Pennsylvania Ice Storm 02-03 January 1999
Introduction:
Radar images showing brightband and refreezing. In the cross sections, note the brigthband near 7000 ft and the second weaker sleet/rain band below 5000 ft. All cross sections have an "x" or xsect in the name. We will add comments to these images and annotations soon. A few vertical wind profiles are included to show the strong winds above the surface based arctic air. See the vwp images.
An unseasonably cold arctic air mass moved over Pennsylvania during the day on 01 January 1999. At the same time, a surface low began to develop in the southern plains. This low would move northeastward into southern Michigan during the day on 02 January 1999. With extremely cold air in place over Pennsylvania, the warm air ahead of this storm was forced to rise over the dome of cold dry arctic air. This resulted in ideal conditions for a significant ice storm over Pennsylvania.
This page will focus on the meteorological conditions associated with the ice storm of 02-03 January. Heavy snow in Pennsylvania was confined to two locations in Somerset County. Most other locations received only 1-3 inches of snow before the precipitation changed to sleet and freezing rain. Many locations in central Pennsylvania received 1-3 inches of sleet accumulation. In the lower Susquehanna Valley, reports of up to 1 inch of freezing rain were reported before the precipitation changed to rain.
In addition to being a significant ice storm over the State during a major travel weekend ending the 1998 Holiday season, several large gravity waves moved across the State during the event. One of these gravity waves brought a period of heavy rain to the lower Susquehanna Valley along with strong winds around 5 AM Sunday morning (03 January). Temperatures, which remained in the teens soared to the upper 40s in Harrisburg and into the 50s in Lancaster after the gravity wave moved through. Wind gusts over 40 mph were observed over the lower Susquehanna Valley.
Over the next several days, we will provide detailed meteorological information on this event. A total snowfall map is included. Values on the map are in inches multiplied by 10. The 82 a Mount Davis is 8.2 inches and the 25 in State College is 2.5 inches. Almost all the "snow" in State College represents sleet accumulation. Note most of the significant snow was in southwestern Pennsylvania.
Satellite images showing the scope of the storm are provided from 2115 UTC on 02 January through 0915 03 January. These are large images averaging about 170K each. A few select images are worth noting. First, the image at 0015 UTC 03 January 1999 which shows the large upper low over the Midwest and a band of enhanced sleet and freezing rain moving out of West Virginia toward Pennsylvania. The are of cold "blue" cloud moved over Pennsylvania spreading moderate sleet across central sections and moderate freezing rain to the lower Susquehanna Valley. The image at 0915 UTC show the sharp back edge to the precipitation shield moving into western Pennsylvania.
Method:
All data were collected in real-time at the National Weather Service Office in State College. All data, included WSR-88D data have been archived to 8-mm tape. In addition to standard meteorological data, 5-minute data were retrieved from ASOS sites to analyze the gravity wave.
Model soundings, showing the warm intrusion and favorable profile to support gravity wave development will be added to this page.
Eta Model Forecasts:
Hourly profile data are displayed to show how well the Eta model forecast the event. The first Eta model run to show the potential for a mixed precipitation event was the 1200 UTC 31 December 1998 run o the model. This run showed light snow changing to ice pellets in the 42 to 48-h period (Saturday afternoon and evening.
The key to the success of the Eta forecasts included the entrenched low-level cold air and the strong warm advection aloft. A time sections at Altoona, Harrisburg, and State College from the 0000 UTC 01 January 1999 Eta shows this warm intrusion. The warm air aloft is depicted by the red colors. The contours are drawn every 4 C.
The corresponding Eta cumulative precipitation and precipitation type forecasts for Altoona, Harrisburg, and State College are also shown. The Altoona chart showed that the model forecast light snow to arrive around 1200 UTC (7 AM) Saturday. Spotter reports of verify that snow began in and south of the Altoona area around 0600 AM local time (1100 UTC). The model forecast the snow to change to ice pellets around 1800 UTC. The change over to ice pellets occurred closer to 0000 UTC on 03 January 1999. The model forecast approximately 2 inches of snow (using a 10 to 1 ratio) and 0.2 inches of ice pellets (about 0.6 inches of ice pellets).
