ITHACA, N.Y. — Normally, when September rolls around, the weather begins to take a turns towards the autumnal. The days are a little cooler, the nights are a little crisper, and the air’s a bit drier, as the humid summer days begin to wane and the leaves start to turn.
Well, keep waiting. More hot, humid days are in store for the week ahead, though perhaps we’ll get a taste of late summer coolness by next weekend.
With August in the record books, it looks like the month finished out warmer and wetter than usual. According to the Northeast Regional Climate Center, the average temperature of 70.4 °F was 3.0 °F above normal, and 4.97″ were recorded at the NRCC’s Game Farm Road weather station, well above the normal of 3.63″ for the month.
In terms of averages, neither of the values stand out very much. 70.4 °F is good enough for 20th place in the 127 years of Ithaca/Cornell temperature records, and the precipitation total ranks 18th in the same period. Where things get a little more interesting is when you look at minimum temperature averages for the month of August. This summer has been among the most humid on record for much of the Northeast, with dewpoints regularly over 70 °F. When the humidity is high, the air can’t cool down as well – as the air temperature approaches the dewpoint, water vapor condenses (forming dew, hence the term), and it also releases heat into the air as it goes from a gaseous state to a less energetic liquid state – a little bit of physics known as “condensational warming”. That warming prevents the air from cooling, and in the case of 2018, the result was an average minimum temperature of 61.2 °F in August, a tie for the seventh highest on record and the highest since 1955.
This first week of September is looking to be more of the same, so don’t put away the shorts and box fans just yet.
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The main driver of our weather in the first half of the week will be a large and deep ridge of high pressure over the mid-Atlantic states, strengthening as we start the week, and moving slowly eastward as the week progresses. This ridge will tap into hot, humid air over the Southeast and bring it northward in its clockwise flow, resulting in unusually hot temperatures for the start of September.
For your Labor Day, take a rest from the labor if you can. With a light southerly wind, oppressive humidity and partly cloudy skies, the highs in the low 90s in Ithaca proper will feel more like lows 100s with the heat index, and heat advisories, already in effect for the Thruway Corridor, may be extended to the valley areas of Tompkins County as well. As we’ve stressed frequently during this hot, humid summer, drink plenty of cool fluids, stay in cooler and preferable air conditioned spaces if possible, take frequent breaks if working outdoors, and be aware of the signs of heat-related illness. Monday night will be a sticky one, best to keep the windows closed as temperatures only fall back to the low 70s under partly cloudy skies, and the dewpoint remains near 70 °F.
Tuesday might be the first day of school for the Ithaca City School District, but the weather will still be very summery. Temperatures will top off around 90 °F with the heat index near 100 °F as dewpoints swelter in the mid 70s under partly cloudy skies. An isolated thunderstorm or two may manage to pop up late in the day, but most areas should stay dry. Tuesday night will be humid and only bottom out in the upper 60s, with only a few passing clouds overhead.
Wednesday will be yet another muggy, stifling hot day. Temperatures will top out in the low 90s in Ithaca and near 90 °F up in the hills, with partly cloudy skies and the heat index once again flirting with triple digits as sweltering mugginess continues. Wednesday night will once again be a hot and humid affair, with lows only in the low 70s, and clouds thickening as morning approaches.
It would appear that Thursday will be the day that the all-natural sauna finally ends, as a cold front makes its way in from the north. Showers and thunderstorms will sweep across the area ahead of the front during the mid and late morning hours, with the front itself pushing through by early afternoon. Temperatures should make it up into the low and mid 80s as the sun will works its way in between the showers, so overall Thursday will still feel fairly humid, if not quite as oppressive as the first half of the week. Thursday night will be mostly cloudy, with some scattered showers in the vicinity, and lows in the mid 60s.
Friday will definitely be a change of pace, as the front slows its southward movement, and the position of the jet stream aloft will be favorable to pulses of atmospheric instability and unsettled conditions. It will be mostly cloudy with scattered showers, but it will definitely feel cooler, with highs only in the mid 70s. Friday night will be fairly comfortable for sleeping with the windows open, as temperatures slip back into the upper 50s under mostly cloudy skies.
Taking a long view out towards next weekend, early indications are for some unsettled conditions with a chance for rain showers on both Saturday and Sunday, and highs seasonably in the low to mid 70s for Saturday, and mid to upper 70s for Sunday. Lows will be in the mid to upper 50s.
Taking a look into the middle of September, the NOAA climate prediction center is suggesting a ridge that will set itself up over the Ohio River Valley and Western Great Lakes, and to its east, where atmospheric disturbances will ride over the ridge and spill out into lower latitudes, will be the Northeast. Higher than normal precipitation amounts are expected as we move towards the end of astronomical summer, with temperature near or a little bit above normal.
There is some divergence in the models, with the typically more accurate European model (ECMWF) favoring a warmer September, as does the American (GFS) ensemble run (the ensemble is a group of model runs with slightly different initial conditions to account for uncertainty), while the GFS operational model is favoring a cooler outlook. As one of my grad school meteorology professors once quipped, “GFS stands for ‘Good For s–t,’” but we’ll keep you posted just in case.