PHILADELPHIA (CBS) — A strong cold front is slowly tracking across Pennsylvania this morning and is expected to slam into the Delaware Valley later in the day on Monday.
A severe thunderstorm watch is in effect until 8 p.m. for Philadelphia, Bucks, Chester, Delaware and Montgomery Counties in southeastern Pennsylvania, and throughout South Jersey.
A tornado warning is in effect until 2:15 p.m. for northwestern Lancaster County, and has since expired.
The National Weather Service’s Storm Prediction Center (SPC) has issued an Enhanced Risk for severe weather for the immediate I-95 corridor with areas along the Jersey coastline and up near the Poconos in a Slight Risk. We should be on the lookout for strong winds and heavy rains as the main concerns, but large hail and even the very small chance for a brief spin-up tornado is not totally ruled out. Due to the potential for the heavy rain, there is a Flash Flood Watch in effect for a vast majority of the region until midnight tonight.
Let’s take a look at how the atmosphere is set up for today’s severe weather event. As we all know it has been hot and humid for the last couple days and today is not going to be any different. The high Dew Point temperatures (70+) mean that there is a lot of moisture for the potential storms to work with and when you couple this with the sunshine, we are likely to see early in the day today, you end up with a lot of energy and instability in the atmosphere.
This is called Convective Available Potential Energy or CAPE. When expecting severe weather, we usually see CAPE values that are in to the 1500+ range at minimum and we are bordering on that this afternoon.
The second ingredient we look for when forecasting severe weather is the change in winds with height, both in speed and direction. This is all “Wind Shear.” You need to have wind shear to get the rotation needed in the atmosphere for supercell thunderstorms to develop. When you have supercells with high shear values that will tend to spin a lot with height, you tend to get the strongest storms, either large hail events or tornadic storms.
The good news about the severe potential today is shear values are going to marginal at best (30-40 kts or 35-46 mph). This type of value in the shear department usually means tornadoes and hail risks are low but the threat for very strong winds is high.
Finally, we need some kind of lifting mechanism to get all this potent and energetic air moving. We usually look for a feature like a surface cold front to lift the air parcels and get them on the path to becoming thunderstorms. So when we take a look at how these ingredients stack up for this afternoon, we can forecast a decent chance for severe weather, due to the high energy or CAPE values, decently strong wind changes with height and finally we have our lifting mechanism, a cold front, to get all these storms moving. Due to all of these ingredients, the SPC has placed the Philadelphia area under an Enhanced Risk of severe weather this afternoon.
Let’s break down the Enhanced Risk of severe weather for today a little further. An Enhanced Risk means that we are going to be looking for a 30 percent chance for a severe storm to form within 25 miles of a point. So if we take Philadelphia and make a 25-mile radius around the city, there is a 30 percent chance for a severe storm to form in that circle. When we talk about severe weather threats we can break it down into three different categories: Hail, Wind, and Tornadoes.
The SPC will give a breakdown for each severe threat as well. Today’s breakdown is as follows: Tornado Chance: 2 percent, Hail Chance: 5 percent, and Wind Chance: 30 percent. Due to the linear nature of the storms that are going to form along the cold front and the shear values in the 30-40 kt range, strong winds are the highest threat for today. While the hail and tornado threats are the lowest they can be, it does not mean that those types of severe storms can’t occur so we need to be weather aware today for all types of severe weather not just strong winds even though that is going to be the main threat. To add a little more context to what severe winds would be like, they are wind gusts within a thunderstorm that are 58 mph or greater.
Heavy rainfall, while not a component of severe weather forecasting or of severe weather, technically, it still can pose just as much of an issue as strong winds or anything else. Today, there is the potential for widespread areas of 1-2 inches of rain across the region with locally much heavier amounts, especially if you are caught in one of heavier thunderstorms that we could see. There is a Flash Flood Watch in effect for the I-95 corridor from Noon today until Midnight tonight. Low lying areas and areas with poor drainage are likely to be flooded if heavy rains occur in the area. Always remember that you never want to drive across a flooded road, “Turn Around Don’t Drown.”
Finally, we will time out when we can expect the severe weather to possibly move into the region today. We can look for thunderstorms to move into the far west and mainly the far northwest sections of the area, like the Poconos, in the early parts of the afternoon, likely in the 1 p.m. to 3 p.m. time frame. The front then tracks east and is in areas such as the immediate suburbs in Pennsylvania, like Bucks, Montgomery, and Delaware Counties around 6 p.m. We should see the strongest storms in the Center City Philly area around 7 p.m. to 8 p.m., and the front clears the city the moves the storms into Central and South New Jersey by 9 p.m. tonight.
While a few showers could linger into the morning hours along the immediate Jersey Shore coastline, we should be done with almost all if not all of the precipitation in the 12 a.m. to 2 a.m. time frame.
Make sure you are staying tuned to the Eyewitness Weather Team throughout the day today as we continue to keep you updated on what to expect with the possible severe weather today and your weather forecast for the coming week. Stay safe and weather aware everyone.