Jan 24-26, 2000 winter storm -- effect on ACARS soundings: a first look

A severe snowstorm effected the East Coast on Jan 24-26, 2000. This is a brief look at its effect on ACARS soundings. This is a brief first look intended to elicit comment. Thanks to Tom Schlatter, Randy Baker, Cecilia Girz, Joe Golden, James Ott, and Rich Mamrosh for helpful comments already.

This comparison is based on two weekly summaries of ACARS soundings, for the weeks starting Sunday, Jan 9, 2000, and Sunday, Jan 23, 2000, and hourly METAR reports for the latter week. (The intervening week that started on Jan 16 is not used because we had network problems here at FSL, so the sounding statistics are artificially low.)

For an overview of the storm, here are some surface data plots with radar superimposed, from the Unisys archive:
1/23-00Z , 1/23-12Z , 1/24-00Z , 1/24-12Z , 1/25-00Z , 1/25-11Z , 1/26-00Z , 1/26-12Z , 1/27-00Z , 1/27-12Z

I looked at several airports that were impacted by the storm. Some of these showed weather-related decreases in soundings and some did not. Here is a summary of what I found. Airports with significantly reduced soundings are shown in red.

Airport	Week starting		Notes
	Jan 9	Jan 23
Greensboro, NC (GSO)	10	11	Aircraft fly as scheduled in < 0.5 mi visibility on 1/25
Greenvile-Spartenbrg, SC (GSP)	4	6	Poor weather on Mon 1/24. (However, no soundings occur here on Mondays (UTC time) because UPS doesn't generally fly.)
Charlotte/Dougls, NC (CLT)	8	13	Poor weather on Mon 1/24. (However, no soundings occur here on Mondays (UTC time) because UPS doesn't generally fly.)
Baltimore Intl, MD (BWI)	13	12	One sounding missed at 04Z on 1/26 due to inclement weather.
Raleigh-Durham, NC (RDU)	10	4	No soundings from 15Z on 1/24 through 1/28. Airport closed from 1/25-0815Z to 1/27-2105Z. (RDU was the only major East Coast airport closed for an extended period by the storm.)
Philadelphia, PA (PHL)	46	37	No soundings before 1/26-1300Z, probably due to snow accumulation. (5 soundings on Wed 1/26 vs. 11 soundings on Wed 1/12)
La Guardia, NY (LGA)	49	42	No soundings between 1/25-0700Z and 1/26-1325Z, probably due to freezing rain on the 25th and possible snow accumulation.
Newark Intl, NJ (EWR)	86	60	No soundings between 1/25-15Z and 1/26-01Z. Freezing rain reported. About half the normal number of soundings on 1/25-26 (18 soundings on 1/25-26 vs. 31 on 1/11-12).
Denver (DIA) (DEN)	521	482	8% decrease for the week, even though no local delays were reported. This shows the effect of flights canceled due to East Coast weather.
All airports	4611	4237	8% decrease for the week overall. This shows how the inclement weather on the East Coast affected the entire route system.

Here is a look at the two weeks in question by day.

week	Total	Sun	Mon	Tue	Wed	Thur	Fri	Sat
9 Jan	4611	456	493	712	806	717	766	661
23 Jan	4237	503	542	649	670	677	637	559

Sunday and Monday, the week of the 23rd actually has more soundings. But by Tuesday, the winter storm begins to show its effect, and we have 136 missing soundings on Wed (with respect to the earlier week). For Tue-Thur, the number of soundings is down 11% nationally. We are reminded by this that we lose two soundings for every cancelled flight. Only one end of the planned flight may be experiencing inclement weather.

This fact is relevant when comparing missed ACARS soundings to missed radiosondes. A draft NAOS report states that most missed radiosondes are not weather-related, being caused instead by staffing or equipment problems. The same report expected most missed ACARS soundings to be weather-related. Our results partially corroborate this: we do see missing soundings when airports experience inclement weather, but we also get missed soundings (the other end of cancelled flights) at locations where the weather is not necessarily bad. Moreover, general backups in the flight system may cause flights to be cancelled between two airports when neither airport is experiencing bad weather.

To assess the availability of traditional radiosondes during the storm, I looked at three sites in the storm area, GSO (Greensboro, NC), IAD (Washington Dulles), and OKX (New York City), and found that no soundings were missing at synoptic times during the storm, and 2 (GSO and IAD) or 1 (IAD) additional soundings were taken on 1/25.

Some tentative conclusions:

• Low visibility doesn't necessarily decrease soundings by much, even at small airports such as GSO, if the airport, crew, and equipment are certified for low-visibility approaches. (I believe most ACARS-equipped aircraft are so certified.)
• IFR weather, even if only rain, can slow throughput at busy airports, such as ORD (but we already get a lot of soundings from these airports).
• Heavy snow cover or severe ice can close airports, as was the case at RDU, or slow throughput, as was the case at EWR.
• When inclement weather effects a portion of the country, route delays and cancelled flights can decrease soundings that are in good weather regimes as well as in locations directly affected by the weather. (This could limit data upwind of inclement weather locations, making it more difficult to predict the end of the bad weather.)
• Missed radiosondes do not seem to be a function of weather, and even when they are, radiosondes in other parts of the country are likely to take place as scheduled.

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Prepared by Bill Moninger, moninger@fsl.noaa.gov