Large parts of the Mid-Atlantic and Northeast are enjoying an unusually pleasant Independence Day after a week of flooding rains and severe weather. A post-frontal high-pressure system has swept out the humidity, leaving sunny skies and comfortable afternoon highs in the low- to mid-80s from Washington, D.C. to Boston and Pittsburgh. Conditions are expected to stay favorable for evening celebrations, with most locations cooling to the upper 60s or low 70s by the time fireworks begin. Washington, D.C. officials have issued a Code Orange air-quality alert, warning sensitive groups that smoke from the displays could briefly degrade air conditions. The respite will be short-lived. Meteorologists in several cities expect heat and stickiness to rebuild over the holiday weekend, pushing temperatures toward 90°F on Sunday and Monday and bringing a chance of showers or thunderstorms early next week. On the West Coast, Seattle is also marking the Fourth with dry weather and highs in the mid-70s; forecasters there likewise see a warmer spell taking hold by mid-week.