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 Influences Gulf Stream Influences Warming also produces pressure falls

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.

Offshore TrackOffshore Track

Offshore TrackOffshore Track

Near Shore TrackNear Shore Track

Near Shore TrackNear Shore Track

Inland TrackInland Track

Inland TrackInland Track

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.

Two Classes of StormsTwo Classes of Storms

One well organized low Two lows…coastal dominates

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

Sample Effects of Dual Low SystemSample Effects of Dual Low System

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.