The arctic surge snow and ice event of
13-15 January 1999
1. Introduction:
During the morning hours of 13 January 1999, a shallow arctic front moved from north to south across Pennsylvania. This shallow arctic air was associated with a large area of high pressure which was moving slowly across Ontario and Quebec Provinces in Canada. This arctic air would set a new State record low of –55 F in Alogash, Maine. This may be the all time record low of all of New England.
Pennsylvania State University student Curtis Alexander examined several aspects of this event as part of the 418W course. Papers covering the arctic frontal passage on the 13 January, the convection over the frontal boundary on the 13-14th and finally the heavy sleet and snow that fell overnight on the 14th into the 15th is examined.
Warm moist air was forecast to move over this low-level arctic and produce mainly a mixture of sleet and freezing rain over most of the Mid-Atlantic region. In reality, most of central Pennsylvania received mainly sleet which ended as snow. Many locations of southeastern Pennsylvania, Maryland, and Virginia received significant amounts of sleet and freezing rain.
Overall, no single model forecast all aspects of this event well. The Eta, as will be shown, captured the arctic surge into the region. But, model forecasts showed too much warm air and too quick an erosion of the low-level warm air, suggesting mainly a sleet and freezing rain event. NGM guidance was too cold, forecasting too much snow over Pennsylvania.
Eventually, this page will show many aspects of the event which spanned several days.
The page shows how the arctic front moved across central Pennsylvania during the late morning hours of 13 January 1999. Eta model profiles are used to show how well the model forecast this intrusion of arctic air. The radar imagery show how shallow the initial surge of the arctic air mass was. NGM soundings did not capture this feature as well and are not shown.
Eta profile data are used to show the precipitation forecast
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 All data, included WSR-88D data have been archived to 8-mm tape.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 a gravity wave which moved through the lower Susquehanna Valley on 15 January 1999.
Model soundings, showing the initial intrusion of shallow arctic air and favorable profiles to support mixed precipitation during most of the event are shown. Baltimores sounding is shown to see how well the Eta forecast shallow cold air which produce freezing rain well south of the Mason-Dixon line.
The observed snowfall as of 1200 UTC 15 January 1999 is shown (note all values are inches *10 so 35 is 3.5 inches). Snowfall amounts ranged from 6-8 inches along the New York border to 1-2 inches in southeastern Pennsylvania. More important was the observed liquid equivalent, or the water content of the snow. In State College, like many other central Pennsylvania locations, 4 inches of "snow" was reported, with a liquid equivalent of 1.31 inches. A large portion of this "snow" was actually accumulated sleet. Local observations suggest about 1-3 inches of snow fell upon 2 inches of sleet. This was true of many other locations. These maps are valid from 7 AM 14 – 7 AM 15 January. An additional inch or two of snow fell at many locations after 7 AM and will be included in the 16 January data.
The temperatures observed over the region from 7 AM 14 – 7 AM 15 January are also shown. The lows were observed during the morning hours on Thursday and most of the highs occurred at the time of observation on Friday morning. Stations that were not reporting show "0" which may confuse the viewer.
Snow fall and rainfall data for the first surge of sleet and freezing drizzle will be included by early next week.
The low-level arctic front moved into northern cia during the early morning hours. Around 0900 UTC an area of snow and rain was observed in northern Pennsylvania. The WSR-88D winds showed southwesterly flow over the region. Temperatures were mainly in the 30s near the New York border and the in the 40s near the Maryland border at this time.
The first hint of the arctic front could be seen in the 1152 UTC WSR-88D velocity data. Looking up approximately the 340 degree radial, there is a weak notch in the zero isodops across McKean and Potter Counties, just south of the New York border. By 1438 UTC, this feature was clearly evident in the velocity data. At this time, the arctic front extended across Clearfield, northern Centre and Clinton counties. A weak are of green "inbound" velocities showed the convergence along the frontal zone about 15 miles from the radar. A cross section, taken along the white line in the figure, clearly showed the approaching arctic front with northerly winds about 15 miles north of the radar. By 1456 UTC, the front was closing in on the radar. Note the areas of no data in this image, areas of "black" within data rich regions. These areas show the ridges of central Pennsylvania, which play a significant role in latter images.
A series of zoomed in images were produced, from 1438 through 1750 UTC. These images show the approach and rapid southward progression of the arctic front. The 1450 UTC image shows the front about 5 miles north of the radar. The front past through the radar site around 1525 UTC. By 1628 UTC, the front has moved south, there appears to be some interaction between the terrain and the front beginning at this time. To the southeast, the front is clearly through State College and Park Forest Village, but not through Tyrone to the west.
By 1623 UTC, the front was past the radar and moving southward. The velocity cross section showed the shallow arctic front, about 6 miles south of the radar. Note the shallow north to south flow at low-levels and the general south to north (color reversal) flow above this shallow front. The radar vertical wind profile (VWP) is included to show the strong wind shift at the radar.
By 2200 UTC, the front has moved well south of the radar. The convergence of the front and the warm air can be seen near Altoona. The long thing white line was used to cut a cross section through these data. Note the strong low-level jet just to the right of this line. The line closely follows the Allegheny front. A low-level jet has developed in the valley, accelerating the cold air southward. The VWP confirmed both how shallow the front was and how the winds had increased in velocity. At the surface, cold air has already arrived in Altoona, despite the radar image.
Reflectivity data showed the associated areas of snow over northern Pennsylvania (1953 UTC), freezing drizzle over central Pennsylvania, and scattered rain showers to the south. By 2001 UTC freezing drizzle and sleet were reported over much of the region. The reflectivity cross section showed the melting level, near 5000 ft.
This intrusion of arctic air play a significant role in the weather events for the next 36 hours. Why this air moved so far south so fast and how it deepened and maintained itself played a critical role in the weather over the mid-Atlantic region from 13-15 January.
A time section of the 0000 UTC Eta forecast for State College (UNV) of temperatures is shown. The time is read from right to left. Note the strong temperature drop forecast between 12 and 18 UTC on 13 January. This is the arctic frontal passage as forecast by the model and shown in the radar sectoin above.
The Eta forecast an intrusion of warm air with time, over the arctic air, leading to forecasts of sleet on the afternoon of the 14th into the evening hours. This is better shown in the Eta hourly precipitation plot.
Several hourly soundings were cut to show the forecast of the arctic front, soundings including for Altoona:
1700 UTC
1800 UTC
1900 UTC
and State College
1400 UTC
1500 UTC
1700 UTC
1900 UTC
14/0600 UTC – near the peak of the cold surge in the model forecasts
These data show how well the model forecast the arctic frontal passage. A pesky warm bias in the lower section of the soundings is clearly evident.
ii. Eta model Soundings
How we forecast this event:
A winter storm warning was issued during the afternoon of 13 January 1999 for a large portion of central Pennsylvania. The warnings called for snow and ice accumulations. Most of the forecasts emphasized accumulating sleet which would make driving difficult.
