Area Forecast Discussion

WHAT HAS CHANGED

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

KEY MESSAGES

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.

2: Above normal temperatures expected through Sunday, warmest readings today and 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.

This morning: Nothing significant before noon. May see some morning showers along the western slopes of the Appalachians where westerly upslope flow and mid level instability may pop off a few showers around daybreak.

This afternoon: There is a slight risk of severe storms along and north of highway 460 and a marginal risk south of 460 into the piedmont of North Carolina.

Environment will support strong to severe storms this afternoon, capable of wind damage. The convective available potential energy (CAPE) is forecast near 2500 j/kg across much of the CWA. Challenge will be to overcome the downslope westerly wind and breaking the cap. Mid level warming is forecast with 700 mb temps increasing to about 11 deg C. This capping inversion and downslope west wind will have to be overcome in order to release the CAPE. The forecast basically hinges on getting enough lift to punch through the cap, the difference being a hot/dry forecast or "severe clear" vs. actually developing a healthy strong to severe thunderstorm.

Watching two areas where enough lift may overcome the cap... the obvious being a shortwave disturbance and associated MCS passing across the Ohio Valley...the energy forecast to move east or downstream into the northern Mid-Atlantic. The outflow from this MCS would be the the most likely place for new thunderstorm initiation as it propagates east into WV/PA this afternoon...our northern CWA falling under its influence. Another area to watch will be the lee trough, east of the appalachians where the westerly downslope wind encounters a more southerly induced and more moist wind field over the piedmont. This feature may also provide enough convergence to trigger isolated storms there, favoring our NC/VA piedmont counties more so toward evening...5PM-10PM time frame.

Though shear will be generally weak, moderate to strong instability will support strong updrafts coinciding with an environment that will also support moderate to strong downdrafts from DCAPE that is forecast to be AOA 1000 j/kg...ingredients for damaging wind. Weak shear also suggests slower storm cell moving and with PWATs above 1.50 the storms will also be rain efficient and capable of localized flash flooding. Friday: Anticipating better forcing to go along with another day of moderate to strong instability. A cold front will act as the lifting mechanism for storms Friday, arriving late in the day and during a time of peak heating. The front is forecast to advance southeast across the Ohio Valley and into the Mid- Atlantic and merge with the lee trough east of the Appalachian mountains late in the day. Storms are expected to be more organized with a mix of multicells and line segments with the primary hazard being damaging wind gusts and marginally severe hail.

Key Message 2: Above normal temperatures expected through Sunday, warmest readings today and Friday.

West winds today and Friday will lead to adiabatic warming, a process which compresses and warms the air as it passes over the mountains. This will be in addition to the increasing warm air advection aloft. 85H temperatures are forecast near 21 deg C both days. With less cloud cover, this should allow surface temperatures to become quite warm east of the mountains with highs in the mid 90s today and possibly the upper 90s Friday. Readings (although not as oppressive) are expected to reach well into the 80s across the mountains and near 90 in the New River Valley.

These high temperatures combined with dewpoint temperatures in the upper 60s to near 70 degrees across the region will lead to Heat Index values in the 100-105 range across much of the Piedmont on today and Friday. These Heat Index values are just below heat headlines for the Piedmont of Virginia and North Carolina; nevertheless, folks should still consider taking frequent breaks, moving indoors or the shade, as well as drinking plenty of fluids.

While a frontal passage is expected Friday, temperatures do not look to decrease that much on Saturday or Sunday; however, dewpoint temperatures will decrease significantly enough that Heat index values will be closer to actual temperatures for the area. The lower dewpoints should make it feel less muggy, which will in turn provide some relief from the heat.

AVIATION /12Z THURSDAY THROUGH MONDAY/

Patchy fog through sunrise otherwise VFR. There may be some isolated showers along the western slopes of the Appalachians this morning but they are expected to remain confined to the windward slope.

Looking at a near full day of sunshine with temperatures warming into the 90s east of the mountains. Instability may lead to some isolated thunderstorms late in the afternoon along and east of the mountains. A more organized area of thunderstorm activity is expected to develop across the northern Mid-Atlantic this afternoon with the potential for strong to severe storms to occur mainly across areas north of Roanoke/Lynchburg. That said, any storm that develops this afternoon will contain strong updrafts and downdrafts due to the moderate to strong instability that is being forecast (CAPES of 2500 J/KG).

EXTENDED AVIATION OUTLOOK

A cold front will cross the area Friday with potential for organized strong to severe storms during the afternoon and evening. 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.