Plain-language summary:
After last weekend's record warmth, it is much cooler today with a hard freeze tonight. A brief warm surge will follow for Thursday and Friday. Nicole will bring heavy rain and/or strong winds, depending on location, on Friday evening through Saturday morning. It turns much colder afterwards, with some chances of snow next week, aside from lake-effect snow in northwestern New York.
Meteorological discussion:
After last weekend's record warmth, it is temporarily much cooler as a cold high pressure temporarily settles to the north, with a direct cold flow out of the north-northwest from Quebec unmodified by the Great Lakes. With the high pressure moving overhead tonight, ideal radiational cooling conditions will occur for most with clear skies and little wind, leading to a hard freeze even in broader valleys and perhaps urbanized areas. The cold hollows could drop to 14F (-10C). However, the high pressure will quickly move to the southeast over the western Atlantic, allowing for a southerly flow to bring back warmer air for Thursday and Friday, though not nearly as warm as last weekend.
Meanwhile, Tropical Storm Nicole is strengthening east of the Bahamas, and due to an unusually strong mid-level ridge to the north, will take an unusual (for November) westward through Florida and into the far northeastern Gulf of Mexico. Afterwards, a deep trough approaching from central North America will pick Nicole up and cause it to race north-northeastward and undergo extratropical transition. The interaction will lead
to a predecessor rain event (PRE), an area of heavy rain to the northwest of Nicole's center. Nicole will be undergoing extratropical transition and moving
quickly, but tropical moisture will lead to heavy rainfall rates for ~8-12 hours. Over the last few model runs, the track has trended farther north, now expected to be over northern New England. This would lead to heavy rain, possibly 50-75 mm, over southern Quebec on Saturday. Depending on the exact track, terrain channeling of northeasterly winds along the St. Lawrence Valley converging with southerly winds along the Champlain Valley could lead to enhanced rainfall in the St. Lawrence Valley. Lesser but still significant rainfall, in the form of shorter-lived but heavier rain bands, would occur over central and northern New England, accompanied by strong southerly winds. Tropical systems rarely, if ever, affect our area this late in the year, but anomalously warm water temperatures surrounding Florida and the unusual westward track initially are contributing.
After Nicole passes off to the east, much colder air will gradually sweep in from the west, somewhat modified by the Great Lakes. In my last post, I said that I was skeptical of it turning really cold persistently, but I am no longer skeptical. The EPO index just flipped from very positive to negative, which will lead to a pattern with ridging over Alaska and a trough over central and eastern North America, allowing for arctic cold to dominate, despite the neutral PNA and NAO indices. With the broad trough expected to stay in that position, with associated cold over our region, for most of next week, almost everyone will see the first snow. At least little disturbances passing through our area will produce at least brief light snows. Given the favored westerly or west-southwesterly low-level flow expected, the western Adirondacks and parts of the St. Lawrence Valley close to Lake Ontario will also receive lake-effect snow.
Outside of lake-effect snow, whether we get a big snowstorm or it stays mostly dry heavily depends on the timing and strength of disturbances rotating around the southern periphery of the broad trough through the southern U.S., which we will not know until a few days beforehand. These disturbances will be able to pick up more moisture from the Gulf of Mexico and the Atlantic. The broad trough position is favorable for the disturbances to turn northeastward upon reaching the Atlantic coast and turning into Nor'easters and potential snowstorms over New England and/or southern/eastern Quebec, but they still need to be strong enough and timed well with any disturbances riding along the polar jet farther north. Also, since it is still November, especially given the anomalously warm water temperature off the East Coast, it is easy for enough warm air to be pulled in to cause a changeover to rain, especially close to the coast. As such, climatology suggests that interior New England is probably favored, but I would not rule out snow in coastal areas either. The first, albeit small chance is this Sunday, as a disturbance starts to strengthen after moving offshore into the western Atlantic. For now at least, it looks like it will be not strong enough and rather disconnected from the cold air until it reaches northern Maine or beyond. The true cold air arrives behind this system, with a perhaps better chance of a snowstorm around the 17th.
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