textproduct: Raleigh

WHAT HAS CHANGED

As of 320 AM Saturday...

* Forecast liquid equivalent precipitation amounts for tonight and Sunday have increased. * Locations in which a coating of snow is expected have been reduced and are focused across the northern Piedmont. * The probabilities and risk of a higher end/worst case event with snow totals of 1 or 2 inches have decreased greatly.

KEY MESSAGES

As of 320 AM Saturday...

1) A decaying band of precipitation moving from the NC mountains into the Foothills and western Piedmont could bring some spotty light rain and a few sleet pellets to the Triad around daybreak this morning.

2) A strong upper-level disturbance will move across the region late tonight and Sunday producing a shield of precipitation across central NC. The rain will mix with snow at times, mainly across northern areas, and then change to all snow before ending from west to east during the afternoon. A slushy coating of snow is possible, mainly on elevated surface with the greatest chance across the northern and northwestern Piedmont.

3) A modified, Arctic cold air mass is expected early-mid next week, with temperatures in the teens and 10 to 15 degrees below normal for consecutive mornings.

DISCUSSION

As of 320 AM Saturday...

KEY MESSAGE 1... A decaying band of precipitation moving from the NC mountains into the Foothills and western Piedmont could bring some spotty light rain and a few sleet pellets to the Triad around daybreak this morning.

The precipitation band across the NC mountains is expected to continue to weaken and diminish as it moves into the western Piedmont and Triad area around daybreak. Any precipitation that falls in the Triad will be light and short lived. With a dry airmass in place, a few sleet pellets may be observed but no accumulation is expected. Temperatures in the Triad will range near or just above freezing early this morning and warm into the mid 30s by mid morning. Dry conditions are expected this morning across the remainder of central NC.

KEY MESSAGE 2...A strong upper-level disturbance will move across the region late tonight and Sunday producing a shield of precipitation across central NC. The rain will mix with snow at times, mainly across northern areas, and then change to all snow before ending from west to east during the afternoon. A slushy coating of snow is possible, mainly on elevated surface with the greatest chance across the northern and northwestern Piedmont.

A 100+ kt jet at 500 mb across Montana and Wyoming early this morning will dive south and aid in carving out a vigorous upper- level trough extending from the western OH Valley into the lower MS Valley late tonight. The trough will advance east across the southern Appalachians around midday Sunday and then become negatively tilted as it reaches the southeast coast by Sunday evening. A strong southwesterly jet at 250 mb will develop across the eastern U.S. with a 175+ kt jet core developing across the mid Atlantic early Sunday morning. Moisture will stream north along the southeast coast on Sunday with PW values exceeding 135% of normal. Global and mesoscale models depict this scenario but differences in the trough configuration and the availability of cold air are driving critical differences in the potential for accumulating snow.

We expect patches of light rain to develop across the Piedmont of GA/SC/NC during the late evening and grow into a southwest to northeast oriented precipitation shield after midnight and toward daybreak. Given the jet configuration, a sharp cut-off to the precipitation shield with little to now precipitation observed to the west is expected with this line stretching from Charlotte northeast to the Triad. The precipitation shield will translate eastward on Sunday with precip ending in the Triad around midday, ending in the Triangle toward mid afternoon and the Coastal Plain toward late afternoon. All of the precipitation should have exited by dinner time.

Liquid equivalent precipitation amounts have generally trended up during the recent cycles and with more mesoscale models capturing the event, a few outliers are showing some notably higher amounts. WPC guidance and ensembles of various modeling systems generally provide a consensus 0.05-0.10 inches in the Triad, 0.2 to 0.3 inches across the rest of the Piedmont including the Triangle, and 0.3 to 0.5 across the Coastal Plain and Sandhills.

Most of the guidance notes that the mid and upper levels are sufficiently saturated and cold enough to become glaciated and generate snow aloft. A key difference lies in how warm the boundary layer temperatures will be. So it will be snowing across much of the area just a few thousand feet above the surface but the mild boundary layer will melt much of the snow. As noted in previous discussions, the cold air high is not in a favored location which raises doubts about the availability of sufficiently cold and dry low-level air for snow. Another concern is how warm it will get today with cloud cover and a southerly wind this evening keep temperatures mild, another inauspicious element ahead of any potential snow event.

