July 29: It’s Gonna Be a Hot One Today!

Jonathan Carr
By Jonathan Carr July 29, 2015 10:07

July 29: It’s Gonna Be a Hot One Today!

The National Weather Service has issued Excessive Heat Warnings and Air Quality Alerts with good reason. Today’s temperatures should easily soar into the 90s for many with heat indices approaching 100. When the low-to-mid levels of the atmosphere are this warm, pollution has trouble rising and escaping the lower-levels where most of us live. This is especially a problem along the heavily-populated and industrialized I-95 corridor running through Philadelphia and New York City (ultimately extended to DC-Boston). That’s why you’ll generally see air quality alerts favor such urban areas during extreme heat. This struggle for pollutants/exhaust to rise is similar in physics to the capping inversion which prevents thunderstorm formation on a warm day. With that said, the cap is always vulnerable to breaking as we saw yesterday in parts of SENJ. That’s also very possible today.

Let’s look at some high-res guidance. Here’s the latest NAM showing peak afternoon surface temperatures (around 2PM). Keep in mind that heat indices will be 5-10 degrees higher when factoring humidity:

heat nam

This much diurnal surface heating coupled with high dew points creates a lot of potential fuel for any thunderstorms that do form. The question is, will the atmosphere provide a match to light it? What I mean by that is a trigger such as a cold front, warm front, surface low disturbance, etc. Well none of those are scheduled to move through today so much of this fuel will be wasted. Here are the projected surface instability numbers:

hires_cape_harrisburg_13

See that higher region along the coast? If a sea breeze front can back in off the ocean with enough force, it could provide the trigger needed for thunderstorm formation. With that said, I think the Jersey Shore has the best potential for storms. The rest of the region will be fighting that capping inversion which traditionally means very isolated pop-up thunderstorms. Here’s the expected precipitation shown on a 850mb map. As you can see, just a few small pop-up cells in the afternoon hours today:

hires_ref_harrisburg_15

In English: Expect a hazy, hot and humid day with temperatures approaching/breaking 90 in many locations. Heat indices should heavily flirt with 100, especially interior urban locations (Philly-Trenton). Watch out for an isolated shower or thunderstorm towards afternoon-evening hours (very hit-or-miss). I still think a sea breeze front could provide enough of a trigger to spark storms along the coast but we’ll see. Most will miss out with such a localized storm threat. I’ll be watching the radar for 1) That sea breeze front and 2) Any showers or storms that form. Stay hydrated, cool and be safe! JC

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Jonathan Carr
By Jonathan Carr July 29, 2015 10:07

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