The high temperature hit 90 degrees Sunday in Lancaster County for the first time this month, according to data from Millersville University's Weather Information Center.
That trend could continue early this week with highs in the low 90s forecast through Wednesday, according to the National Weather Service.
Three consecutive 90-degree days constitute a heat wave. The last heat wave in Lancaster County was July 19, 20 and 21, when highs were in the low 90s. The heat index made temperatures feel like well into the 100s, leading to excessive heat warnings and canceled events.
MU meteorologist Eric Horst said in a tweet Sunday that warm, muggy conditions will prevail through Wednesday with pop-up showers and storms possible each day.
Today expect a high near 93 with heat index values as high as 100 degrees, according to the weather service. There's a chance of showers and thunderstorms late this evening.
On Tuesday look for a high near 92 with heat index values as high as 101. There's a 50 percent chance of showers and thunderstorms during the day.
Wednesday's forecast calls for showers and thunderstorms with a high near 91 degrees.
Horst said a cold front will usher in a "much cooler, drier air mass for week's end."
On Thursday the 90-degree stretch could break to the upper 80s. By the weekend temperatures could dip to closer to 80 degrees.