The heat is forecast to build into the region as a massive heat dome sprawled over the central United States shifts east.
On multiple occasions, we may match or best the summer’s highest temperature registered so far of 98 degrees (on July 14). And, for the first time this summer, we should string together consecutive days above 95 degrees.
But three key questions remain: Will the heat wave break records, just how humid will it be, and how long will it last?
Model data are unanimous that highs will reach at least the mid-90s between Friday and Monday. The average model forecast produces highs in the upper 90s each of those four days.
Some models are advertising triple-digit highs on Saturday and Monday. If the District manages to hit 100, it will mark the first occasion since July 26, 2012.
As it’s the hottest time of the year, record highs — which are at least 100 degrees — are difficult to break. Based on the current forecast, it would appear as though Monday has a chance to soar above 100 degrees and set a record, but it’s by no means certain.
It’s also worth noting that overnight low temperatures will also be unusually high. When it doesn’t cool off substantially at night, the risk of heat-related illness increases for those without air conditioning (for instance, the poor and the homeless).
Overnight lows in the city are likely to only be in the muggy upper 70s to near 80. Historical record-high minimum (or low) temperatures during the Friday-to-Monday stretch are mostly in the low 80s, so they probably won’t be threatened. But the record-high minimum (or low) temperature on Monday is just 79 and could be challenged.
Humidity throughout the heat wave will be moderate to high but probably not off the charts. Still, moderate to high humidity will frequently make air temperatures feel at least five to 10 degrees hotter than they actually are. Saturday and Monday are likely to be the most humid days. Humidity may ease slightly Sunday.
Heat-index values, measures of how hot the air actually feels, should mostly range between about 100 and 105 degrees, but some 105- to 110-degree or even higher values can’t be ruled out Saturday and Monday.
The National Weather Service issues heat advisories when the heat index has the potential to reach 105 degrees, and we expect such advisories will be issued on most, if not all, of the days between Friday and Monday.
Monday would be the one day to look out for the possibility of an exceptional combination of heat and humidity. The European model is forecasting dew points, a measure of humidity, to reach the upper 70s late Monday.
If you combine dew points in the upper 70s with air temperatures in the upper 90s to low 100s, that would produce a heat index of more than 110 degrees. Should the heat index be predicted to approach such levels, the National Weather Service would issue an excessive-heat warning.
The latest model data suggest this heat wave may evolve into a long-duration event. High temperatures in the mid-90s or higher are now predicted to persist through Thursday or Friday next week.
But model forecasts of more than three or four days into future are generally less reliable. So adjustments to the forecast are highly likely.
The longer the heat wave persists, the more dangerous it potentially becomes. For individuals without access to air conditioning, heat stress accumulates and the risk of heat-related illness increases.