textproduct: Raleigh

WHAT HAS CHANGED

* There is increased confidence for cold air damming and below average temperatures during the latter half of the weekend, but the extent of sensible weather impacts will be influenced by the timing and likelihood of steadiest and heaviest rain that remains more uncertain.

KEY MESSAGES

As of 300 AM Wednesday...

1) Patchy dense fog and low clouds will reduce visibility early this morning across portions of central NC, with the lowest visibilities and most persistent fog focused across the Sandhills and Coastal Plain.

2) Temperatures will trend well above average through Friday, with unseasonably hot, but not humid, afternoons.

3) Saturday will mark a transition to cloudy, much cooler, and periods of wet conditions that will continue through the weekend.

DISCUSSION

As of 300 AM Wednesday...

KEY MESSAGE 1... Patchy dense fog and low clouds will reduce visibility early this morning across portions of central NC, with the lowest visibilities and most persistent fog focused across the Sandhills and Coastal Plain.

Calm winds and high low level moisture early this morning will help fog and low cloud development through daybreak. While patchy fog is possible farther west in the Piedmont, the thickest fog and lowest cloud cover will settle over eastern areas. Visibilities across the Coastal Plain could drop to a mile or less at times during the morning commute, creating localized travel delays. Morning sunlight will quickly burn off the moisture, returning clear skies to the entire region by mid morning.

KEY MESSAGE 2... Temperatures will trend well above average through Friday, with unseasonably hot, but not humid, afternoons.

The models indicate a mid-level sub-tropical ridge over the srn Appalachians will continue to strengthen while progressing across and offshore the Carolinas during the next couple of days.

At the surface, a Canadian high and continental airmass now centered over the Middle Atlantic will progress offshore today, then extend from near Bermuda to the South Atlantic coast through Fri. Sly flow around the Bermuda high, and subsidence beneath the ridging aloft, will contribute to warming temperatures through the week and what will likely be the last few days of summerlike temperatures of the year. While hotter, and gradually more humid/muggy by night, moisture return will be shallow and susceptible to vertical advection/mixing of lingering dry/continental within the ridge axis over NC, such that mixed/ afternoon dewpoints will probably only increase from mid/upr 50s today to mostly mid 60s by Fri (with afternoon RH in the 30s to 40s percent).

KEY MESSAGE 3... Saturday will mark a transition to cloudy, much cooler, and periods of wet conditions that will continue through the weekend.

A positively-tilted srn stream trough/shear axis will progress slowly across the Southeast and Carolinas this weekend, just downstream and equatorward of a series of prominent shortwaves that will progress through a broad trough that will become centered over ern Canada.

At the surface, high pressure extending from near Bermuda to the South Atlantic coast will be displaced by a wavy frontal zone forecast to settle swd and across cntl NC on Sat (~3-6 hours faster than previous model cycles). Following, progressive but favorably strong and located high pressure for cold air damming initially over the Great Lakes at 1028 mb, will ridge swd across the Middle Atlantic and Carolinas this weekend.

Given both earlier diurnal timing of the frontal passage on Sat (prior to peak heating) and otherwise limited low-level moisture and mid-level lapse rates, instability will be weak and unsupportive of deep convection. A band of frontal showers and areas of light, anafrontal rain will instead probably result Sat. A better chance of steadier rain will result Sat night-Sun in cntl NC, when and where sly and strengthening 850 mb flow will maximize isentropic lift atop the CAD surface ridge. However, the lack of any substantial perturbations forecast within the aforementioned srn stream trough axis suggests frontal wave development will be limited, as will the risk of any heavy rain. The isentropic upglide regime and saturated CAD airmass will nonetheless favor periods of at least light rain and drizzle late Sat through the weekend. As confidence in that pattern (probably) increases in the coming days, forecast high temperatures and diurnal ranges for the weekend will probably require substantial adjustments downward from currently forecast ones. In fact, if the previously noted pattern occurs, temperatures on Sun will probably be similar to the 60s F that occurred in the recent cold air damming episode on the 23rd.

A reinforcing cold front and following cP/Canadian surface high will progress across the srn Middle Atlantic early next week.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

As of 645 AM Wednesday...

24 hour TAF period: Patchy fog mainly across the Northern Piedmont and Coastal Plain will lift mid to late morning. After that, VFR conditions are expected for the rest of the day across Central NC. Another round of sub VFR conditions are expected late tonight early Thursday morning, mainly at the eastern Terminals.

Outlook: VFR conditions are likely to hold through at least Friday, though areas of sub VFR fog and stratus remain possible each morning (especially across the eastern terminals, with the highest potential early Friday morning). The chance for showers and thunderstorms producing sub VFR conditions will increase Saturday and remain elevated through most of Monday as a cold front approaches and sweeps across the area.

CLIMATE

Record High Temperatures:

September 30: KRDU: 89/1954

October 1: KGSO: 90/1986 KRDU: 90/1986

RAH WATCHES/WARNINGS/ADVISORIES

None.