Plaincast

What has changed

Air Quality Alerts in effect for the Triangle and Triad through Friday.

Heat indices trending slightly downward for Friday.

Messages

Air Quality Alerts in effect through Friday for the Triangle and Triad.

Hot temperatures and potentially dangerous heat expected into Saturday.

Becoming unsettled this weekend into early next week with increased precipitation chances.

Discussion

KEY MESSAGE 1. Air Quality Alerts in effect through Friday for the Triangle and Triad.

A hot and nearly stagnant air mass over the region today is favorable for the development of dangerous levels of ozone, primarily surrounding the main metropolitan areas of North Carolina. This has prompted a Code Orange from the NC Division of Air Quality around the Triangle and the Forsyth County Office of Environmental Assistance and Protection for the Triad. Although there is still lingering Canadian wildfire smoke over the area, especially over northeast NC, these fine particulates remain well above the surface and are not a contributing factor to the poor air quality over the area.

A similarly stagnant and hot air mass will remain over the area Fri and result in additional Code Orange Air Quality Alerts. We will also be watching for the addition of fine particulates currently streaming across NY/PA to work down into VA and potentially into the Piedmont of NC by Fri morning. However, the latest runs from the RRFS and to a lesser degree the the high-res rapid-refresh model, which both simulate near- surface smoke, have trended to keeping the highest concentrations mainly confined along and north of the NC/VA border. How far south dangerous levels of smoke will advect remains a point of uncertainty. Stay tuned for additional forecasts to come later this afternoon and potentially again Fri morning if the forecast changes.

KEY MESSAGE 2. Hot temperatures and potentially dangerous heat expected into Saturday.

An elongated mid-lvl ridge axis stretching from the lower Ohio Valley, across the Carolinas, and into the western Atlantic will continue to provide background subsidence over the region before breaking down on Sat. H850 temperatures of 19-22C observed in the 12z RAOBs over VA and into NC will continue to support hot temperatures over the region with minimal thermal/moisture advection into Sat. Overwhelming model support via low-lvl thickness and dry-adiabatic technique support statistical guidance as well as the National Blend to result in high confidence in 2m temperatures reaching the upper 90s to around 100 this afternoon and once again on Fri. However, confidence is lower with respect to dew points during peak heating and whether they will support multiple hours of heat indices in the mid/upper 100s, especially west of the US-1 corridor.

Forecast challenges: Model guidance remains consistent with northerly winds persisting over the Piedmont into peak heating where continued dry air aloft and some light downsloping component west of a weak pressure trough may help mix dew points into the low/mid 60s on Fri. Additionally within those northwesterly winds, haze/smoke from Canadian wildfires may be deposited into our area, which may further the temperature forecast. Finally, hi-res guidance is beginning to develop isolated to even locally scattered showers/storms Fri afternoon with cool outflow potentially limiting the window to reach heat indices around 105. Regardless on whether heat indices of 105 are met, light to nearly stagnant surface winds, abundant sunshine and hot temperatures will still provide a favorable air mass for heat related illnesses to occur among any individual who does not have access to adequate hydration or cooling.