north carolina coastal lows rick neuherz service hydrologist nws wilmington nc
TRANSCRIPT
North Carolina Coastal LowsNorth Carolina Coastal Lows
Rick NeuherzRick Neuherz
Service HydrologistService Hydrologist
NWS Wilmington NCNWS Wilmington NC
Development of East Coast StormsDevelopment of East Coast Storms In the US, the onset of winter In the US, the onset of winter
coastal cyclogenesis or re-coastal cyclogenesis or re-developmentdevelopment most commonly most commonly occurs off the mid-Atlantic occurs off the mid-Atlantic Coast south of 38N and west Coast south of 38N and west of 70W.of 70W. (Sanders, 1980; (Sanders, 1980; Sanders and Gyakum, 1980)Sanders and Gyakum, 1980)
East Coast Winter Storms…Why Here?East Coast Winter Storms…Why Here?
The Gulf Stream current The Gulf Stream current is closest to landis closest to land within the within the same same general cyclogenesisgeneral cyclogenesis
or re-development or re-development
offshore regionoffshore region
East Coast Winter Storms…Why Here?East Coast Winter Storms…Why Here?Cold air frequently drops into the region during the winter Cold air frequently drops into the region during the winter
monthsmonths
Gulf Stream Influences Gulf Stream Influences It warms the air, creating large temperature contrast over a short distance…lows feed on that contrast
Gulf Stream InfluencesGulf Stream InfluencesThe 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?East Coast Winter Storms…Why Here?Upper levels of the atmosphere often support cyclone development as Upper levels of the atmosphere often support cyclone development as disturbances in the northern and/or southern jet streams pass over the areadisturbances in the northern and/or southern jet streams pass over the area
East Coast Winter Storms…Why Here (summary)
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?How many East Coast lows per season?
Typically in a winter season (10/15- 3/31) Typically in a winter season (10/15- 3/31) we have we have between 11-15between 11-15 coastal winter storm events (Cione, coastal winter storm events (Cione, 1993; Sanders, 1980)1993; Sanders, 1980)
Most of these storm events occur from Most of these storm events occur from December December through February through February with with JanuaryJanuary being the month being the month with the most storm events.with the most storm events.
The bright side? Most of these storms do their The bright side? Most of these storms do their intensifying from Cape Hatteras northward meaning intensifying from Cape Hatteras northward meaning that NC avoids the worst effects in most cases.that NC avoids the worst effects in most cases.
Precipitation with East Coast StormsPrecipitation 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
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 StormsPrecipitation Amount vs. Intensity for East Coast Storms
Effect would be similarfor slow moving vs. fast moving storms
Precipitation Amount vs. Intensity for East Coast StormsPrecipitation 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 TooWhere a Storm Intensifies is Important Too
This storm was already intense as it moved byWilmington
This storm did most of itsintensification after moving north of Wilmington’s latitude
Let’s Muddy the WaterLet’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.
PrecipitationPrecipitationOne 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 FloodsA 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
ExamplesExamples
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 Web Sites National Weather Service WilmingtonNational Weather Service Wilmington
http://www.erh.noaa.gov/ilm/http://www.erh.noaa.gov/ilm/ Detailed coastal low informationDetailed coastal low information
http://www2.ncsu.edu/eos/service/pams/meas/sco/research/nws/training/bomb/index.htmlhttp://www2.ncsu.edu/eos/service/pams/meas/sco/research/nws/training/bomb/index.html http://www.erh.noaa.gov/ilm/science/ascii/ascintro.htmlhttp://www.erh.noaa.gov/ilm/science/ascii/ascintro.html
Summary Summary Nor’easters develop and/or rapidly intensify off the North Nor’easters develop and/or rapidly intensify off the North
Carolina coast dueCarolina coast due to the proximity of the Gulf Stream and its modifying effects on to the proximity of the Gulf Stream and its modifying effects on
cold air andcold air and to upper level conditions which frequently support low pressure to upper level conditions which frequently support low pressure
development during the winter seasondevelopment during the winter season Typically in a winter season (10/15- 3/31) Typically in a winter season (10/15- 3/31) we have we have
between 11-15between 11-15 coastal winter storm events (Cione, 1993; coastal winter storm events (Cione, 1993; Sanders, 1980)Sanders, 1980)
Most of these storm events occur from Most of these storm events occur from December through December through February February with with JanuaryJanuary being the month with the most being the month with the most storm events.storm events.
Precipitation type and amount varies with storm track, Precipitation type and amount varies with storm track, storm intensity and storm speed.storm intensity and storm speed.