NCFR -- January 18, 1999 -- Rare Winter-time Severe Thunderstorms

Banded precipitation, in the form of rain, started well before dawn over Pennsylvania. The periods of rain, some on the moderate to heavy side, continued through the morning hours. During the late morning, a Narrow Cold Frontal Rainband (NCFR) started to get its act together in western PA. It intensified quickly into a damaging squall line, producing damage before noon-time in western PA. The NCFR continued its movement in an easterly direction across central and eastern PA, then into New Jersey and off the Middle-Atlantic coast by early evening. It produced damage along its entire path. Although this damage was widespread in that it covered a fairly large amount of real-estate, the damage in that large area was not what you could call widespread, compared to spring and summer-time severe weather events. There was sizable gaps between damage reports. Nevertheless, very rare January severe thunderstorm and even tornadic activity occurred.

This NCFR was expected, more-or-less. Damage occurred a bit farther west than SPC thought, as shown in their day-1 convective outlook, but they had a good handle on things. The overnight forecast discussion written by the NWS-CTP, quite a few hours before the severe thunderstorms occurred, gave indication that there was fairly high confidence with forecasting severe weather in January. As you might expect, instability was not very great. LI's were well above zero, as high as 10-15 in some cases. CAPE values were generally up to 500, which is not very high at all. The strong winds (60+ knots) just a short distance above the ground is what caused the thunderstorms to be severe. Any convection that formed was able to drag, or "mix", these strong winds down to the surface. Temperatures varied across the state. Upper 30's to low 40's was all that could be reached (despite WAA) in parts of the Susquehanna valley, which is where I live. The high temp at this house just north of Watsontown was 39 degrees. Some areas of the Susquehanna valley managed to get to the upper 40's, but it was brief and most of the time it was cooler. Temperatures in eastern PA, however, soared into the 50's. Philadelphia came in with 59 and Allentown 56! Dew points were just a few degrees, if that, lower than temperatures... so the air was very moist for January.

SPC did not issue any severe thunderstorm watches for the region during the event. Local NWS offices of course issued warnings though, as needed. Radar images will be provided below. Generally, you will not notice widespread, exceptionally high DBZ values along the NCFR. This is because hail was kept to a minimum, and of course hail causes high radar returns (echoes). There was only one report of large hail, of dime size, which fell in eastern Centre county. There was several other reports of small hail, the size of peas or marbles, throughout the state... but even that was kept to a minimum. The main feature was wind damage.

When looking at the reflectivity radar image from 1:39 PM, you'll note the NCFR just west of the radar dome, or just about in the center of the picture. Although it doesn't look impressive, an F1 tornado was produced minutes ago in central Bedford county (bottom part of the line)! Actually, this part of the line looks the least impressive. DBZ values along the line were mainly in the 35-45 range, but you'll note some yellow colors embedded within the line, indicating DBZ's of 45-50. This is where small hail was falling.

The 1:51 PM velocity radar image showed weak convergence along the southern part of the line, where the tornado was produced. Since this tornado was a gustnado, the rotation was very shallow and the radar beam was a few thousand feet above the ground at that location, which is why it didn't pick up the rotation well. But the white colors you see next to the greens would indicate that weak, and shallow, convergence. With lines of thunderstorms showing this type of velocity pattern, it is common for short-lived gustnadoes to spin up at random along the line with no warning. This is why the phrase "severe thunderstorms can and sometimes do produce tornadoes with little or no advance warning" is included in some severe thunderstorm warnings. Rotation with mesocyclone tornadoes is usually detected sometimes 20 or more minutes before the tornado actually forms.

Reflectivity data from 2:08 PM shows the line at its weakest again over extreme southern PA, in central and southern Franklin county. Yet, another gustnado was sighted a few miles east of Waynesboro, in Washington township (Franklin county). Windows were blown out and trees were down... with winds up to 70 mph estimated. Dime size hail was being produced around 2:20 PM in Millheim, Centre county. Radar at about that time showed the strongest DBZ values over over Millheim... indicating where the largest hail was likely.

The severe thunderstorms continued moving east into the Susquehanna valley, where they weakened somewhat, but not entirely. This weakening is likely due to much cooler temps in the valley than outside the valley, like in eastern PA. Once the storms moved east of the valley into warmer air once again, they regained their strength. Still, in the mid Susquehanna valley, almost everyone experienced wind gusts to 40 mph... with some areas having gusts over 50 mph. On I-180 in southern Lycoming county, I received reports of several cars almost being blown off the road by thunderstorm winds around 50 mph. In this backyard (just north of Watsontown) I recorded winds to 40 mph. Trash cans were airborne for several feet, and then rolled even farther. One of them narrowly missed seriously injuring a dog standing nearby.

The severe thunderstorms moved through New Jersey and Long Island causing more damage, but it was isolated in nature and few people saw severe wind gusts... although most people saw wind gusts to around 40 mph as the line of thunderstorms passed through.

Due to unexpected problems, radar images after 2:20 PM are not available. LSR's and PNS's are available from the NWS below:

LSR from PIT, CTP, PHL, and NYC
PNS from CTP, PHL, and NYC

Flash flooding also became a problem in several areas, with flash flood warnings being needing to be issued for much of the Mid Susquehanna valley and portions of the lower Susquehanna valley, for rapid flooding of streams and creeks. Numerous roads were closed throughout these areas, with quite a large number of basements flooded. In some cases, basements were largely destroyed, with water up to the ceiling and going up the stairs to the first floor. A mobile home park in central Columbia county, between Bloomsburg and Berwick, was flooded, with several residents evacuated. Farther east, flooding was more isolated, but still caused problems. The Dunmore borough building in Lackawanna county had water come through its roof. Extensive damage occurred to much of the building, with new computers ruined. A school in Monroe county was closed for a day following the flooding because of water in the first floor of the school. Several roads were closed in northeast PA, with some drivers needing to be towed out of standing water which they thought they could cross safely.

Go here for a listing of all images/text available for this event.