8:45 AM (Thurs.) | **Higher heat and humidity on the way…an increasing chance of showers and storms…any storm can produce heavy rain, damaging wind gusts**

The upper-level ridge centered over the northeastern states in recent days has allowed for a low-level flow out of the east and southeast across DC, Philly and southern New Jersey and this has kept a cap on high temperatures to near the 90 degree mark. This will change in coming days as the low-level flow becomes more continental in nature allowing for higher heat and humidity. Map courtesy NOAA, tropcialtidbits.com

Overview

While quite warm this week in places like Washington, D.C. and Philadelphia, the worst of the heat and humidity is still to come. In fact, humidity levels have been rather manageable in these areas during the past few days limiting “heat indices” which are meant to reflect how weather conditions “feel” to humans given the heat and humidity. Large-scale upper-level ridging has been centered overhead during the past few days allowing for a bit of an onshore flow of air in places like D.C., Philly, and southern New Jersey, but that pattern will change in the Friday, Saturday, Sunday time frame resulting in higher heat and humidity. In addition, the chance of showers and storms will increase beginning later tomorrow and the threat of rain will continue this weekend and any storm that forms can produce locally heavy rainfall and damaging wind gusts.

The upper-level ridge that has been centered over the northeastern states during the past few days will retrograde (i.e., shift west-to-southwest) over the next couple of days. This change in positioning will allow for low-level flow to become more “land-based” than “ocean-based” and this will result in higher heat and humidity in places like D.C. and Philly. Map courtesy NOAA, tropicaltidbits.com

Details

The atmospheric setup during the past few days has not been one that brings the highest heat and humidity to the southern Mid-Atlantic region with large-scale upper-level ridging centered overhead allowing for a bit of an onshore low-level flow of air in places like D.C., Philly, and southern New Jersey. This has capped afternoon highs around the 90 degree mark which will be the case again today in D.C. and Philly (New York City makes it into the low-to-mid 90’s later today).

By later tomorrow, this upper-level ridge will begin to break down, shift to the west-to-southwest, and low-level flow of air will become more continental in nature. This change in the strength and positioning of the upper-level high pressure ridging will result in an increase in heat and humidity on Friday, Saturday, and Sunday resulting in the most uncomfortable weather conditions during this heat spell. Temperatures could climb into the middle 90’s on Friday afternoon in parts of the DC-to-Philly-to-NYC corridor and then the mid-to-upper 90’s are on the table for Saturday and Sunday.

The hottest and most humid stretch of weather in this heat spell will come in the Friday, Saturday, Sunday time period for much of the DC-to-Philly-to-NYC corridor. Map courtesy ECMWF, tropicaltidbits.com

In terms of the potential for rain, the last few days have been largely rain-free in the DC-to-Philly-to-NYC corridor and today will likely be as well. However, as heat and humidity build on Friday and with a back door cool front getting into the picture, the chance of showers and thunderstorms will be on the increase. The threat of showers and storms will continue this weekend perhaps reaching a peak late Sunday/Sunday night at which time a cool front will be approaching from the northwest. Any thunderstorm that forms in the Friday, Saturday, Sunday time period can produce locally heavy rain and damaging wind gusts.

An upper-level ridge will re-establish itself late next week and the heat will likely return to much of the central and eastern US as we end June and begin the month of July. Map courtesy ECMWF, tropicaltidbits.com

The cool front that approaches late in the weekend should have enough push behind it to usher in a more comfortable air mass into the Mid-Atlantic region as we begin a new work week. As a result, humidity levels should become noticeably lower on Monday and temperatures will be more comfortable. It does likely turn hot again by mid-week - ahead of the next cool front - with highs back up in the 90’s on Wednesday in the DC-to-Philly-to-NYC corridor.

Meteorologist Paul Dorian
Arcfield
arcfieldweather.com

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