Forecast: Heat wave begins - may reach excessive, dangerous levels late this week

Code orange air quality: unhealthy for sensitive groups

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Heat wave day 1. Steamy, but going to get much, much worse. Being generous to start.
 
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Today: Hot and humid. 90-94. | Tonight: Partly cloudy, warm - possible storm. 67-75. | Tomorrow: Hot, humid, late storm chance. 91-95. | Get Express Forecast by E-mail

A long duration heat wave, likely to span the entire week (at least), begins today. Through Wednesday, the heat is fairly ordinary by D.C. standards - with low-to-mid 90s, and heat index values near 100. But Thursday through Saturday brings the chance of triple digit heat and heat index values at dangerous 105-110 degree levels. A weak cold front may allow for some slight moderation by Sunday. For the most of the week, thunderstorm chances will be low, with the best chance late Tuesday and Saturday.

Today (Monday): After four straight pleasant days, the heat bounces back. With lots of sunshine, highs reach the low-to-mid 90s. And, with winds coming from the southwest (at 5-10 mph), humidity levels rise too. Peak afternoon heat index values range from 96-100. Confidence: High

Tonight: Just another muggy D.C. night. Lows range from the upper 60s in the cooler suburbs to the mid-70s downtown. Our northern suburbs, primarily, have a slight (20%) chance of thunderstorms late at night. Confidence: Medium-High

Keep reading for the forecast through the weekend...

Tomorrow (Tuesday): Very warm and humid (dew points around 70) to start the day. A weak cool front slipping south into the region increases cloud cover in the afternoon and offers a 30% chance of storms. Any storms that form could produce very heavy rain and dangerous lightning. If we see a lot of clouds, highs may only reach 90 whereas more sunshine would boost highs towards the mid-90s Peak heat indices are again 95-100. Confidence: Medium-High

Tomorrow night: There’s a slight chance of evening storms. Otherwise, it’s partly cloudy, with lows from the upper 60s in the cooler suburbs to mid-70s downtown. Confidence: Medium-High

Wednesday sees a (very) slight down tick in humidity levels (dew points mid-60s) relative to Tuesday, but probably more sunshine. So it feels about the same in the end, with highs in the low-to-mid 90s, and maxium heat indices 95-100. Humidity starts to climb back up Wednesday night, with muggy lows from near 70 in the cooler suburbs to the mid-to-upper 70s downtown. Confidence: Medium-High

Thursday through Saturday are most likely excessively hot and humid with highs Thursday 98-102, Friday 99-104, and Saturday 99-103. (Record highs those three days are 104, 103, 101). Dewpoints near 70 will make it feel like 105-110, with some locally higher values not out of the question. Any thunderstorms are isolated Thursday (10% chance) and Friday (15% chance), and potentially more widespread late Saturday (30-40% chance) as a weak cold front approaches.Overnight lows provide limited relief, ranging from only near 80 downtown to the low 70s in the coolest suburbs. Confidence: Medium

Saturday’s front may not really clear, so the hot and humid airmass may well linger into Sunday. So Sunday still looks hot, with highs in the mid-90s - maybe upper 90s - with a 30-40% chance of storms, mainly late in the day. Confidence: Low-Medium