Area Forecast Discussion

WHAT HAS CHANGED

Two hot and humid days on tap. Not confident for much rain today due to capping inversion aloft, but higher probability Friday associated with frontal passage.

Temperatures were lowered slightly for Friday areawide.

KEY MESSAGES

1: Isolated severe storms will be possible this evening, with a better likelihood of severe storms on Friday.

2: Above normal temperatures expected Friday through Sunday, warmest readings are forecast for Friday.

DISCUSSION

Key Message 1: Isolated strong to severe storms possible late this afternoon. Better chance for strong to severe storms Friday. Main threats would be wind damage localized flash flooding.

A few isolated brief showers have developed across the mountains thanks to gentle upslope westerly flow. Aside from brief heavy downpours, these should have little impact through mid afternoon. The bigger concern is the chance of severe storms later this afternoon and into this evening. There is a Marginal Risk across the entire area, with a Slight Risk generally north of US-460. Instability will be fairly high today, due to the lack of cloud cover and increase in temperatures. Sufficient moisture and a leeside trough is in place to help create lift, along with a shortwave upper-level trough that will pass through the northern part of our area this evening. Despite these factors, storm coverage today is expected to remain isolated to widely scattered. Although CAPE will be at least 2000 J/kg, additional forcing will be needed to break the temperature inversion aloft, aka the cap, due to the downsloping westerly winds. This additional forcing will occur when the upper trough passes through later today. Any storms that do break through the cap will easily have the potential to cause damaging wind gusts, due to DCAPE values well over 1000 J/kg. Large hail cannot be ruled out, but will be much more isolated in the strongest storms in the northern part of our forecast area. Storms continue into tonight before tapering off around midnight. With all of the above factors mentioned, a severe thunderstorm watch may be issued for the northern part of the area.

For Friday, a cold front will approach the area from the west, which will provide additional forcing. Instability will remain equally high to today, with temperatures in the 80s/90s. Due to these factors, more widespread thunderstorm development is likely as the front moves through during peak heating mid-afternoon, which is why a Slight Risk of severe storms is across the entire area. Damaging winds will once again be the main threat, and will increase further in likelihood tomorrow. Large hail is also a possibility in the strongest storms.

Both later today and for Friday, isolated flash flooding will remain a concern. Most of the area is in a Marginal Risk for excessive rainfall through tonight, while the entire area is included for Friday. Rain rates of 2-4" per hour will be possible in the heaviest storms, and with the steering flow/shear not being very strong, could allow for some slow moving storms to dump heavy rainfall over a small area. Any storms that repeatedly move over the same locations could also cause flash flooding, especially in low-lying, flood-prone, or urban areas.

Key Message 2: Above normal temperatures expected Friday through Sunday, warmest readings are forecast for Friday.

850mb temperatures are forecast to surge to around 20-24 degrees Celsius. Temperatures aloft this high typically translate to upper 90 temperatures to near 100 degree temperatures at the surface with adiabatic warming. With storm activity expected on Friday, there is some uncertainty if temperatures will reach the high temperatures that the NBM is currently forecasting across the area; therefore, some Consshort was blended in to lower high temperatures region wide. Forecast highs in the upper 90s are widespread across the Piedmonts of Virginia and North Carolina on Friday, with mid to upper 80s forecast for mountain locations. One caveat that was mentioned above is that if storm activity pushes into the region in the early afternoon, high temperatures for the day might be capped at whatever stations end up reaching prior to storm activity pushing into the region.

While temperatures look to reach the hottest of the year thus far, dewpoint temperatures are also expected to be elevated across the region in the mid 60s to low 70s. This will lead to heat index values in the low 100s across the Piedmont, and upper 80s to low 90s across mountain locations. While the air will feel muggy, deep boundary layer mixing needed to reach these high temperatures in the upper 90s to near 100 will likely mix down some drier air aloft, and inevitably lower surface dewpoints slightly into the low to mid 60s Friday afternoon. This will likely prevent Heat Index values from climbing to Heat Advisory criteria levels; however, folks should still continue to take frequent breaks from the outdoor heat, and drink plenty of fluids. Sensitive groups should also remain indoors during the peak heat of the day.

The frontal passage on Friday will provide some relief as dewpoints are lowered areawide, which will reduce the muggy feeling outside; however, highs will still reach the low to mid 90s in the Piedmont, and upper 80s across the mountains. Mid to upper 90s look to return in the Piedmont, and mid to upper 80s in the mountains on Sunday before another frontal boundary pushes through the area late Sunday.

AVIATION /18Z THURSDAY THROUGH MONDAY/

Scattered showers and thunderstorms this afternoon look to mainly impact piedmont terminals ROA, LYH, and DAN; however, some isolated showers and thunderstorms still look possible at BLF, BCB, and LWB. These showers and thunderstorms will be able to produce strong gusty and erratic winds, and MVFR to IFR restrictions due to reduced VSBYs and CIGs. Prob 30 groups were maintained due to the low likelihood, but high end impact potential for these storms today. Outside of these storms, VFR conditions are predominantly expected to remain at all terminals through the TAF period. Some reduced VSBYs may be possible at BCB and LWB due to river valley fog early Friday morning; however, these restrictions will be brief and should quickly dissipate after sunrise if they do develop.

Beyond the early morning hours, winds look to remain out of the west/southwest at around 5-10 knots, with gusts up to 15-20 knots across the area starting around 15 UTC.

EXTENDED AVIATION OUTLOOK

A cold front will cross the area Friday during the afternoon and evening hours with potential for organized strong to severe storms. Passage of the front will bring a period of drying Saturday before going back into a general summer time pattern where scattered showers and storms will become common during afternoon and evening hours, followed by clearing at night with patchy fog.

RNK WATCHES / WARNINGS / ADVISORIES

VA...None. NC...None. WV...None.