Friday May 20 travel

Late Spring snowstorm disrupts DEN operations by afternoon; moisture surging into Florida likely to produce AFPs.

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. These feature several recurring themes that we recognize may be unfamiliar or intimidating, so we’ve written explainers that tackle airport arrival rates, queuing delays in the airspace and different tools to distribute those delays. If there’s a topic or mechanism you’d like to see unpacked, please let us know (same goes for special travel occasions).

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After a shallow post-Easter recession, passenger volumes have surged in the last week or so to pandemic highs by at least one measure. Following yesterday, the 7-day moving average of TSA throughput broke through 2.2 million screened for the first time since Feb. 28, 2020. We’ll concede, however, that single-day checkpoint volumes are a more tangible indicator—and that high-water mark still belongs to the Sunday after Thanksgiving 2021, when 2.45 million travelers were screened. For those readers more inclined towards one-day records, our Holt-Winters model suggests both tomorrow and Sunday have about a 2 in 5 chance to surpass 2.45 million screened. Taken together, there’s a 2 in 3 chance that Friday or Sunday sets a new pandemic single-day record¹.

While Sunday looks to be the more challenging setup—with a strong cold front spreading rain and thunderstorms through the Gulf and East Coasts—we couldn’t help but write about Friday. Variety makes these a bit more enjoyable to author (and hopefully to read) and tomorrow provides probably our last chance to write about snow for a bit. Plus thunder. Maybe even at the same airport.

Denver (DEN)

What a difference 24 hours will make. Near record-breaking warmth today will give way to a significant late spring snowstorm tomorrow. An elongated area of low pressure in the upper atmosphere will be the antagonist in this story. Rain showers are possible across the Front Range urban corridor by sunrise Friday morning, with snow mixing in² by midday. The TAF calls for a transition to all snow by 2 p.m. There’s a considerable spread around snowfall amounts owing to, in part, uncertainty about how much melting will occur. That having been said, the Denver/Boulder weather service office’s expected accumulation is 5”; at the high end of the snowfall range, DEN could see 11”. Not all of that comes down in time to impact Friday’s operation, however: By midnight MDT, mean snowfall from the short range ensemble forecast (SREF) is 2.5” (with 7” at the high-end).

The ceilings that accompany the plain rain are enough to introduce a roughly 1 in 10 chance for a 64 arrival rate. Once the snow arrives, we’re prompted to drop our arrival rate floor to 48. Scheduled arrival demand³ peaks at 74, in the 10 a.m. hour. The question will be when would a step down occur. The answer, though still unspecific, is when snow accumulates to the point that runway closures are required for snow removal. The plain rain that precedes snow will undermine any chemical pretreatment of runways, though that’ll be balanced by warm ground temperatures and the May sun angle. If all snow is falling by 2 p.m., we’ll [unscientifically] say the first runway closure is required 2 hours later at 4 p.m. We’ve modeled an all-day ground delay program, opening at a 64 rate before stepping down to stepping down to 48 at 4 p.m. While arguably too pessimistic on the front end, it produces average delays of 18 minutes⁴ for all arrivals. Arrivals during the three hours starting 6 p.m. are especially impacted, when delays approach 70 minutes. Delays on either side of this window approach 30 minutes (as would delays in 8, 9 and 10 a.m. hours).

The late season nature also introduces some uncertainty with respect to readiness, though there’s still nearly a 2% probability for measurable snow on May 20, per daily climate normals. That’s hardly a tail risk, so we’d bet the airport community is generally still prepared for winter operations. If readiness issues were to manifest themselves anywhere, we’d look to deicing (not runway/taxiway snow removal). To be clear, deicing delays should be expected anytime snow is falling (whether mixing in or all snow), but a less-than-normal complement of deicers could exacerbate queues.

Florida

Shower and thunderstorm chances increase alongside a surge of moisture tonight into tomorrow. A strong thunderstorm or two cannot be ruled out, especially in the afternoon and evening hours. We typically focus our attention on connecting hubs, of which there’s really only one⁵ in Florida—Miami (MIA). We think a terminal initiative (i.e. ground stop or ground delay program) is unlikely here, though could foresee some airborne holding (and diversions) if a storm trains over the field.

What’s more concerning, both in terms of probability and consequence, are airspace flow programs (AFPs). Modeling airspace, not airport, delays is unfortunately challenging—historical data on airspace capacity and demand is buried a bit deeper than their airport analogs. We’re digging, but in the interim there’s regrettably little useful information we can provide. We will mention that unconstrained rates very by sector, though range from 31 to 99 aircraft per hour. In a moderately constrained environment, rates can be cut by roughly 30%; in a severely constrained environment, it’s more like a 45% reduction in capacity.

Recommendations

United and Southwest have both issued waivers covering DEN tomorrow. If you have a layover in DEN that is scheduled to start in the 6, 7 or 8 p.m. hour and cannot absorb a delay of 70 minutes, we’d encourage to check out alternate itineraries.

We’re also keeping an eye DFW, where there’s still considerable model disagreement about a line of late afternoon thunderstorms. We’ll wait for one more round of models then post an update on Twitter for those interested!

1. There’s also a 3 in 10 chance that today exceeds 2.45 million screened; considering today, there’s a 3 in 4 chance that at least one of Thu, Fri or Sun breaks the record.

2. Though the forecast discussion doesn’t mention thunder, the hourly forecast does include a slight chance for much of the late day. We’re admittedly not sure why—CAPE values look to be pretty modest, though shear is a little more impressive (winter storm considered). Perhaps a weather geek among our readers can chime in.

3. Cargo airlines as well as private jets are not included in scheduled demand and only become apparent when they file a flight plan (generally day-of). This unforeseen demand introduces the risk that delay probabilities/intensities are under-forecast. For the 30 days ended yesterday, unscheduled demand has added 14.8% to scheduled demand for Core 30 airports. For DEN, unscheduled demand added 6.9%.

4. While our modeling is aimed at tackling arrival delays, there's a strong correlation to departure delays (albeit with some lag and/or possible alleviation). Consider a scheduled "turn" at an airport: the inbound flight is scheduled to arrive at 2:19 p.m. and departs at 3:30 p.m. (71 minutes of turnaround time). Let's say the inbound is delayed by 40 minutes and instead arrives at 2:59 p.m. We'll further assume that the airline doesn't need the full 71 scheduled minutes to turn the aircraft and can accomplish the turn in 45 minutes if they hustle - the departure will push back from the gate at 3:44 p.m. (delayed by 14 minutes). In this example, a 40 minute arrival delay in the 2 p.m. hour is partially passed through to a departure in the 3 p.m. hour. Had the turnaround been scheduled at 45 minutes instead (i.e. no turnaround buffer), the lag between arrival and departure delay would still exist, however the delay would be fully passed through.

Additionally, our efforts are aimed at diagnosing air traffic delays (i.e. those that result from an imbalance between capacity and demand). Though not the focus of our efforts (yet), delays owing to aircraft servicing, airline staffing, network effects, etc. are always lurking.

5. Don’t @ us about FLL or MCO. We like JetBlue too, but c’mon.