Public Analysis: Hot Again Today, Then Unsettled with T-Storms Possible Tonight thru Monday

Good morning and happy Saturday! After a few weeks of more pleasant, warm weather, we finally got a reminder that it’s still mid-July late this week. We had three consecutive day of 90°+ temperatures in many of the climate reporting sites around the region, such as Central Park, NY and Newark, NJ. Which officially makes these last few days of hot and humid weather a heat wave. Some locations will add 4th day to the heat wave today, before more unsettled and cooler weather arrives tonight and Sunday.

A weak frontal boundary will be sagging to will come through early this morning. But lift and instability will be weak over the region. So only a few isolated showers or thunderstorms are possible. Otherwise, weak high pressure will provide more sunshine and dry weather for much of the day. It will not be hot as past days,  High temperatures will likely be several degrees above normal again with highs in the upper 80s to lower 90s over much of the region. Near the shores, some sea-breezes could keep temperatures from rising out of 80s this afternoon.

Humidity will be little higher than on Friday, due to more southwest winds. Therefore heat index values will be higher in the middle 90s. For this reason, the National Weather service has issued a Heat Advisory again for today, New York City and urban areas of Northeast New Jersey. While this heat won’t be particularly dangerous and will be typical for the summer here, it is still advised to avoid any unnecessary strenuous activity and extended exposure to heat. Some heat-related illnesses are still possible, especially for the elderly.

NAM high temperatures for today

High-resolution NAM model with forecast high temperatures for today

Then some clouds will be increasing later this afternoon as a wave of low pressure develops along frontal boundary that will be over Southern PA and Central/Southern New Jersey. Our mesoscale models suggest complex of showers and thunderstorms will form this disturbance, into what is known as a MCS (mesoscale convective system). This complex will move through the during evening and overnight hours tonight with some heavy downpours possibly. Not everyone will necessarily see rainfall. But we recommend taking an umbrella if you are going out. Overnight lows will be in the middle 60s to lower 70s across the region.

There is potential with some stronger, direction shear and instability especially over parts of Southern PA and Southern NJ that some thunderstorms could become strong-severe, with main threat being gusty or damaging winds. For this reason, the Storm Prediction Center has issued a slight risk for severe weather for southern parts of the region for late today and tonight. However, latest indications suggest much of this system may pass just south of NYC. Also the stronger instability will likely be able to build further to the south, as the airmass will be more moist with surface dewpoints in the lower to middle 70s.

However, model guidance suggest strong moisture convergence, mid-level frontogenesis and some elevated instability just north this low this low track. This result in band of heavy rainfall with some isolated or embedded thunderstorms. Precipitable water values which are indicate the amount of moisture available the atmosphere, will be very high between 2.00 and 2.50”. These values are what you typical see with tropical systems. This mean that any thunderstorms will be also capable of produce some torrential downpours with some localized flash flooding.

NAM mdoel showing high instability (CAPE) juxtaposed with 35-45kts of bulk shear and precipitable water values over 2.00" over SE PA, Central and Southern NJ this evening

NAM model showing high instability (CAPE) juxtaposed with 35-45kts of bulk shear and precipitable water values over 2.00″ over SE PA, Central and Southern NJ this evening

On Sunday, this system moving offshore with any linger showers or thunderstorms likely tapering off during the morning hours. Then clouds may break for sunshine during the afternoon But high northeast of northeast region, will support more onshore winds on Sunday.  This will likely temperatures much cooler that past few days, with highs in the upper 70s to lower 80s near the coast in the middle 80s further inland. So Sunday may turn out be a little dreary for any outdoor plans, especially early in the day. But it doesn’t look like a washout.

Then by Sunday night and Monday an area of low pressure associated deep shortwave trough will be moving through Great Lakes. This will cause the frontal boundary begin returning slowly northward as warm front. Some isentropic lift, mid-level frontogenesis and elevated instability will cause more likely to more showers and thunderstorms that could arrive early as Sunday afternoon, especially over Western parts of the region. But are more likely to be numerous Sunday night into Monday morning.

Some models suggest another secondary or triple-point low pressure may form along the frontal boundary to the south by Monday morning, which could enhance more directional shear and moisture convergence. More severe weather appears likely to stay southwest of NYC metro where instability will likely be more sufficient. But precipitable water values will be very high again. So there is potential for locally heavy rainfall and flash flooding some areas.

NAM model forecast for Monday

NAM model forecast for low developing along frontal system and shortwave moving through with some showers and possible thunderstorms on Monday

 Temperatures could warm up again on Monday afternoon into the 80s again. But that will largely depend on how far north the warm moves through the region. More scattered showers and thunderstorms are still possible Monday afternoon and night, as shortwave trough and cold front move through region. Finally some cooler, drier weather will likely arrive by Tuesday and Wednesday, as high pressure from Canada builds into the Northeast. Temperatures may actually be slightly cooler than normal.

We will continue to monitor the potential for severe weather and heavy rainfall in the region through weekend and into early next week. Stay tuned for more public updates through our social media accounts, and client/member updates, as necessary.  For more information and posts like this one, make sure you sign up for Premium Forecasts — where multiple detailed articles, videos, and interactives are posted each day. Also, come interact with our staff and many other weather enthusiasts at!

Have a great day!

Miguel Pierre