The Heavy Rainfall Patterns of Winter
by Jan Jackson
The weather focus of winter is often on the threat of snow, sleet or freezing rain. However, flooding also remains a threat through the winter months. In fact, based on a local study, the majority of widespread areal floods, including river flooding, occurs during the late cool season, from January through March. More importantly, the 3 month period from January through March also accounts for roughly half of all significant river flood events, 25 percent of all significant areal floods, and even 20 percent of all significant faster response flash floods.
Significant flooding is defined as causing considerable damage and/or posing a high threat to life and property. Some of the more memorable significant flood events for the area occurred during the cool season and were not associated with tropical systems, such as January 19, 1996, February 22, 2003, and November 19, 2003. Even the granddaddy of significant flood events for the area in recent memory was in the cool season- November 4, 1985.
High numbers of significant floods still occur in the winter months because vegetation is dormant and less evaporation allows soils to remain moist, so it takes less rainfall to produce runoff into creeks and streams. In addition, melting snow cover may add to the runoff. Since the area is located close to the moisture sources of the Gulf of Mexico and the Atlantic ocean, storm systems can sometimes tap into abundant moisture. (See below)
Pattern recognition is an important tool for early forecasts of heavy rainfall. Meteorologists look for weather patterns which have brought widespread heavy rainfall to the area in the past. A study of heavy rainfall events over the Blacksburg County Warning Area (CWA) during a 46 year period, (1964 – 2010), showed that there are basically 6 weather patterns which bring widespread heavy rainfall to the area during the cool season (September through April). One of those patterns- remnants of tropical storms, have their highest frequency in September, and then trail off through October, so do not affect the winter months. The following are brief descriptions of the other 5 heavy rainfall patterns of winter:
Upslope Flow- this pattern is characterized by a prolonged period of a southeast flow of moist unstable air into the Blue Ridge foothills and mountains. The southeast flow is perpendicular to the mountains, and the moist air rises as it moves into higher terrain. This provides additional lift for shower, and sometimes thunderstorm development, while also focusing the heaviest rainfall right along the Blue Ridge. Below are composite surface charts of the 17 cases observed in the study, and also the location of deep moisture and low level flow. The upslope heavy rain pattern occurred most often in September and October, with 14 cases. The other 3 occurred in November, February and April.
This particular pattern has brought very heavy rainfall events to the Blue Ridge mountains and foothills in the past, from the northern mountains and foothills of northwest North Carolina, northeast into southwest Virginia. The two 24 hour rainfall maps below from an October 7 - 8, 2005 event illustrate this well.
Upslope Flow with Strong Dynamics- this pattern is similar to the Upslope Flow pattern, with a southeast flow increasing the rainfall intensity, but also has strong dynamical lift provided by an upper level disturbance. Below are composite surface charts of the 15 cases observed in the study, and also the location of deep moisture and low level flow. Note that the main difference from the Upslope surface charts is the strength of the low pressure that tracks out of the Gulf of Mexico. The Upslope Strong Dynamics events occurred most frequently in October and November, (7 of 15 events), but have occurred in every month of the cool season.
This pattern, along with tropical remnants, has brought most of the historically extremely heavy rainfall events to the area. It is similar to Upslope, with the heaviest rainfall along the Blue Ridge, but often has more widespread heavy rainfall amounts across the Blacksburg CWA, and can also bring a period of very high rainfall rates, which produces flash flooding. Below is a rainfall map of a recent Upslope with Strong Dynamics heavy rain event from January 24th into the early morning of January 25, 2010.
Cyclogenesis- in this pattern, heavy rainfall is generated by the strong vertical motion associated with a rapidly developing low pressure system. Below are composite surface charts of the 15 cases observed in the study, and also the location of deep moisture and low level flow. This is the same type of pattern than can bring heavy snowfall to the area in the winter, except that the high pressure prior to the event is centered off the Mid-Atlantic coast, and is pumping warm moist air into the Blacksburg CWA. In addition, the surface low tracks further north, generally directly over the Blacksburg CWA. Cyclogenesis heavy rain events occur mainly from December onward, (13 of 15 events), with a peak in March (6 events). The events have been rare in the recent past, with the last event identified in February 22, 2003. The heavy rainfall in this pattern is usually widespread, with the heaviest amounts varying along and south of the track of the surface low pressure.
Overrunning- in this pattern, the heavy rainfall is generated mainly from the strong vertical motion of very warm moist air rising over cooler air at the surface. Below are composite surface charts of the 12 cases observed in the study, and also the location of deep moisture and low level flow. Note that it is similar to Cyclogenesis, except the surface high pressure is centered over the Great Lakes and northeast U.S. prior to the event, and a weaker surface low tracks out of the Gulf of Mexico and across the CWA. A warm front develops south of the Blacksburg CWA, and strong south winds bring plentiful moisture over the warm front, resulting in a cold rain. The events occurred in all months of the cool season, but the majority (8 of 12) occurred in February through April.
While heavy rain is generally widespread in the Overrunning events, the heaviest rainfall tends to develop in one or more bands oriented with, but north of, the warm front. The rainfall map below from an event on January 26, 2009 illustrates this banding well.
Cold Frontal Passage- in this pattern, the heavy rainfall is generated from training thunderstorms along a slow moving cold front. Below are composite surface charts of the 6 cases observed in the study, and also the location of deep moisture and low level flow. Note the deepening low pressure moving northeast into the eastern Great Lakes. This brings a slow eastward progression of the cold front through the Blacksburg CWA. Meanwhile, high pressure off the Mid-Atlantic coast pumps deep moisture into the frontal boundary. The result is thunderstorms continuing to develop and moving northeastward along the slow moving front. This was the least common heavy rain pattern in the study, with only 6 events.
Heavy rain in the Cold Frontal Passage events occurs in a band oriented with the cold front. Because the deeper Atlantic moisture is generally in the piedmont sections of Virginia and North Carolina, that is where the heavy rain bands tend to be located. The rainfall map below from an event early on New Years Day, 2007 shows the common locations of these heavy rain bands. This particular event caused a rapid rise on the Dan River, and significant flooding in Danville.