Oppressive combination of heat and humidity this weekend

We have all experienced high temperatures before. In fact, most of us sat underneath a large ridge of high pressure earlier this summer which brought us temperatures in the upper 90’s. This weekend, however, temperatures will again surge into the upper 90’s — and combine with dew points in the middle to upper 70’s to bring Heat Index values near 110 F for periods of time each afternoon. This air, not surprisingly, will feel overly oppressive.

The culprit? The positioning of the aforementioned ridge in the mid levels of the atmosphere. Earlier in July, our area experienced a prolonged period of heat. But the ridge axis was centered in the Plains States. A clockwise flow around that ridge axis brought the heat into our area from the Northwest, meaning dew points were lower and the heat was of the “drier” variety. This time, the ridge is centered off the Southeast US Coast, with the clockwise flow around the high pressure bringing the heat into our area from the southwest.

Dews

Dew points on Saturday are expected to reach into the middle and upper 70’s, to combine with temperatures in the upper 90’s.

This southwesterly flow in the lower and mid levels of the atmosphere draws air into our area from the Gulf of Mexico and tropics. Higher dew points and highly anomalous atmospheric moisture content (Precipitable Water values) will combine with an unusually hot airmass, then, to produce oppressive conditions. Heat Index values on almost all forecast models are expected to surge over 105 F during the afternoon, and occasionally exceed 110 F. This will be especially true in the urban corridor from Philadelphia to New York City and Northeast New Jersey.

Accordingly, the National Weather Service has issued Excessive Heat Watches and Warnings (See your local office for more information), for the oppressive conditions that are anticipated on both days. These are mainly centered near the urban areas of Philadelphia and New York City where the heat and oppressive dew points are expected to be maximized.

In addition to the heat, scattered strong thunderstorms and torrential downpours will be possible each day this week. Mid level atmospheric disturbances are expected to meander about, occasionally providing enough lift for thunderstorms to form in a highly unstable environment. The main mitigating factor to a more organized event, at least early this weekend, will be a lack of wind shear. Most of the favorable wind fields are pushed well north of the area, due to the large ridge overhead. Any thunderstorm that forms won’t have much wind shear to help it’s organization — meaning storms will “pulse” more instead of become a more organized severe weather complex.

The main story will continue to be the heat. Be sure to stay hydrated and keep a close eye on children, the elderly and pets. Avoid prolonged exposure to the sun if possible and spend plenty of time in a cool, air conditioned place. And as always, check the Dashboard for the latest information throughout the day and the weekend ahead.