Many of us woke up to dense fog this morning, and are still experiencing dense fog as we speak. Normally, fog forms in the early-morning hours and then dissipates after sunrise when the atmosphere warms up and the fog can lift. However, this dense fog has lingered all morning and will continue to linger throughout the entire day, though somewhat diminish in intensity during the afternoon, but may become a bit more dense again during the evening hours. The cause of the fog is a warm front that is approaching the area, helping to advect lots of moisture into the region.
To the west of the warm front is a low pressure system and a cold front, and this front will eventually bring us rain for tonight and Friday night. In between the cold front and warm front are southwest winds, which is a warm wind; so south and west of the warm front, there is more warmth, which helps to lift any fog away. Closer to the low pressure itself and the cold front is rain in western PA, which will stay to our west throughout the day. However in our area, we are further ahead of the low pressure, which brings us southeasterly winds at the surface. This brings in warm air for winter standards, but still relatively chilly Atlantic Ocean air, as well as abundant moisture to form the fog. The warm front is the boundary between the warmer southwest winds, and the cooler, but still mild and moist/Atlantic contaminated southeast winds.
Additionally, thanks to the approaching warm front, strong southwest winds aloft have brought in lots of warm air as well, which helps to create a very stable atmosphere — when the air aloft is similar in temperature to the air at the ground (as opposed to cooler), the atmosphere is very stable, and thus the fog at the ground cannot lift very much. This is why it has lingered so long, and will continue to linger throughout the day. As temperatures at the ground slowly warm up, some of this fog will lift, so the fog will be less dense after noon. However, the clouds will prevent too much warming, so fog will linger and still reduce visibilities. In very dense fog, it is recommended for drivers to use low-beam headlights and obviously try to keep more distance between you and the driver ahead of you. When temperatures cool this evening, the fog may become a bit more dense again, but not as dense as it was this morning.
Temperatures will tend to hold in the mid 50s this afternoon, and potentially rise to the upper 50s as the warm front approaches. Due to all of the low-level moisture, drizzle may occur throughout the day, but the organized rain will remain to our north and west. The warm front will cross the region tonight, which will help to hold temperatures into the lower 50s, as a band of rain from the cold front crosses the region between 1am and 7am tomorrow morning. The rain will not be all that heavy, but still take caution on the roads and have your umbrellas with you during the overnight. Patchy fog will linger into the overnight as well.
There will then be a bit of a break in the action on Friday once the cold front crosses, and northerwesterly winds behind this front start to usher in chillier air. Fortunately, fog does not appear to be a big issue for Friday, as any early-morning patchy fog will dissipate quickly; though caution may still be needed for some locations during the early-morning commute. Temperatures will still initially be warm, as they will hold in the low 50s in the morning, but then gradually fall through the 40s during the afternoon and evening hours. The front will briefly stall just to our east, and another wave of low pressure will move along this front and give the area more rain on Friday night into Saturday morning.
Since this precipitation will be behind the cold front, it will be much chillier this time around than during the precipitation tonight. This leads to a chance of some of the rain turning into a wintry mix and snow, especially north and west of the city. Most of the precipitation will have moved out by the time the coastal areas are cold enough for snow, but we cannot rule out a wintry mix or even a brief period of non-accumulating snow along the coast and areas near the city. Inland areas, primarily NW Jersey and the Hudson Valley may have minor accumulations from a coating to as much as 2″ of snow and sleet. We will be sure you keep you posted on this.
As far as the wintry mess on Sunday night is concerned, stay tuned as we will be publishing another article later this afternoon addressing that potential.
Have a great day, everyone!