ITHACA, N.Y. — Officially, astronomical fall begins at 3:50 AM on Monday morning. Typically, we don’t ring in autumn with temperatures of 85 °F at the airport, and dewpoints more typical for July, yet here we are. It certainly makes for an unusually summery Porchfest.

Temperatures will pull pack a bit as a weak cold front enters the region Monday, but generally, apart from some minor unsettled weather Monday and Thursday, it looks to be a mild, comfortable and mostly sunny work week ahead.

Your Weekly Weather

This unusually warm late September is likely delay the changes in fall foliage by a few days, though it’s also based on total hours of sunlight, so the dropping of leaves can’t be put off forever. In order to get the most vibrant reds, oranges and yellows, one typically needs cool temperatures at night in combination with mild but not hot daytime temperatures, along with reasonably dry conditions. Sunshine, dry weather and cool overnight temperatures allow the chlorophyll in leaves (the chemical that keeps them green and photosynthesizing) to break down more efficiently, showing the warm-toned pigments they’ve been hiding in their cells. If it stays too warm, then the cholorphyll sticks around and the leaf is more likely to drop without fully changing color.

We’ve got a few weeks before peak color, and it looks like in the short-term we can expect temperatures to stay on the warm side overall, with average temps anywhere from near normal to well above normal depending on the day.

At present, high pressure centered off the Carolinas is keeping things mostly dry, though a weak upper-level disturbance is providing just enough instability to fire off a few widely scattered rain showers (one of which tracked across Tompkins County earlier this evening). With southerly flow on the backside of the high (weekly reminder, air flows clockwise around highs in the Northern Hemisphere), a strong ridge has been built over the Eastern U.S. and temperatures are well above normal for late September, by as much as twenty degrees.

This warm spell will come to a close, if briefly, when a cold front moves into the area late Monday. However, temperatures should rebound into the 70s by Wednesday, with 80 °F once again possible by the weekend.

Tonight will be dry as the disturbance exits and the high regains full control. It will be quite warm and a bit muggy. Plan for partly cloudy skies and a low in the upper 60s in Ithaca and along Cayuga Lake, and mid 60s in the outlying towns. Monday will start mostly cloudy and dry, but by late morning, showers will begin moving in ahead of the front. Anywhere from about noon to a bit after sunset could see rain, but 2-6 PM is looking to be the most likely period for rain in Ithaca proper, probably in the form of a line of showers with some thunderstorms in the mix. The atmosphere will have only modest amounts of energy available but a high amount of shear, which means that the thunderstorms are unlikely to be severe, but could produce gusty winds. Highs will be a bit cooler than today, in the upper 70s.

The front should pass through during the early overnight hours Monday, with a slow but steady shift of the winds from the southwest to the northwest, where cooler air lies. It’ll dry out after the last showers press through in the late PM, with mostly cloudy skies and lows in the mid 50s by daybreak Tuesday.

With Lake Ontario as warm as it is, the northwest winds could pick up moisture and destabilize the air downstream as they pick up heat energy transmitted from the lake surface, so a few lake-enhanced rain showers are possible for Tuesday. Otherwise, it will be mostly cloudy with highs in the upper 60s. Tuesday night will be drier as the winds shift to a more westerly angle (and direct the showers towards the Syracuse-Watertown I-81 corridor instead), and it will be partly cloudy with lows around 50 °F in the usual warmer locales (urban areas, lakeshore), and upper 40s elsewhere.

High pressure to the south shifts eastward over the Mid-Atlantic by Wednesday morning, and the loss of the northerly wind component will allow temperatures to warm up. It will be mostly sunny with highs in the mid 70s. Wednesday night will follow the same formula, though winds will shift to the southwest as a low pressure system and weak cold front approach. It will be mild and dry early, with thickening clouds and showers approaching during the AM hours. Lows will be in the upper 50s.

On Thursday, the rain that does develop will be light and brief, and should push out as the front passes during the mid to late morning hours. Mostly cloudy skies will start to break up during the afternoon hours, and while there could be some gusty winds, the actual drop in temperature won’t be much – highs will still get into the low 70s. High pressure returns for Thursday night, with partly cloudy skies and lows around 50 °F.

Friday will be dry with generous amounts of sunshine, thanks to the latest high pressure system to the south. Temperatures will top out in the low to mid 70s. Friday night will be a little milder as the wind takes on a more southerly component from the backside of the high, with lows in the mid 50s and partly cloud skies.

The weekend is looking good at this point, though there’s some indication in the models for some showers Saturday night into Sunday. During the day, it will be partly cloudy with highs in the low 80s Saturday, and mid 70s Sunday, and lows in the mid to upper 50s.

Graphic courtesy of the NOAA Climate Prediction Center.

Extended Outlook

Wow. It’s pretty common to have a warm east / cool west setup, or vice-versa, but typically not to this extreme. The ridge in the jet stream over the Eastern United States will be intensely strong, which will keep temperatures well above normal going into October, as well as abnormally dry conditions. This poses also poses some potential problems, because this is the kind of synoptic-scale setup that can cause tropical storms and hurricanes to veer into the coast instead of out to sea – something national interests will want to keep an eye on once Tropical Storm Karen clears Puerto Rico in a few days, giving it the chance to strengthen and potentially take a westward track. Meanwhile, early indications are in place for some snowy weather in parts of the Mountain West, and cities like Denver swing from 90 °F to the upper 40s in just a day and a half.

Brian Crandall

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