textproduct: Raleigh

WHAT HAS CHANGED

As of 320 AM Sunday...

* Minor adjustments to precipitation amounts today. Confidence has increased that areas roughly along and southeast of Hwy 1 will see storm total liquid precip amounts of 0.50-0.75", tapering down to a tenth to quarter inch in the Triad. * The most likely snow amounts remain at a trace to a half inch, over all but the southern row or two of central NC counties; the reasonable worst-case/higher-end scenario is slightly higher, at 1.0-2.0" with isolated 3" amounts, and these low-probability higher-end totals have shifted slightly east. * There is a small possibility of a strip of higher snow totals (0.5- 1.5") over portions of the Sandhills into the far W Coastal Plain just E of the Triangle region, but confidence in this is very low.

KEY MESSAGES

As of 320 AM Sunday...

1) A strong storm system will move through the region today, bringing widespread rain, heaviest in the SE half of the forecast area. This rain will likely mix with or change over to a period of wet snow before ending, with light accumulations possible, mainly over the northern Piedmont.

2) Cold arctic air expected early next week. Temperatures moderate through mid to late next week.

3) Next good chance for precip is next weekend.

DISCUSSION

As of 320 AM Sunday...

KEY MESSAGE 1... A strong storm system will move through the region today, bringing widespread rain, heaviest in the SE half of the forecast area. This rain will likely mix with or change over to a period of wet snow before ending, with light accumulations possible, mainly over the northern Piedmont.

Overview: A deep positively-tilted mid-upper shortwave trough noted on current WV imagery moving through Louisiana will swing E then NE today, moving through the Carolinas with a slight negative tilt during the afternoon hours. This tilting will cause the upper jetting to the E of this shortwave to accelerate, resulting in a focused upper level divergence that will shift through the central Carolinas just ahead of the mid level DPVA, with peak forcing for ascent occurring over our area from very late morning through the afternoon. In the low levels, colder air will spill in gradually from the NW as the first in a series of fronts shifts through. Weak surface low pressure centers developing just off the Southeast US coast will strengthen slowly today while congealing into one or two lows, but they will remain rather flat and somewhat weak as they push well to our ENE and offshore later today.

Precip amounts: Models are in good agreement on the timing and amounts, with lesser totals in our NW and the heavier totals over the E Sandhills through the Coastal Plain/I-95 corridor, esp in the far SE CWA. Isolated totals near 1" liquid remain possible, but overall peak rainfall should be around 0.75".

Precip type: While the coldest surface air will hold W of the mountains through today, CAA just aloft will ensure that the entire column from about 950 mb up through the mid levels will cool to be below freezing as the column saturates with increasing forcing for ascent amidst above normal PW (highest SE of Raleigh). We're likely to see the last few hours of precip as a mixture with or changeover to wet snow, likely lasting for 1-3 hours in any given location. (There is no threat of freezing rain or icing.) The key will be the surface (or near-surface) wet bulb temps, which are expected to drop as cooler air arrives but hover near or just above 32F through the end of the precip. A rain/snow mixture would result in minimal to no accumulation as any snow melts as it falls, however sufficient concentrated dynamic lift and subsequent cooling to produce a period of full changeover to all snow remains possible, which would cause a dusting to a half inch of accumulation, mainly on grassy surfaces, and perhaps even isolated snow islands of higher totals.

Alternate scenarios: A scenario of almost no accumulation would happen if the surface temps simply stay too warm through the event, resulting in a complete melting of any snow aloft as it reaches the ground. For the higher-end scenario, very strong lift and intense dynamic cooling in conjunction with cooler-than-expected surface air (even by just a couple of degrees, making this quite tricky) could lead to a longer duration of mostly snow, occasionally moderate in intensity. In particular, if the surface low just off the coast deepens more than expected with sloping fgen just aloft, bands of heavier totals may occur, including in our interior southeast from the Sandhills up through the W Coastal Plain. While this has a low probability of happening overall, we'll be closely monitoring observations upstream over GA and SC to see if such banding comes to fruition.

Temps: 24-hr highs today in the mid 30s to lower 40s are likely to occur earlier in the day than the typical diurnal peak, with readings steady then slipping during the afternoon hours.

Black ice potential tonight: With temps areawide likely to be below freezing by late evening and lows tonight in the upper teens to mid 20s, any puddles or snow runoff will freeze, and lingering wet spots on roads may turn into patchy black ice. A Winter Weather Advisory for Black Ice may be needed late today for tonight.

KEY MESSAGE 2... Cold arctic air expected early next week. Temperatures moderate through mid to late next week.

Mid/upper troughing will persist on Monday, while swly sfc flow allows temps to moderate a bit into the mid to upper 40s. A secondary upper trough will then amplify and push across the eastern US Monday into Tuesday. An associated dry reinforcing cold front will push across central NC Monday night. This pattern will promote well below normal highs Tuesday in the upper 30s. Overnight lows Tuesday night into Wednesday will dip into the lower to mid teens.

The cold sfc high/air mass will shift offshore mid to late week which should promote some moderation to nearer normal temps Wednesday/Thursday (highs in the mid to upper 40s Wed; upper 40s/lower 50s Thursday). A weak short-wave will move across the southeast on Thursday, but ensembles continue to trend drier.

KEY MESSAGE 3... Next good chance for precip is next weekend.

The next chance for any appreciable precipitation will likely be next weekend ahead of another potentially strong cold front. There still a fair amount of uncertainty with this next system at this time, given timing, track and strength differences (bears watching).

AVIATION /12Z SUNDAY THROUGH THURSDAY/

As of 625 AM Sunday...

As a storm system moves into the area, MVFR conditions are likely to develop at all terminals prior to 14z, with areas of rain increasing in coverage and becoming more steady. Then, by around 14z, central NC terminals are expected to drop to IFR, and briefly LIFR. These conditions will last until ~18z-21z at INT/GSO and until 21z-01z at RDU/RWI/FAY, after which time a cold front sweeping in from the west will clear out the clouds from NW to SE, resulting in a trend back to VFR. The areas of rain are expected to be mixed or briefly change to mostly wet snow for a few hours, mainly midday/early afternoon at INT/GSO, and mid to late afternoon at RDU/RWI, with FAY likely seeing all rain or just a brief rain/wet snow mixture in the late afternoon before precip ends. Vsbys will periodically drop to MVFR and IFR within the rain/snow mixture. There is a chance for patchy shallow fog in the SE (including FAY) 07z-12z early Mon morning. Surface winds will be light and variable, mainly from the N or NW, for much of the day, then become uniformly from the NW in the afternoon at 5-10 kts, before shifting to more W after nightfall under 5 kts.

Looking beyond 12z Mon, dry/VFR conditions are likely to hold through Thu, although a period of clouds and slight rain chances are possible Thu.

RAH WATCHES/WARNINGS/ADVISORIES

None.