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  • Tim Donohue

[Thanksgiving 2021] Wednesday Nov 24 update

We're back to discuss the latest weather forecast and its implications for air travel tomorrow; if you didn't read our first look, you can do so here. The large scale weather features highlighted in yesterday's outlook remain intact, with the main culprit for unsettled weather being a disturbance that tracked across the Northwest this morning. That disturbance heads east, causing an area of low pressure to form over the Great Lakes: a cold front follows that produces scattered showers (and some thunderstorms in southern areas) from the Great Lakes to South Central. What does this mean for travelers planning to fly tomorrow, when air travel is set to recover nearest to pre-pandemic levels? Let's read on to find out! (tl;dr delays possible for DFW mid-morning and late-evening.)

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.

Seattle (SEA) will be staring down their next incoming frontal system by Sunday evening, which delivers yet another round of rain. The good news is conditions look hold up through the 6 p.m. hour, which is the evening demand peak (with 45 scheduled arrivals). The 8 p.m hour (with 43 scheduled arrivals) will be somewhat more marginal - if we did our conditional probabilities correctly, we estimate chances for a 42 arrival rate during this time are between 1 in 20 and 1 in 10. Stakes are relatively low in the 8 p.m. hour, when a 42 rate would produce arrival delays averaging less than 5 minutes. We'll also mention some scheduled bunching in the first half of the 10 p.m. hour (24 of the hour's 28 arrivals) that would produce arrival delays approaching 15 minutes if the rate drops to 42 by then; given that we'd be two hours later in the frontal system overspreading the area, probabilities are nearer 1 in 10 for this outcome. While we aren't modeling departure delays yet, there's a strong correlation to arrival delays (albeit with some lag and/or possible alleviation).

The most interesting story to follow at Denver (DEN) will not be related to air traffic delays, which we expect to be zero, but whether they receive their first measurable snowfall. Denver typically receives their first snowfall during mid to late October and last night set the record for latest into a season without snow. The air mass moistens tonight as the cold front mentioned in the opening paragraph moves through the region. Any snow will be fighting dry lower levels as well as warming temperatures tomorrow, however, and NWS Boulder puts the chance at just 9% that DEN will receive 0.1" of snow. As far as airport capacity goes, even if that 9% chance verifies and they receive a dusting, runway closures for snow removal won't be required. With that said, we anticipate DEN will deliver a 96 rate, which comfortably accommodates the hourly peak of 67 scheduled arrivals. Even if snow doesn't accumulate, de-icing delays should be expected if snow is actively falling (probabilities for snow - while still less than 20% - are highest during late afternoon). We'll also take the opportunity to reiterate that our modeling is aimed at simulating air traffic delays (i.e. those delays that result from an imbalance between capacity and demand): separately, delays resulting from aircraft servicing, airline staffing, network effects, etc. can (and will) occur.

Let's move over at Chicago O'Hare, where we'd expressed some initial concern yesterday at strengthening south-southwesterly winds ahead of the cold front. Even yesterday, however, we'd backed off our concern; today, with a refined forecast in hand, we think there's less than a 1 in 50 chance that ORD is forced to relinquish a west flow [and its 5 accompanying runways] in favor of just 2 "crosswind" runways. If you're uninterested in some technical discussion, you can skip to the next paragraph... a reasonable worst case scenario for ORD tomorrow would be winds from a direction of 190 degrees, sustained at 20 knots and gusting to 32 knots. This results in a crosswind component of 24 knots for a runway heading of 270 degrees. This may flirt with allowances for the E175 especially, so we wouldn't be surprised to see some regional arrivals request one of the 22's. Otherwise, we expect most will accept 27/28.

One more note on Chicago, though this one concerns with Midway (MDW). Because private jet operations make up 9% of demand at MDW (compared to 4% at other core airports), we won't really have a clear picture of demand until tomorrow (as flight plans are filed). We'll be monitoring how it takes shape in real-time and you can follow along with us on Twitter (@aerology_ai). You can also consult to the FAA's airport arrival demand chart directly!

Cloud cover will be abundant at Dallas-Fort Worth (DFW) tomorrow as rich moisture is transported north in to the region. Clearing is possible by afternoon, but some damage may already be done in the 9 a.m. hour: we think there's a 1 in 5 chance that morning arrival rates will be at or underneath 78. If a 78 rate is realized in the 9 a.m. hour, we've modeled average delays of 20 minutes for the 105 scheduled arrivals (with delays lingering for scheduled arrivals in the 10 a.m. hour). We'd bet the FAA will take a wait-and-see approach, which would leave a ground stop to manage a potential demand overage - this would serve to essentially exempt longer flights (i.e. those originating beyond a radius of approximately 600 miles) from delays, leaving arrivals originating from closer in to absorb a disproportionate delay. In this case, for those closer-in arrivals, we expect delays would average nearly 70 minutes.

The other time of day to keep an eye on at DFW will be late evening, when the cold front arrives. Latest model guidance suggests isolated thunderstorms become more likely only after the front has pushed south and east of the airport (into a more moist/unstable environment). While probabilities are low (10-20%), if thunderstorms were to set up west of or over the airport, another reduction in arrival rate to 78 is possible. Airborne holding will be a slightly more plausible tool to manage the overage in the 9 p.m. hour (94 flights) than earlier in the day, in which case we estimate delays will average about 10 minutes (max of 29 minutes) and be distributed in a last-come, last-serve type basis; if the FAA instead relies on a ground stop, longer flights will again be exempted and we model an average delay of approximately 24 minutes for closer-in flights. More probable are departure delays for eastbound flights in the 10 p.m. hour whose departure fixes may be constrained by thunderstorms.

In terms of recommended actions, our only real area of concern will be DFW. We think it'd be prudent to at least explore re-booking options if you're originating in the South Central US and connecting through DFW around 10 a.m. tomorrow. The good news is airlines are increasingly offering rebooking flexibility (including no change fees on most tickets): we've linked to American's same-day change policy. We feel like we have some time (and are waiting for the 00Z HREF) before needing to make a recommendation to passengers transiting DFW during the late-evening, but generally less concerned for such trips. Elsewhere, passengers arriving SEA during the late evening may want to nudge their delay expectations up, but unless they have a short layover (and we don't think there should be many such itineraries), we don't believe rebooking is called for.

Happy Thanksgiving to our readers! We'll pick up coverage later this week with a focus on Sunday.

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