(WGHP) – The Piedmont Triad can’t seem to catch a break when it comes to rainy weekend weather, and Memorial Day weekend was no exception.
Dreary, cold and soggy conditions stuck around for the holiday weekend.
From a light mist to heavy downpours, let’s take a look at how much rain we observed Saturday, Sunday and Monday.
How much rain fell over the holiday weekend?
Rain began falling as early as Saturday morning in southeastern parts of the Piedmont Triad, but rainfall took over the majority of the area by Saturday evening.
With persistent periods of mist and heavy rainfall, rainfall totals quickly began to add up with totals ranging from under an inch over the three days to nearly four inches in some areas.
The above graphic depicts the highest rainfall totals by county released by the National Weather Service.
Alleghany County picked up the highest rainfall totals in the Piedmont Triad. Two miles northeast of Sparta measured nearly four inches of rain.
With nearly 3.5 inches, six miles west of Winston-Salem in Forsyth County came in second for the highest three-day rainfall totals.
Alamance, Montgomery, Rowan, Randolph, Davie and Iredell counties received the “least” amount of rainfall with less than two inches of rain falling over the holiday weekend.
Not only did we observe significant rainfall over the three-day period, but it was also unusually cold.
Saturday’s high temperature was 61 degrees which was the record coldest high temperature for the day. It broke the previous record of 62 degrees that was set on May 27, 1963.
On Sunday afternoon, temperatures also reached a high of 61 degrees. However, this did not set the record coldest high temperature for the date.
Normal high temperatures in the Triad at the end of May are in the low 80s, so temperatures were nearly 20 degrees colder than normal.
The warmest temperature over the three-day period was 72 degrees on Monday afternoon, still eight degrees cooler than normal for the end of May.
Why did it rain so much over Memorial Day Weekend?
The reason so much rain fell over the holiday weekend is because of a coastal low-pressure system, also referred to as an extratropical cyclone or a mid-latitude cyclone.
Low-pressure systems are known as the “weather makers” and some people even associate the “L” that depicts low pressure on a weather map as “L for lousy weather.”
According to the University of Illinois Urbana-Champaign, a coastal low is a low-pressure system that rotates counter-clockwise in the mid-latitudes, 30ºN to 55ºN. That ring around the glob includes everything from just north of St. Augustine, Florida, up to the Alaskan panhandle.
The system was able to form because there was a boundary between warm air in the North and the contrasting colder air in the South. Along that boundary, a counter-clockwise circulation formed at the surface, sending the warm air from the South to the North and bringing the cold air from the North down to the South.
The rotation of the warm and cold air creates a “whirlpool” effect where both the warm and cold air reach a center point. At that center point, the air begins to rise since it has nowhere else to go.
The upper levels of the atmosphere over the weekend were considered favorable for cyclone development because the winds in the upper atmosphere were stronger. The strong upper-level winds pulled the warm and cold air up from that center point, allowing the coastal low to continue developing.
As the low developed, it continued to bring the warm air north and the cold air south, which is also part of the reason our temperatures were significantly cooler this weekend. The Carolinas experienced well-below-normal temperatures over the holiday weekend, while those in the Northeast experienced warmer weather.
The movement and transfer of energy between the warm and cold air allowed the coastal low to continue to strengthen. Once the low fully formed, a well-defined cold front and warm front appeared on the Weather Prediction Center’s surface map.
The strong coastal low from Memorial Day weekend ended up taking on the appearance of a hurricane, with what looked like an eye forming as well as rain rotation around the low pressure.
While it may have looked like a hurricane on satellite images, there is a big difference between a hurricane and a coastal low-pressure system.
What is the difference between a hurricane and a coastal low-pressure system?
There are a few ways that hurricanes and coastal lows are different. According to the National Hurricane Center, a hurricane is a type of warm-core cyclone and a coastal low is a kind of cold-core cyclone.
Coastal low-pressure systems occur where cold fronts and warm fronts meet while hurricanes don’t have fronts or any contrast between cold and warm air masses.
According to NOAA, a hurricane is described as a warm-core cyclone that does not have any frontal boundaries and forms over warm ocean temperatures, has organized storms and a closed area of winds around a well-defined center.
Once a hurricane forms, it maintains its strength from the heat of the ocean and the cold temperatures in the upper levels of the troposphere, the level of the atmosphere where weather occurs.
Basically, hurricanes maintain their strength and energy from a vertical difference in temperatures at the different levels of the atmosphere while coastal lows maintain their strength and energy from a horizontal difference, or surface difference, in temperatures.
Hurricanes are not associated with strong upper-level winds or the jet stream, so the wind speed at the surface is not much different from the wind speeds in the upper levels of the atmosphere. Coastal lows are typically associated with strong upper-level winds or the jet stream, which makes the wind speeds at the surface very different from the strong upper-level winds.
The structures also typically appear different. Hurricanes are typically more symmetrical due to the wind speeds being similar at different levels of the atmosphere. Coastal lows are typically not as symmetrical and almost look more like a “comma” around the center of the low pressure with long “tails” extending from the center because of clouds and storms forming along the cold front and warm front.
Because satellite imagery showed what looked like a hurricane even though it was actually a coastal low-pressure system, the question is: Was there ever a transition from a cold-core coastal low-pressure system to a warm-core tropical cyclone?
That will be up to the National Hurricane Center to decide in their post-storm survey.
