textproduct: Raleigh

WHAT HAS CHANGED

* An Air Quality Alert has been issued for the Triad counties, including Forsyth, Guilford, Alamance, Davidson and Randolph counties until 8 PM this evening after collaboration with the Forsyth County Office of Environmental Assistance and Protection in Winston-Salem.

* Another Heat Advisory has been issued for Friday from 11 AM until 8 PM.

* Increasing chances for post-frontal showers on Sunday, mainly across the west.

KEY MESSAGES

As of 215 PM Thursday...

1) A Heat Advisory has been issued for Friday 11 AM until 8 PM. Anomalously hot temperatures are expected to persist into Sat at least for a portion of central NC.

2) Storm chances will increase on Friday and peak on Saturday ahead of a cold front. This will be followed by drier weather and a return to more seasonal temperatures early next week.

DISCUSSION

As of 215 PM Thursday...

KEY MESSAGE 1... A Heat Advisory has been issued for Friday 11 AM until 8 PM. Anomalously hot temperatures are expected to persist into Sat at least for a portion of central NC.

A lower mid-latitude/sub-tropical ridge centered over the srn Appalachians, observed around 595 dam and 324 dam in the 500-700 mb layer this morning, will persist in a favorable location for excessive heat in cntl NC for the next couple of days.

Persistence surface temperatures, to 1-2 F hotter ones especially over the nrn Piedmont, will likely result today and combine with mixed/afternoon dewpoints in the upr 60s to lwr 70s. Related heat index values will range between 100-105 F over the nw Piedmont to mostly 105-108 F in the Advisory area. Any convective chances continue to decrease throughout mid-afteroon to provide any relief from dangerous heat.

Mean mixing ratios and surface dewpoints are forecast to increase slightly on Fri, when mixed/afternoon dewpoints are likely to be in the lwr 70s. That uptick in dewpoints will likely yield the highest heat index values of this stretch on Fri, when values near 110 F will be possible from the nrn Sandhills to the ne Piedmont and Coastal Plain. As such, a Heat Advisory has been issued for Fri (11 AM until 8 PM). The Heat Advisory for today, Thurs, remains in effect until 8 PM this evening. Additional Heat Advisories will likely be needed, but perhaps over a smaller area, on Sat.

KEY MESSAGE 2... Storm chances will increase on Friday and peak on Saturday ahead of a cold front. This will be followed by drier weather and a return to more seasonal temperatures early next week.

Mid to upper-level northwest flow will increase Friday and peak on Saturday. The anomalous ridge will gradually be shunted westward by Saturday as an East Coast trough becomes more dominant. This pattern shift will increasingly favor showers and thunderstorms through the first half of the weekend.

Friday: The dangerous heat dome will remain in place. Model guidance indicates a morning disturbance tracking across the Great Lakes and northern Mid-Atlantic, which may sustain ongoing early-day convection. By the afternoon and evening, several 12z HREF members initiate new storms along the southern Virginia border, spreading into northern portions of central and eastern North Carolina. Organization may be enhanced by an upstream disturbance, a surface trough, or lingering outflow boundaries. These storms could grow upscale as they track east-southeast into the Piedmont and Coastal Plain through the mid-to-late evening. Damaging winds and large hail are the primary hazards. A Slight Risk for severe weather remains in place for much of the northern Piedmont, the Triangle, and the central/northern Coastal Plain, with a Marginal Risk covering the remaining southwestern and western areas.

Saturday: The greatest coverage of showers and thunderstorms is expected on Saturday. A well-defined, convectively-induced mid- level disturbance over the lower Ohio Valley will interact with a backdoor cold front sliding south through the Mid-Atlantic. Deep-layer shear will increase compared to Friday 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 all of central North Carolina. Ahead of this convective activity, expect another day of dangerous heat.

Sunday & Beyond: The cold front is expected to push through by Sunday morning. While post-frontal stratus, lingering moisture, and upslope flow could support isolated to scattered showers across the western Piedmont, the severe threat will have greatly diminished. Drier air and more seasonal temperatures will arrive in time for Labor Day and persist into Tuesday. Unfortunately, this reprieve from the heat will be brief, with high temperatures expected to rebound into the low-to-mid 90s by Wednesday and Thursday. While some ensemble guidance hints at another frontal passage and a renewed chance for precipitation late next week, confidence remains low at this range.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

As of 150 PM Thursday...

Prevailing VFR conditions are expected to persist through the 24 hour TAF period. An isolated sprinkle can't be ruled out around RDU, FAY or RWI today, but widespread or even scattered deep convection is not expected. Mostly shallow convection continuing to develop along the VA mountains may continue build to shift south to southeast. Latest time-of-arrival calculations and available hi-res guidance keeps these showers/storms north of the forecast area until after sunset, with an expected weakening trend 1-2 hours thereafter. With probs of any restrictions from convection decreasing around RWI, have opted to remove the PROB30 to later assess with the 00z TAFs.

Outlook: Deepening/richer low-level moisture by Fri will favor the development of areas of fog and low stratus especially over the ern Carolinas Fri morning, including near and probably just southeast of FAY. Scattered convection will again focus from near RWI and possibly slightly swd across the Piedmont Fri afternoon and evening, then throughout cntl NC with the approach and passage of a backdoor cold front Sat afternoon-evening with accompanying post-frontal stratus.

CLIMATE

Records within reach this forecast period:

Record High Temperatures:

September 3: KRDU: 99/2010 KFAY: 101/2010 September 4: KRDU: 100/1925 KFAY: 100/1944 September 5: KGSO: 97/1954 KRDU: 98/2023 KFAY: 99/1932

Record High Minimum Temperatures:

September 3: KGSO: 73/2020 KRDU: 78/1933 KFAY: 77/2020 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- 023>028-040>043-074>078-083>086-088-089. Heat Advisory from 11 AM to 8 PM EDT Friday for NCZ011-024>028- 040>043-074>078-083>086-088-089. Air Quality Alert until 8 PM EDT this evening for NCZ021>023- 038-039.