textproduct: Raleigh

WHAT HAS CHANGED

* No major changes.

KEY MESSAGES

As of 300 PM Wednesday...

1) Hazy skies from Canadian wildfire smoke mainly over the northeast Piedmont and a portion of the Coastal Plain, and otherwise hot, but with low afternoon RH the next couple days

2) Increasing moisture will fuel potentially hazardous heat Fri and Sat as well as isolated diurnal convection until an effective front brings more appreciable chances for storms on Sun.

DISCUSSION

As of 300 PM Wednesday...

KEY MESSAGE 1... Hazy skies from Canadian wildfire smoke mainly over the northeast Piedmont and a portion of the Coastal Plain, and otherwise hot, but with low afternoon RH the next couple days

Wildfire smoke originating in srn ON and nrn MN will manifest as continued hazy skies and apparent high-level ceilings mainly over the ne Piedmont and a portion of the Coastal Plain for the next couple of days. Aside from a minimal restriction to visibility (eg. 7-9SM) around the Richmond Metro and Hampton Roads, there have been no observed visibility restrictions from the portion of the smoke plume now maximized over nern NC and the Virginias. Given these observational trends and also HRRR and RRFS simulations of near- surface smoke, no appreciable restrictions to surface visibility are expected at the latitude of cntl NC at this time.

It will otherwise turn hotter, though perhaps tempered slightly by wildfire smoke, as a mid-level ridge centered over the OH Valley progresses across and offshore the srn Middle Atlantic, while weakening. Related subsidence and adiabatic warming will add another few degrees C to the observed 850 mb temperature of 19C at GSO this morning. That warming would translate to high temperatures in the upper 90s to around 100 F, assuming no appreciable impact from wildfire smoke to insolation/diabatic heating. Based on surface observational trends this afternoon beneath the aforementioned densest portion of the smoke plume now over VA and nern NC, there may be a couple of degree impact to surface temperatures over the ne Piedmont and a portion of the Coastal Plain on Thu.

Both the 12Z-observed soundings at GSO and MHX also sampled a layer of markedly dry air between 900-800 mb, with dewpoint depressions of 30-40C, atop a shallow, surface moist layer and with related mean mixing ratios of 10-11 g/kg. Surface dewpoints today have consequently "mixed out" through the 60s, with some 50s F noted over the Piedmont; and this process will result again on Thu, though with surface dewpoints probably a few degrees higher owing to a slight uptick in forecast mean mixing ratios.

As such, heat index values should be tempered on Thu and remain mostly in the 100-105 F range and shy of Advisory criteria of 105 F or greater for two consecutive hours.

KEY MESSAGE 2... Increasing moisture will fuel potentially hazardous heat Fri and Sat as well as isolated diurnal convection until an effective front brings more appreciable chances for storms on Sun.

The mid-level ridge and area of associated subsidence from the Ohio Valley into the Carolinas begins to break down late this week and will provide an opportunity for increasing surface moisture with upper 60s to around 70 dew points during peak heating Fri and Sat. Convective coverage may remain isolated at best until the late afternoon/evening hours as the upper-lvl anticyclone remains positioned over the Ohio Valley on Fri and over the Carolinas on Sat. If surface moisture is able to remain elevated with limited dry air aloft to tap into during diurnal heating, air temperatures in the upper 90s to around 100 would support heat indices in the 105- 109 range for consecutive hours, primarily across the eastern Piedmont, Sandhills and Coastal Plain. Although some acclimation is most likely beginning to set in, individuals especially sensitive to the heat as well as anyone partaking in strenuous activity during peak heating will be susceptible to heat related illness if precautions are not taken.

Our perhaps better chance for more scattered to locally numerous showers/storms comes Sat night into Sun. A shortwave trough progressing through Ontario and trailing trough axis sliding across the Mid-Atlantic into the Carolinas will likely provide weak synoptic forcing along with any embedded convective perturbations to drive precipitation chances. Most likely 24 hour rainfall totals range from minor amounts to around 0.75" through Mon morning (greatest in the east). However, model guidance is hinting at lingering surface high pressure over the northwestern Gulf as well as an area of low pressure that may develop tropical characteristics in the northeastern Gulf during this time. Both of these features may prevent Gulf moisture from lifting through the Southeast and into the Carolinas, which could limit rainfall amounts in magnitude and aerial coverage.

AVIATION /00Z THURSDAY THROUGH MONDAY/

As of 700 PM Wednesday...

24 hour TAF period: Generally expect VFR conditions to prevail through the TAF period. Winds will mainly be swly-sswly through Thu, light overnight, then increasing to 5-10 kts across the ern terminals Thu aft/eve. Wildfire smoke originating in srn ON and nrn MN will manifest as continued hazy skies and apparent high-level ceilings mainly from RDU and RWI and points newd for the next couple of days. No restrictions to surface visibility are expected at the latitude of cntl NC at this time.

Outlook: Increasing low-level moisture/humidity levels this weekend will support a chance of stratus/mist/fog and showers/storms, with the latter favored on Sun.

CLIMATE

Record temperatures within 3F of forecast ones...

Record Highs:

July 16: KGSO: 99/1915 KRDU: 101/1887

July 17: KRDU: 100/1887 KFAY: 102/1932

Record High Mins:

July 17: KGSO: 75/2025 KFAY: 77/2019

July 18: KGSO: 76/1986 KFAY: 77/2025

July 19: KGSO: 75/2016 KFAY: 77/2023

July 20: KRDU: 76/2025

RAH WATCHES/WARNINGS/ADVISORIES

None.