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Weather: Rain at the start of the week, beautiful in the back half

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ITHACA, NY – It’s going to be a rainy start to the week, but don’t pack up the sun hats and hiking boots. The second half of the week promises to be quite pleasant, with seasonal heat and sunny skies. Fall may be approaching, but let’s enjoy the last gasps of summer while they’re in town.

Your weekly weather forecast

The first half of the week will be largely under the influence of a large, slow-moving low-pressure storm system currently forming over the Western Great Lakes. With weak steering flow aloft and high pressure over the western Atlantic Ocean, the system will essentially oscillate in place for the next day or so before eventually finding an exit via the eastern Great Lakes and river valley. Saint Laurent.

What this means in terms of sensitive weather is that it is going to be quite stormy and choppy over the next two days, although temperatures are expected to remain seasonally mild as Tompkins County will reside in the warm sector of the depression turning anti-clockwise.

Today is just the beginning as the warm front of the low shifts over the southern portion and the Finger Lakes. Expect periods of light to moderate rain, usually between 0.10 and 0.25″, with highs in the 70s. The lack of atmospheric energy this morning will keep thunderstorm activity to a minimum. As the rain shield moves northeast and Ithaca is firmly in the warm sector, there will be a break in the rain tonight, although it will remain cloudy. Overnight lows will be in the 60s.

Monday will see those warmer temperatures build up as highs approach 80 across most of the county with near overcast skies. As the low and its cold front begin to move eastward, there will be enough instability and thermal energy to trigger a few isolated showers in the morning, followed by much more persistent showers and a few weak thunderstorms during the afternoon hours. New precipitation amounts on Monday will be 0.10 to 0.25″. Periods of rain will continue overnight Monday through Tuesday as the cold front moves toward sunrise. Quantified Precipitation Forecast (QPF) models show another 0.10″-0.25″ likely overnight, with lows around 60.

Tuesday will be a little cooler with highs in the mid 70s, but with the front past and the low slowly rising northeast into Canada, the rain will lessen as the day progresses and cloud cover will start to dissolve. Another 0.10-0.25″ is likely, although mostly in the morning; by early evening the weather will be drying out and partly cloudy. On Tuesday evening, drying and clearing will continue as the high builds up from the northwest. Skies will be mostly clear and it will be a kind of “windows open” night to sleep with lows in the mid 50s.

With the high pressure continuing to build on Wednesday, it will be a pleasant September day, although there will be a rather strong northwesterly breeze. Skies will be mostly sunny with highs in the mid to high 70s. On Wednesday evening the winds will subside as daytime heating is lost and the core of the anticyclone approaches. This with largely clear skies will result in an optimal radiant cooling setup, and if you’re out late Wednesday night you’ll need a jacket, as lows will be around 50 at the lake and in the town of ‘Ithaca, and the upper 40 on the summits and in the outlying towns.

On Thursday, direct northerly winds will bring cooler Canadian air. This will result in a pleasant but cooler day than we have seen in a while, with mostly sunny skies and highs in the upper 60s. Thursday evening will be another optimal cooling environment, so with mostly clear skies and calm air, temperatures will drop into the mid 40s across the county for lows. Friends, we are approaching autumn, get used to it.

As the heart of the high skirts eastward on Friday, the southerly flow on the trailing flank of its clockwise circulation will raise temperatures a few degrees, up to the low 70s for the highs with a sunny sky. Friday night will welcome partly cloudy skies with lows around 50.

Medium-range patterns show a west-east frontal trough over northern New York on Saturday, and this will likely pass near or through Tompkins County later on Saturday or early next Sunday. At this time, daylight hours on Saturday look pleasant, with partly cloudy skies and highs in the upper 70s. Sunday looks a bit riskier with some showers depending on the timing of the trough, although temperatures are again in the mid to high 70s for the highs. Lows will be between the mid-50s and around 60s.

Graphics courtesy of NOAA Climate Prediction Center.

Extended Outlook

For the second half of September, the large-scale pattern favors a pronounced jet ridge and above-normal temperatures over the central and eastern United States, moving farther east more near October, with a jet trough further upstream over the continental western United States. Over the northeast, precipitation is expected to be near normal for the period. I will note that the only real concern with a setup like this is that it tends to be favorable to hurricanes that make landfall. However, in the tropics things are fairly calm at the moment so any hurricane activity seems to be out of the picture for the next few days.