There are a lot of atmospheric conditions that are changing as you read this article. These changes will impact Virginia's weather for the Fall and Winter of 2021. Here's what you should know.
Storms, Tropical Storms, & Hurricanes
The Climate Adaptation Center has changed its 2021 hurricane model as the chances of La Niña weather patterns occurring have now increased just weeks before peak hurricane season in the Atlantic (Late August - Early November). This is not good news for the Atlantic Southeast.
So far, in 2021, eight tropical storms have formed. Compared to 2020's record-breaking hurricane season (30 storms and four Category 4 hurricanes), this season is looking mild so far.
Three major factors, however, may change this perspective pretty quickly. La Nina, Atlantic sea surface water temperatures (SSTs), and changes in a measurement called the Madden-Julian Oscillation (MJO) are stirring things up.
Sea Surface Temperatures are perhaps the most important factor in determining the number and strength of storms formed in the Atlantic. Hurricanes feed off warm seawater energy and convert that energy into wind and rain. In the image below, the colored sea surface temperatures range from blue (cooler than normal) to red (way above normal).
The SSTs in the Gulf of Mexico, Caribbean, and tropical Atlantic are above normal nearly everywhere - the ideal incubator for hurricanes. Should a storm form and move up the east coast, the red areas along the New England and Canadian coasts will mean even stronger storms for those areas.
When La Niña occurs, the SSTs in the mid-Pacific Ocean are colder than normal (as you can see in the map above), decreasing wind shear in the Atlantic and Gulf of Mexico.
There are two types of wind shear - vertical and horizontal.
When a tropical system encounters strong vertical wind shear, the top of the tropical storm or hurricane can be blown hundreds of miles downstream. This can cause the storm to become lopsided, pull in drier air, and eventually weaken dramatically.
Horizontal wind shear is the change in direction or speed of winds over the surface of the ocean. When a storm encounters strong horizontal wind shear, the storm can be pushed into another direction or even ripped apart by the strong winds. This is most typically experienced when a strong cold front is encountered.
Decreased or low-to-moderate wind shear can push a tropical cyclone into warmer waters, which are more favorable for development. It’s even possible that low to moderate wind shear blowing from the same direction as the rotation of the storm could cause the storm to spin faster and strengthen.
Wind shear typically must be 20 knots or less for intensification to occur, with most instances of rapid intensification of hurricanes occurring when the wind shear is 10 knots or less.
NOAA models show that the ocean water in the mid-Pacific area is forecast to cool even further over the coming months. If it does, a La Niña event will be underway with Atlantic wind shear conditions ideally suited to storm formation.
The MJO (Madden-Julian Oscillation), in simple terms, is an eastward-moving "pulse" or disturbance of clouds, rainfall, wind, and pressure near the equator that typically recurs every 30 to 60 days.
Half of the MJO is a low-pressure area with a vertical/rising motion that encourages storm formation. The other half is a high-pressure system with a vertical/sinking air motion that promotes good weather. When the low-pressure cycle of the MJO is over the Atlantic Basin, as it is for the next two months, the number of hurricanes likely to form increases.
What This Means For Virginia and The Rest of The U.S. This Winter
All three conditions make the 2020 forecast of 20 storms more likely even after a mild start to the season. The map below shows how La Nina will affect Virginia and the rest of the country while it remains in effect through next Spring. Good news for parts of the dry upper West and Ohio Valley, but not the already water-deprived Southwestern U.S.
Bonus: If Farmers' Almanac is correct, for the rest of this season we should, "Watch for a hurricane in early to mid-August." And expect that"September and October will be cooler and rainier than normal."