The Effect of Hurricane Floyd on ACARS and RAOB Soundings: A First Look

Bill Moninger
NOAA, Forecast Systems Laboratory

Hurricane Floyd impacted the East Coast between Sept 15th and 17th, 1999. There has been considerable interest in how this may have affected the number of ACARS soundings at airports in the region.

Because the number of soundings, particularly at smaller airports, varies by day and week as equipment is shifted among routes, I thought it best to study this question by looking at a three week period bracketing the hurricane. I looked at several airports along the east coast: Jacksonville and Orlando, FL, Greensboro, NC, La Guardia, NY and Newark, NJ.

When Floyd was near major airports such as Orlando or Newark the daily number of soundings was cut roughly in half, but this effect didn't appear to linger after the storm moved away. At regional airports, such as Jacksonville, effects of the hurricane were more long-lasting. The number of soundings were lower than normal even when the storm was not nearby, presumably because flights had been cancelled or diverted due to current or recent (previous day) weather problems elsewhere in the route system.

Regional Overview

The storm track shows that Floyd moved from the Jacksonville area on Sept 15th to north of New York on the 17th. It reached its maximum intensity between 12Z on the 15th and 12Z on the 16th as it moved up the GA, SC, NC coast. Here are some images of Floyd at various times during its history, taken from NOAA's Operational Significant Event Imagery Server. (Further information about Floyd may be found at the USGS Hurricane Floyd site.)

Radiosondes

Several radiosonde sites along the east coast were effected by Floyd. I looked at 12 sites, 11 of which normally report at 0 and 12 UTC and one of which (APB) reports at only 12 UTC. The table below shows what soundings were taken.

+ indicates a sounding was released at a synoptic time.
M indicates a missing sounding at a synoptic time.
e indicates an extra sounding, launched at 6 or 18 UTC.
P indicates a partial sounding.
A number indicates the hour of the sounding, if it was not 0, 6, 12, or 18. In generating summary statistics, these soundings are associated with the hour of the column in which they appear.

Date 9/14        9/15        9/16        9/17        9/18
Hour 0  6 12 18  0  6 12 18  0  6 12 18  0  6 12 18  0  6 12

MIA  +  e  +  e  +     +  e  +     +     +     +     +     +
XMR  M     M     M     M    22    10    22    10 15 22    11
JAX  +  e  +  e  +  e  +  e  +     +     +     +     +     +
CHS  1  e  +  e  +  e  +  e  1  e  +     +     +     P     +
RNK  +     +     +  e  +  e  +  e  +     +     +     +     +
GSO  +     +     +  5  +  e  P  5  +     +     +     +     +
MHX  +     +     +  e  +  e  1  5  +     P     +     P     +
IAD  +     +     +  e  +  e  +  P  +     +     +     +     +
WAL  +     +     +     +  e  +  e  +     +     +     +     +
APG        +           M           M           M           M
OKX  +     +     +     +     +  e  +  P  +     +     +     +
CHH  +     +     +     +     +  e  +  e  +     +     +     +
GYX  +     +     +     +     +  e  +     +     +     +     +

To summarize this table: Of the 125 possible radiosondes at synoptic times, 8 were missing and 4 were partial (only ascended to a limited altitude or had missing humidity information). However, 31 additional soundings were taken at non-synoptic times when the storm was nearby.

ACARS

ACARS flights were affected most obviously on Thursday, Sept 16th in North Carolina):

ACARS data for Thursday, Sept 9th, show a typical pattern:

Below is a detailed look at soundings at individual airports.

JACKSONVILLE, FL:

JAX appears to have been directly impacted by Floyd on the 15th. However, the hurricane had moved substantially to the north by the 16th, yet JAX still has no soundings on the 16th and 17th.

A look at three Fridays: the 10th, 17th and 24th, reveals the following. The previous Friday, the 4 soundings were by flights to and from O'Hare and Atlanta; the following Friday, the 3 soundings at JAX were by flights from or to O'Hare and Philadelphia. On Fri the 17th, Philadelphia was experiencing bad weather but Atlanta and O'Hare were not. I therefore surmise that the absence of any soundings at JAX on the 17th was due mostly to general backups in the route system more than to specific weather problems in any particular region.

