textproduct: Raleigh

WHAT HAS CHANGED

* Another Heat Advisory has been issued for Sunday for much of south central NC plus the Triangle. But morning stratus could limit strong heating in some areas tomorrow.

* Scattered strong to severe thunderstorms remain possible through this evening, with a focus on the W and S Piedmont and far SE.

* Confidence increasing that Mon will be the hottest day of this stretch, with several 100F air temps possible.

KEY MESSAGES

As of 315 PM Saturday...

1) Another Heat Advisory has been issued for parts of the Triangle and south central NC for Sun. The hot weather will continue into mid week, peaking on Monday when most of the area will have heat index values near or over 105F, and our southeast could see 110F or higher.

2) A Marginal (level 1 of 5) risk of severe thunderstorms continues through this evening over all of central NC, and over the north Sunday. Damaging winds are the primary threat.

DISCUSSION

As of 315 PM Saturday...

KEY MESSAGE 1... Another Heat Advisory has been issued for parts of the Triangle and south central NC for Sun. The hot weather will continue into mid week, peaking on Monday when most of the area will have heat index values near or over 105F, and our southeast could see 110F or higher.

Strong heating across southern portions of central NC along with high dewpoints have pushed heat index values to around 105F today, while over NE and far N sections, this morning's sct-bkn stratus curbed heating a bit, keeping heat indices in the 90s to around 100F. These clouds were along and NE of a surface boundary evident in recent analyses stretching over NW into E NC, with slightly more stable air noted earlier today NE of this boundary. This boundary should wash out and dissipate late tonight, putting us back into a light flow from the SSE to SW Sun and pushing high temps well into the 90s most areas. Despite some modest mixing of surface dewpoints Sun afternoon, readings should still hold in the low-mid 70s, with some upper 70s persisting over parts of the Coastal Plain, and this will result in heat index values peaking around 105 to 109 over much of our southern half and Triangle region, where heat island effects could further boost local temps. We've issued another Heat Advisory for Sun for this area. But there is some uncertainty with this, as our confidence is increasing that we'll have areas of low stratus around the area Sun morning, not dissipating substantially until late morning or the noon hour, which could put a damper on strong heating in some areas. If this occurs, heat index values could be a bit lower than expected, although this could be balanced by less mixing of surface dewpoints. Otherwise, thicknesses are projected to be around 10 m above normal, so highs well into the 90s remain plausible.

This hot stretch is likely to be its worst on Mon, when highs are expected to reach the mid 90s to around 100F areawide, with elevated dewpoints persisting, pushing the heat index over 105F over all but the far N, and perhaps over 110F over our far SE. If confidence in this continues to grow, additional Heat Advisories will be needed, and an Extreme Heat Watch or Warning may be considered in some areas for Mon.

Temps will remain above normal with high humidity through mid week, but with slightly lower intensity. The mid level ridge to our S will gradually retrograde in response to a southward progression of the flat NW flow from the Midwest through VA/NC, nudging thicknesses down a bit, although our SW areas may stay atypically hot into late week.

KEY MESSAGE 2... A Marginal (level 1 of 5) risk of severe thunderstorms continues through this evening over all of central NC, and over the north Sunday. Damaging winds are the primary threat.

For another day, we're in a regime featuring high PW aoa 2" and mdt- high SBCAPE of 1500-3000 J/kg, highest in the S and far W, but with modest mid level lapse rates and poor deep layer bulk shear, particularly in the area of highest CAPE, resulting in low overlap of the most favorable CAPE/shear. Predictability of precise location and timing of convection remains somewhat low beyond the first 24 hours, as the primary features that are focusing convection remain multiple weak outflows from upstream convective systems, along with the sea breeze and terrain-favored storms rolling into the W Piedmont. The sub-20-kt deep layer shear will continue to limit storm organization and longevity, but a few strong storms producing water-loaded downdrafts remain possible through mid evening. This overall regime will hold for Sun as well, primarily (but not solely) across the north as outflows drift in from the NW and N, and where PWs are expected to be a bit higher as compared to our SW sections.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

As of 800 PM Saturday...

Rich low-lvl moisture within the northeast flow in the lowest 100mb remains over the area after being deposited earlier this morning. As the boundary layer cools, widespread IFR/LIFR stratus is expected, and persisting longest around RDU/FAY. On the periphery, there is less forecast confidence in onset and duration as drier air filters in off the western Atlantic in the east, and westerly weakly downsloping flow in the west. Slow rise in cigs is expected in the heart of the stratus coverage Sun morning while quicker erosion is expected on the periphery. Scattered showers/storms are possible, with highest confidence around RDU/RWI between 21z and 03z.

Looking beyond 18z Sun, scattered late-day showers and storms are possible again Mon, with the better chances then shifting to just south and southeast areas for Tue-Thu. Outside of storms, generally VFR conditions are expected, although a few hours of patchy early- morning sub-VFR fog/stratus may occur each day, with the highest chances in areas that see rain.

CLIMATE

Record High Temperatures:

August 17: KRDU: 99/1997 KFAY: 104/1954 August 18: KFAY: 98/1988

Record High Minimum Temperatures:

August 15: KGSO: 77/1995 KRDU: 77/1995 KFAY: 78/2017 August 16: KGSO: 75/2016 KRDU: 76/2007 KFAY: 78/2005 August 17: KGSO: 75/2010 KRDU: 76/2024 KFAY: 78/2007 August 18: KGSO: 75/2017 KRDU: 77/1995 KFAY: 78/2021

RAH WATCHES/WARNINGS/ADVISORIES

Heat Advisory from 11 AM to 8 PM EDT Sunday for NCZ025-040>042- 073>077-083>086-088-089.