Unusually strong spring storm to bring heavy rain in many areas and strong winds to some others tonight into tomorrow morning; showery few days afterward before gradual warming and drying through next week though possibly with an interruption

Plain-language summary:

 

A strengthening storm will bring heavy rain to eastern Ontario, northwestern New York, and the southeastern slopes of the Laurentians and White Mountains tonight into early tomorrow. Strong downslope winds with much less rain is expected are northern Vermont and southeastern Quebec. The storm will merge with another storm over the Great Lakes and move slowly southeastward, leading to cool and showery weather to continue through Wednesday, with the coolest air in western areas. Gradual clearing and warming is expected later this week. At least periods of relatively warm and dry weather are likely the following week, though there may be one more interruption, and slow-moving weather patterns like this one are challenging to forecast for.

Meteorological discussion:

An complex and impressive storm for this time of year, akin to a more inland Nor’easter, already started to impact the region yesterday with a band of rain. That band of rain has moved well into Quebec. A vertically stacked storm is currently over Lake Michigan and stalling, leading to broad southerly flow in our area. At the same time, another storm is over the southeastern U.S. and will pinwheel around the upper-level low and strengthening as it initially heads northeast up the U.S. East Coast and the north and northwestward into Quebec tomorrow.

Source: TropicalTidbits

 

Source: TropicalTidbits

As this occurs, southeasterly winds will strengthen ahead of the storm, leading to strong downslope winds of up to 60 mph (100 km/h) on the western slopes. The strong winds will also lead to a pronounced orographic influence on the precipitation. Moist Atlantic air will be forced up the southeastern slopes of the Green and especially White Mountains, leading to very heavy rain of up to 4” (100 mm), with lesser upslope amounts in the Laurentians. A frontogenetic band on the western side of the storm will bring heavy rain in northwestern New York and eastern Ontario, with up to 2” (50 mm) likely. Most of the rain will fall tonight into tomorrow morning. While the storm will have enough cold air to produce snow, even a lot of snow, in the Upper Midwest, warm southerly flow on our region will ensure that all precipitation falls as rain, even on the mountain tops, with temperatures rising to slightly above average briefly, especially from the Adirondacks eastward. The same winds will downslope on the northwestern sides of the mountains, leading to much less rain initially for northern Vermont and southeastern Quebec. The downsloped areas will get most of the lesser amount of rain along and just behind the cold front moving from the southwest Sunday night into Monday morning, when the winds will switch to westerly and southwesterly, shortly before the steady rain ends and rain turns into more intermittent showers.

Source: PivotalWeather

This new storm will become the dominant storm and merge with the Great Lakes storm, forming one broad, occluded, vertically stacked low-pressure that will move slowly southeastward through Wednesday and Thursday and gradually weaken with no more energy to feed off. Little pieces of energy rotating around the broad low combined with daytime-induced instability with any clearing will lead to frequent showers, especially Monday and Tuesday afternoons and evenings. A few higher elevations spots in the Adirondacks will have snow mixed with the rain, but accumulation is expected to be minimal with the coldest air staying to the west and weakening. It is also possible that the higher terrain of northern Maine receives a little snow Tuesday night if the upper-level low can move close enough and generate enough precipitation and dynamic cooling. Temperatures will be below average across the region but especially over eastern Ontario.

Source: TropicalTidbits

High pressure will gradually move in from the northwest by late Thursday into Friday, with showers decreasing and skies clearing. Despite the northwesterly flow, the air mass will actually be warmer due to the clearer skies, high May sun angle, and Greenland blocking routing out the cold air masses. Daytime temperatures will be at least near average in areas with decent sunshine, though nighttime frosts and freezes are possible in the usual cold spots. Scattered afternoon showers, especially over higher terrain, are still possible if there is enough daytime-induced instability and if the cold air aloft moves out slower than expected.

Source: TropicalTidbits

The long-range outlook then gets blurry, with a shortwave possibly reinforcing the trough over Atlantic Canada, cold air aloft, and showers one more time next Sunday into the following Monday. These blocked, slow-moving patterns are always very tricky to forecast, as any speed error in the modeling has a great effect on timing. Nonetheless, it appears likely that there will at least be periods of ridging, high pressure, and relatively warm and dry weather starting next weekend or just after.

Source: TropicalTidbits