Soundings at JAX (Summary):
         09/05    09/06    09/07    09/08    09/09    09/10    09/11 
 tot:        0        0        3        3        6        4        4
         09/12    09/13    09/14    09/15    09/16    09/17    09/18
 tot:        1        2        7        0        0        0        2
         09/19    09/20    09/21    09/22    09/23    09/24    09/25
 tot:        1        2        2        8        3        3        4

GREENSBORO, NC:

GSO was north of Floyd on the 15th, and shows a normal number of soundings that day. It appears to have been most impacted on the 16th and 17th. Although Floyd had moved considerably to the north by the 17th, GSO still shows fewer soundings than normal.

For comparison, on the following Friday, the 24th, GSO's soundings are taken by flights to and from O'Hare, Louisville, and Roanoak. None of these airports was affected by Floyd on the 17th, so it is hard to see why there are so few soundings on the 17th. As with JAX, I surmise the storm had so disturbed air travel by the 17th that airlines were moving equipment to major hubs rather than small airports in order to serve the maximum number of stranded passengers.

Soundings at GSO (Summary):
         09/05    09/06    09/07    09/08    09/09    09/10    09/11
 tot:        2        2        4        7        7        6        2
         09/12    09/13    09/14    09/15    09/16    09/17    09/18
 tot:        2        2        3        4        1        2        3
         09/19    09/20    09/21    09/22    09/23    09/24    09/25
 tot:        2        1        4        4        5        4        6

ORLANDO:

At a major airport like Orlando, secondary effects are small. The soundings were cut in half on the 15th. but on the 16th and 17th were back to normal.

Soundings at MCO (Summary):
         09/05    09/06    09/07    09/08    09/09    09/10    09/11
 tot:        6        7        6        9       10        5        5
         09/12    09/13    09/14    09/15    09/16    09/17    09/18
 tot:        5        7        9        4       11        9        6
         09/19    09/20    09/21    09/22    09/23    09/24    09/25
 tot:        5        6        8       10       12        9        7

LA GUARDIA, NY

On the 15th and 16th, Floyd is considerably to the south, yet the number of soundings at LGA is approximately half the normal number. Perhaps, LGA, being more of a feeder airport for the East Coast than, for instance, Newark (see below), is affected by the problems to the south. On the 17th, LGA is impacted directly by Floyd, with approximately half the normal number of soundings.

Soundings at LGA (Summary):
         09/05    09/06    09/07    09/08    09/09    09/10    09/11
 tot:        5        7       13       14       15       12        9
         09/12    09/13    09/14    09/15    09/16    09/17    09/18
 tot:        8       13       12        9        5        6        9
         09/19    09/20    09/21    09/22    09/23    09/24    09/25
 tot:        7       12       16       14       11       11        9

NEWARK, NJ

This major hub is only affected by Floyd on the 17th when it is nearby, even though the storm had lost force by then, with maximum sustained winds of only 50 kts. Problems further south on the East Coast on the 15th and 16th have no effect on EWR, presumably because aircraft from airports in that area do not represent an appreciable fraction of flights into EWR. (Currently, I have no explanation for the large number of soundings on the 23rd.)

Soundings at EWR (Summary):
         09/05    09/06    09/07    09/08    09/09    09/10    09/11
 tot:       12        9       10       19       15       14       19
         09/12    09/13    09/14    09/15    09/16    09/17    09/18
 tot:       13       13       17       17       15        7       15
         09/19    09/20    09/21    09/22    09/23    09/24    09/25
 tot:       14       10       14       16       26       13       13

Final remarks

The number of ACARS soundings at an airport can vary for several reasons.

Day of the week: both package and passenger carriers have different flight patterns on weekends than on weekdays.
Changes of equipment: even when the daily number of flights to an airport remains constant, the number of ACARS-equipped aircraft will change due to operational equipment rotation patterns.
Weather-related problems at other airports can disrupt the entire airline route system, and this study suggests that regional airports remain disrupted longer than do major hubs.
Finally, weather-related problems at the airport itself can of course limit the number of arriving and departing flights.

This study suggests strongly that weather at a single airport alone is not a sufficient predictor of whether that airport will have a decreased number of soundings. Rather, the impact of weather on entire regions must be considered.

When this is done, the data presented here suggest that a major weather event such as Hurricane Floyd can decrease the number of soundings in a region for two or three days, with the decrease being more pronounced and longer lasting at regional airports than at major hubs.

I consider these results provisional. I'm sure airline meteorologists and weather forecasters can point out many effects that I have missed. I solicit comments to the ACARS forum (restricted to NOAA and other authorized users of real-time ACARS data), or directly to me, and I hope that together we can expand and develop this study.