[Spring Break] Thursday Apr. 14 travel

South wind and rain showers for SFO, decaying thunderstorm activity across Gulf and Florida panhandle

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).

---

That felt weird to write. Thursday? Thursday.

We typically aim our blogging efforts at the most heavily traveled days, which has meant Friday or Sunday for the last 6 weeks or so. But Easter looks to redistribute demand. In 2019, when Easter fell on the same equivalent weekend, the Thursday before was actually the second busiest day of the entire Spring Break period. TSA throughput increased by 5.2% versus the previous Thursday in 2019: the same percentage increase would net 2.33 million screened this year, which would be good for second-most during Spring Break. Easter fell earlier in April last year—and the underlying trend was a bit different—but Holy Thursday checkpoint volumes were seemingly piqued in 2021 as well, increasing by 8.1% week-over-week. If this year behaves more like last year, we’d expect 2.40 million screened, which would be the highest throughput during Spring Break¹.

Fortunately for travelers tomorrow, the powerful low pressure system that produced significant impacts across the central U.S. on Tuesday and Wednesday is forecast to fizzle at least somewhat by midday Thursday. Meanwhile, another storm system approaches the Pacific Northwest and Northern California.

West Coast

Let’s get started in San Francisco (SFO), where shower chances should increase through the first portion of Thursday. Though rain chances taper off during the evening, a lingering shower can’t be ruled out into the overnight. More notable will be the southwesterly winds. Winds look to start the day with a stronger southerly component (200°), though sustained at under 10 knots; as the day goes on, winds will become gustier, but veer to a more southwesterly direction (230°). That the strongest winds do not coincide with the strongest southerly component should help capacity. Even so, we’re prompted to set our arrival rate floor for SFO at 28 (at least 1 in 3 chance for equal or lower rates). Scheduled arrival demand² peaks at 40 in the 9 a.m. hour, with a secondary peak of 36 in the 9 p.m. hour.

Though arguably a bit pessimistic, we’ve modeled³ an all-day 28 arrival rate and delays exceed 30 minutes in the 9 a.m., 11 a.m., noon, 3 p.m. and 9 p.m. hours, before peaking at nearly 60 minutes in the 10 p.m. hour. That assumes all inbound flights are captured by the ground delay program, however there’s some question as to whether they use a fence. We could foresee a scope that captures only those flights originating within 1,650 miles of SFO (essentially Chicago and west). In the latter case—with delays distributed across a smaller number of arrivals—we’d expect delays for captured flights to increase by more than 50%.

Snow levels plummet tonight around Seattle (SEA), allowing for wintry precipitation during tomorrow morning. Temperatures—and snow levels—rebound by late morning, permitting any shower activity to changeover to plain rain. The forecast discussion from the NWS Seattle/Tacoma office mentions light accumulations, i.e. less than 1/10th inch. The short-range ensemble forecast (SREF) is a little more bullish, but generally keeps accumulations underneath half an inch. Unless an outlier SREF member⁴ that tries to dump 4” of snow early tomorrow morning verifies, we don’t anticipate impacts to airport capacity resulting from snow removal. We do wonder about how SEA might handle mid-April deicing, however. Snow climatology suggests that the airport and airlines could have reasonably concluded their winter readiness season: After March 31, probabilities for measurable snow are less than 0.5%. We wouldn’t be surprised if deicing staffing is leaner than normal, which could result in longer queues (read: delays) for deicing. Otherwise, ceilings are forecast to stay at or above 6,000’ and we think there’s only a 1 in 20 chance that airport capacity will be insufficient to accommodate a scheduled demand peak of 46.

Valid 24 hours ending midnight (MT) tomorrow night. Note 95th percentile!

As we head east, we want to mention Salt Lake City (SLC). A stationary front trailing the central U.S. system will allow snow showers to linger across the Rockies. The good news is forecasted accumulations⁵ won’t pressure airport capacity, so while deicing delays should be expected if snow is falling, we can’t foresee a capacity/demand imbalance that would result in air traffic delays. While it will be windier in Minneapolis-St. Paul (MSP)—and some other details differ—the headline there should be the same: deicing delays possible, air traffic delays not anticipated⁶.

