textproduct: Raleigh

WHAT HAS CHANGED

* Heat Advisory has been expanded to cover all of central NC until 8 PM this evening.

* A new Heat Advisory has been issued for Saturday where forecast confidence was greatest. This Advisory will be in effect from 11 AM until 8 PM.

KEY MESSAGES

As of 215 PM Friday...

1) A new Heat Advisory has been issued for Saturday from 11 AM until 8 PM.

2) Scattered to locally numerous showers/storms will be possible through this afternoon into the evening hours.

3) A conditional severe risk persists on Saturday ahead of a cold front.

4) A cold frontal passage Sunday will bring cooler and drier air early next week, but not before some isolated to scattered showers remain possible late this weekend.

DISCUSSION

As of 215 PM Friday...

KEY MESSAGE 1... A new Heat Advisory has been issued for Saturday from 11 AM until 8 PM.

The Heat Advisory was expanded to all of central NC for today until 8 PM. Surface observations across the northern Piedmont up into Virginia have shown heat indices rising into 102 to around 105 and this will likely continue until convective outflow from developing showers/storms cool the area or until 8 PM as nocturnal cooling begins.

A cold front is expected to remain well to our north through the daylight hours on Sat. However, the hot and humid air mass that has plagued the southern Mid-Atlantic and Carolinas this week will likely be convectively contaminated by cloud debris, lingering outflows, and pockets of rain cooled/moistened soils from convection this afternoon. Additionally, A canopy of mid- level convective debris cloudiness from the (probably weakening) remnants of an overnight convective cluster from OH to the Virginias may limit insolation and curb the heat over especially the nrn half of cntl NC on Sat. And if that convective cluster were to maintain intensity more than currently forecast, which they oftentimes do east of the Appalachians, then a more- prominent cold pool/outflow and possible ongoing convection would further limit early day heating there. The airmass will nonetheless have the potential to be as hot (or nearly so) as previous days in the absence of those lower predictability influences from convection/clouds. A Heat Advisory has been issued for Saturday across the southern half of central NC where confidence is greatest that at least two hours of +105 is probable before convection/cloud cover move into the area. This area may need to be adjusted with the overnight forecast update.

KEY MESSAGE 2... Scattered to locally numerous showers/storms will be possible through this afternoon into the evening hours.

The leading edge of a cold front is beginning to work down into northern VA this afternoon, but is expected to stay well north of the forecast area. The primary triggering mechanism for convection is expected to be along a leading pre-frontal trough acting as a low-lvl convergence boundary and is draped near or just south of the VA/NC border. The air mass in place remains hot and humid with temperatures regularly in the upper 90s to around 100 degrees and surface dew points in the upper 60s to low 70s. When combined with slightly cooler 500-700mb temperatures on the northeast side of the lower/mid level subtropical ridge over the southern Appalachians and extending into SC, moderate to high instability is expected (1500- 3500 MLCAPE) around the initiating boundary and locations to its south in central NC. Storm coverage and intensity remains uncertain given the lack of a strong forcing mechanism and minimal mid/upper lvl support from any upstream MCV's given our location relative to the strong ridge.

If deep convection is able to develop, mean cloud-layer winds of 10- 25 kts over the southern Mid-Atlantic oriented about 30 degrees off the initiating boundary should allow for convection to shift off the boundary and track generally east- southeastward. Deep layer shear of 25-35 kts should support storm organization along a common cold pool within small scale bowing clusters, although splitting cells and transient mid-lvl rotation will be possible with short, straight hodographs if storm collisions can remain at a minimum (00z HREF highlights 10-30% of short segments of either right or left moving storms in the 6-11 PM timeframe).

Steep low-lvl lapse rates, high LCLs, and strong DCAPE, will favor strong diabatic cooling within collapsing deep-convection and result in the primary risk coming from damaging straight- line winds, especially within any storm clusters oriented along a common cold pool. Isolated large hail can't be ruled out, but given the hot temperatures, it will likely require an incredibly deep storm and/or some mid-lvl storm rotation to have the hail survive its journey to the surface.

KEY MESSAGE 3... A conditional severe risk persists on Saturday ahead of a cold front.

