North Carolina Coastal Lows Rick Neuherz Service Hydrologist NWS Wilmington NC
Development of East Coast Storms • In the US, the onset of winter coastal cyclogenesis or re-developmentmost commonly occurs off the mid-Atlantic Coast south of 38N and west of 70W.(Sanders, 1980; Sanders and Gyakum, 1980)
East Coast Winter Storms…Why Here? The Gulf Stream current is closest to land within the same general cyclogenesis or re-development offshore region
East Coast Winter Storms…Why Here? Cold air frequently drops into the region during the winter months
Gulf Stream Influences It warms the air, creating large temperature contrast over a short distance…lows feed on that contrast
Gulf Stream Influences Warming also produces pressure falls
Gulf Stream Influences The addition of heat and moisture to low levels of the atmosphere primes the area for low development by creating an unstable atmosphere
East Coast Winter Storms…Why Here? Upper levels of the atmosphere often support cyclone development as disturbances in the northern and/or southern jet streams pass over the area
East Coast Winter Storms…Why Here (summary) • Due to its proximity to land here, the Gulf Stream results in the formation of an area off the coast favorable for cyclone development or intensification by: • Creating large temperature contrasts over a short distance • Producing an elongated area of pressure falls along the gulf stream • Producing an unstable atmosphere • Upper levels of the atmosphere frequently conducive to cyclone development in the area
How many East Coast lows per season? • Typically in a winter season (10/15- 3/31) we have between 11-15 coastal winter storm events (Cione, 1993; Sanders, 1980) • Most of these storm events occur from December through February with January being the month with the most storm events. • The bright side? Most of these storms do their intensifying from Cape Hatteras northward meaning that NC avoids the worst effects in most cases.
Precipitation with East Coast Storms • Precipitation amounts and types dependent on several factors including: • Temperature and relative humidity over land • Speed of system • Faster system – generally lower amounts • Slower system – generally higher amounts • Intensity • Less intense system – generally lower amounts • More intense system – generally higher amounts • Storm Track • Under similar conditions, track is the main determining factor in precipitation type
Precipitation Type vs. track for East Coast Storms The following sequence of slides shows the effect storm track can have on the precipitation type associated with a coastal low. Shown are hypothetical storm tracks and possible precipitation types along with real world examples showing what happened in similar cases.
Precipitation Amount vs. Intensity for East Coast Storms Effect would be similar for slow moving vs. fast moving storms
Precipitation Amount vs. Intensity for East Coast Storms Note differences in areas where snow fell in the intense storm case vs. idealized near shore case Difference results from intense low pulling in cold air in its wake and wrapping moisture back into cold air.
Where a Storm Intensifies is Important Too This storm was already intense as it moved by Wilmington This storm did most of its intensification after moving north of Wilmington’s latitude
Let’s Muddy the Water The previous information covered coastal lows where one well organized storm moves through the area. But, there is a second class of coastal low where there is initially a low to the west that gradually dissipates while a second low develops off the coast and becomes the main storm.
Two Classes of Storms One well organized low Two lows…coastal dominates
Precipitation One well organized low Two lows...coastal dominant • Narrow transition zone from rain to snow with a small or no zone of ice accumulation in between • Broad transition zone from rain to snow with a large zone of ice accumulation in between
A Word about Floods • Winter storms can occasionally produce rainfall induced flooding. Both short term (flash) and river flooding are possible. • Flash Flooding possible due to • thunderstorms • terrain enhanced precipitation • relatively warm air (IE little or no wintry precipitation) • River flooding tends to be more common • series of storms back to back • one prolific precipitation producer
Examples • A slow moving, late season nor’easter produced up to 13 inches of rainfall over southeast North Carolina April 30 through May 1, 1999. • Another slow moving winter storm caused flash flooding in lower elevations of the mountains while snow accumulated at higher elevations February 3, 1998. Wilmington 88-D Storm Total Precipitation
Web Sites • National Weather Service Wilmington • http://www.erh.noaa.gov/ilm/ • Detailed coastal low information • http://www2.ncsu.edu/eos/service/pams/meas/sco/research/nws/training/bomb/index.html • http://www.erh.noaa.gov/ilm/science/ascii/ascintro.html
Summary • Nor’easters develop and/or rapidly intensify off the North Carolina coast due • to the proximity of the Gulf Stream and its modifying effects on cold air and • to upper level conditions which frequently support low pressure development during the winter season • Typically in a winter season (10/15- 3/31) we have between 11-15 coastal winter storm events (Cione, 1993; Sanders, 1980) • Most of these storm events occur from December through February with January being the month with the most storm events. • Precipitation type and amount varies with storm track, storm intensity and storm speed.