Figures showing the forecasts from the 1200 UTC 01 January show a similar trend.
Altoona temperature time section 01/1200 02/0000 02/1200
Altoona precipitation Type
01/1200 02/0000 02/1200Harrisburg temperature time section
01/1200 02/0000 02/1200Harrisburg precipitation Type
01/1200 02/0000 02/1200State College temperature time section 01/1200 02/0000 02/1200
State College precipitation Type
01/1200 02/0000 02/1200A model profile from the Eta valid at 0600 UTC 03 January from the hourly
profile data and the gridded data are also shown. The soundings for State College show the intrusion of warm air above 900 mb. This layer would melt any falling snow, changing it to rain. However, the deep cold layer would act to refreeze the rain, creating sleet. The Lancaster sounding showed a shallower cold layer near the surface, indicative of freezing rain. The model forecast freezing rain in Harrisburg a few miles to the northwest of Lancaster.Planview Maps
A few traditional planview maps are shown to provide and overview of the case. The 1200 UTC 02 January 1999 mean sea-level pressure analysis and forecasts are
shown. These data show that the model analyzed the surface low over Arkansas at 1200 UTC 02 January and forecast the low to move over Indiana by 0000 UTC 03 January. Note the strong low-level ridging along the East Coast indicative of cold air damming. By 1200 UTC 03 January, the model forecast the main surface low to be over Michigan with a secondary low forecast to develop over Virginia and Maryland.The track of a surface low to the west of Pennsylvania rarely produces heavy snow. The track of a strong surface low to our west almost always results in a change over from snow to sleet and often to rain. With low-level arctic air in place, this storm track favored a mixed precipitation event.
Traditional thickness and thermal maps, valid at 0600 UTC 03 January 1999, used to forecast precipitation type is also
shown. These charts showed the strong intrusion of warm air, which indicated the potential for sleet and freezing rain. It was difficult to view the thermal structure using these charts; however, they forecast the event quite well.
The corresponding Eta cumulative precipitation and precipitation type forecasts for Altoona, Harrisburg, and State College are also shown.
How we forecast this event:
The first Special Weather Statement on this event was issued at 0500 AM Wednesday morning 30 December 1998. This statement, and several follow on statements (SPS) emphasized the differences in model forecasts with the event. The National Centers for Environmental Predictions (NCEP) ensemble models showed several solutions. One suggested a coastal storm, which favored snow. Others suggested a track west of the Appalachian Mountains, suggesting snow to sleet and freezing rain. The European Centers Medium Range forecast model (ECMWF) forecast the low to move into the eastern Great Lakes.
The ensemble forecasts pointed to the uncertainty with the event. The two forecast storm tracks suggested significantly different precipitation events. One emphasized snow and the other emphasized a mixed precipitation event. All the forecasts emphasized and intrusion of unseasonably cold arctic air to move over the region on 01 January, ensuring an icy start to the event. Long range forecasts emphasized cold conditions New Years day and at the onset of the precipitation event.
Based on these forecasts, SPS's emphasized a winter storm of snow, sleet and freezing rain. As the event drew near, forecasts continued to emphasize mixed precipitation. The first Winter Storm Watch was issued in the afternoon hours of 01 January. This watch emphasized sleet and freezing rain potential based on forecasts of above freezing air about 3000 feet above the surface. A follow-on winter storm warning was issued Saturday morning at 0400 AM. Again, the emphasis was on mixed precipitation. Snowfall was not forecast to exceed 6 inches of accumulation in a 12-hour period.
During the event, a non-precipitating warning was issued for high winds along the ridge of Cambria and Somerset Counties. A few reports of winds of 50 mph were reported at the ski resorts in the Laurel Highlands of Cambria and Somerset Counties Saturday evening. The heaviest snow was also observed at the higher elevations with Mt Davis reporting 8.2 inches of snow and sleet.
