We're back with our second disruption outlook, this time examining late week flying into Philadelphia with an eye towards Andrew McMahon's Saturday concert. We're also relieved that our second outlook is relatively favorable (tl;dr we don't think rebooking is called for, though adjusting expectations to include delays of approximately 45 minutes is appropriate). As we mentioned at the top of our first NYC marathon outlook, we're building algorithms that will automate and democratize the insights provided herein; until then, we're happy to synthesize things manually!
Let's start with the weather forecast to set the table: low pressure across the upper Midwest will send a strong cold front across the area on Friday. We'll keep linking to the NWS explainer on large scale weather patterns until somebody tells us otherwise. Out ahead of the front, rain will push into the region Thursday night then overspread the region on Friday. Rainfall could be heavy at times and a rumble of thunder is possible earlier in the day. Some clearing is expected as the day wanes on, however the front won't be offshore until early Saturday. What does this weather forecast mean for airport capacity? Before we answer that part of the question, if you're wonder what airport capacity is, we might suggest you read our blog post on the topic (as well as the follow-on post that explains how delays are created).
We wouldn't be surprised if PHL opens up at a 32 arrival rate, which is significantly off it's optimal rate of 60. Fortunately, an hourly supply of 32 slots meets demand for the most part, though we are keeping an eye on a demand overage in 8 a.m. hour. There are 39 scheduled arrivals in the 8 a.m. hour, which by itself would be enough to create delays. These potential delays are compounded by the fact that the arrival activity is heavily backloaded, with 31 of the 39 arrivals scheduled in the back half of the hour. How does that translate to some specific delay estimates? While delays would have lingered into the beginning of the 9 a.m. hour even if arrivals were more evenly spread out, the backloading will result in delays persisting for the entirety of the 9 a.m. hour. We've modeled delays averaging 17 minutes for arrivals scheduled in the 8 a.m. and 9 a.m. hours. As we illustrated in the above-linked delay explainer, delay minutes can be shifted around between flights: that means we're confident in the average figure we predicted, however the maximum delay is likely to exceed our estimate if delays are are not equitably distributed. Assuming delays are equitably distributed (i.e. allocated strictly by order of scheduled arrival), our modeling did include a maximum delay of 38 minutes. And as long as we're making disclaimers, we should also share that the flight schedules we used do not include cargo activity, most of which is concentrated outside of peaks for passenger flying. Nevertheless, there also exists some risk that demand from unseen cargo flying will increase delays beyond those predicted.
Anyways - enough with caveats! Next we have to answer how the FAA will dispense these delays. The demand overage is likely too short-lived that they'll reach for the GDP option (like that which was simulated in the delay explainer). That leaves airborne holding (i.e. circling prior to landing) or a ground stop as the next most likely options. 38 minutes is quite lengthy for an airborne hold and introduces the possibility of a diversion, so we'd bet on ground stop. We're working on a new post that surveys the rest of the FAA's traffic management toolbox, including ground stops - for now, we'll describe a ground stop as an initiative that applies the delay at the flight's origin airport (like a GDP). Unlike a GDP, flights are not given an individual wheels up time but instead released all at once upon the expiration of the ground stop. Ground stops are most likely to impact those flights that are shorter (i.e. under 600 miles or so). Before we move on, we'll circle back to that mention of thunder. Lightning is liable to create delays separate from those modeled above: employees who work outside (e.g. baggage handlers, fuelers) are pulled off the ramp when lightning is in the area, which halts aircraft servicing.
We're most concerned about arrivals in that mid-morning window, though there are 2 more periods that we should highlight. Clearing is forecast to have just begun by the noon hour, when 30 arrivals are scheduled. Again, we observe some backloading here (22 of the 30 scheduled to arrive in the back half of the hour). The good news is even if capacity remains constrained at a 32 rate, we predict average delays will be less than 8 minutes (and peak at 21 minutes). This level of delay should be manageable with airborne holding.
We anticipate the arrival rate will have increased to somewhere between 40 and 48 by early evening, which will help the airport handle the 46 scheduled arrivals between 6:15 p.m. and 7:15 p.m. If improved weather conditions allow for the increased arrival rate, any delays are likely to be almost imperceptible and absorbed in the air. Should clearing not occur as forecast, the early evening will play out very similarly to mid-morning (including a likely ground stop to manage delays averaging 16 minutes). That about does it for potential air traffic delays on Friday; we should of course mention that things like aircraft servicing, airline staffing, network effects, etc. can - and will - produce some delays.
Where does this leave concert goers flying in on Friday? The predicted level of disruption isn't so acute that airlines would consider a schedule reduction to mitigate delays, so let's not worry about cancelled flights. With delays underneath an hour in all likelihood, we're skeptical that rebooking options would be sufficiently attractive, so our recommendation is to adjust your delay expectations. That said, we understand if a delayed arrival is not the particular type of angst you're after this weekend. If you're inclined to rebook away from the windows we highlight, airlines are progressively offering some rebooking flexibility. Airlines have eliminated change fees for most tickets, though a fare difference may still apply; additionally standby is increasingly free. Same-day confirmed change (SDC) policies are a bit too varied to generalize, so we've linked to respective policies for Alaska, American, Delta, JetBlue, Southwest and United.
Flying in Saturday? We'll be back tomorrow with an updated forecast. As a preview: clearing should generally allow for increased airport capacity while demand relaxes somewhat relative to Friday. This is a good combination and we expect fewer air traffic delays on Saturday!
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