textproduct: Raleigh

WHAT HAS CHANGED

* Another Heat Advisory has been issued for Monday for southern and eastern portions of central NC.

* The chance for a few strong to severe storms has increased for much of the Piedmont of NC for Monday.

KEY MESSAGES

As of 245 PM Sunday...

1) A Heat Advisory remains in effect today for parts of the Triangle and south-central NC, and we've issued another Heat Advisory for the Triangle to the S and E for Mon. The hot weather will continue into mid week, will gradually lowering heat indices mid through late week.

2) The threat for a few strong to severe storms continues through Mon. We have a Marginal (level 1 of 5) risk of severe thunderstorms across the northern half of central NC for the rest of today, and a Slight (level 2 of 5) risk in the Piedmont on Mon. Damaging winds are the primary threat.

3) Rain and thunderstorm chances will increase from late Thursday through the weekend as a slow-moving cold front stalls over the region, though exact timing and impacts remain uncertain.

DISCUSSION

As of 245 AM Sunday...

KEY MESSAGE 1... A Heat Advisory remains in effect today for parts of the Triangle and south-central NC, and we've issued another Heat Advisory for the Triangle to the S and E for Mon. The hot weather will continue into mid week, will gradually lowering heat indices mid through late week.

Heating has been curbed somewhat today by this morning's areas of low clouds as well as the early-onset of convection across parts of the Sandhills and far N CWA. But several locations have still managed to reach a 105F heat index today, as pockets of strong heating combine with high dewpoints in the mid-upper 70s. We should see a bit less cloudiness Mon morning, and with low level thicknesses expected to reach 20 m above normal, highs should reach the mid 90s to near 100F. The small bit of good news is that the dewpoints tomorrow are expected to be slightly lower than today, dipping into the 70-76 range with mixing in the afternoon, with these lower readings in the west, and this should help keep heat indices from reaching criteria for an Extreme Heat Warning, although certainly this will be assessed further tonight and monitored closely on Mon. But a Heat Advisory is warranted for all but NW and VA border portions of the CWA, with heat index values expected to reach 105-109F. NWS Heat Risk, which is based on how unusual and/or long-lived this heat is, as well as on past CDC heat-illness data, reaches a level 3 (out of 4) nearly everywhere Mon, suggesting that this heat could be particularly dangerous.

Temps may be a bit less hot on Tue, but still elevated over the S and E CWA, where the long duration of this hot stretch will add to the already-high heat stress.

KEY MESSAGE 2... The threat for a few strong to severe storms continues through Mon. We have a Marginal (level 1 of 5) risk of severe thunderstorms across the northern half of central NC for the rest of today, and a Slight (level 2 of 5) risk in the Piedmont on Mon. Damaging winds are the primary threat.

A few strong to severe storms remain possible today and Mon. Deep layer bulk shear remains poor today, under 20 kts, which will limit the intensity and duration of any updraft. But SBCAPE has reached 2500-4000 J/kg thus far (according to RAP-based mesoanalysis) with a DCAPE over 800 J/kg much of the area, highest SW, and with PWs still around 2" or up to 130%-140% of normal, isolated water-loaded downdrafts remain possible, leading to localized strong to severe gusts into early this evening. On Mon, the SBCAPE may be a bit lower, but our deep layer shear is expected to be around 25 kts, with added forcing for ascent provided by an approaching MCV expected later in the day from the NW. In particular, convection over the NC mountains may maintain strength with better organization as they track ESE into the NC Piedmont, again with damaging winds as the primary threat, from mid afternoon through mid evening.

Storm coverage is expected to decrease and push to our SE sections on Tue, as a slug of drier air with lower PW spreads in from the WNW.

KEY MESSAGE 3... Rain and thunderstorm chances will increase from late Thursday through the weekend as a slow-moving cold front stalls over the region, though exact timing and impacts remain uncertain.

As we head into late week, a stronger cold front is expected to move into the region. Forecast guidance shows precipitation chances increasing late Thursday ahead of a slow moving trough. The frontal boundary is expected to stall over the region before eventually washing out. This lingering boundary will act as a focus for daily rain and thunderstorm chances returning Friday afternoon/evening and persisting through the weekend. Given how far out this is, there are still several details to look into. Will closely monitor forecast trends over the coming days for any adjustments in the timing, amounts, and exact placement of this frontal boundary which will then result in a better idea for how isolated or widespread precip chances will be over the weekend.

AVIATION /06Z MONDAY THROUGH FRIDAY/

As of 200 AM Monday...

24-hour TAF period: Mainly VFR conditions will prevail across central NC through this morning. Guidance continues to suggest patchy fog can't be totally ruled out around 09z-12z, so retained the TEMPO groups for MVFR visibilities. This could be most likely at INT and GSO where some rain fell over the last couple hours. However, most locations should stay VFR. The cluster of showers and storms that did produce the rain at the Triad sites earlier this evening is moving east but diminishing considerably, and dry conditions should also largely prevail overnight, with calm to very light SW winds.

A complex of showers and storms will move into the NW Piedmont in the late afternoon or early evening hours on Monday, potentially spreading east through the rest of the evening and early overnight hours. However, high-res guidance generally shows the convection weakening with time especially once daytime heating is lost. The best chance for gusty winds and sub-VFR restrictions is at INT/GSO, followed by RDU and then RWI. It appears likely the showers and storms will stay north of FAY, so don't have any mention in the TAF there, but it still can't be entirely ruled out. Outside of any storms, WNW winds may still gust up to 15-20 kts at times in the afternoon.

Looking beyond 06z Tue: The better chances of storms should shift to mainly south and southeast of the terminals for Tue-Thu. Higher rain chances are expected areawide Fri ahead of an approaching upper level low pressure trough. 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: KGSO: 99/1988 KRDU: 99/1997 KFAY: 104/1954

August 18: KFAY: 98/1988

August 20: KFAY: 102/2007

Record High Minimum Temperatures:

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

August 19: KRDU: 77/1988 KFAY: 76/1940

August 20: KRDU: 75/2007 KFAY: 78/2005

August 21: KRDU: 76/2009 KFAY: 77/2005

August 22: KFAY: 75/2018

RAH WATCHES/WARNINGS/ADVISORIES

Heat Advisory from 11 AM to 8 PM EDT Monday for NCZ024>028-040>043- 073>078-083>086-088-089.