Southeast

We were a bit concerned by the location of the cold front relative to Atlanta (ATL) as well as the thunderstorm hatching, but the setup is not as intrusive as the national forecast chart might suggest. The front should cross the Mississippi River early this evening, make steady eastward progress overnight and reach northwest portions of Georgia by 8 a.m. tomorrow morning. Thunderstorm activity associated with the front looks to weaken as it approaches Thursday morning and the forecaster elected to include rain showers, not thunderstorms, in the TAF⁷. Some daytime heating-driven thunderstorms are also possible during the afternoon, though potential is higher for areas south of Atlanta. While not convective, the showers are still forecast to drop visibilities to 3 miles and ceilings to 800’. Fortunately, we think 96-98 is a good arrival rate floor⁸ in those conditions, which should accommodate the scheduled arrival demand peak of 83 flights (8 a.m. hour).

Before we move on from the Southeast, we also want to highlight Jacksonville Center (ZJX). This is the parcel of airspace that caused so much disruption during the first weekend of April. Mercifully, the weather set-up is not nearly as injurious: Whereas a stalled front produced slow-moving storms that bisected the Peninsula two weekends ago, tomorrow’s storms should be focused on on the Panhandle and diminish into the late afternoon. Nonetheless, some impacts look likely to Gulf routes and perhaps the western half of Florida through early afternoon. Historical data on airspace—rather than airport—capacities is buried a bit deeper (we’re digging), so it’s difficult to put even rough probabilities around capacity tomorrow. That said, unconstrained hourly rates for these sectors are approximately 31 and 99 along the Gulf and western half of Florida, respectively. More modest constraints can be expected to reduce capacity by approximately 15-25%.

Valid noon ET.

Recommendations

United has weather waivers published for Chicago and the Rocky Mountains: While tomorrow is included in terms of original travel dates, the primary intent it to address disruption today and applicability to tomorrow is low. As always, we will take the opportunity to remind readers that airlines have meaningfully improved general rebooking flexibility by eliminating change fees for most tickets (though a fare difference may still apply). We’ve linked to the same-day change policy for United (who operates an SFO hub). For the moment, we’ll confine any suggestions about exploring alternate itineraries to those trips that include an SFO layover that cannot absorb a delay of at least 30 minutes.

Northeast

We’re also keeping an eye on the Washington-Boston corridor, where the later arrival of the cold front will allow instability to increase with daytime heating. A broken line of strong to locally severe thunderstorms will be possible during the afternoon and early evening. We want the benefit of one more TAF cycle before sharing our thoughts on the Northeast, but don’t want to be clingy towards our West Coast and Southeast outlooks. So we’ll hit publish now on this post and craft a tweet-sized Northeast outlook later today.

And as long as we’re tweeting, we’ll also keep an eye on a southerly component to ORD winds.

1. The Holt-Winters model, though it can’t “see” the effects of Easter, gives Thursday a 5% chance to set a new pandemic high-water mark. The current record is 2.45 million and belongs to the Sunday after Thanksgiving 2021.

2. 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. From March 1 to April 6, unscheduled demand has added 14.6% to scheduled demand for Core 30 airports. For SFO, SEA and ATL, unscheduled demand added 14.6%, 4.9% and 4.7% respectively.

3. 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.

4. Go home, MBN3—you’re drunk.

5. Even at the 95th percentile of the Weather Prediction Center’s forecast, Thursday accumulations at SLC are less than 1”.

6. The 95th percentile of the Weather Prediction Center’s forecast keeps MSP accumulations at under 2”. Winds may be the more worrisome variable, as a direction of 260° splits the difference between their 30L|30R complex and [single] Runway 22. If a Runway 22 operation is required—at a 26 rate—the only sizable overage that exists would be in the 2 p.m. hour (49 scheduled arrivals).

7. Terminal aerodrome forecast (TAF) is a format for reporting weather forecast information, particularly as it relates to aviation. TAFs are issued at least four times a day, every six hours, for major civil airfields: 0000, 0600, 1200 and 1800 UTC, and generally apply to a 24- or 30-hour period, and an area within approximately five statute miles from the center of an airport runway complex.

TAFs complement and use similar encoding to METAR reports. They are produced by a human forecaster based on the ground. TAFs can be more accurate than Numerical Weather Forecasts, since they take into account local, small-scale, geographic effects. Source: Wikipedia

8. In this case, that means a 1 in 50 chance for lower rates, 1 in 20 chance for equal rates.