textproduct: Raleigh

WHAT HAS CHANGED

* A Frost Advisory has been issued for roughly the northern half of central NC for Tue morning, after patchy to areas of frost result in the usual cold spots this morning.

KEY MESSAGES

As of 330 AM Monday...

1) Patchy to areas of frost will result in the usual cold spots this morning, then again for much of the Piedmont and nrn Coastal Plain Tue morning.

2) Weather parameters will be supportive of Increased Fire Danger today, with brief Red Flag conditions possible over the northern Piedmont and northern Coastal Plain.

3) Temperatures will be above normal through the extended forecast period. Generally dry through the end of the work week, with the next chance of rain over the weekend.

DISCUSSION

As of 330 AM Monday...

KEY MESSAGE 1... Patchy to areas of frost will result in the usual cold spots this morning, then again for much of the Piedmont and nrn Coastal Plain Tue morning.

Downslope drying over the Piedmont, of a Pacific airmass and 1026 mb surface high centered over the Deep South and Southeast, has combined with calm within the ridge to support excellent radiational cooling this morning. Indeed, temperatures in the usual cold spots over the Piedmont are already supportive of frost at this hour.

A reinforcing, polar cold front, now stretching across the nrn Middle Atlantic and lwr OH Valley, will progress swd and across cntl NC late this afternoon through evening - a few hours slower than previously forecast. The slower frontal progression and passage will yield delayed CAA and decoupling potential, as the center of a following, 1027 mb cP high over the upr MS Valley settles over VA at around 1030 mb by 12Z Tue. A blend of statistical guidance, favored in radiational cooling regimes, has trended slightly less chilly in recent days but still well below the warm-biased NBM in such regimes. Nonetheless, a few hours of decoupling and calm to very light nely stirring nearest the center of the high over VA (ie. over the nrn half of cntl NC) will likely result and support strong radiational cooling into the lwr to mid 30s; and as such, a Frost Advisory has been issued for those areas. Local policy is for issuance of Frost Advisory when forecast temperatures are in the 33- 3 6F range under good radiational cooling conditions, regardless of RH. And while surface (2-meter) dewpoints will likely be in the 20s F over the Advisory areas tonight, with associated marginal RH values for frost development, ground-level RH should be higher and adequate for at least patchy frost development.

KEY MESSAGE 2... Weather parameters will be supportive of Increased Fire Danger today, with brief Red Flag conditions possible over the northern Piedmont and northern Coastal Plain.

At the base of a trough that will progress across ern Canada and the nrn Middle Atlantic and Northeast through tonight, an observed 55 kt speed max at ILX last evening is forecast to strengthen in excess of 60 kts across the lwr OH Valley and Virginias this morning, then weaken through the 40s kts while progressing across and offshore NC this afternoon and evening. Dry, wnwly flow in the lee of the srn Appalachians will favor the development of a deeply-mixed boundary layer up to 8-10 thousand ft AGL over cntl NC today, which will increase the likelihood of momentum transfer from the aforementioned 700 mb speed max. So while wly surface winds will already be strong and gusty surrounding the reinforcing, moisture-starved cold front noted above, into the 20s kts and strongest around 30 kts across the nrn Piedmont and nrn Coastal Plain, momentum transfer from stronger flow above may support a few higher peaks in the 30-40 kt range. Indeed, similar gusts were noted in proximity to the surface front and 700 mb speed max across IL, IN, and OH Sun afternoon, with some in excess of 40s kts where weak convection diabatically-accelerated gusts. Additionally, RH values are expected to decrease below 30% by noon and reach 15-25% minimums this afternoon, lowest over the srn Piedmont and Sandhills. While brief Red Flag conditions cannot be ruled out, especially where winds are forecast to be strongest and longest over the nrn Piedmont, and where RH will likely be around 25%, a relative lack of spatial overlap of highest winds (expected over the nrn Piedmont and nrn Coastal Plain) and lowest RH (over the srn Piedmont and Sandhills), should preclude widespread Red Flag conditions.

KEY MESSAGE 3... Temperatures will be above normal through the extended forecast period. Generally dry through the end of the work week, rain possible over the weekend.

Aloft, a s/w disturbance will slide sewd across the region Wed/Wed night as the longwave ridge progress ewd across the Midwest. The ridge will continue slowly ewd to over the East Coast where it will flatten out over the weekend as another s/w disturbance moves ewd across the region. At the surface, the high will sit off the Southeast/Carolina coast Wed/Wed night as a low tracks ewd across the nrn mid-Atlantic. A backdoor cold front may approach from the northeast Fri/Fri night, but should stay well northeast of the area as another low tracks sewd across the OH Valley and mid-Atlantic Fri and Sat. However, there are some timing differences amongst the available guidance and forecast uncertainty increases beyond Fri. The weather should largely remain dry through the week, with no significant rainfall expected. The best (but still uncertain) chance for rain will be over the weekend, however there could be a fleeting shower with the s/w passage Wed/Wed night, mainly across the northeast Piedmont/nrn Coastal Plain. Above normal temperatures expected through the extended forecast period.

AVIATION /06Z MONDAY THROUGH FRIDAY/

As of 230 AM Monday...

Shallow radiation fog and associated vertical visibility restrictions have developed over portions of ern NC this morning, where low level moisture characterized by surface dewpoints in the upr 30s to around mid 40s F has lingered. Some has developed as far west as the Coastal Plain, including at and especially just east of RWI, where variable MVFR to LIFR conditions will be likely through 12Z. VFR conditions are expected otherwise and elsewhere in cntl NC through tonight. Wswly surface winds will develop and strengthen with daytime heating and ahead of a reinforcing, moisture-starved cold front that will move swd and across cntl NC this afternoon through evening. Winds will gradually veer through wly and nwly with its passage, before lessening around sunset.

Outlook: Dry, continental air will favor continued, VFR conditions over cntl NC this week.

RAH WATCHES/WARNINGS/ADVISORIES

Frost Advisory from 2 AM to 8 AM EDT Tuesday for NCZ007>011-021>028- 038>043-076.