textproduct: Raleigh

WHAT HAS CHANGED

* No significant changes from the earlier forecast, but confidence is increasing in a threat for localized heavy rainfall today and Sat.

DISCUSSION

As of 315 AM Friday...

KEY MESSAGE 1... Another round of slow-moving storm clusters will bring a risk for pockets of heavy downpours and flooding today. Isolated damaging wind gusts are also possible. High rain chances will extend into Sat.

The moist and unsettled pattern will hold through late Sat, as baggy mid level troughing holds from Quebec down through W NC into the NE Gulf, sandwiched between two anticyclones, one centered over NM and another centered E of Bermuda. Numerous factors, those in place and those noted by model guidance, are indicated today that support having sufficient lift, instability, moisture, and slow cell motion for a threat of localized heavy downpours. First, large scale forcing for ascent will be provided by small but multiple perturbations aloft tracking up the E side of the mean trough axis and through central NC, as well as by the presence of a sharpening upper jet's RRQ over our area today, boosting upper divergence. Second, while morning clouds should somewhat hinder strong heating today, the warm boundary layer and high humidity will allow for moderate afternoon buoyancy, with SBCAPE reaching 1000-2500 J/kg, albeit with small deep-layer bulk shear. Third, the column will remain quite moist with PWs of 1.75-2.25", or 110-130% of normal, highest E of Hwy 1, and the LCL-0C depth will be aoa 4 km, favoring efficient warm rain processes. And fourth, weak winds from the surface up through the mid levels will lead to slow and meandering cell motions, with the potential for storm clusters to sit over the same area along with training cells. All of these factors support pockets of heavy downpours and high localize rain totals today. The expected limited coverage of heavy rain clusters precludes consideration of a flood watch, but at least a few flood advisories may be needed. Rain chances will remain elevated through Sat as we stay on the E side of the mean trough, but as PWs dip slightly and shift E into coastal sections with a decrease in upper divergence, the coverage of heavy downpours should be a bit lower and focused more on the Coastal Plain. Pops head back down by Sun and especially next week, as the mid-upper level shear axis just to our W pivots clockwise to a W-E orientation and pushes to our S, allowing mid- upper ridging to build from the mid Miss Valley and lower Ohio Valley through the Mid Atlantic.

Regarding the potential for strong storms, a few wet microbursts will be possible today, when SPC has us in an area of Marginal Risk (level 1 of 5) for severe storms. Again, the bulk shear won't be especially strong, but the moderate CAPE and possible dynamic contributions from upstream MCVs and RRQ upper divergence could tip the scales to a few mature storms this afternoon and evening.

KEY MESSAGE 2... A hot start to September is expected across central NC.

After one slightly cooler than normal day on Saturday (highs in the lower-to-mid-80s), a warming trend will commence on Sunday that continues into late next week. This is due in large part to a strong ~595 dam mid-level ridge initially centered over the Southern Plains that shifts east into the TN Valley and Southeast US. At the surface, a Bermuda High will bring warm SW flow to the region through the period. With mostly sunny skies and very low (if any) precipitation chances, heat will be a concern across central NC as we start the workweek and especially by mid to late week.

By Monday, forecast highs are mainly in the lower-90s, with mid-to- upper-90s by Wednesday and Thursday. The one silver lining is dew points don't look overly oppressive and are forecast to mix out to the upper-60s to lower-70s each afternoon. Still, with the exception of the far northern and northwestern Piedmont, this is enough for heat indices in the lower-to-mid-100s on Wednesday and Thursday, which is when the WPC Experimental HeatRisk is in the Major to Extreme categories for much of the region. This means the heat is expected to affect anyone without adequate cooling/hydration. Heat Advisories may eventually be needed for a portion of the area, especially given the expected prolonged nature of this heat and overnight lows only dropping into the lower-to-mid-70s.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

As of 630 AM Friday...

Central NC terminals will see periods of adverse aviation conditions over the next 24 hours, especially through mid morning, from mid afternoon through evening due to showers/storm activity, and after 07z tonight.

Areas of IFR conditions have passed over much of central NC overnight, including all TAF sites, where vsbys and cigs have varied widely and frequently overnight, and this will persist into mid morning, with most places trending to predominantly VFR in the 14z- 15z time frame. The mostly dry morning should ramp up to high chances of showers and storms by mid afternoon, developing first in the SE and expanding to the N and NW in late afternoon and early to mid evening. These clusters of storms will be slow-moving, with high potential for spots of torrential downpours through mid evening before coverage slowly diminishes after 04z. Periods of MVFR conditions and briefly IFR conditions are possible in and near any storms. A short period of VFR conditions are expected during the late evening with the decrease in shower/storm coverage, however another round of MVFR to IFR cigs/vsbys is expected to develop tonight, starting after 06z. Surface winds at all sites will remain light, under 8 kts, mainly from the SW.

Looking beyond 12z Sat, sub-VFR conditions will remain likely through much of Sat morning, with a slow lifting of low cigs and patchy fog. Another day of high shower/storm chances is expected Sat afternoon and evening, greatest coverage near RDU/RWI/FAY, followed by another round of sub-VFR conditions Sat night into Sun morning. Best rain chances will be in the E and SE Sun afternoon (RWI/FAY), then as upper level high pressure builds in from the W, rain chances will decrease for Mon/Tue, with lower chances for overnight/morning stratus/fog and hotter temps.

RAH WATCHES/WARNINGS/ADVISORIES

None.