A decaying, convectively-induced mid-level disturbance over the lower Ohio Valley will interact with a backdoor cold front sliding south through the Mid-Atlantic on Saturday. Deep-layer shear will increase compared to Friday, with mid-level flow strengthening as the East Coast trough digs further west, supporting multicell storms capable of producing damaging wind gusts and large hail. A Slight Risk remains in place for the majority of central North Carolina. This severe threat, however, remains conditional. Upstream decaying convection out of western VA could focus debris cloudiness across our northern tier of counties. Realistically, a gradual thinning of these clouds will take shape through the late morning and early afternoon. A southward moving effective front/outflow may then trigger afternoon and evening convection. Where that convection initiates will depend on conditional factors mentioned, but could be most focused from along and south of US-64 per the latest 12z HREF. WPC has much of the area in an excessive rainfall risk, but this is also conditional. Nevertheless, brief heavy rainfall in some slow- moving storms could favor isolated flash flooding given the anomalous moisture in place.

KEY MESSAGE 4... A cold frontal passage Sunday will bring cooler and drier air early next week, but not before some isolated to scattered showers remain possible late this weekend.

The cold front is expected to push through by Sunday morning. However, the trend since Monday has been for a slow erosion of the low-level moisture, coupled to the fact that a moist northeasterly flow behind the front could favor upslope stratus in the morning and early afternoon. The guidance is now honing in on a weak mid-level impulse diving down out of WV/VA during the day Sunday. Coupled to the still high dewpoints in the upper 60s for most areas, weak instability could support low-topped isolated to scattered showers, especially along and west of the Triangle into the western/southern Piedmont.

Drier air and more seasonal temperatures will arrive in time for Labor Day and persist into Wednesday. Monday and Tuesday should feature much lower humidity, with dewpoints expected to range from the upper 50s to lower 60s, and highs largely in the 80s. Unfortunately, this reprieve from the heat will be brief, with high temperatures expected to rebound into the low-to- mid/upper 90s by Wednesday and Thursday. The ridge is expected to extend from the central US this time late next week as opposed to over the southern Appalachians, which may limit our overall heat risk. And ensembles continue to suggest a late-week front sometime Thu-Fri. There is a large degree of spread, however, in the trough/ridge pattern at this time range, so confidence on storm chances are low, though highest late Thu- Fri.

AVIATION /23Z FRIDAY THROUGH WEDNESDAY/

As of 720 PM Friday...

TAF period: While GSO had gusts as high as 30 kt with a thunderstorm that passed just to the east, other terminals have not been affected by thunderstorms this afternoon, although the potential cannot be ruled out for some gusty winds and restrictions in thunderstorms this evening, with RDU the only one with enough confidence to keep in the forecast. VFR conditions are expected everywhere overnight, with another round of scattered showers and thunderstorms forecast for Saturday afternoon. The coverage of precipitation Saturday afternoon should be greater than it was this afternoon, and confidence was high enough to go with prevailing showers at RDU/RWI along with PROB30s for thunderstorms at INT/GSO/RDU/RWI. While scattered showers are likely at FAY, the most widespread precipitation is likely to reach FAY after 00Z Sunday.

Outlook: The highest chance for showers and thunderstorms will shift to FAY/RWI Saturday evening, with another round of scattered showers/storms on Sunday. Northeast flow behind a backdoor cold front may support areas of IFR-MVFR ceilings Sun morning and Mon morning.

CLIMATE

Records within reach this forecast period:

Record High Temperatures:

September 4: KRDU: 100/1925 KFAY: 100/1944 September 5: KGSO: 97/1954 KRDU: 98/2023 KFAY: 99/1932

Record High Minimum Temperatures:

September 4: KGSO: 72/2018 KRDU: 74/1921 KFAY: 75/2020 September 5: KGSO: 73/2018 KRDU: 74/2012 KFAY: 74/1935

RAH WATCHES/WARNINGS/ADVISORIES

Heat Advisory until 8 PM EDT this evening for NCZ007>011- 021>028-038>043-073>078-083>086-088-089. Air Quality Alert until 8 PM EDT this evening for NCZ021>023- 038-039. Heat Advisory from 11 AM to 8 PM EDT Saturday for NCZ042- 073>078-083>086-088-089.