ITHACA, N.Y. — As the old adage goes, “don’t let the sun fool you”. The unseasonable cold continues, but at least the sun will be out for most of this chilly week.
The killing frost has taken hold. As of last Friday morning (the 19th), temperatures fell into the mid and upper 20s throughout the area, including the more sheltered locations by Cayuga Lake. Technically, 25 °F to 28 °F is better described as a “hard freeze” or “moderate freeze”. Since it’s destructive to most vegetation (ice crystal growth damages sensitive plant tissue) and shuts down any further plant growth, it’s the true end of the growing season. It also gets the ominous moniker of “the killing frost”.
Climatologically speaking, it’s pretty much right on schedule. According to the National Weather Service, the first light freeze (28 °F to 32 °F) typically occurs between October 1st and October 10th, and the first moderate freeze / killing frost takes place between October 11th and October 20th. Up in the Adirondacks, the end of the growing season strikes around late September, while down in New York City and Long Island, it is often mid November or even close to Thanksgiving Day before the killing frost hits, thanks to their more southerly latitude, lower elevation and close proximity to the coast. Locally, the first moderate freeze has been as early as late September, and as late as the second week of November.
We’ll have more chilly night in store this week, so if an “Indian Summer” is coming, it won’t be arriving anytime soon. But it could be worse -at least there’s no accumulating snows expected this week.
Amazing how steady the signal has been for a coastal storm *next* weekend into early the following week. Really not much of any significant weather until then. pic.twitter.com/kvE9nccTFZ
— Eric Fisher (@ericfisher) October 19, 2018
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You can lay blame for our recent cold spell on an unseasonably deep (high amplitude) trough enveloping much of the Northeast and Mid-Atlantic since the frontal passage on Saturday. On the bright side, apart from some lake effect rain and snow showers the weather has been fairly quiet, as an area of high pressure currently situated over the Ohio River Valley is keeping the atmosphere relatively stable.
This high pressure area will quickly shift eastward during the day Monday, and as it does, its clockwise flow will draw air from the south and southwest, moderating our temperatures (though still below normal highs in the mid 50s). It’ll be a quiet autumn day Monday, with partly cloudy skies and high temperatures near 50 °F.
On Monday night, our next disturbance will start moving in from the northwest, a clipper-type system more common to the heart of winter than mid-autumn. There won’t be much moisture for this system top tap into, so rainfall (and it is expected to be rainfall, not snow) will be light. Expect increasing clouds, with overcast skies and scattered rain showers by mornings. Lows will be in the low 40s.
Tuesday will be rather dreary, with some gusty winds, scattered rain showers, and mostly cloudy skies. Chances for rain will be greatest during the afternoon. Highs will get into the low 50s ahead of the system, and then as the cold air starts to seep in by late afternoon, temperatures will steadily decline as the rain tapers off Tuesday evening, leaving mostly cloudy skies and lows in the mid to upper 30s.
Wednesday will be brisk, as temperatures only make into the low40s in the more elevated areas, and mid 40s near Ithaca city. It will remain rather unsettled behind the front, with mostly cloudy skies lingering showers in the region. Add in the northwest breeze and it will be a rather unpleasant day outside. Wednesday night will be mostly cloudy, with an isolated rain shower or flurry towards morning, and lows around 30 °F.
Thursday will settle down a bit as high pressure builds in once again, but the cold air advection will be strong, so expect only low 40s under partly cloudy skies. Thursday night will be mostly cloudy with a low around 30 °F, and Friday should moderate a bit with a reduced flow of cold air from the northwest, allowing temperatures to top out in the upper 40s. Friday night will be partly cloudy with lows in the low 30s.
At the moment, this weekend is a question mark. Multiple model runs have indicated a strong coastal storm forming by late this weekend – potentially the first Nor’Easter of the 2018-19 winter season. Right now it’s about 50-50 on whether or not the system is too far east to affect Tompkins County, so keep an eye out for any changes, as the stormy half of the runs are split between a rainy Sunday, and potentially a snowy Sunday. For now, expect a partly cloudy Saturday with highs in the low 50s, and a more cloudy Sunday with scattered rain or snow showers on the fringe of the storm system, and highs in the upper 40s.
For those looking for a reprieve from the chill, it looks like there will be some moderation in the long haul. The deep and prolonged trough in the jet stream is expected to retrograde westward by the start of November, which will shift the brunt of the cold air to the Great Plains states, and give us a chance to moderate out to seasonable temperatures in the lower 50s. Unfortunately, this would set the jet stream to a location nearly overhead, and as major weather disturbances often track along the jet stream, it means an above average chance for precipitation during this period. Most of that should come as rain, but timing (i.e. overnight) or smaller-scale shifts in the jet could open us up to the possibility of snow. But for now, chances of a wintry start to November are looking pretty low.
On a seasonal note, the latest three-month products are expecting above-normal temperatures and normal precipitation totals for the November-January period, thanks in part to a strengthening El Niño pattern in the Pacific Ocean. The added heat from the unusually warm waters tends to adjust the polar jet, and like moving a rope up and down, it has downstream impacts – for us, its usually a northward adjustment in the jet, meaning fewer storms and cold snaps. However, it’s just one component of many in the atmospheric system that defines our winter weather. Other parts, such as the North Atlantic Oscillation, an atmospheric pressure differential in the North Atlantic, play a bigger role but are difficult to forecast on seasonal time scales.