So, you ask, why does a National Weather Service (NWS) office in the Northeast United States issue a Severe Thunderstorm Warning in the cold season (Fall, Winter, early Spring), when there is no lightning with the "storm"? Well the answer is simple, yet at the same time its not. Severe thunderstorm warnings are issued for convective cells. And since convection does not necessarily have to be a thunderstorm, a shower (remember, showers are convective) can produce damaging wind gusts without lightning ever being present! In the Northeastern U.S., most cold season severe weather events contain little or no lightning. The ones that do contain lightning usually do not have a significant amount of it, and typically come and go with little fanfare in the lightning department. However, some NWS offices will still decide to issue a severe thunderstorm warning with or without lightning present.
Typically, most severe weather events in the NE US during the cold season are produced through what is termed a Narrow Cold Frontal Rainband... or NCFR for short. These occur virtually every year during the cold months in the NE US. On Doppler radar, they appear as a very narrow line of convective echoes. Winds just above the surface may be blowing at very fast speeds. Since convective cells along this narrow line show noteable vertical development in the clouds, they can transfer this wind down to the surface. Imagine a column of air standing vertically in the atmosphere, extending thousands of feet up. Picture it as a tall but relatively narrow cloud. There is air (transporting heat and moisture) moving up and down within this column. Now remember we have strong horizontal winds blowing just aloft. As the air from the upper portions of the vertical column falls back toward the ground, it can drag (or mix) some of the wind from aloft back to ground-level. In addition, the evaporation of some of the rain as it falls creates gusty winds by itself. The combination of the two can result in strong and even damaging wind gusts at the surface. All this can happen without lightning being present.
The functions of a thunderstorm are much the same... containing a cumulus cloud (vertical column of moving air as described above), updraft, and downdraft. Knowing that the basic structure of a shower and thunderstorm are similar overall, and both can produce damage, it is easier to understand why a severe thunderstorm warning is issued for some events where lightning is not present. However, this can become extremely confusing to the general public. Perhaps a better warning could have been developed for "severe showers", verses "severe thunderstorms". Depending on the situation, some NWS offices will issue a High Wind Warning for NCFR events. But these are generally kept for large scale, non-convective, high wind events... persisting for hours and caused by large scale conditions (even under sunny skies sometimes). At this time, severe thunderstorm warnings are what is issued for convective events in most situations.
In some NCFR events, where only a small amount of lightning is present (or none at all)... not only are strong downburst winds a threat... but even tornadoes! Yes, even in Winter in the NE US. Now, granted, these "tornadoes" are virtually always gustnadoes. And as described in the weather glossary on this website, a gustnado is not a true tornado. A good number of meteorologists (and other meteorologically advanced people) use the term tornado to describe a gustnado, however. While incorrect, such a statement is often accepted among the meteorological community while explaining to the public. Either way, multiple gustnadoes can sometimes occur in association with an NCFR... due to a sharp change in wind direction at the surface which can be present along the leading edge of the NCFR. Both downbursts and gustnadoes with an NCFR can produce damage... to both trees and structures. However due to the very random occurance of the gustnadoes with NCFR's, tornado warnings are almost never issued. A severe thunderstorm warning would be your only warning. Additionally, since gustnadoes do not originate from rotation within a convective cloud, the rotation within a gustnado is typically very shallow near the ground and is not detected by Doppler radar.
Below are some examples of showers which were severe but did not contain lightning.
Email: BowEchoCPA@suscom.net
