Millions across the northeastern United States will experience another round of weather whiplash this week as a sharp surge of cool air will wipe away the unseasonable heat that was ushered into the region over Memorial Day weekend.
Weather this May across the region has at times felt more like late winter or summer, with very few days of near-average temperatures. And days on which temperatures have fallen well below the normal mark have been far outpaced days with unseasonable warmth this spring. Average temperatures were 2 to 4 degrees below normal for the month as of Wednesday.
Earlier this month, record cold gripped the eastern U.S. and lows plunged to freezing and below as some cities even set a record for the latest recorded snowfall.
The most recent spike in temperature has delivered the hottest conditions of the season so far to some areas with more than a dozen locations from Ohio to Pennsylvania and Vermont tying or smashing record highs on Tuesday alone. A few spots broke long-standing records set back in 1939, including Mansfield and Youngstown, Ohio, both of which recorded highs of 90 on Tuesday.
In a few cases, such as in Burlington, Vermont, all-time May record highs are forecast to be set amid the hot spell lasting through Thursday. The May record in Burlington is 93 set in 2017 and 1977. Burlington already tied a daily record high temperature of 92 on Tuesday, which was last set in 2010.
However, the heat will be fleeting. The most dramatic change to cooler conditions will be felt from the lower Great Lakes to the St. Lawrence Valley and the Appalachians this weekend. In this area, high temperatures in the 80s and 90s F will be swapped with highs in the 70s, 60s and even the upper 50s in some cases.
Average highs during late May and early June range from the upper 60s in northern New England and areas just across the border in Canada to the upper 70s to near 80 around the Chesapeake Bay.
Rain advancing from west to east across the region will be a sign of the pattern change coming to the Northeast. Some of the rain from Bertha, which made landfall as a tropical storm along the South Carolina coast on Wednesday morning, will become intertwined with rain from an advancing cold front from the Midwest.
Cloud cover, rain and a breeze for a time will add to the dramatic temperature drop. AccuWeather RealFeel® Temperatures are likely to dip to 10 degrees lower than the actual temperature due to rainfall on Friday and Saturday then breezy conditions on Saturday and Sunday.
"We are concerned about frost in the normally cold spots from Virginia and West Virginia northward to New York state and northwestern New England from later Sunday night to early Monday morning," said AccuWeather Senior Meteorologist Randy Adkins.
The frost may not be widespread, but for the thousands of people who have started their vegetable and flower gardens for the season, it is a reason to take preventative measures. Simply bringing potted plants indoors for the night or covering them with plastic or old blankets will help to offer protection to these sensitive plants.
Agricultural interests, such as vineyards and orchards, in the region may need to utilize wind machines, bonfires and water to reduce the chance of damage.
"In order for frost to form, winds must drop off and the sky will need to remain clear," Adkins explained. "This allows the atmosphere to 'decouple' and cold air to collect in the low spots, such as in valleys and meadows over the interior."
Often when there is frost, the temperature near the ground is several degrees lower than at eye level. Official temperatures are taken at approximately 6 feet above the ground. Low temperature forecasts are presumed for this 6-foot level. That is why frost can occur under certain conditions even when low temperatures are forecast to bottom out in the middle to upper 30s.
Temperatures may hover just above frosty levels over the interior valleys, where there are large bodies of water such as lakes and rivers. Temperatures are not expected to drop to frosty levels in the urban areas of major cities, especially those along the Interstate 95 corridor from Boston to New York City and Washington, D.C.
Where a breeze and/or cloud cover linger over the region Sunday night, frost will not develop, even in the chilly spots.
As the pattern continues to evolve and a large southward dip in the jet stream develops over the Northeast, the chilliest air is likely to settle over New England on Monday. Clouds and spotty showers may continue to hold temperatures well below average over the central Appalachians.
A significant temperature contrast is likely to unfold during the middle days of next week with building warmth and sunshine over the Ohio Valley and lingering chill and even rain over New England. Temperatures could spike well into the 80s to near 90 over Ohio and southwestern Pennsylvania, even as parts of New England fail to climb past the 50s.
In addition to the sharp southwest to northeast temperature contrast zone, AccuWeather meteorologists will also be monitoring for the potential for complexes of thunderstorms, which tend to move around the edge of a dome of building heat. The core of that heat is likely to take up shop over the middle Mississippi Valley next week.
"The pattern next week may favor thunderstorm complexes that erupt over the Upper Midwest and travel hundreds of miles to the Ohio Valley or Northeast states," Carl Erickson, AccuWeather forecaster, said.
Such patterns in the past have also created strong, long-lived thunderstorm complexes, known as derechos.
In lieu of a derecho, there could be more than one cluster of thunderstorms developing on a given day, which means that some areas could be hit with storms multiple times next week.