Weather Watch

By Dave Nicosia

Late winter and early spring:
flood season

REGION — Historically, late winter and early spring is flood season in central New York and northeast Pennsylvania. On average, this is the time of year that streams and rivers have the highest levels and flow the strongest. About half of the annual runoff into river basins occurs in the spring months.

There are several factors that lead to higher river and stream levels this time of year. First, snowpack, as well as water frozen in the soil, begins to melt. The snowpack and frozen water accumulate for much of the winter and are released with the arrival of warmer temperatures during late winter and early spring. This water is released, along with any rain that occurs, leading to extra runoff into waterways. In addition, deciduous trees are still leafless and, therefore, evapotranspiration is lacking. Evapotranspiration is the process by which trees draw up water from the soil and give it off as water vapor through the leaves. This process soaks up plenty of water from the soil, cutting down on runoff each time it rains.

Another factor is that evaporation of water from the soil is still low. Temperatures are still chilly and the sun is not quite as strong as it is late in spring or summer. This cuts down on evaporation of water from the soil. Evaporation rates peak in the summer when the highest temperatures occur along with abundant sunshine.

The increased runoff into area streams and creeks is responsible for higher water levels this time of year on average. The occurrence of flooding reaches a yearly maximum during March. One of the worst floods on record in the region occurred in March 1936. That winter saw deep snowpack. When warm, moist winds developed and brought several inches of rain to the region, the results were devastating. The warm temperatures that melted the snowpack combined with the heavy rainfall to send huge quantities of water into streams and rivers. The combination led to major flooding. Some of the high river-level records from this flood still stand today.

Another reason for the increased occurrence of flooding during late winter and early spring is ice break-up on rivers and streams. During most winters, there is significant ice formation on waterways. With warmer temperatures arriving by the end of winter and early spring, the ice breaks up and, at times, can clog streams and rivers leading to flooding upstream from the “jam.” This type of flooding is known as ice jam flooding and can lead to rapid rises on streams and rivers. Flash flooding is often the result with the more serious ice jams.

Flash flooding is the most dangerous kind of flood since the water rises very quickly, often taking people by surprise. Although flash flooding tends to be more localized than river flooding, it claims more lives than any other natural hazard in the U.S. River flooding tends to be more widespread and takes longer to develop. Therefore, there is often advanced warning. However, river flooding typically takes a much larger toll on property.

The National Weather Service (NWS) issues both flash flood and river flood watches when flooding is imminent and/or occurring. Warnings mean actions should be taken immediately to protect life and property. When flooding threatens your area, keep in mind these safety tips: 1) Never cross a flooded roadway in your vehicle. 2) Know where you live in relation to the flood plain. If you live in a flood prone area, know the best routes to escape to higher ground in case of flash flooding. When flood watches are issued, you should begin making preparations in case flooding occurs. Be ready to evacuate at a moments notice.

For more information on flood potential during winter and early spring, the NWS in Binghamton issues a Flood Potential Outlook every two weeks through March. The latest flood potential outlook can be found at www.atm.ucdavis.edu/~wxauto/fos/fgus/FGUS71.KBGM.

For more information on flood safety, check out www.nws.noaa.gov/er/bgm/prepare.html. You can also send an e-mail to me at david.nicosia@noaa.gov.

[Dave Nicosia is a Warning Coordination Meteorologist at the National Weather Service in Binghamton.]