textproduct: Raleigh

WHAT HAS CHANGED

As of 315 AM Friday...

Refined snow accumulation forecast which notes a coating to half inch of snow possible on Sunday with the greatest amounts focused across the northern Piedmont.

KEY MESSAGES

As of 315 AM Friday...

1) Unseasonably cold this morning, with wind chill values in the teens and temperatures not much higher

2) Increased fire danger this afternoon mainly over the Piedmont and western Sandhills

3) A cold front along with a strong upper-level disturbance will move across the region late Saturday/Sunday and likely produce a period of precipitation including rain and snow which could result in a minor accumulation of snow.

DISCUSSION

As of 315 AM Friday...

KEY MESSAGE 1...Unseasonably cold this morning, with wind chill values in the teens and temperatures not much higher KEY

MESSAGE 2...Increased fire danger this afternoon mainly over the Piedmont and western Sandhills

Very strongly rising heights forecast to peak around 25 dam at 500 mb, and related deep subsidence and warming, will progress across and offshore the Middle and South Atlantic states today. They will do so between a powerful shortwave trough now pivoting across New England and the wrn Atlantic and a couple of upstream ones that will pivot through the ern Great Lakes and OH through mid MS Valleys.

At the surface, ~1019 mb cP high pressure now centered over the nrn Gulf --and with related surface dewpoints in the upr teens F as far south as n-cntl FL and single digits ones over cntl NC-- will migrate across and offshore the South Atlantic states today. The MSL pressure (and low-level height) gradient between the high, and a now ~995 mb cyclone that will weaken/fill while progressing from wrn ON to the Great Lakes, will cause sswly flow to strengthen throughout the South and Middle Atlantic states through this evening. That strengthening flow will manifest as 8-12 kt surface winds that will gust through the teens to around 20 kts over cntl NC this afternoon. These marginally strong, and at least occasionally gusty sswly winds, will combine with the aforementioned very low surface dewpoints that will yield min RH around 20 percent, with diurnal, diabatic warming into 40s. Those meteorological conditions, though relatively uniform throughout cntl NC, will create another day of increased fire danger that may be greatest over the wrn half of cntl NC per coordination with NCFS, where they indicated fuels were most critical (in districts 3, 10, 11, and 12).

KEY MESSAGE 3...A cold front along with a strong upper-level disturbance will move across the region late Saturday/Sunday and likely produce a period of precipitation including rain and snow which could result in a minor accumulation of snow.

A vigorous upper trough extending from the OH Valley into the Deep South late Saturday night will swing east and become negatively tilted as it reaches the southeast coast by Sunday evening. Various sets of NWP guidance all depict this feature but with differences in the timing and location of the trough configuration. The upper trough will produce strong forcing for ascent that will generate a southwest to northeast oriented precipitation shield that develops across AL/GA/SC on Saturday evening and then expands northeast across the Piedmont of NC Saturday night and continues into Sunday as the shield translates eastward during the day. Liquid equivalent precipitation amounts generally range around 0.05 inches in the Triad, 0.10 to 0.20 inches across the rest of the Piedmont, and 0.25 inches across the Coastal Plain and Sandhills. Most of the guidance is consistent with this scenario, but details including the amount and intensity of the precipitation and the extent of the available cold air are handled differently and result in differences in the amount of snow and how much, and where it will accumulate.

Most of the guidance notes that the mid and upper levels are sufficiently saturated and cold enough to become glaciated and generate snow. A key difference lies in how cold the boundary layer temperatures will be. A proven old rule for winter weather/snow forecasting is to forecast the cold first. The cold air high for this event will be centered over TX, no where near the favored Great Lakes/NY location for NC which raises doubts about the availability of sufficiently cold and dry air for snow. A key item to monitor is how mild temperatures get on Saturday. A southerly flow and highs in the 50s less than 24 hours before a potential NC snow event is another concern for accumulating snow as cloud cover and a light southerly flow on Saturday evening should retard a rapid drop in temperatures as precipitation breaks out. It is worth noting that as this events draws closer, mesoscale models are beginning to capture it and the NAM, which often handles the thermal profile well, notes that the stable cold airmass across western and northern Piedmont may be slow to warm on Saturday which would decrease the amount of cooling needed for snow on Sunday.

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 melting snow above the boundary layer cools the surface. With temperatures starting off colder in the Triad, the rain will mix with wet snow well after midnight. By daybreak, patches of light snow and rain 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 during the day as cooler and drier air filters in from the northwest as well as diabatic cooling from melting snow. This will favor an eastward transition to a snow and rain mix during the day and eventual transition to mostly snow before the precipitation tapers off from west to east during the afternoon.

Still have doubts about how efficient the cooling process will be but were pretty confident that many locations will see at least a period of conversational snow falling during the day Sunday with temperatures a few degrees about freezing. This scenario should limit accumulations to a coating up to a half inch of snow accumulation, with these accumulation favored across the northern Piedmont and in an arc near and a little east of the I-85 corridor. The accumulations will be most notable on elevated surfaces. The potential for accumulating snow will be much less across the Sandhills and southern Coastal Plain where the cold air will have a hard time arriving in time and most of the precipitation will just be rain. It's worth noting that if the colder air is more aggressive or if precipitation amounts or rates are greater, then accumulation amounts could be more impactful and range closer to an inch.

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 upper teens to lower 20s which may result in some refreezing and icy patches on roadways Sunday night. -Blaes

AVIATION /08Z FRIDAY THROUGH TUESDAY/

As of 120 AM Friday...

Continental Polar (very dry) high pressure will maintain VFR conditions in cntl NC through this evening, as it drifts across and offshore the South Atlantic states. Sswly flow, and surface winds around it, will strengthen and gust this afternoon into the teens to around 20 kts, before lessening at sunset. An intense low-level jet will develop across the srn Appalachians and overspread the srn Middle Atlantic/Carolinas tonight into Sat, during which time low- level wind shear and/or mechanical turbulence will become likely.

Outlook: A band of rain and MVFR ceilings will also overspread particularly the Piedmont of NC Sat morning, with VFR conditions and the development of a gusty swly breeze otherwise and elsewhere through the day Sat. Another area of precipitation, including some snow over the Piedmont, will overspread cntl NC late Sat night through Sun, when additional flight restrictions will be likely area- wide.

RAH WATCHES/WARNINGS/ADVISORIES

None.