pressure over New England will also strengthen as it nudges east, moving over Nova Scotia on Friday, and this sets up a tricky forecasting scenario for all of New England. At the center of high pressure is cool and dry air, able to warm on Thursday thanks to ample sunshine, but ready to cool Thursday night with mainly clear skies and light winds for at least the first half of the night before clouds spilling northeast from the developing storm center and its associated Gulf of Mexico moisture tap move through the New England skies. The cooling for the first half of the night will mean a cool start Friday morning, and though conditions will begin dry, clouds will quickly thicken Friday morning which will limit how much warming can be achieved by the sun's warmth. As the strengthening high pressure center and strengthening low pressure system come closer together with the battlezone right over New England, the difference in barometric pressure between them will result in an increasing wind both at the surface and aloft late Thursday night, but more so during the day Friday.
There are a few major forces that govern wind speed and direction, but the biggest factor is always pressure gradient force - the difference between high and low pressure, with air tending to flow from high to low pressure, and in this case meaning a tendency for air to flow from Nova Scotia to Upstate New York - an east-northeast or even northeasterly wind flow over New England. This is the wind direction that will filter across New England early Friday, holding cool air in place, and this wind direction is likely to hold for most of the day in Northern New England. As a result, the North Country is expected to see prolonged wintry precipitation. Farther south, however, the counter-clockwise flow of air around the strengthening storm will take over, meaning a southeast wind is more likely in Southern New England, with an easterly wind between. All the while, the wind speed will increase across the region on Friday, picking up to 10-20 mph Friday afternoon, and 15-30 mph by Friday evening with higher gusts along the coastline. Of course, with an onshore flow, the relatively warm ocean waters will have an impact on the type of precipitation that falls - first at the coastline and far Southern New England, and eventually farther inland. Also of importance is the slug of tropical warmth and moisture streaming northward out of the Gulf of Mexico. The moisture will be important in providing efficient precipitation production - with around an inch of melted liquid equivalent precipitation for many spots, and locally higher amounts - but the warmth will make its impact known as it rides over the top of the cold dome, about 7,000 feet above the ground. With the less dense, warm air riding up and over the top of the heavier, more dense cold dome, this will leave a subfreezing layer of air in the lowest several thousand feet of the atmosphere, with an above-freezing layer a few thousand feet thick above it, across the interior of Central New England, and eventually across much of Northern New England, as well. This scenario of cold air up high, a layer of warmth, then cold air low in the sky, is exactly the setup needed for an ice storm - if the surface cold is shallow with more warmth aloft, snowflakes will melt to raindrops, then freeze upon contact with the ground, known as freezing rain. If the surface cold layer is thicker, the raindrops will completely refreeze upon descent and turn to ice pellets, known as sleet. Though most spots will start with some form of frozen precipitation - a quick burst of sleet before changing to rain and freezing rain south and south-central and snow to wintry mix north - the trend will be for warming in the atmosphere as the storm center moves over New England and the strengthening southerly flow aloft carries the Gulf warmth northward. The setup on Friday afternoon and evening will be favorable for freezing rain in Central New England, sleet a bit farther north after a burst of snow, and mostly snow in Northern Maine, though some sleet is likely to mix in for a few hours Friday evening.
Of all the spots in New England, the greatest concern comes in the locations that are favored for prolonged freezing rain. Though travel will be difficult in the snow and sleet areas, with sleet pellets acting like ball-bearings beneath tires, freezing rain freezes on contact and creates a glaze on surfaces ranging from untreated roadways to walkways, stairs, trees and power lines. At this point, it appears as though the spots indicated as significant icing in the map above favor a heavy glaze of ice developing Friday afternoon into evening, with the potential for a quarter to half an inch of ice accretion. There is always the potential that just a bit thicker cold air dome will bring sleet instead, which actually poses less of a problem considering winds behind the passage of the storm center will be the most fierce - shifting to blow from the west and increasing to 20-30 mph sustained with gusts to 45 mph - and these winds will put tremendous stress on any lines or limbs with ice on them, resulting in potential power outages.
Nonetheless, the storm continues to move and drier air streams across New England around dawn Saturday, bringing precipitation to an end in most areas and allowing the clouds to break up. Thereafter, Saturday should bring a fast wind from the west under the belly of the departing low, still gusting to 40 mph but sloping down the mountains and hills, meaning air will continue to dry in "downsloping flow," leaving a relatively pleasant day for most. The atmosphere will continue to dry on Sunday with plenty of sunshine and similar temperatures to Saturday. To our north, a high pressure dome will build southward across Quebec, forcing cooler air southward into New England again, but the bleed of cold air will be quite gradual given that the high pressure dome will remain centered to our north, and the colder air should be better established across Northern New England on Monday. Of course, by that point the jet stream trough over the Western United States will have reloaded, sending another storm strengthening northeast out of the Plains and toward the Great Lakes, increasing clouds later Monday and bringing the potential for another round of wintry mix changing to rain Monday night and early Tuesday, before record warmth is possible Tuesday afternoon regionwide. A cold front on Wednesday would deliver cooler air for the middle and end of next week, but even that would still be above normal for most spots until an intense but brief shot of cold air next weekend. The overall pattern, though, would favor warmer than normal temperatures battling back for most of the Eastern United States through at least the first half of the month, though deep arctic cold will be reloading over Northern and Central Canada, meaning the potential for a substantial U.S. cold outbreak will build during the second half of the month.
Have a great Thursday evening!
Matt