For the snow lovers, the NAM provides some hope as it holds onto the stable cold airmass across the western and northern Piedmont and provides temperatures in the mid 30s ahead of the arrival of precipitation. In addition, it is very generous with the precipitation amounts, nearly double the consensus amounts noted above, which provides enough diabatic cooling from melting snow to get temps down to 33 and allow for more generous accumulations.

The most likely scenario with this event is for patchy light rain to develop across southern NC late Saturday evening and then expand northeast overnight with a sharp edge/cut off to the west. Temperatures will likely be in the upper 30s and lower 40s as the precipitation starts and then slowly cool as colder air filters in from the northwest and diabatic cooling from melting snow cools the boundary layer. With temperatures starting off colder in the Triad, the rain will mix with wet snow toward daybreak. During the early and mid morning, mixed rain and snow will be falling across the Triad with more widespread rain to the east which could mix with a little wet snow near the VA border and I-85 region. Surface temperatures will gradually cool as colder air filters in from the northwest as well as diabatic cooling from melting snow. The mixed rain and snow in the Triad will likely end as a brief period of wet snow during the late morning and midday hours. The transition to mixed rain and snow and eventually mostly snow will shift east reaching the Triangle and VA border counties during the early to mid afternoon. Still think it is doubtful that much if any snow will be observed in the Sandhills.

While much of the Piedmont and VA border counties should observe some mixed rain and snow or even a change over to mostly snow for a time, surface temperatures just above freezing and a daytime event should limit the accumulation efficiency. But a coating of snow, mainly on elevated and grassy locations seems possible/reasonable north of U.S. route 64 with a few spots hitting jack pot amounts of a half inch along the VA border and extending into the I-85 corridor east of the Triad.

All of the precipitation should wrap up by early Sunday evening. Temperatures should fall below freezing by dinner time Sunday and then crash into the lower 20s which may result in some refreezing and icy patches on roadways Sunday night. -Blaes

KEY MESSAGE 3...A modified, Arctic cold air mass is expected early-mid next week, with temperatures in the teens and 10 to 15 degrees below normal for consecutive mornings.

Downstream of a highly amplified ridge over the ern Pacific and AK, the latter where 500 mb standardized height anomalies of 3-4 sigma are forecast, a persistent polar vortex over Hudson Bay will anchor a broad trough over the rest of cntl and ern NOAM. A series of shortwave perturbations will navigate the associated cyclonic flow and amplify most sharply from the Northwest Territories to the Middle Atlantic through Tue, with relaxation of the trough and increasing interaction from the srn stream mid-late week.

At the surface, a clipper type low, one related to one of the aforementioned shortwave perturbations in highly amplified, nwly flow aloft, will sweep an Arctic cold front across cntl NC Mon night. A following Arctic high, initially ~1045 mb over the Northwest Territories, will weaken about 10 mb as it settles across the Middle Atlantic through mid-week. Modification of the associated airmass will occur most markedly as it progresses beyond snow cover surrounding the Great Lakes, from the nrn Plains and upr MS Valley to the interior Middle Atlantic and Northeast. The result for cntl NC will be an unseasonably cold Tue, with highs about 15 F below average and mostly in the 30s (to around 40 across srn counties), flanked by a pair of mornings with temperatures mostly in the teens.

AVIATION /08Z SATURDAY THROUGH WEDNESDAY/

As of 145 AM Saturday...

Moisture transport and lift, focused along the axis of a strong low- level jet, will result in a good chance of MVFR ceilings over the NC Piedmont later this morning, especially at INT/GSO. Very light rain, with minimal to no visibility restrictions, will also probably accompany the MVFR cloud band. Otherwise, the aforementioned low- level jet will favor low-level wind shear and/or mechanical turbulence until surface winds strengthen, and the nocturnal temperature inversion breaks, with daytime heating between 14-16Z. Sswly winds may then gust into the low-mid 20s kts at times through the afternoon, then substantially lessen/diminish around sunset.

Outlook: A precipitation band, and associated flight restrictions, will likely blossom along and equatorward of an intense upr-level jet streak over the Southeast tonight, including over cntl NC. The precipitation band will then gradually edge and pivot ewd and out of cntl NC through Sun-afternoon-evening. While a cold rain will be the predominant precipitation type, a mixing with or changeover to snow at times will be possible within heavier intensity portions of the band on Sunday, most likely at RDU, and near and east of GSO.

RAH WATCHES/WARNINGS/ADVISORIES

None.