moisture sources for frontal cyclones (hurricanes, tropical storms, and nor 'easters) in Virginia
Source: National Weather Service, The Heavy Rainfall Patterns of Winter
Almost every weather report will highlight if rain is predicted. There are two primary types of storms that generate rain in Virginia. Big storms (frontal cyclones) sweep in from the Atlantic Ocean, affecting the eastern edge of the state, while localized summer thunderstorms provide most of the rain in the mountains.
However, a few frontal cyclones, such as Hurricane Camille, can migrate all the way north from the Gulf of Mexico before drenching Virginia. Such storms can drop one to two feet of rainfall in a day, generating massive floods and stripping the soil off the Blue Ridge in dramatic landslides. The little thunderstorms in the western part of Virginia may be more common, but the rare frontal cyclones can cause more erosion and damage to roads/houses in just a day or two than several decades of thunderstorms.
As described in a Virginia State Climatologist report, the major storms such as hurricanes are more important in recharging groundwater, providing rain to put out fires in Great Dismal Swamp, and raising water levels in drinking water reservoirs east of the Blue Ridge:1
The air heated at the Equator travels north, cools, and descends in our latitudes. The Coriolis effect steers such winds to the right - so in Virginia, most winds and storms usually come from the west. A "west wind" is a wind that blows from the west. Yesterday's weather in Missouri and Ohio will often be our weather today in Virginia.
In the winter, polar air masses from Canada are pushed south and then towards the east by the jet stream. In the summer, when the jet stream moves north and the low pressure systems from the Gulf of Mexico travel northeast, we also pay close attention to the weather in Arkansas and Tennessee.
When the wind patterns blow northwest across the Gulf Stream, the air over south-central Virginia ends up relatively warm and moist. A long day of sunlight will heat the ground, and that heat is radiated into the atmosphere where it can create local thunderstorms. The same local heating west of the Blue Ridge and in northwestern Virginia, far from the Chesapeake Bay and the Atlantic Ocean and the wider sections of the rivers, will not have enough moisture in the air to generate the same number of thunderstorms.
Rainstorms in the winter may last up to three days, while summertime storms are typically over in just one day. Summertime vacationers at Virginia Beach enjoy clear skies and warm weather due to the Bermuda High, a high-pressure system in the North Atlantic that creates stable weather along the coast for much of the summer.
A northeaster is a storm where the wind comes from the northeast. Wintertime nor 'easters, when the wind and the currents at Virginia Beach push the sand in the same direction with great energy, can cause great erosion of the Atlantic and Chesapeake shorelines. One or more strong storms can undo the efforts of the Big Beach project, which widened the Atlantic Ocean beach at Virginia Beach about 300 feet to provide more recreational space in front of the hotel strip along Atlantic Avenue.
Hurricanes, typically in August-November, threaten lives as well as property and get much more attention that nor 'easters. They also have very obvious impacts, both short term and long term. Storm surges between 1799-1806 pushed enough sand onshore to built Willoughby Spit, now the southern terminus in Norfolk of the Hampton Roads Bridge-Tunnel on I-64.2
The threat of a storm surge creates great concern in the Hampton Roads area. Between Hampton and Williamsburg, Interstate 64 has special sections of the median already paved and painted for reversing traffic on the eastbound lanes. That would double the capacity of the highway for evacuating the Hampton Roads area - if people are willing to leave as soon as warning are issued...
1. "Advisory 01/04: Keeping A Radar Eye On Local Moisture," University of Virginia Climatology Office, January 2004, http://climate.virginia.edu/Climate/ad01-04.htm (last checked September 13, 2014)
2. "The Formation Of Willoughby Spit," Willoughby on the Web, http://www.willoughbyontheweb.com/celebrate250/formation_of_willoughby_spit.htm (last checked May 31, 2013)
