High pressure means great weather, right ???
That was the number one question posed to me from WBTV viewers last week.
If high pressure "is controlling our weather," why is it wet and cold for days on end ?
The devil is in the details.
The reality is, when high pressure weather systems are directly over the Carolinas, we most often enjoy clear, sunny skies.
But if the high is positioned north of us, say over New York or New England, we receive a steady fetch of North Atlantic marine-influenced air shoved our way on chilly north to northeast breezes.
And when the water off New England is cold – as it is this time of the year – we typically find ourselves "wedged in," with cloudy, cold conditions.
And when everything comes together – meteorologically speaking – as it did last week, we can add rain (often times heavy) to the mix.
That's a classic "wedge!"
A classic wedge forms in the Southeast when high pressure over the Northeast forces cold, damp air southward, along the eastern seaboard, while a Midwest or Gulf Coast storm forces warmer air, northward.
Because warm air is less dense than cold air, it rises slowly, but gradually, over the cold layer, forming a dense stratiform cloud shield – the slate, gray overcast we experienced most of last week.
The newly formed stratus deck blocks the otherwise warming effects of the sun, maintaining the cool pool of air that settles in from just east of the Appalachians extending over to the coastal plain. Evaporative cooling from the rain and drizzle beneath the stratus deck further cools the air near the surface, therefore strengthening the wedge.
At this point, we're usually "locked in" for a good 2-3 day period. Forecast models can usually predict the onset of a wedge, but commonly underestimate their duration, and I've experienced Carolina wedges that have lingered four or even five days.
They can be rough!
So, if you're an avid weather-watcher and it's sunshine, dry air and light winds you seek, hope for high pressure cells to build southward from the Great Lakes, not eastward. If they slide over us, we're on easy street: sinking air under the high promotes drying air, which often times equates to a sunny forecast.
If the system builds east, look out! At that point, you can usually skip the sunglasses for a few days and be sure to keep your umbrella close at hand!