ITHACA, N.Y. — Here’s an example of taking the extended forecast with a grain of salt. When putting together last week’s forecast, it was showing a fairly significant cold air mass on our doorstep from Monday on, so that was noted in the write-up.

That did not pan out for a couple different reasons – over the past week, a pair of strong typhoons (Maysak and Haishen) caused a major realignment in the jet stream upstream over the Pacific. These tropical cyclones injected a lot of energy into the jet stream, realigning its troughs and ridges and supercharging its amplitude. The results are jaw-dropping: downstream, this is leading to record-breaking West Coast heat, and a freakish cutoff low in the Mountain West that will take temperatures from the mid 90s in the Denver area, to snow for Tuesday. Meanwhile, even further downstream over the Eastern United States another massive ridge is building thanks to all that displaced warm air from the interior west. That means the next few days will be quite hot for September. The extended forecast from last week is a bust, though it’s doubtful anyone locally will be that upset.

Your Weekly Weather

It’s been a pleasant Labor Day weekend thanks to an area of high pressure over the Southeastern United States. Skies are partly cloudy, with some patchy fair-weather cumulus and streaming cirrus overhead, providing a little contrast on a comfortable Sunday with generous amounts of sunshine. Temperatures have already topped out in the mid 70s in most of the Southern Tier and Tompkins County, and it should be a calm evening as well, with the cumulus clouds withering away after the loss of diurnal heating, and leaving but a few passing clouds overhead as lows slide back to the upper 50s to around 60°F, one of those perfect nights for sleeping with the windows open or letting the kids camp in the backyard.

Heading into Monday, a strong storm system will be passing well to our north across Canada. The impacts on Ithaca and Tompkins County will be limited. We’re on the periphery of the system, and the air aloft is quite dry, which will keep shower activity to only a few scattered cells. In fact, the tip of the front will stall out for a little while over the Eastern Great Lakes as it tries to slip past the edge of the high. However, while stalled the low’s counterclockwise flow will enhance the southerly flow of the clockwise-spinning high pressure system to the south. This will will allow for warmer temperatures, and the interaction of the high and the low will tighten the pressure gradient and conjure up some gusty winds, up to 30 MPH. Expect mid 80s for highs with partly cloudy skies, with breezy conditions and a few showers west of Ithaca. Monday night will see the winds calm as the low drags eastward and the systems’ flow decouples. Lows will be in the low to mid 60s with mostly cloudy skies.

Looking into Tuesday, the high pressure area, now sitting off to the southeast over the coastal Atlantic, will drive some serious late summer heat into the Southern Tier. It will be hot and humid, with highs in the upper 80s and dewpoints in the upper 60s. The air should stay stable, however, so pop-up storms aren’t expected. Tuesday night will be partly cloudy with lows in the mid 60s.

Wednesday will likely be even hotter and more humid, as that cutoff low pushes southward over Colorado and the downstream ridge over the eastern U.S. expands with the influx of warm air. A disturbance over the Mid-Altantic will supply the moisture to make it very muggy here as we flirt with 70°F dewpoints and 90°F for highs. However, while moist air is inherently more unstable than dry air, it still won’t be enough to overwhelm the stability of our local column of atmosphere, so it will remain partly to mostly sunny through the day. Wednesday night will be partly cloudy, maybe a few showers south of Ithaca, with lows in the mid 60s.

Thursday will be an unsettled day as a cold front pushes in from the northwest against the ridge of hot, humid air. This should be enough to shunt the ridge back to the southeast, but it will take a day. By sunrise Thursday, scattered showers and a few thunderstorms will begin to fire off west of Ithaca as southerly winds between the ridge and the low pressure storm system funnel in unstable air from the southwest. It won’t be a washout, but you’ll want an umbrella. Between the showers and storms will be drier periods with mostly cloudy skies. Highs Thursday will get into the mid 80s. Thursday night will remain stormy as the front continues to grind forward, with scattered showers and storms, mostly cloudy skies otherwise, and lows in the low 60s.

Cooler air should begin to works its way in Friday as the front finally pushes past Tompkins County, but with the remnant instability, scattered showers will continue through the day, including thunderstorms in the afternoon and evening. Between periods of rain we can expect mostly cloudy skies and highs in the mid 70s. Drier and more stable air should begin to work its way in after sunset Friday, with Friday night turning partly cloudy with lows in the mid 50s.

Next weekend isn’t looking bad, though it appears there will be enough instability in place to allow a few pop-up showers and storms late in the day Saturday, and possibly a cold front later in the day Sunday. Both days will be in the mid to upper 70s with mostly cloudy skies, with lows in the upper 50s. While cooler, that’s still a few degrees above normal for this time of the year.

Graphics courtesy of the NOAA Climate Prediction Center.

Extended Outlook

Looking ahead towards the middle of September, the large-scale weather patterns will be dependent on that super-charged, reconfigured jet stream pattern. The energy injections from those Western Pacific typhoons will result in a deep ridge on the West Coast, a deep trough in the central U.S., and a second ridge on the edge of the East Coast – if you imagine those typhoons as stones thrown into a pond, these are the ripples. In Tompkins County, the jet stream will be practically overhead, not really favoring either the trough or the ridge, so temperatures will be about normal.

Now, you might notice that only the eastern flank of the country is expected to have above-normal precipitation, even though there are two ridge. For that explanation.consider the source of the air feeding those ridges in trough. For the West Coast, the air comes from Mexico, it’s dry, hot continental air. For the trough over the Central U.S., it’s Canadian air, or in other words dry, cold air. Meanwhile, over the East Coast, the source of warm air is going to be the Gulf of Mexico – a body of water that will provide moisture into the air moving northward, and since moist air is more unstable, and moist air allows for heavy rains, this should translate to above-average precipitation.

After this period, the models show an eastward (downstream) propagation of the jet stream, meaning cooler and drier air by about September 20th. But if another strong typhoon goes poleward and hits the jet stream (there are no other typhoons in the tropical Western Pacific at the moment), the forecast could be very different, so we’ll see what happens.

Brian Crandall

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