Heat with afternoon thunderstorms today and Wednesday

A warm and muggy airmass has its grip on our area, and combined with a slowly approaching cold front and a disturbance aloft, some showers and thunderstorms will be triggered. For most of the day today, the cold front will still be a bit far to our west, which will generally lead to mostly sunny skies, hot temperatures, and a delay in any thunderstorm activity.

NAM model valid for 2:00pm shows temperatures in the low-to-mid 90s in our area, with 80s along the shores.

850mb temperatures of 18C with sunshine support high temperatures in the low-to-mid 90s, which will generally be commonplace throughout the area. The one exception will be for most of Long Island and the immediate Jersey Shore beaches, where temperatures will stay in the 80s, due southerly winds from the ocean bringing in slightly cooler temperatures.

Initially, the highest 500mb vorticity and core of the trough are still to our west, as evidenced in the image below. The cold front is associated with the low pressure system well to our northwest, and we have a warm, southwesterly flow out ahead of it. However, there is a weak surface pressure trough in the Mid Atlantic — which combined with a weak 500mb vorticity maximum — may act to try and force some convection during the evening.

NAM model showing a weak surface trough and weak area of vorticity, which may help to trigger some thunderstorms. The core of the trough will remain to the west for most of the day, which will delay the thunderstorms somewhat. Additionally, 18C 850mb temperatures will lead to quite the hot afternoon.

As far as the type of thunderstorms that can be expected, some may produce some some strong winds and very heavy rain, due to sufficient wind shear, moderate instability, and very high precipitable water values. 

Moving forward to this evening, high precipitable water, moderately high surface-based CAPE, and increasing wind shear all lead to the potential for strong to severe thunderstorms.

By 8:00pm, we’ll have around 1,500 J/KG of surface-based CAPE and around 40 knots of 0-6km wind shear, which supports relatively organized thunderstorms with gusty winds and small hail. Additionally, precipitable water values are around 2.00″, which supports torrential downpours and flash-flooding with any storms that do occur. The cold front remaining well to our west with relatively weak triggering mechanisms will prevent a widespread severe weather outbreak, but some strong to severe thunderstorms are still possible; particularly west of the city where it is a bit closer to the front and the atmosphere will be less capped.

Moving forward to Wednesday, the airmass becomes a bit cooler and drier as winds shift to the west and 850mb temperatures cool off a tad. However, with a bit of downsloping flow, the surface temperatures will not be all that much cooler. High temperatures should be in the upper 80s to around 90, but again cooler along the shores. What will be different is that there will be more clouds, as the cold front and vigorous trough will be much closer to our area.

The surface, 500mb, and 300mb trough all become much more evident tomorrow afternoon, as well as a stronger area of vorticity. This will help to trigger thunderstorms early in the afternoon.

A more potent surface trough is evident late tomorrow afternoon, as well as a strong 500mb vorticity maximum in eastern Pennsylvania. This will help to trigger showers and thunderstorms, potentially several hours earlier than they will be triggered today. Further evidence of the dynamics in the atmosphere is the strong approaching trough and the 300mb winds in excess of 70 knots — which can also help for thunderstorm development.

Instability will not be off the charts, and lower precipitable water values may make any downpours not as significant. That being said, there is still enough heat and humidity in the atmosphere for CAPE values near 1,000 J/KG, and combine that with 0-6km wind shear between 30-40 knots, as well as the aforementioned dynamic atmosphere leading to strong lifting and a weaker CAP, strong thunderstorms are still possible on Wednesday.

Moderate surface-based CAPE, low Convective Inhibition, 30-40 knots of 0-6km shear, and ~1.5″ of precipitable water all support thunderstorms on Wednesday.

The cold front should pass sometime on Wednesday night, but the question becomes whether the front completely clears the region, or if it stalls just offshore. As of now, it looks like Thursday will be a sunnier, drier, and pleasant day compared to Wednesday. However, with the cold front still close-by, weak systems may ride along it and re-enhance some lift, creating some widely scattered showers — but nothing to worry about. Friday looks to be a gorgeous day.