textproduct: Raleigh

WHAT HAS CHANGED

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KEY MESSAGES

As of 210 AM Wednesday...

1) Another above normal day today, followed by a mostly dry cold frontal passage on Thursday

1) The chilliest air of the season will arrive and support the first widespread frost/freeze risk this weekend, followed by continued below average temperatures and dry conditions that will persist through most of next week.

DISCUSSION

As of 210 AM Wednesday...

KEY MESSAGE 1... Another above normal day today, followed by a mostly dry cold frontal passage on Thursday

Ahead of Thursday's cold frontal passage, this afternoon should be another above normal day as pre-frontal southerly winds usher warm air into the region. Maximum temperatures this afternoon look to be similar to Tuesday's, with highs expected in the lower 70s north, to the mid/upper 70s south.

The cold front appears to move through the region Thursday afternoon into the evening. The chance for pre-frontal showers continues to trend down, with most models suggesting that most of the rain will dry up as the front crosses the mountains. Some very light rain may still be possible early Thursday morning ahead of the front, mainly in the Triad, and Thursday afternoon in the north. The REFS is suggesting that only up to trace amounts of rain are likely, with a lesser chance of 0.01-0.02 inches in a few locations.

KEY MESSAGE 2... The chilliest air of the season will arrive and support the first widespread frost/freeze risk this weekend, followed by continued below average temperatures and dry conditions that will persist through most of next week.

A mid/upr-level cyclone will develop along the srn ON/QC border and lwr Great Lakes by Fri, then progress slowly across and offshore the Northeast and Atlantic Canada through the weekend, during which time a series of shortwave perturbations will progress through the associated cyclonic flow across and offshore the Middle Atlantic.

At the surface, a cold front trailing a deepening/redeveloping clipper low will have moved across cntl NC on Thu, with following cold and dry air advection that will initiate a step down to below average temperatures for Friday. A following, reinforcing cold front will slow as it encounters the srn Appalachians Thu night-early Fri, then move across cntl NC later Fri-early Fri night. High pressure will follow and build from central Canada to the Middle Atlantic this weekend through early next week, where it will remain while gradually weakening/modifying through most of next week.

As temperatures aloft cool beneath the aforementioned cyclone and shortwave perturbations, a shallow layer of scant instability at the top of adeeply-mixed boundary layer may accompany the reinforcing cold front across cntl NC late Fri-early Fri night. Point forecast soundings indicate that buoyancy will coincide with a similarly shallow, 3-4 thousand ft deep saturated layer centered around 800 mb that will extend to near minus 8-10C, such that mixed phase clouds and precipitation production may result. While the aforementioned deeply-mixed boundary layer will cause most of that precipitation to sublimate/evaporate to virga, a few sprinkles will be possible especially over the ne Piedmont and nrn-cntl Coastal Plain in the several hours centered around sunset Fri.

That reinforcing front will otherwise usher even chillier and drier air across cntl NC for the weekend. Point forecast soundings continue to depict relatively steep low-level lapse rates in CAA behind it Fri night, which may include a potential for gustiness between 20-30 kts as a surge of nwly momentum in the residual mixed layer arrives during the evening. While related mixing should gradually weaken through Sat morning, it will hinder further cooling potential via radiative processes. As such, the coldest temperatures are expected when the CAA wanes and the MSL pressure gradient relaxes by Sun and Mon morning, when sub-freezing temperatures appear likely and a hard freeze will be possible especially in outlying/rural Piedmont locations.

While dewpoints and relative humidity values have been adjusted lower than both the previous forecast and the latest NBM (with its systematic high bias), forecast minimum RH for both Fri and Sat are mostly between 30-40%. Although those values may continue to trend lower and nearer IFD criteria (25-30%), the greater mitigating parameters for increased fire danger will be wind/gusts, given only teens kts momentum transfer indicated in the mixed layer both days, and cool temperatures in the 50s.

While some moderation will result as the high gradually weakens and airmass modifies through most of next week, its continued presence and influence will yield continued below average temperatures and multiple, consecutive mornings of frost/freeze conditions.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

As of 1245 AM Wednesday...

VFR conditions are generally expected over the 24 hour TAF period. There is another very low, but non-zero chance of some shallow, patchy fog overnight in the south and east. If restrictions do occur the best chance would be at FAY and RWI. Otherwise, just some upper level clouds are expected through the day, with light, generally southerly winds.

Outlook: VFR conditions should mainly prevail through the weekend. There is a low chance of rain Thursday and Friday, but it appears to only be light, with no restrictions expected. Gusty NW to N winds will still be possible Friday and Saturday.

RAH WATCHES/WARNINGS/ADVISORIES

None.