ITHACA, N.Y. — The start of astronomical autumn is at 9:03 PM Thursday night. As if on cue, a potent cool air mass presses into the region, sending temperatures plummeting and creating an early-season chill for fall’s commencement.

Your Weekly Weather

Currently, a high pressure system sits over the Mid-Atlantic and Atlantic coast. A warm front has worked around the high and currently sits over Ontario and Southern Quebec. As the high sinks further south and its grip over New York State weakens, the frontal boundary will be able to assert itself and push southward as a cold front. The parent low of the front will sweep across New York state later Monday, created a widespread, substantial rain.

It’ll still feel summer-like this Sunday, as Tompkins County remains in the warm sector between the low and the high pressure systems. The moisture nearby will create a rather muggy feel today, as highs make it into the low 80s with dewpoints in the mid 60s. This muggy environment will also make it easier to fire up some showers and thunderstorms later in the day, though these will be widely scattered. Most areas will remain dry, and partly to mostly cloudy. Sunday night will see the storms fed by daytime heating die off early, but as the front sinks closer and the core of the low moves towards Ithaca, rain chances will build back up by sunrise. Expect mostly cloudy skies, some showers to start off and to end the night with a dry spell in-between, and lows in the mid 60s.

Monday’s going to be a grey, rainy day as that low moves overhead. Expect showers and thunderstorms throughout the day, winding down after sunset, with highs in the mid 70s. There will likely be a few breaks in the clouds during the evening, but generally, plan for overcast skies with that rain, which could be heavy at times. Rainfall amounts are generally forecast in the 0.25-0.50″ range, though where thunderstorms occur, total rainfall could end up closer to 1.00″. Monday night sees rain ending, with partly cloudy skies after midnight and lows in the upper 50s.

Great Lakes high pressure briefly builds in for Tuesday. It will be quiet and pleasant. Skies will be mostly sunny with highs in the mid and upper 70s. However, a very large and strong low pressure storm system begins to approach after midnight Tuesday night. A few pre-frontal showers are possible before sunrise, but otherwise expect dry conditions with lows in the mid 50s.

That large low pressure storm system extends from its core over Northern Quebec all the way to the Mid-Atlantic, and will practically demolish the ridge of warmer air to its east as it marches eastward. This is going to lead to a major change in the weather pattern, in that a period of below-normal temperatures is likely, lasting for several days before more seasonable conditions return. If there’s a silver lining to this, it’s that this large frontal boundary will deflect Hurricane Fiona away from the coast and out into the open Atlantic.

Wednesday will be an unsettled day as that low approaches. A few showers will be present in the morning, but the bulk of rain holds off until the PM hours. Scattered showers and thunderstorms are likely through sunset, with highs in the low 80s thanks to enhanced southerly flow ahead of the counterclockwise-circulating low. Wednesday night will host scattered showers and thunderstorms with lows in the mid 50s.

There’s some discrepancy with Thursday, in that the model’s still haven’t settled on a time frame for the cold front’s passage and for winds to turn gusty from the northwest. The prevailing thought is early AM hours before sunrise, but the models are differing by several hours, which could ultimately effect how warm Thursday is. Right now, the consensus is mid 60s with partly cloudy skies and a few lingering showers. But I’m seeing a range from upper 50s to about 70, so don’t be surprised if the forecast for Thursday swings rather widely over the next day or so. Regardless, all runs have northwest winds in place for Thursday night. Remnant moisture will keep temperatures from plunging, but it’ll be sweater/coat weather with lows in the lower to mid 40s. A little frost might even be possible over by Connecticut Hill.

As strong high pressure builds in from Canada, Friday will be sunny but cool, with highs around 60. Friday night will see partly cloudy skies with lows in the low 40s around Cayuga Lake and in the urban core of Ithaca, and some upper 30s possible in the outlying towns and on the hilltops.

Looking into next weekend, some moderation in temperature is likely as the high moves into the Mid-Atlantic and northerly flow is tampered off. With partly cloudy skies, highs will be in the mid 60s Saturday and upper 60s Sunday, with lows in the upper 40s to around 50.

Graphics courtesy of the NOAA Climate Prediction Center.

Extended Outlook

Looking into the end of September, the large scale pattern favors an extended ridge of warm air over the Western United States, and a jet stream trough over the Great Lakes. To be more precise, the first few days in the period are likely to be below normal in average temperature for Tompkins County as we work through the colder air mass in place, with temperatures likely to become slight above normal as we begin October. Precipitation is expected to be near normal for the period.

Brian Crandall

Brian Crandall reports on housing and development for the Ithaca Voice. He can be reached at bcrandall@ithacavoice.com.