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We've transitioned our blog to Substack

  • Writer's pictureTim Donohue

[Thanksgiving 2021] Wednesday Nov 24 first look

We pick up our Thanksgiving coverage focused on Wednesday. We've anchored our passenger volume expectations to last year's TSA throughput and, while there's been some flip-flopping (Friday was narrowly busier than yesterday), we still anticipate Wednesday will be the busiest day of the "go" period.

Welcome to any new readers - we're grateful that you're here! We're building deep learning algorithms to democratize flight delay predictions; until we launch, we're eager to synthesize things manually in our outlooks. We'll frequently refer to airport arrival rates, queuing delays in the airspace and different tools to distribute those delays. If these concepts are unfamiliar (or intimidating), we'd encourage you to check out our explainers.

Another frontal system looks to reach the Pacific Northwest coast by late Wednesday night, though dry conditions prevail for much of the day at Seattle (SEA). Chances for rain start to increase as we head into the evening, however, and some lowering of ceilings/visibility should be expected. While we'll hold off on putting chances on it, for the moment we think an arrival rate of 42 is a reasonable floor once rain arrive. With that said, there are notable arrival demand peaks in the 6 p.m. (45 scheduled arrivals) and 8 p.m. (43) hours. If a 42 rate is realized, we modeled average delays of less than 10 minutes (max delay of about 25 minutes) for arrivals in these hours; there's also some scheduled bunching in the first half of the 10 p.m. hour that could produce a similar level of delay. We should add that our modeling is aimed at arrival delays, however there's a strong correlation with departure delays (albeit with some lag and/or possible alleviation). We bet the FAA will rely on metering (possibly airborne holding) to manage the demand overage in such a case, which would unfortunately obfuscate delays somewhat (i.e. not included in the FAA's handy - and newly redesigned - airspace system dashboard, though may be referenced in their ops plan).

Let's move southeast to Denver (DEN), where there's a slim chance that they may see their first measurable snowfall of the season. As an aside, DEN typically sees their first snow of the season during the third week of October and will actually set the record tonight for latest snowfall. The system that could potentially deliver some flakes on Wednesday, however, appears to be setting up too far south - a continued southward trend threatens to remove even the slight chance for snow from the forecast. Regardless, any coating of snow should not prompt the type of snow removal efforts that close runways [and reduce arrival rates]. In the absence of those runway closures, we struggle to envision an arrival rate less than 64 (and that's quite pessimistic - we'll put chances on it tomorrow with a refined forecast in hand). Fortunately, even a 64 rate will largely accommodate the schedule - whose peak is at 67 scheduled arrivals in the 10 a.m. hour - without air traffic delays. As always, delays on account of aircraft servicing, airline staffing, network effects, etc. can (read: will) produce some delays.

We'll continue east to Chicago O'Hare (ORD), where at first glance we were admittedly a bit worried. South-southwesterly winds begin to ramp up Tuesday night into Wednesday ahead of a broad low pressure area. ORD is optimized for the prevailing east-west wind, with 5 runways oriented correspondingly; just 2 "crosswind" runways are available otherwise. Thankfully, ORD appears to be quite successful at hanging onto a west flow, even with a southerly wind component introduced: we looked at over 1,000 hours with comparable wind conditions and conclude there's less than a 1 in 20 chance that the arrival rate will be materially reduced. We should highlight that while the risk is low, the consequence is quite high - resulting arrival delays would average at least 30 minutes in several hours and approach 60 minutes in the 7 p.m. hour.

We're also watching the arrival time of some precipitation at ORD. Much of the day looks to remain to dry before rain chances tick up during the early evening. We think there's a 8 in 10 chance that ORD maintains a 100 arrival rate even once clouds thicken/lower, which comfortably accommodates the evening scheduled arrival peak of 79 flights (for that matter, even the worst case arrival rate of 92 accommodates demand). Before we move on from Chicago, we should also touch on nearby Midway (MDW). MDW's arrival rate floor should be 28 on Wednesday: given that scheduled arrival demand peaks at just 22, we'd understand if you dismissed the possibility of air traffic delays. Unfortunately, MDW is a somewhat unique case given elevated levels of private jet operations (make up 9% of demand versus 4% at other core airports). These private flights are not reflected in demand numbers until day-of (cargo flights present a similar problem) and make any MDW modeling exercise at this point a bit futile; we'll monitor the developing demand picture.

Let's round out this post with a look at Dallas-Fort Worth (DFW), where a cold front is expected to move across North & Central Texas late Wednesday. It appears the highest chances for rain and thunder occur after midnight, which should thankfully mitigate impacts to air travel. Nonetheless, there's a fairly hefty late-evening bank (94 arrivals in the 9 p.m. hour and 58 departures in the 10 p.m. hour) that could see impacts, especially if the approaching front were to speed up. While we're not ready to put chances on it, a 78 arrival rate - and resulting ground stop - seems like a reasonable worst case scenario; more probable are some departure delays for westbound flights whose departure fixes are constrained.

Where does that leave readers planning to travel on Wednesday? We're monitoring ORD and DFW most closely, though risk is too low at ORD and uncertainty too high at DFW to recommend rebooking at the moment. We're also keeping a curious eye on DEN's nascent snowfall record, though don't anticipate impacts to air travel. We'll be back tomorrow with the latest weather forecast and updated delay modeling!

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